_BACTERIA IN WATER_
In entering upon a consideration of such a common article of use as
water, we shall do well to describe in some detail the process by which
we systematically investigate the bacteriology of a water, or, indeed,
of any similar fluid suspected of bacterial pollution.
The collection of samples, though it appears simple enough, is
sometimes a difficult and responsible undertaking. Complicated
apparatus is rarely necessary, and fallacies will generally be
avoided by observing two directions. In the first place, the sample
should be chosen as representative as possible of the real substance
or conditions we wish to examine. Some authorities advise that it
is necessary to allow the tap to run for some minutes previous
to collecting the sample; but if we desire to examine for lead
chemically or for micro-organisms in the pipes biologically, then
such a proceeding would be injudicious.[11] Hence we must use common
sense in the selection and obtaining of a sample, following this one
guide, namely, to collect as nearly as possible a sample of the exact
water the quality of which it is desired to learn. In the second
place, we must observe strict bacteriological cleanliness in all our
manipulations. This means that we must use only sterilised vessels or
flasks for collecting the sample, and in the manipulation required
we must be extremely careful to avoid any pollution of air or any
addition to the organisms of the water from unsterilised apparatus.
A flask polluted in only the most infinitesimal degree will entirely
vitiate all results.
Accompanying the sample should be a more or less full statement of
its source. There can be no doubt that, in addition to a chemical and
bacteriological report of a water, there should also be made a careful
examination of its source. This may appear to take the bacteriologist
far afield, and in point of fact, as regards distance, this may be so.
But until he has seen for himself what "the gathering-ground" is like,
and from what sources come the feeding streams, he cannot judge the
water as fairly as he should be able to do. The configuration of the
gathering-ground, its subsoil, its geology, its rainfall, its relation
to the slopes which it drains, the nature of its surface, the course of
its feeders, and the absence or presence of cultivated areas, of roads,
of houses, of farms, of human traffic, of cattle and sheep--all these
points must be noted, and their influence, direct or indirect, upon the
water carefully borne in mind.
When the sample has been duly collected, sealed, and a label affixed
bearing the date, time, and conditions of collection and full
address, it should be transmitted with the least possible delay to
the laboratory. Frequently it is desirable to pack the bottles in a
small ice case for transit. On receipt of such a sample of water the
examination must be immediately proceeded with, in order to avoid, as
far as possible, the fallacies arising from the rapid multiplication
of germs. Even in almost pure water, at the ordinary temperature of a
room, Frankland found organisms multiplied as follows:--
No. of Germs
Hours. per cc.
0 1,073
6 6,028
24 7,262
48 48,100
Another series of observations revealed the same sort of rapid increase
of bacteria. On the date of collection the micro-organisms per cc. in
a deep-well water (in April) were seven. After one day's standing at
room temperature the number had reached twenty-one per cc. After three
days under the same conditions it was 495,000 per cc. At blood-heat the
increase would, of course, be much greater, as a higher temperature
is more favourable to multiplication. But this would depend upon the
degree of impurity in the water, a pure water _decreasing_ in number
on account of the exhaustion of the pabulum, whereas, for the first
few days at all events, an organically polluted water would show an
enormous increase in bacteria.
Furthermore, it is desirable to remember that organisms, in an ordinary
water, do not continue to increase indefinitely. There is a limit
to all things, even to numbers in bacteriology. Cramer, of Zurich,
examined the water of the Lake after it had been standing for different
periods, with the following results:--
Hours and Days of No. of Micro-organisms
Examination. per cc.[12]
0 hours 143
24 " 12,457
3 days 328,543
8 " 233,452
17 " 17,436
70 " 2,500
The writer's own experience is entirely in agreement with this
cessation of multiplication at or about the end of a week, and the
later decline.
_Method of Examination._ At the outset of a systematic study of a water
it is well to observe its physical characters. The colour, if any,
should be noted. Suspended matter and deposit may indicate organic
or inorganic pollution. If abundant or conspicuous, a microscopic
examination of the sediment may be made. The reaction, whether acid,
neutral, or alkaline, must be tested, and the exact temperature taken.
Any and every fact will help us, perhaps not so much to determine the
contents of the water as to interpret rightly the facts we deduce from
the entire examination.
[Illustration: LEVELLING APPARATUS FOR KOCH'S PLATE]
At the beginning of the bacteriological work the water should be
examined by means of the gelatine plate method. This consists in
drawing up into a fine sterilised pipette a small quantity of the water
and introducing it thereby into a test-tube of melted gelatine at a
temperature below 40° C.[13] It will depend upon the apparent quality
of the water as to the exact quantity introduced into the gelatine;
about .5 or .1 of a cubic centimetre is a common figure. The stopper
is then quickly replaced in the test-tube, and the contents gently
mixed more or less equally to distribute the one-tenth cubic centimetre
throughout the melted gelatine. A sterilised sheet of glass (4 inches
by 3) designated a _Koch's plate_ is now taken and placed upon the
stage of a levelling apparatus, which holds iced water in a glass jar
under the stage. The gelatine is now poured out over the glass plate,
and by means of a sterilised rod stroked into a thin, even film all
over the glass. It is then covered with a bell-jar and left at rest
to set. The level stage prevents the gelatine running over the edge
of the plate; the iced water under the stage expedites the setting
of the gelatine into a fixed film. When it is thus set the plate is
placed upon a small stand in a moist chamber, and the whole apparatus
removed to the room temperature incubator. A _moist chamber_ is a glass
dish, in which some filter paper, soaked with corrosive sublimate, is
inserted, and the dish covered with a bell-jar. By this means the risks
of pollution are minimized, and moisture maintained. In all cases at
least two plates must be prepared of the same sample of water, and it
is often advisable to make several. They may be made with different
media for different purposes, and with different quantities of water,
though the same method of procedure is adopted. In a highly polluted
water extremely small quantities would be taken, and, _vice versâ_, in
pure water a large quantity.
[Illustration: MOIST CHAMBER IN WHICH KOCH'S PLATES ARE INCUBATED]
When we come to discuss the relation of disease organisms to water,
particularly those causing typhoid fever, we shall learn that they
are both scarce and intermittent. This point has been dwelt upon
frequently by Dr. Klein, and it is clear that such a state of things
greatly enhances the difficulties in detecting such bacteria, and he
has proposed a simple procedure by which the difficulty of finding the
_Bacillus typhosus_ in a large body of water may be met.
[Illustration: HOT AIR STERILISER
For the Sterilization of Glass Apparatus, etc.]
One or two thousand cubic centimetres of the water under examination
are passed through a sterilised Berkefeld filter by means of siphon
action or an air-pump. The candle of the filter retains on its outer
surface all, or nearly all, the particulate matter contained in the
water. The matter thus retained on this outer surface is brushed by
means of a sterile brush into 10 or 20 cc. of sterilised water. Thus
we have all the organisms contained in two litres of the water reduced
into 10 cc. of water. From this, so to speak, concentrated emulsion of
the bacteria of the original water, phenol-gelatine plates or Eisner
plates (both acid media) may be readily made. In this way we not only
catch many bacteria which would evade us if we were content with the
examination merely of a few drops of the water, but we eliminate
by means of the acid those common water bacteria, like _Bacillus
fluorescens liquefaciens_, which so greatly confuse the issue.
[Illustration: HOT AIR STERILISER
For the Sterilizatin of Glass Apparatus, etc.]
In the course of two or three days the film of gelatine on the plate
becomes covered with _colonies_ of germs, and the next step is to
examine these quantitatively and qualitatively. We may here insert a
simple scheme by which this may be most fully and easily accomplished:--
1. _Naked-Eye Observation of the Colonies._ By this means at the
very outset certain facts may be obtained, viz., the size, elevation,
configuration, margin, colour, grouping, number, and kinds of colonies,
all of which facts are of importance, and assist in final diagnosis.
Moreover, in the case of gelatine plates (it is otherwise in agar)
one is able to observe whether or not there is present what is termed
_liquefaction of the gelatine_. Some organisms produce in their
development a peptonizing ferment which breaks down gelatine into a
fluid condition. Many have not this power, and hence the characteristic
is used as a diagnostic feature.
2. _Microscopic Examination of Colonies_, which confirms or corrects
that which has been observed by the naked eye. Fortunately some
micro-organisms when growing in colonies produce cultivation features
which are peculiar to themselves (especially is this so when growing
in test-tube cultures), and in the early stages of such growths a low
power of the microscope or magnifying glass facilitates observation.
3. _Make cover-glass preparations_: (_a_) unstained--"the hanging
drop"; (_b_) stained--single stains, like gentian-violet, methyl
blue, fuchsin, carbol fuchsin, etc.; double stains--Gram's method,
Ziehl-Neelsen's method, etc.
[Illustration: THE HANGING DROP]
This third part of the investigation is obviously to prepare specimens
for the microscope. "The hanging drop" is a simple plan for securing
the organisms for microscopic examination in a more or less natural
condition. A hollow ground slide, which is a slide with a shallow
depression in it, is taken, and a small ring of vaseline placed
round the edge of the depression. Upon the under side of a clean
cover-glass is placed a drop of pure water, and this is inoculated
with the smallest possible particle taken from one of the colonies
of the gelatine plate on the end of a sterilised platinum wire. The
cover-glass is then placed upon the ring of vaseline, and the drop
hangs into the space of the depression. Thus is obtained a view of the
organisms in a freely moving condition, if they happen to be motile
bacteria. As a matter of practice the hollow slide may be dispensed
with, and an ordinary slide used.
[Illustration: DRYING STAGE FOR FIXING FILMS]
With regard to staining, it will be undesirable here to dwell at length
upon the large number of methods which have been adopted. The "single
stain" may be shortly mentioned. It is as follows: A clean cover-glass
is taken (cleaned with nitric acid and alcohol, or bichromate of potash
and alcohol), and a drop of pure sterilised water placed upon it. This
is inoculated with the particle of a colony on the end of a platinum
needle, and a scum is produced. The film is now "fixed" by slowly
drying it over a flame. When the scum is thus dried, a drop of the
selected stain (say gentian-violet) is placed over the scum and allowed
to remain for varying periods: _sarcinæ_ about thirty seconds; for
many of the bacilli three or four minutes. It is then washed off with
clean water, dried, and mounted in Canada balsam. The organisms will
now appear under the microscope as violet in colour, and will thus be
clearly seen.
The "double staining" is adopted when we desire to stain the organisms
one colour and the tissue in which they are situated a contrast colour.
Some of the details of these methods are mentioned in the Appendix.
