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Transcriber's note: Words and sentences in
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THE SCIENCE SERIES
1. +The Study of Man.+--By A. C. HADDON. Illustrated, 8^o, $2.00.
2. +The Groundwork of Science.+--By ST. GEORGE MIVART. 8^o, $1.75.
3. +Rivers of North America.+--By ISRAEL C. RUSSELL.
Illustrated, 8^o, $2. 00.
4. +Earth Sculpture.+--By JAMES GEIKIE. Illustrated, 8^o, $2.00.
5. +Volcanoes.+--By T. G. BONNEY. Illustrated, 8^o, $2.00.
6. +Bacteria.+--By GEORGE NEWMAN. Illustrated, 8^o, $?
G. P. PUTNAM'S SONS, NEW YORK AND LONDON
The Science Series
EDITED BY
Professor J. McKeen Cattell, M.A., Ph.D.
AND
F. E. Beddard, M.A., F.R.S.
BACTERIA
[Illustration]
BACTERIA
ESPECIALLY AS THEY ARE RELATED
TO THE ECONOMY OF NATURE
TO INDUSTRIAL PROCESSES
AND TO THE PUBLIC HEALTH
BY
GEORGE NEWMAN
M.D., F.R.S. (EDIN.), D.P.H. (CAMB.), ETC.
DEMONSTRATOR OF BACTERIOLOGY IN KING'S COLLEGE, LONDON
ILLUSTRATED
NEW YORK
G. P. PUTNAM'S SONS
LONDON
JOHN MURRAY
1899
COPYRIGHT, 1899
BY
G. P. PUTNAM'S SONS
The Knickerbocker Press, New York
PREFACE
The present volume is not a record of original work, nor is it a
text-book for the laboratory. Theoretical and practical text-books of
Bacteriology plentifully exist both in England and America. There are
two large works widely used, one by Professor Crookshank, entitled
_Bacteriology and Infective Diseases_, the other by Dr. Sternberg,
_A Manual of Bacteriology_. There are also, in English, a number of
smaller works by Abbott, Ball, Hewlett, Klein, Macfarland, Muir and
Ritchie, and Sims Woodhead. This book is of a less technical nature. It
is an attempt, in response to the editor of the series, to set forth
a popular scientific statement of our present knowledge of bacteria.
Popular science is a somewhat dangerous quantity with which to deal. On
the one hand it may become too popular, on the other too technical. It
is difficult to escape the Scylla and Charybdis in such a voyage.
I am much indebted to Professor Crookshank, who, in reading the
manuscript, has helped me by many valuable criticisms. My thanks are
also due to Sir C. T. D. Acland, Bart., for many kind suggestions,
and to Mr. E. J. Spitta, M.R.C.S., who has been good enough to take a
number of excellent photo-micrographs for me. Some other illustrations
have been derived from the _Atlas of Bacteriology_, brought out jointly
by Messrs. Slater and Spitta. For these also I am glad to have an
opportunity of expressing my thanks. It should be understood that the
outline drawings are only of a diagrammatic nature.
GEORGE NEWMAN.
LONDON, 1899.
CONTENTS
PAGE
INTRODUCTION ix
CHAPTER I
THE BIOLOGY OF BACTERIA 1
CHAPTER II
BACTERIA IN WATER 37
CHAPTER III
BACTERIA IN THE AIR 96
CHAPTER IV
BACTERIA AND FERMENTATION 111
CHAPTER V
BACTERIA IN THE SOIL 137
CHAPTER VI
BACTERIA IN MILK, MILK PRODUCTS, AND OTHER FOODS 178
CHAPTER VII
THE QUESTION OF IMMUNITY AND ANTITOXINS 240
CHAPTER VIII
BACTERIA AND DISEASE 264
CHAPTER IX
DISINFECTION 322
APPENDIX 337
ILLUSTRATIONS
[Illustrations starred (*) are reproduced by permission of the
Scientific Press from Drs. Spitta and Slater's _Atlas of Bacteriology_.]