4. _Sub-culture._ The plate method was really introduced by Koch in
order to facilitate isolation of species. In a flask it is impossible
to isolate individual species, but when the growth is spread over a
comparatively large area, like a plate, it is possible to separate the
colonies, and this being done by means of a platinum wire, the colonies
may be replanted in fresh media; that is to say, a _sub-culture_ may be
made, each organism cultivated on its favourite soil, and its manner of
life closely watched. We have already mentioned the chief media which
are used in the laboratory, and in an investigation many of these would
be used, and thus _pure cultures_ would be obtained. Let us suppose
that a water contains six kinds of bacteria. On the plate these six
kinds would show themselves by their own peculiar growth. Each would
then be isolated and placed in a separate tube, on a favourite medium,
and at a suitable temperature. Thus each would be a _pure culture_;
_i. e._, one and only one, species would be present in each of the six
tubes. By this simple means an organism can be, we say, _cultivated_,
in the same sort of way as in floriculture. From day to day we can
observe the habits of each of our six species, and probably at an early
stage of their separated existences we should be able to diagnose
what species of bacteria we had found in the water. If not, further
microscopic examination could be made, and, if necessary, secondary or
tertiary sub-cultures.
5. _Inoculation of Animals._ It may be necessary to observe the action
of supposed pathogenic organisms upon animals. This is obviously a last
resource, and any abuse of such a process is strictly limited by law.
As a matter of fact, an immense amount of bacteriological investigation
can be carried on without inoculating animals; but, strictly speaking,
as regards many of the pathogenic bacteria, this test is the most
reliable of all. Nor would any responsible bacteriologist be justified
in certifying a water as healthy for consumption by a large community
if he was in doubt as to the disease-producing action of certain
contained organisms.
[Illustration: TYPES OF LIQUEFACTION OF GELATINE]
By working through some such scheme as the above we are able to detect
what quantity and species of organisms, saprophytic or parasitic, a
water or similar fluid contains. For, observe what information we
have gained. We have learned the form (whether bacillus, micrococcus,
or spirillum), size, consistence, motility, method of grouping, and
staining reactions of each micro-organism; the characters of its
culture, colour, composition, presence or absence of liquefication or
gas formation, its rate of growth, smell, or reaction; and lastly, when
necessary, the effect that it has upon living tissues. Here, then, are
ample data for arriving at a satisfactory conclusion respecting the
qualitative estimation of the suspected water.
As to to the quantitative examination, that is fulfilled by counting
the number of colonies which appear, say by the third and fourth day,
upon the gelatine plates. Each colony has arisen, it is assumed, from
one individual, so that if we count the colonies, though we do not
thereby know how many organisms we have upon our plate, we do know
approximately how many organisms there were when the plate was first
poured out, which are the figures we require, and which can at once
be multiplied and returned as so many organisms per cubic centimetre.
There is, unfortunately, at present no exact standard to which all
bacteriologists may refer.
Miquel and Crookshank have suggested standards which allow "very pure
water" to contain up to 100 micro-organisms per cc. Pure water must not
contain more than 1000, and water containing up to 100,000 bacteria
per cc. is contaminated with surface water or sewage. Macé gives the
following table:
Very pure water 0- 10 bacteria per cc.
Very good water 20- 100 " "
Good water 100- 200 " "
Passable (mediocre) water 200- 500 " "
Bad water 500- 1,000 " "
Very bad water 1,000-10,000 and over "
Koch first laid emphasis on the quantity of bacteria present as an
index of pollution, and whilst different authorities have all agreed
that there is a necessary quantitative limit, it has been so far
impossible to arrive at one settled standard of permissible impurity.
Besson adopts the standard suggested by Miquel, and, on the whole,
French bacteriologists follow suit. They also agree with him, generally
speaking, in not placing much emphasis upon the numerical estimation of
bacteria in water. In Germany and England it is the custom to adopt a
stricter limit. Koch in 1893 fixed 100 bacteria per cc. as the maximum
number of bacteria which should be present in a properly filtered
water. Hence the following has been recognised more or less as the
standard:
0- 100 bacteria per cc. =a good potable water,
100- 500 " " =a suspicious water.
500-1000 or more " =a water which should have
further filtration before
being used for drinking
purposes.
The personal view of the writer after some experience of water
examination would favour a standard of "under 500" being a potable
water, if the 500 were of a nature indicating neither sewage pollution
nor disease. Miquel holds that not more than ten different species of
bacteria should be present in a drinking water, and such is a useful
standard. The presence of rapidly liquefying bacteria _associated
with sewage or surface pollution_ would, even though present in fewer
numbers than a standard, condemn a water. Thus it will be seen that it
is impossible to judge alone by the numbers unless they are obviously
enormously high.
[Illustration: WOLFHÜGEL'S COUNTER]
When we are counting colonies upon a Koch's plate, _Wolfhügel's
counter_ may be used. This is a thin plate of glass a size larger than
Koch's plates, and upon it are scratched squares, each square being
divided into nine smaller squares. The Wolfhügel plate is superimposed
upon the Koch's plate, and the colonies counted in one little square or
set of squares and multiplied.
[Illustration: PETRI'S DISH]
By using flat, shallow, circular glass dishes, generally known as
_Petri's dishes_, instead of Koch's plates, much manipulation and time
is saved, and, on the whole, less risk of pollution occurs. Moreover,
these are easily carried about and transferred from place to place.
When counting colonies in a Petri's dish it is sufficient to divide the
circle into eight equal divisions, and counting the colonies in the
average divisions, multiply and reduce to the common denominator of one
cc. For example, if the colonies of the plate appear to be distributed
fairly uniformly we count those in one of the divisions. They reach,
we will suppose, the figure of 60; 60 × 8=480 micro-organisms in the
amount taken from the suspected water and added to the melted gelatine
from which the plate was made. This amount was .25 cc. Therefore we
estimate the number of micro-organisms in the suspected water as 60 ×
8=480 × 4= 1920 m.-o. per cc., which is over standard by about 1500.
A water might then be condemned upon its quantitative examination
alone or qualitative alone, or both. If the quantity were even that
of an artesian well, say 4-10 m.-o. per cc., but those four or ten
were all _Bacillus typhosus_, it would clearly be condemned on its
quality, though quantitatively it was an almost pure water. If, on the
contrary, the water contained 10,000 m.-o. per cc., and none of them
disease-producing, it would still be condemned on the ground that so
large a number of organisms indicated some kind of organic pollution
to supply pabulum for so many organisms to live in one cc. of the
water. It is not the number _per se_ which condemns. The large number
condemns because it indicates probable pollution with surface water or
sewage in order to supply pabulum for so many bacteria per cc.
It should always be remembered that a chemical report and a
bacteriological report should assist each other. The former is able
to tell us the quantity of salts and condition of the organic matter
present; the latter the number and quality of micro-organisms. Neither
can take the place of the other and, generally speaking, both are more
or less useless until we can learn, by inspection and investigation
of the source of the water, the origin of the organic matter or
contamination. Hence a water report should contain not only a record
of physical characters, of chemical constituents, and of the presence
or absence of micro-organisms, injurious and otherwise, but it should
also contain information obtained by personal investigation of the
source. Only thus can a reasonable opinion be expected. Moreover, it
is generally only possible to form an accurate judgment of a water
from watching its history, that is, not from one examination only,
but from a series of observations. A water yielding a steady standard
of bacterial contents is a much more satisfactory water, from every
point of view, than one which is unstable, one month possessing 500
bacteria per cc. and another month 5000. It is obvious that rainfall
and drought, soil and trade effluents, will have their influence in
materially affecting the bacterial condition of a water.
[Illustration: BERKEFELD FILTER
In Position for Filtration of Water to be Examined.]
It is perhaps scarcely necessary to add that we have not in the
above account of the examination of water included all, or nearly
all, the various methods adopted for acquiring a knowledge of the
bacterial contents of the water. Many of these are of too detailed
and technical a nature to enter into here. Three points, however, we
may touch upon. In the first place, as we have said, the particulate
matter out of a large body of water should be concentrated in a small
quantity. Accordingly it has become the custom to pass 2000 or 3000
cc. of the suspected water through a Berkefeld filter. When this
has been accomplished, by means of a sterile brush the particulate
matter on the candle of the filter is brushed off into 10 or 15 cc. of
sterilised water. This simple arrangement is analogous to the use of
gravity or centrifugal methods of securing the solid matter in milk.
The smaller quantity of water is then readily examined, and scanty
germs more readily detected. A second point elaborating the scheme
of water examination is the choice of media for sub-culturing. Mere
examination on gelatine is not sufficient. Even in making the primary
plate cultivations it is well to vary the media--agar, carbol-gelatine,
Elsner, etc. But when colonies have appeared upon these plates it is
important to sub-culture with accuracy and good judgment upon all or
any media--gelatine, agar, broth, potato, milk, blood serum, glucose
agar, glycerine agar, etc.--that will reveal the real characters of the
bacteria present. A method proposed by Professor Sheridan Delépine is
to place some of the suspected water in sterilised test-tubes without
further treatment, and incubate at 37° C. for twelve or eighteen hours,
and then plate out and estimate the number of bacteria as in the
ordinary course. "In polluted water, containing an excess of organic
matter," he says, "an extremely rapid multiplication of bacteria is
observed. In unpolluted water, containing only water bacteria and a
very small amount of organic matter, very little or no multiplication
takes place, and the growth of the water bacteria liquefying gelatine
is checked to a remarkable extent." Thirdly, by none of these methods
should we be able to isolate anaërobic bacteria, and to furnish a
complete report these also must receive careful attention.
[Illustration: APPARATUS FOR FILTERING WATER TO FACILITATE ITS
BACTERIOLOGICAL EXAMINATION]
_The Bacteriology of Water._ In many natural waters there will be found
varied contents even in regard to flora alone: _algæ_, _diatoms_,
_spirogyræ_, _desmids_, and all sorts of vegetable detritus. Many of
these organisms are held responsible for divers disagreeable tastes
and odours. The colour of a water may also be due to similar causes.
Dr. Garrett, of Cheltenham, has recorded the occurrence of redness
of water owing to a growth of _Crenothrix polyspora_, and many other
similar cases make it evident that not unfrequently great changes may
be produced in water by contained microscopic vegetation.
With the exception of water from springs and deep wells, all unfiltered
natural waters contain numbers of bacteria. The actual number roughly
depends upon the amount of organic pabulum present, and upon certain
physical conditions of the water. As we have already seen, bacteria
multiply with enormous rapidity. In some species multiplication does
not appear to depend on the presence of much organic matter, and,
indeed, some can live and multiply in sterilised water: _Micrococcus
aquatilis_ and _Bacillus erythrosporus_. Again, others depend not
upon the quantity of organic matter, but upon its quality. And
frequently in a water of a high degree of organic pollution it will
be found that bacteria have been restrained in their development
by the competition of other species monopolising the pabulum.
Probably at least one hundred different species of non-pathogenic
organisms have been isolated from water. Some species are constantly
occurring, and are present in almost all natural waters. Amongst
such are _B. liquefaciens_, _B. fluorescens liq._, _B. fluorescens
non-liquefaciens_, _B. termo_, _B. aquatilis_, _B. ubiquitus_, and
not a few _micrococci_, etc. Percy Frankland[14] collected water from
various quarters at various times and seasons, and some of his results
may here be added:
RIVER THAMES WATER COLLECTED AT HAMPTON
Number of Micro-organisms Obtained from 1 cc. of Water.
----------------------+--------------+--------------+--------------
MONTH. | +1886.+ | +1887.+ | +1888.+
----------------------+--------------+--------------+--------------
| | |
January | 45,000 | 30,800 | 92,000
February | 15,800 | 6,700 | 40,000
March | 11,415 | 30,900 | 66,000
April | 12,250 | 52,100 | 13,000
May | 4,800 | 2,100 | 1,900
June | 8,300 | 2,200 | 3,500
July | 3,000 | 2,500 | 1,070
August | 6,100 | 7,200 | 3,000
September | 8,400 | 16,700 | 1,740
October | 8,600 | 6,700 | 1,130
November | 56,000 | 81,000 | 11,700
December | 63,000 | 19,000 | 10,600
----------------------+--------------+--------------+--------------
Again, another example:
RIVER LEA WATER COLLECTED AT CHINGFORD
Number of Micro-organisms Obtained from 1 cc. of Water.
----------------------+--------------+--------------+--------------
MONTH. | +1886.+ | +1887.+ | +1888.+
----------------------+--------------+--------------+--------------
| | |
January | 39,300 | 37,700 | 31,000
February | 20,600 | 7,900 | 26,000
March | 9,025 | 24,000 | 63,000
April | 7,300 | 1,330 | 84,000
May | 2,950 | 2,200 | 1,124
June | 4,700 | 12,200 | 7,000
July | 5,400 | 12,300 | 2,190
August | 4,300 | 5,300 | 2,000
September | 3,700 | 9,200 | 1,670
October | 6,400 | 7,600 | 2,310
November | 12,700 | 27,000 | 57,500
December | 121,000 | 11,000 | 4,400
----------------------+--------------+--------------+--------------
"During the summer months these waters are purest as regards
micro-organisms, this being due to the fact that during dry weather
these rivers are mainly composed of spring water, whilst at other
seasons they receive the washings of much cultivated land."--Frankland.
Prausnitz has shown that water differs, as would be expected, according
to the locality in the stream at which examination is made. His
investigations were made from the river Isar before and after it
receives the drainage of Munich:
No. of Colonies
per cc.
Above Munich 531
Near entrance of principal sewer 227,369
13 kilometres from Munich 9,111
22 " " " 4,796
33 " " " 2,378
Professor Percy Frankland also points out how the river Dee affords
another example, even more perfect, of pollution and restoration
repeated several times until the river becomes almost bacterially pure.
We cannot here enter more fully into the many conditions of a water
which affect its bacterial content than to say that it varies
considerably with its source, at different seasons, and under different
climatic conditions. An enormous increase will occur if the sediment is
disturbed, and conversely sedimentation and subsidence during storage
will greatly diminish the numbers of bacteria. Sand filtration, plus a
"nitrifying layer," will remove more than 90 per cent. of the bacteria.
Sea-water contains comparatively few bacteria, and the deeper the water
and the farther it is from shore so much less will be the bacterial
pollution.
THE CHIEF DISEASE ORGANISMS FOUND IN WATER
We will now consider several of the more important disease-producing
bacteria found in water.
+Bacillus Typhosus+ (Eberth-Gaffky). In 1880-81 Eberth announced the
discovery of this bacillus in cases of clinical enteric fever. In 1884
it was first cultivated outside the body by Gaffky. Since then other
organisms have been held responsible for the causation of enteric (or
typhoid) fever. In 1885 the _B. coli communis_ was recognised, and it
has been a matter of great debate amongst bacteriologists as to how far
these two organisms are the same species, and the typhoid germ merely
a higher evolution of the _B. coli_. The differentiating signs between
them will be referred to shortly. Bacteriologists generally regard the
Eberth-Gaffky bacillus as the specific cause of the disease, though
complete proof is still wanting.
[Illustration: BACTERIA OF TYPHOID FEVER]
_Microscopic Characters_ (in pure culture). Rods, 2-4 µ long, .5 µ
broad, having round ends. Sometimes threads are observable, being 10 µ
in length. In the field of the microscope the bacilli differ in length
from each other, but are all the same thickness approximately. Round
and oval cells constantly occur even in pure culture, and many of
these shorter forms of typhoid are identical in morphology with some
of the many forms of _Bacillus coli_. There are no spores. Motility is
marked; indeed, in young culture it is the most active pathogenic germ
we know. The small forms dart about with extreme rapidity; the longer
forms move in a vermicular manner. Its powers of movement are due to
some five to twenty flagella of varying length, some of them being much
longer than the bacillus itself, though, owing to the swelling of the
bacillus under flagellum-staining methods, it is difficult to gauge
this exactly. The flagella are terminal and lateral, and are elastic
and wavy. Their active contraction produces an evident current in the
field of the microscope.
_Cultures._ This organism may be isolated from ulcerated Peyer's
patches in the intestine, from the liver, the spleen, and the
mesenteric glands. Owing to the mixture of bacteria found elsewhere, it
is generally best to isolate it from the spleen. The whole spleen is
removed, and a portion of its capsule seared with a hot iron to destroy
superficial organisms. With a sterilised knife a small cut is made
into the substance of the organ, and by means of a sterilised platinum
wire a little of the pulp is removed and traced over the surface of
agar. _Agar_ reveals a growth in about twenty-four hours at 37° C.,
which is the favourite temperature. A greyish, moist, irregular growth
appears, but it is invariably attached to the track of the inoculating
needle. On _gelatine_ the growth is much the same, but its irregular
edge is, if anything, more apparent. There is no liquefaction and no
gas formation. On plates of gelatine the colonies appear large and
spreading, with jagged edges. The whole colony appears raised and
almost limpet-shaped, with delicate lines passing over its surface.
There is an appearance under a low power of transparent iridescence.
The growth on _potato_ is termed "invisible," and is of the nature of
a potato-coloured pellicle, which looks moist, and may at a late stage
become a light brown in colour, particularly if the potato is alkaline.
_Milk_ is a favourable medium, and is rendered slightly acid. No
coagulation takes place. _Broth_ is rendered turbid.
_Micro-pathology._ Typhoid fever is an infiltration and coagulation,
necrosis, and ulceration of the Peyer's patches in the small intestine
of man. The mesenteric glands show the same features, except that
there is no ulceration. The spleen is enlarged, and contains the germs
of the disease in almost a pure culture. The bacillus is present
in the intestinal contents and excreta, particularly so when the
Peyer's glands have commenced ulceration. In the blood of the general
circulation the bacillus is not demonstrable, except in very rare
instances. Typhoid fever is not, like anthrax, a blood disease.
COMPARATIVE FEATURES OF BACILLUS TYPHOSUS AND B. COLI
B. TYPHOSUS B. COLI
_Morphology:_ Cylindrical bacillus Shorter, thicker; filaments rare.
2.4 µ, unequal lengths; some
filaments.
_Flagella:_ Long, wavy, spiral, Shorter, stiffer, fewer; movement
and very numerous; movement very less active.
active.
_On Gelatine and Agar:_ Angular, Even edge, homogeneous; much
irregular, raised colonies; slow larger, quicker growth, and less
growth; translucent; medium translucent than _B. typhosus_;
remains clear. medium becomes turbid or
coloured.
_In Gelatine:_ In ordinary Under the same circumstances
gelatine and in sugar gelatine abundant gas is produced.
no gas is produced.
_Milk:_ Not curdled by the Milk is coagulated (within three
bacillus. days).
_Indol:_ The production of indol Indol is present.
in ordinary broth is _nil_.
_Potato:_ The "invisible growth," Thick, yellow growth.
if potato is acid.
_Lactose:_ Fermentation very slight. Fermentation marked.
_25 per cent. Gelatine at 37° C.:_ Gelatine remains limpid and clear,
Strongly and uniformly turbid but possesses thick pellicle.
(Klein).
_Elsner's Iodised Potato Gelatine:_ Very fast growth; larger, brown,
Slow growth; small, very less transparent colonies.
transparent colonies.
_Widal's Test:_ Bacilli become Bacilli remain actively motile.
motionless and clumped together
when suspended in a drop of blood
serum from a typhoid patient.
_Broth containing 0.3 per cent. Phenol Grows well.
or Formalin_ (1:7000): No growth.
_Thermal Death Point:_ 62° C. for five 66° C. for five minutes (Klein).
minutes (Klein).
_Vitality in Water and Sewage:_ The _B. coli_ retains for a much longer
Typhoid bacillus soon ceases to time its vitality and power of
multiply and readily dies (Klein). self-multiplication (Klein).
The two species, _Bacillus typhosus_ and _B. coli_, agree in possessing
the following characters: no spores, no liquefaction of gelatine; both
grow well on phenolated gelatine, and in Parietti's broth; both act
similarly upon animals, though typhoid fever is not a specific disease
of animals.
The _Bacillus typhosus_, though a somewhat susceptible bacillus,
can when dried retain its vitality for weeks. In sewage it is very
difficult indeed to detect, and is soon crowded out. Dr. Andrews and
Mr. Parry Laws, in their bacterial researches into sewage for the
London County Council,[15] found that when they examined specially
infected typhoid sewage it was only with extreme difficulty they
isolated Eberth's bacillus. In ordinary sewage it is clear such
difficulty would be greatly enhanced.
[Illustration: B. COLI COMMUNIS]
We have pointed out elsewhere the relation between soil and typhoid.
In water, even though we know it is a vehicle of the disease,
the _Bacillus typhosus_ has been only very rarely detected. The
difficulties in separating the bacillus from waters (like that at
Maidstone, for example), which appear definitely to have been the
vehicle of the disease, are manifold. To begin with, the enormous
dilution must be borne in mind, a comparatively small amount of
contamination being introduced into large quantities of water.