PAGE
VARIOUS FORMS OF BACTERIA 9
SARCINA 10
NORMAL AND PLEOMORPHIC FORMS OF TUBERCLE 13
BACILLI, SHOWING FLAGELLA 15
VARIOUS FORMS OF SPORE FORMATION AND FLAGELLA 18
POTATO IN A ROUX TUBE PREPARED FOR CULTIVATION 22
STAPHYLOCOCCUS PYOGENES AUREUS INCUBATOR _to face_ 22
CULTURE MEDIA READY FOR INOCULATION 23
INOCULATING NEEDLES 24
PASTEUR'S LARGE INCUBATOR FOR CULTIVATION AT ROOM
TEMPERATURE _to face_ 24
METHOD OF PRODUCING HYDROGEN BY KIPP'S APPARATUS
FOR CULTIVATION OF ANAËROBES 27
ANAËROBIC CULTURE 28
KOCH'S STEAM STERILISER 31
LEVELLING APPARATUS FOR KOCH'S PLATE 40
MOIST CHAMBER IN WHICH KOCH'S PLATES ARE INCUBATED 41
HOT-AIR STERILISER 42
THE HANGING DROP 44
DRYING STAGE FOR FIXING FILMS 45
TYPES OF LIQUEFACTION OF GELATINE 47
WOLFHÜGEL'S COUNTER 49
PETRI'S DISH 50
BERKEFELD FILTER 52
APPARATUS FOR FILTERING WATER TO FACILITATE
ITS BACTERIOLOGICAL EXAMINATION _to face_ 52
BACTERIA OF TYPHOID FEVER 56
BACILLUS COLI COMMUNIS 60
THE COMMA-SHAPED BACILLUS OF CHOLERA 66
*BACILLUS TYPHOSUS _to face_ 66
*BACILLUS TYPHOSUS " 66
*BACILLUS COLI COMMUNIS " 66
*BACILLUS MYCOIDES " 66
PASTEUR-CHAMBERLAND FILTER 80
PROTEUS VULGARIS 86
BACILLUS ENTERIDITIS SPOROGENES 86
A PLAN OF SEPTIC TANK AND FILTER-BEDS 91
FILTER-BEDS 94
MIQUEL'S FLASK 97
SEDGWICK'S SUGAR-TUBE 99
SEDGWICK'S TUBE 100
SACCHAROMYCES CEREVISIÆ 117
ASCOSPORE FORMATION 120
GYPSUM BLOCK 121
YEAST _to face_ 122
ASCOSPORE FORMATION IN YEAST " 122
NITROGEN-FIXING BACTERIA FROM ROOTLET NODULES " 122
*BACILLUS OF TETANUS " 122
SACCHAROMYCES ELLIPSOIDEUS 126
SACCHAROMYCES PASTORIANUS 126
BACILLUS ACIDI LACTICI 131
BACILLUS BUTYRICUS 133
KIPP'S APPARATUS 140
FRÄNKEL'S TUBE 141
BUCHNER'S TUBE 141
A METHOD OF GROWING CULTIVATIONS IN A VACUUM OVER PYROGALLIC
SOLUTION 143
MICROCOCCUS FROM SOIL 151
NITROUS ORGANISM _to face_ 158
NITRIC ORGANISM " 158
NITROGEN-FIXING ORGANISM FROM SECRETION
OF ROOT-NODULES " 158
ROOTLET OF PEA WITH NODULES 163
NITROGEN-FIXING BACTERIA IN SITU IN NODULE
ON ROOTLET OF A PEA _to face_ 164
NITROGEN-FIXING BACTERIA IN SITU IN ROOTLET-NODULE
OF A PEA " 164
NITROGEN-FIXING BACTERIA IN SITU IN ROOT-NODULE
OF A PEA " 164
BACILLUS OF TETANUS 170
BACILLUS OF SYMPTOMATIC ANTHRAX 172
BACILLUS OF MALIGNANT ŒDEMA 172
A CENTRIFUGE 228
SUSPENDED SPINAL CORD 255
FLASK USED IN THE PREPARATION OF THE TOXIN OF DIPHTHERIA 262
*BACILLUS TUBERCULOSIS _to face_ 280
*BACILLUS TUBERCULOSIS " 280
*STREPTOCOCCUS PYOGENES " 280
*BACILLUS ANTHRACIS " 280
FLASK USED IN THE PREPARATION OF TUBERCULIN 282
BACILLUS OF DIPHTHERIA 289
TYPES OF STREPTOCOCCUS 298
MICROCOCCUS TETRAGONUS 299
DIPLOCOCCUS OF NEISSER 300
BACILLUS OF ANTHRAX AND BLOOD CORPUSCLES 302
THREADS OF BACILLUS ANTHRACIS, SHOWING SPORES 302
BACILLUS OF PLAGUE 306
*BACILLUS OF PLAGUE _to face_ 310
*BACILLUS OF LEPROSY " 310
STREPTOTHRIX ACTINOMYCES " 310
BACILLUS MALLEI " 310
DIPLOCOCCUS OF PNEUMONIA 312
BACILLUS OF INFLUENZA 315
INTRODUCTION
We live in a world that is teeming with life. From the earliest times
of man that life has been studied and the observations recorded. Thus
there has slowly come to be a considerable accumulation of knowledge
concerning the various forms (morphology) and functions (physiology)
of organised life. This we call the science of biology. It has for its
object the study of organic beings, and for its end the knowledge of
the laws of their organisation and activity. Slowly, too, in the midst
of this gradual accumulation of facts, we begin to see incoherence
becoming coherent, chaos becoming cosmos, chance and accident becoming
law. Further, the contemplation and comprehension which built up the
edifice of modern biology is assuming a new relationship to practical
life. Biology can no longer be considered only as an academic
occupation or as a theoretical pabulum upon which the leisured mind
may ruminate. With rapid strides and determined face this giant of