Secondly, the huge group of the _B. coli_ species considerably
complicates the issues, for it copiously accompanies the typhoid, and
is always able to outgrow it. Further, we must bear in mind a point
that is systematically neglected, namely, that the bacteriological
examination of a water which is suspected of having conveyed the
disease is from a variety of circumstances conducted too late to detect
the causal bacteria. The incubation period of typhoid we may take at
fourteen days. Let us suppose a town water supply is polluted with some
typhoid excreta on the 1st of January. Until the 14th of January there
may be no knowledge whatever of the state of affairs. Two or three
days are required for notification of cases. Several more days elapse
generally before bacteriological evidence is demanded. Hence arises the
anomalous position of the bacteriologist who sets to work to examine a
water suspected of typhoid pollution three weeks previously. There can
be no doubt that these difficulties are very real ones. The solution
to the problem will be found in Dr. Klein's dictum that "a water in
which sewage organisms have been detected in large numbers should be
regarded with suspicion"[16] as the vehicle of typhoid, even though no
typhoid bacilli were discoverable. The chief of these sewage bacteria
are believed to be _Proteus vulgaris_, _B. coli_, _P. zenkeri_, and
_B. enteritidis_, and they are all nearly related to _B. typhosus_.
The presence of the _B. coli_ in limited numbers is not sufficient to
indicate sewage pollution, seeing that it is so widely distributed. But
in large numbers, and in company with the other named species, it is
almost certain evidence of sewage-polluted water.
It may occur to the general reader that, as the typhoid bacillus is not
extremely rare, drinking water may frequently act as a vehicle to carry
the disease to man. But, to appreciate the position, it is desirable
to bear in mind the following facts: the typhoid bacillus is only
found in the human excrement of patients suffering from the disease;
it is short-lived; in ordinary waters there exist organisms which can
exert an influence in diminishing its vitality; exposure to direct
sunlight destroys it; and it has a tendency to be carried down-stream,
or in still waters settle at the bottom by subsidence. Even when all
the conditions are fulfilled, it must not be forgotten that a certain
definite dose of the bacillus is required to be taken, and that by a
susceptible person. Into these latter questions of how bacteria produce
disease we shall have an opportunity of inquiring at a later stage.
We must now mention several of the special media and tests used in the
separation of _Bacillus typhosus_ and _B. coli_.
1. _The Indol Reaction._ Indol and skatol are amongst the final
products of digestion in the lower intestine. They are formed by the
growth, or fermentation set up by the growth, of certain organisms.
Indol may be recognised on account of the fact that with nitrous acid
it produces a dull red colour. The method of testing is as follows. The
suspected organism is grown in pure culture in broth, and incubated
for forty-eight hours at 37° C. Two cc. of a 4 per cent. solution of
potassium nitrite are added to 100 cc. of distilled water, and about
1 cc. of this is added to the test-tube of broth culture. Now a few
drops of concentrated sulphuric acid (unless quite pure, hydrochloric
should be used) are run down the side of the tube. A pale pink to dull
red colour appears almost at once, and may be accentuated by placing
the culture in the blood-heat incubator for half an hour. Much dextrose
(derived from the meat of the broth) inhibits the reaction. _Bacillus
typhosus_ does not produce indol, and therefore does not react to the
test; _B. coli_ and the bacillus of Asiatic cholera do produce indol,
and react accordingly. It should be pointed out, however, that the
bacillus of cholera also produces nitrites. Hence the addition of acid
only to a peptone culture of cholera yields the "red reaction" of indol.
2. _Carbolised Gelatine._ To ordinary gelatine .05 per cent. of phenol
is added. This inhibits many common water bacteria.
3. "_Shake Cultures._" To 10 cc. of melted gelatine a small quantity
of the suspected organism is added. The test-tube is then shaken and
incubated at 22° C. If the organism is _Bacillus coli_, the next day
reveals a large number of gas-bubbles.
4. _Elsner's Medium._ This special potassium-iodide-potato-gelatine
medium is used for the examination of typhoid excreta. It is made as
follows: 500 grams of potato gratings are added to 1000 cc. of water;
stand in cool place for twelve hours, and filter through muslin; add
150 grams of gelatine; sterilise and add enough deci-normal caustic
soda until only faintly acid; add white of egg; sterilise and filter.
Before use add half a gram of potassium iodide to every 50 cc. Upon
this acid medium common water bacteria will not grow, but _Bacillus
typhosus_ and _B. coli_ flourish.
5. _Parietti's Formula_ consists of--phenol, five grams; hydrochloric
acid, four grams; distilled water, 100 cc. To 10 cc. of broth 0.1-0.3
cc. of this solution is added. The tube is then incubated in order to
see if it is sterile. If that is so, a few drops of the suspected water
are added, and the tube reincubated at 37° C. for twenty-four hours. If
the water contains the _B. typhosus_ or _B. coli_, the tube will show a
turbid growth.
6. _Widal's Reaction._ Mix a loopful of blood from a patient suspected
of typhoid fever with a loopful of young typhoid broth culture in a
hanging drop on a hollow ground slide. Cover with a cover glass and
examine under 1/6-inch objective. If the patient is really suffering
from typhoid, there will appear in the hanging drop two marked
characteristics, viz., agglutination and immotility. This aggregation,
together with loss of motility, is believed to be due to the inhibitory
action of certain bacillary products in the blood of patients suffering
from the disease. The test may be applied in various ways, and its
successful issue depends upon one or two small points in technique into
which we cannot enter here, but which the reader will find dealt with
in the appendix.
7. _Flagella-staining._ Special methods must be adopted for staining
the flagella of _Bacillus typhosus_ and _B. coli_. The cover glasses
should be absolutely clean, the cultures young (say eighteen hours
old), and a diluted emulsion with distilled water must be made in a
watch-glass in order to get bacilli discrete and isolated enough. _Van
Ermengem's Method_ is as follows:--Place a loopful of the emulsion on
a clean cover glass and dry it in the air, fixing it lastly by passing
it once or twice through the flame of a Bunsen burner. Place films for
thirty minutes in a solution of one part boric acid (2 per cent.) and
two parts of tannin (15.25 per cent.), which also contains four or five
drops of glacial acetic acid to every 100 cc. of the mixture. Wash in
distilled water and alcohol. Then place for five to ten seconds in a
25.5 per cent. solution of silver nitrate. Immediately thereafter, and
without washing, treat the cover glass to the following solution for
two or three seconds: gallic acid, five grams; tannin, three grams;
fused potassium acetate, ten grams; distilled water, 350 cc. After
this place in a fresh capsule of silver nitrate until the film begins
to turn black. Wash in distilled water, dry, and mount. The process
contracts the bacilli somewhat, but the flagella stain well.
The +Bacillus coli communis+ occupies such an important place in all
bacteriological investigation that a few words descriptive of it
are necessary in this place. The "colon bacillus," as it is termed,
appears to be almost ubiquitous in distribution. The idea once held
that it belonged exclusively to the alimentary canal or sewage is
now discarded. It is one of the most widely distributed organisms in
nature, though, as its name implies, its habitat is in the intestinal
tract of man and animals. It is an aërobic, non-sporulating,
non-liquefying bacillus, about .4 µ in thickness, and twice that
measurement in length; hence it often appears oval or egg-shaped. Its
motility is in varying degree, occasionally being as active as _B.
typhosus_, but generally much less so. It possesses lateral flagella.
On gelatine plates at 20° C. _B. coli_ produces non-liquefying,
greyish-white, round colonies; in a stroke culture on the same medium,
a luxuriant greyish band, much broader and less restricted to the
track of the needle than _B. typhosus_. In depth of medium or "shake"
cultures there is an abundant formation of bubbles of gas (methane or
carbon dioxide) in the medium. On potato it produces a light yellow,
greasy growth, which must be distinguished from the growth of _B.
fluorescens liquefaciens_, _B. pyocyaneus_, and several other species
on the same medium. If the potato is old or alkaline, the yellow
colour may not appear. Milk is curdled solid in from twenty-four to
forty-eight hours, and a large amount of lactic acid produced. In broth
it produces a uniform turbidity, with later on some sediment and a
slight pellicle. It gives the reaction to indol.
It is now the practice to speak of the family of _Bacillus coli_
rather than the individual. The family is a very large one, and shows
throughout but few common characters. The morphology readily changes
in response to medium, temperature, age, etc. Fermentation of sugar,
coagulation of milk, or indeed the indol reaction cannot always be
used as final tests as to whether or not the organism is _B. coli_,
for unfortunately some members of the family do not show each of these
three features. Most varieties, however, appear to show some motility,
a small number of flagella, a typical growth on potato, and develop
more rapidly on all media than _B. typhosus_. These characters, plus
one or more of the three features above named, are diagnostic data upon
which reliance may be placed.
+Cholera.+ This word is used to cover more a group of diseases rather
than one specific well-restricted disease. In recent years it has
become customary to speak of Asiatic cholera and British cholera, as
if indeed they were two quite different diseases. But, as a matter of
fact, we know too little as yet concerning either form to dogmatise on
the matter. Until 1884 practically nothing was known about the etiology
of cholera. In that year, however, Koch greatly added to our knowledge
by isolating a spirillum from the intestine and in the dejecta of
persons suffering from the disease.
Cholera has its home in the delta of the Ganges. From this endemic area
it spreads in epidemics to various parts of the world, often following
lines of communication. It is a disease which is characterised by
acute intestinal irritation, manifesting itself by profuse diarrhœa
and general systemic collapse, with cramps, cardiac depression,
and subnormal temperature. The incubation period varies from only
a few hours to several days. In the intestine, and setting up its
pathological condition, are the specific bacteria; in the general
circulation their toxic products, bringing about the systemic changes.
Cholera is generally conveyed by means of water.
The spirillum of Asiatic cholera (Koch, 1884) generally appears, in the
body and in artificial culture, broken into elements known as "commas."
These are curved rods with round ends, showing an almost equal diameter
throughout, and sometimes united in pairs or even a chain (spirillum).
The latter rarely occur in the intestine, but may be seen in fluid
cultures. The common site for Koch's comma is in the intestinal wall,
crowding the lumina of the intestinal glands, situated between the
epithelium and the basement membrane, abundant in the detached flakes
of mucous membrane, and free in the contents of the intestine. They do
not occur in the blood, nor are they distributed in the organs of the
body.
[Illustration: THE COMMA-SHAPED BACILLI OF CHOLERA]
The bacilli are actively motile, and possess at least one terminal
flagellum. The organism is aërobic, and liquefies gelatine. It
stains readily with the ordinary aniline dyes. It does not produce
spores, though certain refractile bodies inside the protoplasm of the
bacillus in old cultures have been regarded as such. The virulence
of the bacillus is readily attenuated, and both the virulence and
morphology appear to show in different localities and under different
conditions of artificial cultivation a large variety of what are
termed _involution forms_. Unless the organism is constantly being
sub-cultured, it will die. Acid, even the .2 per cent. present in the
gastric juice, readily kills it. Desiccation, 55° C. for ten minutes,
and weak chemicals have the same effect. The bacilli, however, have
comparatively high powers of resistance to cold. Unless examined by
the microscope in a fresh and young stage, it is difficult to
differentiate Koch's comma from many other curved bacilli.
[Illustration: BACILLUS TYPHOSUS (SHOWING FLAGELLA)
× 1000]
[Illustration: BACILLUS TYPHOSUS (WIDAL REACTION)
(Agglutination by serum from typhoid patient)
× 400]
[Illustration: BACILLUS COLI COMMUNIS
(From agar culture, 48 hours growth)
× 1000]
[Illustration: BACILLUS MYCOIDES
(Spore formation. From agar culture)
× 1000
_By permission of the Scientific Press, Limited_]
Its cultivation characters are not always distinctive. Microscopically
the young colonies in gelatine appear as cream-coloured, irregularly
round, and granular. Liquefaction sets in on the second day, producing
a somewhat marked "pitting" of the medium, which soon becomes reduced
to fluid. In the depth of gelatine the growth is very characteristic.
An abundant, white, thick growth exactly follows the track of
the needle, here and there often showing a break in continuity.
Liquefaction sets in on the second day, and produces a distinctive
"bubble" at the surface. The liquefied gelatine does not fall from the
sides of the tube, as in the Finkler-Prior comma of _cholera nostras_,
but occurs inside the border where the gelatine joins the glass. In the
course of a week or two all the gelatine may be reduced to fluid. On
agar Koch's comma produces with rapidity a thick, greyish, irregular
growth. On potato, especially if slightly alkaline, an abundant
brownish layer is formed. Broth and peptone water are excellent media.
In milk it rapidly multiplies, curdling the medium, with production
of acid. Unlike _Bacillus coli_, it does not form gas, but, like
_B. coli_, it produces large quantities of indol and a reduction of
nitrates to nitrites. Hence the indol test may be applied by simply
adding to the peptone culture several drops of strong sulphuric acid,
when in the course of several hours, if not at once, there will be
produced a pink colour, the "cholera red reaction." Although it readily
loses virulence, and its resistance is little, the comma bacillus
retains its vitality for considerable periods in moist cultures, upon
moist linen, or in moist soil. In cholera stools kept at ordinary
room temperature the cholera bacillus will soon be outgrown by the
putrefactive bacteria. The same is true of sewage water.
The lower animals do not suffer from any disease at all similar to
Asiatic cholera, and hence it is impossible to fulfil the postulate
of Koch dealing with animal inoculation. In this respect it is like
typhoid. It is, however, provisionally accepted that Koch's bacillus
is the cause of the disease. The four or five other bacteria which
have from time to time been put forward as the cause of cholera have
comparatively little evidence in their support. It is less from these,
and more from several spirilla occurring in natural waters, that
difficulties of diagnosis arise.
Some hold that, however many comma bacilli be introduced into the
alimentary canal, they will not produce the disease unless there is
some injury or disease of the wall of the intestine. It need hardly
be added that cholera acts, like other pathogenic bacteria, by the
production of toxins. Brieger separated cadaverin and putrescin and
other bodies from cholera cultures, and other workers have separated a
tox-albumen.
_Methods of Diagnosis of Cholera_:
1. The nature of the evacuations and the appearance of the mucous
membrane of the intestine afford striking evidence in favour of a
positive diagnosis. Nevertheless it is upon a minute examination of the
flakes and pieces of detached epithelium that reliance must be placed.
In these flakes will be found in cholera abundance of bacilli having
the size, shape, and distribution of the specific comma of cholera.
The size and shape have been already touched upon. The distribution is
frequently in parallel lines, giving an appearance which Koch described
as the "fish-in-stream arrangement." This distribution of comma bacilli
in the flakes of watery stools is, when present, so characteristic of
Asiatic cholera that it alone is sufficient for a definite diagnosis.
But unfortunately it is not always present, and then search for other
characters must be made.
2. The appearance of cultivation on gelatine, to which reference has
been made, is of diagnostic value.
3. The "cholera red reaction." It is necessary that the culture be pure
for successful reaction.
4. Isolation from water is, according to Dr. Klein, best accomplished
as follows: A large volume of water (100-500 cc.) is placed in a
sterile flask, and to it is added so much of a sterile stock fluid
containing 10 per cent. peptone, 5 per cent. sodium chloride, as will
make the total water in the flask contain 1 per cent. peptone and .5
per cent. salt. Then the flask is incubated at 37° C. If there have
been cholera vibrios in the water, however few, it will be found after
twenty-four hours' incubation that the top layer contains actively
motile vibrios, which can now be isolated readily by gelatine-plate
culture.
5. To demonstrate in a rapid manner the presence of cholera bacilli
in evacuations, even when present in small numbers, a small quantity
must be taken up by means of a platinum wire and placed in a solution
containing 1 per cent. of pure peptone and .5 per cent. sodium chloride
(Dunham). This is incubated as in the case of the water, and in twelve
hours is filled with a turbid growth, which when examined by means of
the hanging drop shows characteristic bacilli.
NATURAL PURIFICATION OF WATER
We have already noticed that rivers purify themselves as they proceed.
There are many excellent examples of this self-purification. The Seine
as it runs through Paris becomes highly polluted with every sort of
filthy contamination. But twenty or thirty miles below the city it
is found to be even purer than above the city before it received the
city sewage. In small rivers it is the same, provided the pollution is
less in amount. Whilst authorities differ with regard to the mode of
self-purification, all agree that in some way rivers receiving crude
sewage are able in a marvellous degree to become pure again.
The conditions influencing this phenomenon are as follows:
(_a_) _The Movement of the Water._ It is probable, however, that
any beneficial result accruing from this cause is due, not to any
mechanical factor in the movement, but to the extra surface of water
available for oxidation processes.
(_b_) _The Pressure of the Water._ It is believed that the volume of
water pressing down upon any given area beneath it weakens the vitality
of certain microbes. In support of this theory, it is urged that the
number of bacteria capable of developing is less the greater the depth
from the surface. Yet it must be remembered that mud at the bottom of a
river, or at the bottom of the sea, is teeming with living organisms.
(_c_) _Light._ We have seen how prejudicial is light to the growth of
organisms in culture media. This is so, though to a less extent, in
water. Arloing held that sunlight could not pierce a layer of water
an inch in thickness and still act inimically on micro-organisms. But
Buchner found that the sun's rays could pass through fifteen or twenty
inches and yet be bactericidal. This evidence appears contradictory.
On the whole, however, authorities agree that the influence of the
sun's rays upon water is distinctly bactericidal and causes a marked
diminution in the quantity of organisms after acting for some hours.
Especially will this be so when the water is spread over a wide area
and is therefore shallow and stationary, or moving but slowly.
(_d_) _Vegetation in Water._ Pettenkofer, in his observations upon the
Iser below Munich, has shown how algæ bring about a marked reduction in
the organic matters present in water.
(_e_) _Dilution._ There can be no doubt in anyone's mind that the
pollutions passing into a flowing river are very soon diluted with the
large quantities of comparatively pure water always forthcoming. But
this, whilst it would lower the percentage of impurity, cannot remove
impurities.
(_f_) _Sedimentation._ Whilst Pettenkofer attributes self-purification
to oxygenation and vegetation, most authorities are now agreed that
it is largely brought about by the subsidence of impure matters,
and by their subsequent disintegration at the bottom of the river.
Sedimentation obviously is greatest in still waters. Hence lake water
contains as a rule very few bacteria. "The improvement in water during
subsidence is the more rapid and pronounced the greater the amount
of suspended matter initially present" (Frankland). Tils has pointed
out that the number of micro-organisms was invariably smaller in the
water collected from the reservoir than in that taken from the source
supplying the latter. Percy Frankland has demonstrated the same effect
of sedimentation by storage as follows:
No. of Colonies in
1 cc. of water.
1. Intake from Thames, June 25, 1892 1,991
2. First small storage reservoir 1,703
3. Second " " 1,156
4. Large storage reservoir 464
The large reservoir would of course necessitate a prolonged subsidence,
and hence a greater diminution than in the small reservoirs. Many like
examples might be cited, but a typical one such as the above will
suffice.
(_g_) _Oxidation._ Many experiments and observations have been made to
prove that large quantities of oxygen are used up daily in oxygenising
the Thames water. Oxygenated water will come up with the tide and down
with the fresh water from above London. There will also be oxygen
absorption going on upon the surface of the water, and from these three
sources enough oxygen is obtained to oxidise impurities and produce
what is really an effluent. In many smaller streams the opportunity for
oxidation is afforded by weirs and falls.
Probably all these factors play a part in the self-purification of
rivers, but we may take it that oxidation, dilution, and sedimentation
are three of the principal agencies.
We may here digress to refer in passing to the facts obtainable from
Sir Edward Frankland's report on Metropolitan water supply in 1894,
as they will afford a connecting link between self purification and
artificial purification. Judged by the relatively low proportion of
carbon to nitrogen, the organic matter present in the water was, as
usual, found to be chiefly, if not entirely, of vegetable origin. An
immense destruction of bacteria was found to be effected by storage in
subsidence reservoirs. The bacterial quality of the water might differ
widely from its chemical qualities. These three facts are of primary
importance in the interpretation of water reports, and it will be well
to bear them in mind. Sir E. Frankland also refers to the physical
conditions affecting microbial life in river waters. The importance
of changes of temperature, the effect of sunlight, and rate of flow
had been referred to in previous reports. Respecting the relative
proportion of these factors, he adds:
"The number of microbes in Thames water is determined mainly
by the flow of the river, or, in other words, by the rainfall,
and but slightly, if at all, by either the presence or absence
of sunshine, or a high or low temperature. With regard to the
effect of sunshine, the interesting researches of Dr. Marshall
Ward leave no doubt that this agent is a powerful germicide,
but it is probable that the germicidal effect is greatly
diminished, if not entirely prevented, when the solar rays
have to pass through a comparatively thin stratum of water
before they reach the living organisms."
From which it is clear that evidence favours the effect of
sedimentation and dilution. These two factors in conjunction with
filtration are, practically speaking, the methods of artificial water
purification, with which we are now in a position to deal.
ARTIFICIAL PURIFICATION OF WATER
_Sedimentation and Precipitation._ Naturally, we see this factor in
action in lakes or reservoirs. For example, the water supply of Glasgow
is the untreated overflow from Loch Katrine. Purification has been
brought about by means of subsidence of impurities. Nothing further is
needed. Artificially, we find it is this factor which is the mechancial
purifier of biological impurity in such methods as _Clark's process_.
By this mode "temporary hardness," or that due to soluble bicarbonate
of lime, is converted into insoluble normal carbonate of lime by the
addition of a suitable quantity of lime-water. Carbonates of lime and
magnesia are soluble in water containing free carbonic acid, but when
fresh lime is added to such water it combines with the free CO_{2} to
form the insoluble carbonate, which falls as a sediment:
CaCO_{3} + CO_{2} + CaH_{2}O_{2} (lime-water) = 2 CaCO_{3} + H_{2}O.
As the carbonate falls to the bottom of the tank it carries down with
it the organic particles. Hence sedimentation is brought about by means
of chemical precipitation. It is obviously a mechanical process as
regards its action upon bacteria. Nevertheless its action is well-nigh
perfect, and 300 or 400 m.-o. per cc. are reduced to 4 or 5 per cc.
We shall refer to this same action when we come to speak of bacterial
purification of sewage. _Alum_ has been frequently used to purify
waters which contain much suspended matter. Five or six grains of
alum are added to each gallon of water, with some calcium carbonate
by preference. Precipitation occurs, and with it sedimentation of the
bacteria, as before. But, as Babes has pointed out, alum itself acts
inimically on germs; in such treatment, therefore, we get sedimentation
and germicidal action combined.
As a matter of actual practice, however, sedimentation alone is rarely
sufficient to purify water. It is true that the collection of water in
large reservoirs permits subsidence of suspended matters, and affords
time for the action of light and the competitive suicidal behaviour
of the common water bacteria. Yet, after all, filtration is the most
important and most reliable method.
_Sand Filtration_, as a means of purifying water, has been practised
since the early part of the present century. But it was not till
1885 that Percy Frankland first demonstrated the great difference in
bacterial content between a water unfiltered and a water which had
passed through a sand filter. Previous to this time the criterion of
efficiency in water purification had been a chemical one only, and
the presence or absence of bacteria in any appreciable quantity was
described, not in mathematical terms, but in indefinite descriptive
words, like "turbid," "cloudy," etc. It is needless to say that this
difference in estimation was due to the introduction by Koch of the
gelatine-plate method of examination. As a result of Percy Frankland's
work, he formulated the following conclusions as regards the chief
factors influencing the number of microbes passing through the filter.
It depends upon:
(1) _The Storage Capacity for Unfiltered Water._ This, of course, has
reference to the advantages, which we have noticed above, of securing a
large collection of water previous to filtration for subsidence, etc.
(2) _The Thickness of Fine Sand through which Filtration is Carried
on._ An argument needing no further support, for it is clear, other
things being equal, the more sand water passes through the greater the
opportunity of leaving its impurities behind.
(3) _Rate of Filtration._ The slower filtration will be generally the
more complete in its results.
(4) _Renewal of Filter-Beds._ After a certain time the filter-bed
becomes worn out and inefficient; at such times renewal is necessary.
Not only may the age of the filter act prejudicially, but the extra
pressure required will tend to force through it bacteria which ought to
have remained in the filter.
In 1893, Koch brought out his monograph upon _Water Filtration and
Cholera_, and his work had a deservedly great influence upon the whole
question. He shows how the careful filtration of water supplied to
Altona from the Elbe saved the town from the epidemic of cholera which
came upon Hamburg as a result of drinking unfiltered water, although
Altona is situated several miles below Hamburg, and its drinking
water is taken from the river after it has received the sewage of
Hamburg. Now, from his experience of water filtration, Koch arrived at
several important conclusions. In the first place, he maintained that
the _portion of the filter-bed which really removed micro-organisms
effectively was the slimy organic layer upon the surface_. This
layer is produced by a deposit from the still unpurified water lying
immediately above it. The most vital part of the filter-bed is this
organic layer, which, after formation, should not be disturbed until
it requires removal owing to its impermeability. A filter-bed, as is
well known, consists of say three feet of sand and one foot of coarse
gravel. The water to be filtered is collected into large reservoirs,
where subsidence by gravitation occurs. Thence it is led by suitable
channels to the surface of the filter-bed. Having passed through the
three or four feet of the bed, it is collected in a storage reservoir
and awaits distribution. The action of the whole process is both
mechanical and chemical. Mechanically by subsidence, much suspended
matter is left behind in the reservoir. Again, mechanically, much
of that which remained suspended in the water when it reached the
filter-bed is waylaid in the substance of the sand and gravel of the
filter-bed. Chemically also the action is twofold. Oxidation of the
organic matter occurs to some extent as the water passes through the
sand. Until recently this chemical action and the double mechanical
action were believed to be the complete process, and its efficiency
was tested by chemical oxidation and alteration, and absence of the
suspended matter.
Now, however, it is recognised that the second portion of the chemical
action is vastly the more important, indeed, the only vital, part of
the process. This is the chemical effect of the layer of scum and mud
on the surface of the sand at the top of the filter-bed. The mechanical
part of this layer is, of course, the holding back of the particulate
matter which has not subsided in the reservoir; the vital action
consists in what is termed _nitrification_ of unoxidised substance,
which is accomplished in this layer of organic matter. We shall deal
at some length with the principles of nitrification when we come to
speak of soil. But we may say here that by nitrification is understood
a process of oxidation of elementary compounds of nitrogen, by which
these latter are built up into stable bodies which can do little
harm in drinking water. From what has been said it will be seen that
the action of a filter-bed is of a complicated nature. There is (1)
subsidence of the grosser particles of impurity in the water; (2)
mechanical obstruction to impurities in the interstices of the scum,
sand, and gravel in the filter; (3) oxidation of organic matter by the
oxygen held in the pores of the sand and gravel; (4) nitrification
in the vital scum layer, which is accomplished by micro-organisms
themselves. This latter is now considered to be incomparably the most
important part of the filter. That being so, its removal, except when
absolutely necessary, is to be avoided as detrimental to the efficiency
of the filter. New filters have obviously but little of this action.
Hence it is wise to allow a new filter-bed to act for a short period
(say twenty-four to forty-eight hours) before the filtered water is
used for domestic purposes, in order to allow the organic layer to be
formed. This must also be borne in mind after a filter-bed has been
cleaned.
To maintain this nitrifying action of a filter in efficiency, Koch
suggested, in the second place, _that the rate of filtration must not
exceed four inches per hour_. At the Altona water-works this rate of
filtration was maintained, and the number of organisms always remained
below 100 per cc., which, as we have seen, is the standard. Thirdly,
it is important that _periodic bacteriological examinations should
be made_. Koch's emphasis upon this point is well known, and the
cumulative experience of bacteriologists only further supports such a
course being taken. If it be true that efficient sand filtration is a
safeguard against pathogenic germs like typhoid and cholera, then there
can be but one criterion of efficiency, viz., their absence in the
filtered water, which can only be ascertained by regular examination.
But it is not alone for pathogenic germs that filtration is proposed.
Filtered water containing more than 100 micro-organisms of any kind
per cc. is below the standard in purity, and should on no account be
distributed for drinking purposes. In this country chemical analysis,
with a more or less cursory microscopic examination, has been almost
invariably accepted as reliable indication of the condition of the
water. But such an examination is not really any more a fair test of
the working of the filter than it is of the actual condition of the
water. It is true, the quantity of organic matter can be estimated and
the condition in which it exists in combination obtained; but it cannot
tell us what a bacteriological examination can tell us, viz., the
quantity and quality of living micro-organisms present in the water.
Upon this fact, after all, an accurate conclusion depends. There is
abundant evidence to show that no valuable opinion can be passed upon
a water except by both a chemical and a bacteriological examination,
and further by a personal investigation, outside the laboratory, of the
origin of the water and its liabilities to pollution.
So convinced was Koch of the efficiency of sand filtration as
protection against disease-producing germs that he advocated an
adaptation of this plan in places where it was found that a well
yielded infected water. Such pollution in a well may be due to various
causes; surface-polluted water oozing into the well is probably the
commonest, but decaying animal or vegetable matter might also raise the
number of micro-organisms present almost indefinitely. Koch's proposal
for such a polluted well was to fill it up with gravel to its highest
water level, and above that, up to the surface of the ground, with
fine sand. Before the well is filled up in this manner it must, of
course, be fitted with a pipe passing to the bottom and connected with
a pump. This simple procedure of filling up a well with gravel and sand
interposes an effectual filter-bed between the subsoil water and any
foul surface water percolating downwards. Such an arrangement yields as
good, if not better, results than an ordinary filter-bed, on account of
there being practically no disturbance of the bed nor injury done to it
by frost.
The effect of the remedies we have been discussing upon the number of
bacteria is demonstrated in the results which Sir Edward Frankland
arrived at in his investigation of London waters.[17]
MEAN OF MONTHLY EXAMINATIONS FOR THE YEAR
------------------+---------+----------------------------+---------------
| | M.-O. PER CC. | Average per
| +-------+--------+------- ---+ cent. of
|Source of| | | |Micro-organisms
NAME OF COMPANY. | Supply. | At | After | After | Removed
| |Source.|Storage.|Filtration.| by Filtration.
------------------+---------+-------+--------+-----------+---------------
The Chelsea Co. |Thames at| | | |
| Hampton |16,138 | 1067 | 34 | 98.96
West Middlesex Co.| " |16,138 | 1788 | 58 | 99.40
Southwark & | | | | |
Vauxhall Co. | " |16,138 | .... | 80 | 97.72
| | | | { 623 | }
Grand Junction Co.| " |16,138 | 2500 | { 100 | } 98.46
| | | | { 96 | }
Lambeth Co. | " |16,138 | 7820 | 75 | 99.50
------------------+---------+-------+--------+-----------+---------------
The teaching of these figures could, with great ease, be reproduced
again and again if such was necessary; but these will suffice to show
that sand filtration, when carefully carried out, offers a more or less
absolute barrier to the passage of bacteria, whether non-pathogenic or
pathogenic.
_Domestic Purification of Water._ Something may here be said, from a
bacteriological point of view, relative to what is called _domestic
purification_. There is but one perfectly reliable method of
sterilising water for household use, viz., _boiling_. As we have seen,
moist heat at the boiling point maintained for five minutes will kill
all bacteria and their spores. The only disadvantages to this process
are the labour entailed and the "flat" taste of the water. Nevertheless
in epidemics due to bad water it is desirable to revert to this simple
and effectual purification.
There are a large number of filters on the market with, in many
cases, but little modification from each other. The materials out of
which they are made are chiefly the following: carbon and charcoal,
iron (spongy iron or magnetic oxide), asbestos, porcelain and other
clays, natural porous stone, and compressed siliceous and diatomaceous
earths. From an extended research in 1894 by Dr. Sims Woodhead and Dr.
Cartwright Wood our knowledge of the quality of these substances as
protectives against bacteria has been largely increased. They concluded
that a filter failed to act in one of two ways. It was either pervious
to micro-organisms, or its power of filtering became modified owing to
(_a_) structural alteration of its composition, or to (_b_) the growing
through of the micro-organisms. The conditions which chiefly influence
the growth of bacteria through a filter appear to be the temperature,
the intermittent use of the filter, and the species of bacteria. The
higher the temperature and the longer the organisms are retained in the
filter the more likely is it that they will grow through, and in the
next usage of the filter appear in the filtrate. As to the species,
those multiplying rapidly and possessing the power of free motility
will naturally appear earlier in a filtrate than others. Woodhead and
Wood, from their searching and most able investigation, concluded that
the Pasteur-Chamberland candle filters (composed of porcelain formed
by a mixture of kaolin and other clays) were the only filters out of
the substances named above which were reliable and protective against
bacteria. They tested over three dozen of the Pasteur filters, and
"in every case these gave a sterile filtrate." Pure cholera bacillus
in suspension (5000 bacilli to every cc.) and typhoid bacillus in
suspension (8000 per cc.) were passed through these filters, and
not a single bacillus was detectable in the filtrate. The Berkefeld
filter (siliceous earth) came second on the list as an effective
filter, and had but the fault of not being a "continuous" steriliser.
A certain Parisian filter ("Porcelaine d'Amiante"), made of unglazed
porcelain, rendered water absolutely free from bacteria. Its action
was, however, very slow. Setting aside these three efficient filters,
we are face to face with the fact that most filters do not produce
germ-free filtrates, even though they are nominally guaranteed to do
so. It is professed for _animal charcoal_, which is widely used, that
it absorbs oxygen, and so fully oxidises whatever passes through it.
This may be so at first, but after a little use it probably does more
harm than good. It appears to add nitrogen and phosphates to water,
which are both nutritive substances on which bacteria grow. Moreover it
readily absorbs impurities from the air. As a matter of experiment and
practice, it has been found by Frankland, Woodhead, and others, that
charcoal actually adds to the number of germs after it has been in use
for some days.
[Illustration: PASTEUR-CHAMBERLAND FILTER
Attached to Water Supply]
_Diseases Conveyed by Water._ There are a few preliminary features
to be noticed before we enter in detail upon the characteristics of
several of the chief pathogenic bacteria in water.
In sterilised water, and in very highly polluted water or sewage,
pathogenic bacteria do not flourish. In the former case they die of
starvation, although there are some experiments on record which appear
not to support this view; in the latter case they are killed by the
enormous competition of common bacteria. Even in ordinary water there
is a wide divergence of behaviour. Some bacteria are destroyed in a few
hours; others appear to flourish for weeks. In all cases the spores are
able to resist whatever injurious properties the water may have much
more persistently than the bacilli themselves. These changes in the
vitality of bacteria in water, partly due to the water and partly to
the other micro-organisms, bring about two characteristics which it is
important to remember, viz., that pathogenic germs in water are, as a
rule, _scanty_ and _intermittent_. It is these features in conjunction
with the enormous quantities of common water bacteria which make the
search for the bacillus of typhoid what Klein has called "searching
for a needle in a rick of hay." Not that it cannot be detected, but
its detection is one of the most difficult of investigations. We
shall refer to this matter again when _Bacillus typhosus_ is under
consideration.
In artificial cultivation water bacteria respond very readily to
external conditions. Increase of alkalinity (.01 grams of sodium
carbonate added to 10 cc. of ordinary gelatine) causes the number of
colonies to be five or six times greater than that revealed by using
ordinary gelatine; on the other hand, very slightly increasing the
acidity of a medium as markedly diminishes the number of bacteria.
Advantage is taken of this in culturing the bacillus of typhoid, which
does not object to an acid medium.
Water may become polluted in a variety of ways, and it is helpful to
classify these as pollutions at the _source_, in the _course_, and at
the _periphery_. Gathering-grounds are frequently the locality of the
pollution. The recent Maidstone epidemic is an example. Here some of
the springs supplying the town with water were contaminated by several
typhoid patients. Frequently on the gathering-ground one may find a
number of houses the waste and refuse of which will furnish ample
surface pollution, which in its turn may readily pass into a collecting
reservoir or a well. Only recently the writer investigated the cause
of typhoid in a large country house, and traced it to pollution of the
private well by surface washings from the stable quarters. Leakage of
house-drains into wells is not an infrequent source of contamination.
The same cause is generally operative in cases of pollution of a water
supply in its _course_ from the source to the cisterns or taps at the
periphery, viz., a sewer or drain leaking into the water supply.
Water companies and those responsible for water supply appear to hold
the opinion that so long as there is sand filtration or subsidence
reservoirs it is unnecessary to consider the gathering-ground or
transit. But, as we have seen, a frost may completely dislocate the
efficient action of a filter, and times of flood may prevent proper
sedimentation; then our dependence for pure water is wholly upon the
gathering-ground and source. Hence we find water contaminated at its
source by polluted wells, by sewage-infected rivers and streams, by
drainage of manured fields, by innumerable excremental pollutions over
the areas of the gathering-grounds, and in transit by careless laying,
poor construction, bad jointing, and close proximity of water-and
drain-pipes. In the third place, we may get a water infected at the
_periphery_, in the house itself. Such cases are generally due to one
of two causes: filthy cisterns or suction. Cisterns _per se_ are more
or less indispensable where a constant service does not exist, but they
should be inspected from time to time and maintained in a cleanly
condition. Suction into the tap has been recently emphasised by Dr.
Vivian Poore as a cause of pollution. It is liable to occur whenever a
tap is left turned on, and a vacuum is produced in the supply-pipe by
intermission of the water supply, so that foul gas or liquid is sucked
back into the house-pipe.
One more point requires our attention. It has relation to bacterially
polluted water when it has gained entrance to the body. It has been
known for some time past that not all waters polluted with disease
germs produce disease. As we have before said, this may depend upon the
infective agent, its quantity and quality; the body being able in many
cases to resist a small dose of poison. It is, however, necessary to
infection, especially in water-borne disease, that the tissues shall
be in some degree disordered. The perverted action of the stomach
influences the acid secretion of the gastric juice, through which
bacilli might then pass uninjured. Particularly must this be so in
the bacillus of cholera, which is readily killed by the normal acid
reaction of the stomach. Hence, in this disease at least, it is the
opinion of bacteriologists that the condition of the mucous membrane
of the stomach is of primary importance. Metschnikoff has indeed
demonstrated the presence of the bacillus of cholera in the intestinal
excretion of apparently healthy persons, which shows that they were
protected by the resistance of their tissues to the bacilli. Further
light has been thrown on this question by the researches of MacFadyen,
who has pointed out that suspensions of cholera bacilli in water passed
through the stomach untouched, and were thus able to exert their evil
influence in other parts of the alimentary canal. When, however,
cholera bacilli were suspended in milk, none appeared to escape the
germicidal action of the gastric juice. The explanation of this is
probably the simple one that the stomach reacted with its secretion of
gastric juice only to food (milk), but simply passed the water on into
the lower and more absorptive parts of the alimentary canal. Such a
condition of affairs clearly increases the danger of water-borne germs.
THE BACTERIOLOGY OF SEWAGE AND SEWAGE-POLLUTED WATERS
It will not be needful to insist upon the obvious fact that bacteria
abound in sewage. Such a large quantity of organic matter, in which
decomposition is constantly taking place, will afford an almost ideal
nidus for micro-organic life. There is indeed but one reason why such
a medium is not absolutely ideal from the microbe's point of view, and
that reason is, that in sewage the vast number of bacteria present make
the struggle for existence exceptionally keen. Not only are the numbers
incredibly large, but we also find a very extensive representation
of species, including both saprophytes and parasites, non-pathogenic
and pathogenic. Not infrequently it is from pollution by sewage that
drinking water is contaminated with disease. A patient, we will say,
suffers from typhoid fever. The specific organism has its habitat
largely, though not exclusively, in the alimentary canal. It passes out
in the excreta, and though sometimes partially disinfected, may escape
without hindrance into the drains, and thus to the sewer or cesspool.
How often, by means of direct connection or by percolation, sewage,
from sewers or cesspools, gains access to drinking water, the history
of typhoid outbreaks in this country only too fully records.
It is impossible to lay down any exact standard of the chemical and
bacteriological quality of sewage. The quality will differ according to
the size of the community, the inclusion or otherwise of trade-waste
effluents, the addition of rain-water, and other like physical
conditions. Moreover, sewage itself when, so to speak, fully formed
is liable to undergo rapid changes owing to fermentation and the
competition of micro-organisms. It is clear that these latter are the
chief agents in bringing about the change, because, if sewage be placed
in hermetically sealed flasks and sterilised by heat, it is found that
no change occurs. From facts such as the above it will be apparent
that no exact standard of chemical or bacterial contents is possible.
Respecting the chemical condition we may shortly say that the chief
characteristic of sewage is its enormous amount of contained organic
matter in suspension or solution; respecting the bacterial content we
may say that the chief species of the very numerous organisms are those
commonly concerned in fermentative putrefaction. London crude sewage
contains on an average about four millions of micro-organisms per cc.
Many of these are "liquefying" bacteria; that is to say, they have the
power of liquefying gelatine, which is generally one of the features
of putrefactive species. In considering the quality of the bacteria
present in sewage, a still wider field of research opens before us.
For though we can say that, roughly, all sewage will contain probably
between four and eight millions of bacteria, we cannot even lay down a
rough standard respecting the kinds of bacteria present more than we
have done already in stating that a very large number indeed out of the
total will belong to putrefactive species.
We may, however, make a provisional list of normal sewage bacteria[18]
as follows:
1. _Bacillus coli communis_ and all its varieties and allies.
2. _Proteus vulgaris_ and the various protean species.
3. _B. enteritidis sporogenes_ (Klein).
4. Liquefying bacteria, _e. g._, _Bacillus fluorescens liquefaciens_,
_B. subtilis_, _B. mesentericus_.
5. Non-liquefying bacteria.
6. Sarcinæ, yeasts, and moulds.
[Illustration: PROTEUS VULGARIS]
[Illustration: B. ENTERITIDIS SPOROGENES]
We have not included in the above inventory any pathogenic bacteria.
Doubtless such species (_e. g._, typhoid[19]) not infrequently find
their way into sewage. But they are not normal habitants, and though
they struggle for survival, the keenness of competition among the dense
crowds of saprophytes makes existence almost impossible for them. Nor
can they expect much sympathy from us in the difficulties of life which
fortunately confront them in sewage.
Of those we have named as normally present it is unnecessary to speak
in detail, with the exception of the newly discovered anaërobe,
_Bacillus enteritidis sporogenes_ of Klein.[20] This bacillus is
credited to be a causal agent in diarrhœa, and has been isolated
by Dr. Klein from the intestinal contents of children suffering from
severe diarrhœa, and from adults having cholera nostras. It has
been readily detected in sewage from various localities, and also in
sewage effluents, after sedimentation, precipitation, and filtration.
Its biological characters are shortly as follows: It is in thickness
somewhat like the bacillus of symptomatic anthrax, thicker and shorter
than the bacillus of malignant œdema, and standing therefore between
the latter and anthrax itself. It is motile and possesses flagella,
but has no threads. It readily forms spores, which develop as a rule
near the ends of the rods and are thicker than the bacilli. It is
stained by Gram's method. In various media (particularly milk) it
produces gas rapidly. It is an anaërobe, and is cultivated in Buchner's
tubes. A recent epidemic of diarrhœa affecting 144 patients in St.
Bartholomew's Hospital was traced to milk in which _B. enteritidis_ was
present.
_Sewer Air._ Though not of material importance as regards bacterial
treatment of sewage, this subject calls for some remark. For long it
has been known that air polluted by sewage emanations is capable of
giving rise to various degrees of ill-health. These chiefly affect two
parts of the body; one is the throat, and the other is the alimentary
canal. Irritation and inflammation may be set up in both by sewer air.
Such conditions are in all probability produced by a lowering of the
resistance and vitality of the tissues, and not by either a conveyance
of bacteria in sewer air or any stimulating effect upon bacteria
exercised by sewer air. What evidence we have is against such factors.
(See p. 105.)
Several series of investigations have been made into the bacteriology
of sewer air, amongst others by Uffelmann, Haldane, Laws, and Andrewes.
From their labours we may formulate four simple conclusions:
1. The air of sewers contains very few micro-organisms indeed,
sometimes not more than two organisms per litre (Haldane), and
generally fewer than the outside air (Laws and Andrewes).
2. There is no relationship between the microbes contained in sewer air
and those contained in sewage. Indeed, there is a marked difference
which forms a contrast as striking as it is at first sight unexpected.
The organisms isolated from sewer air are those commonly present in
the open air. Micrococci and moulds predominate, whereas in sewage
bacilli are most numerous. Liquefying bacteria, too, which are common
in sewage, are extremely rare in sewer air. _Bacillus coli communis_,
which occurs in sewage from 20,000 to 200,000 per cc., is altogether
absent from sewer air.
3. Pathogenic organisms and those nearly allied to them are found in
sewage, but absent in sewer air. Uffelmann isolated the _Staphylococcus
pyogenes aureus_ (one of the organisms of suppuration), but such
a species is exceptional in sewer air. Hence, though sewer air is
popularly held responsible for conveying diphtheria and all sorts of
other virulent bacteria, there is up to the present no evidence of a
substantial nature in support of such views. Sewer air neither conducts
pathogenic organisms nor stimulates the virulence of such.
4. Lastly, only when there is splashing in the sewage, or when bubbles
are bursting (Frankland), is it possible for sewage to part with its
contained bacteria to the air of the sewer.
Whilst we cannot here enter more fully into an account of the bacteria
found in sewage or of their functions, it is necessary to remark upon
one distinguishing feature. A very large number of sewage bacteria
are _decomposing_ and _denitrifying_, that is to say, breakers down,
by means of putrefaction, of organic compounds. The knowledge of this
fact has recently been applied, in conjunction with oxidation, to the
biological treatment of sewage. As this illustrates in a marked degree
some of the facts we have dwelt upon in considering the bacteriology of
soil, and as it is likely that the future will witness a still wider
application of these same facts, it will be necessary to refer in some
detail to the matter.
Hitherto there has been adopted one of four methods of treatment of
sewage. In the first place, in towns situated on the coast the sewage
has, by means of a conduit, been _carried out to sea_. It is clear that
such a course, which is in itself open to criticism, is applicable to
but few towns. In the second place, methods of _chemical treatment_
have been practised. This has generally been of the nature of a
"precipitation" process. Six to twelve grains of quicklime have been
added to each gallon of sewage. The process is simple and cheap, but
it does not remove the organic matter in solution. On the one hand, it
does not produce a valuable manure; on the other, it fails to purify
the effluent. A dozen other methods have been tried, but all based
on the addition of chemical substances to precipitate or change the
organic matter of the sewage. Electrolysis, too, has been proposed. The
third mode adopted in the past has been that known as _intermittent
downward filtration_. This may be defined as "the concentration of
sewage at short intervals on an area of specially chosen porous ground,
as small as will absorb and cleanse it, not excluding vegetation,
but making the product of secondary importance" (Metropolitan Sewage
Commission). The action is mechanical and biological, that is to say,
due in part to nitrification by bacteria in the upper layers of soil.
The fourth plan is that of _irrigation_, or "the distribution of sewage
over a large surface of ordinary agricultural ground, having in view a
maximum growth of vegetation (consistently with due purification) for
the amount of sewage supplied." Like the former, there is biological
influence at work here, though in a less degree. About one acre is
required for every hundred persons in the population. These two latter
modes are much to be preferred to chemical treatment, yet on account of
space and management, as well as on account of the non-removal of the
"sludge," their success has not been all that could be desired. Until
comparatively recent times the above methods of treating sewage were
the only ones available.
In 1881 it appears that M. Louis Mouras, of Vesoul (Haute Saône),
published an account of a hermetically sealed, inodorous, and
automatically discharging cesspool, in which sewage was anaërobically
broken down by "the mysterious agents of fermentation." This is the
first record we have of the newly applied treatment of sewage by simply
allowing Nature to fulfil her function by means of bacteria. We shall
most easily arrive at an appreciation of the recent developments of
the process in England by describing the so called septic tank and
cultivation beds.
[Illustration: A PLAN OF SEPTIC TANK AND FILTER-BEDS
As Used at Exeter]
The _septic tank_ is a large underground vault of cemented brick,
having a capacity of thousands of gallons, according to the population.
That at Exeter has a capacity of 53,800 gallons, and takes the average
sewage of 1500 inhabitants in twenty-four hours. Near the entrance is
a submerged wall, seven feet from the entrance and twelve inches below
the surface of the liquid in the full tank. Within this are caught, by
gravity, gravel and such-like deposits. The remaining solid matter of
the sewage becomes deposited in the tank itself. Both in the sediment
at the bottom of the tank and in the thick scum on the surface the
organic compounds are broken down and made soluble. In the former
position this is accomplished by anaërobic bacteria, in the latter on
the surface by aërobic bacteria. It need hardly be added that these are
denitrifying and putrefactive bacteria, and that those at the bottom
of the tank perform greater service than those at the top. When the
liquid sewage passes out of the tank it differs from the crude sewage
which enters the tank in the following particulars: (_a_) The gravel
and particulate débris have been removed; (_b_) the organic solids in
suspension are so greatly diminished that they are almost absent; (_c_)
there is an increase of organic matter in solution; (_d_) the sewage is
darker in colour and more opalescent; (_e_) compounds like albuminoid
ammonia, urea, etc., have been more or less completely broken down,
and reappear in elementary conditions, like ammonia, methane, carbon
dioxide, and sulphuretted hydrogen. These latter bodies may be in
solution or may have escaped as gas.
The _cultivation beds_ are four or five filters, to which the sewage
from the tank flows in such a manner as to produce a weir. By an
automatic arrangement the fluid is distributed to each filter in turn.
When the second filter is full the first is discharged, and remains
empty during the time that the third and fourth are being filled.
Each filter is thus full, say, about six hours, and has from ten to
twelve hours' rest. These filter-beds (at Exeter) have an area of
eighty square yards and a depth of five feet; collecting drains are
laid on the bottom of the filters, joining main collectors, the latter
terminating in discharging wells. The filtrant is broken furnace
clinker or broken coke.
The changes occurring in these filters are of the nature of oxidation,
with the result that the proportion of the oxidised nitrogen increases
(as nitrites and nitrates), the ammonia becomes less, and the total
solids and organic nitrogen almost disappear. It will thus be seen
that the work of these filters is not merely a straining action. It is
true that particulate matter in the effluent from the tank is caught
on the surface by the film (resulting from previous effluents), but
the real work of the bed is _nitrification_, an oxidation of ammonia
into nitrites and nitrates. This change obviously begins when the tank
effluent flows over the "weir" on to the filter-beds, and the oxygen
thus obtained by the effluent is carried down in solution into the
coke-breeze. Upon the surface of the filtrant are oxidising bacteria.
When the effluent is on the bed they oxidise its contained products;
when the bed is empty and "resting" they oxidise carbon. An advantage
arising from the periodical emptying and filling of the filter is that
the products of decomposition which would eventually inhibit the action
of the aërobic bacteria are washed away, and pass into the nearest
stream, where they become absolutely innocuous.
The "filter" is more correctly termed a _cultivation bed_, for its
purpose is to furnish a very large surface upon which the nitrifying
organisms present, as we have seen, in all soils, may flourish, and
thus feeding upon the organic matter of the sewage, may perform their
function of oxidation.
It is not possible to lay down exact limits as to where denitrification
ends and oxidation begins. To a certain extent, and in varying degree,
they overlap each other. But roughly we may say that in the tank
there is a breaking down (denitrification and decomposition) and in
the filter-beds a building up (nitrification). The case is precisely
parallel to similar changes occurring in soil, and which we have dealt
with elsewhere. The advantage indeed of this biological treatment
of sewage is that it exactly follows the processes of nature, in
contradistinction to the mechanical and chemical methods hitherto
adopted.
At Sutton and some other places the same principles are applied,--that
is to say, bacterial filtration,--but there is no tank. A metal screen
in some measure takes its place, and holds back solid matter from being
carried on to the beds. The filtrant is burnt clay, and it is forked
over occasionally to let in oxygen. The crude sewage is run over the
top of the burnt ballast, where it is left for two or three hours. It
is then slowly run off on to a finer filter, where it also stays two
hours. Thence the effluent is run into the stream.
[Illustration: FILTER-BEDS
As Used at Sutton]
It must be admitted that the bacterial treatment of sewage, though
exhibiting such excellent results where it has been given a fair trial,
is still in a probationary stage. It appears to stand on reason. The
sludge of previous methods is avoided. The sewage is entirely broken
down, and the effluent is a comparatively pure one, yet taking back
nitrogen, as nitrate, to the soil. The whole change, indeed, in the
opinion of Dr. Dupré, is more effective and radical than in chemical
treatment. Further, it has been tested as regards its action upon the
pathogenic bacilli--those of tubercle and typhoid--with the result that
these infective bacteria have been completely destroyed. It appears
that such destruction of infective germs occurs in the tank, and
depends in degree upon the rapidity with which sewage is passed through
the tank. The cultivation beds also have an inimical effect upon
infective bacteria. Hence the final effluent is practically germ-free
as regards pathogenic organisms.