knowledge has marched into the arena of practical politics. The world
is opening its eyes to a reality which it had mistaken for a vision.
This application of biology to life and its problems has in recent
years been nowhere more marked than in the realm of bacteriology.
This comparatively new science, associated with the great names of
Pasteur, Koch, and Lister, furnishes indeed a stock illustration of
the applicability of pure biology. Turn where we will, we shall find
the work of the unseen hosts of bacteria daily claiming more and more
attention from practical people. Thus biology, even when clothed in the
form of microscopic cells, is coming to occupy a new place in the minds
of men. "Its evolution," as Professor Patrick Geddes declares, "forms
part of the general social evolution." Certainly its recent rapid
development forms a remarkable feature in the practical science of our
time. Not only in the diagnosis and treatment of disease, nor even in
the various applications of preventive medicine, but in ever-increasing
degree and sphere, micro-organisms are recognised as agents of utility
or otherwise no longer to be ignored. They occur in our drinking water,
in our milk supply, in the air we breathe. They ripen cream, and
flavour butter. They purify sewage, and remove waste organic products
from the land. They are the active agents in a dozen industrial
fermentations. They assist in the fixation of free nitrogen, and they
build up assimilable compounds. Their activity assumes innumerable
phases and occupies many spheres, more frequently proving themselves
beneficial than injurious. They are both economic and industrious in
the best biological sense of the terms.
Yet bacteriology has its limitations. It is well to recognise this,
for the new science has in some measure suffered in the past from
over-zealous friends. It cannot achieve everything demanded of it,
nor can it furnish a cause for every disease. It is a science fuller
of hope than proved and tested knowledge. We are as yet only upon the
threshold of the matter. As in the neighbouring realm of chemistry, it
is to be feared that bacteriology has not been without its alchemy.
The interpretations and conclusions which have been drawn from time to
time respecting bacteriological work have led to alarmist views which
have not, by later investigation, been fully supported. Again, the
science has had devotees who have fondly believed, like the alchemists,
that the twin secret of transmuting the baser metals into gold and of
indefinitely prolonging human life was at last to be known. But neither
the worst fears of the alarmist nor the most sanguine hopes of the
alchemist have been verified. Science, fortunately, does not progress
at such speed, or with such kindly accommodation. It holds many things
in its hands, but not finally life or death. It has not yet brought to
light either "the philosopher's stone" or "the vital essence."
What has already been said affords ample reason for a wider
dissemination of the elementary facts of bacteriological science. But
there are other reasons of a more practical nature. Municipalities
are expending public moneys in water analysis, in the examination of
milk, in the inspection of cows and dairies, in the bacterial treatment
of sewage, and in disinfection and other branches of public health
administration. Again, the newly formed National Association for
the Prevention of Tuberculosis, our increasing colonial possessions
with their tropical diseases, even medical science itself, which
is year by year becoming more preventive, make an increasing claim
upon public opinion. The successful accomplishment and solution of
these questions depend in a measure upon an educated public opinion
respecting the elements of bacteriology. Recently it was urged that
"the first elements of bacteriology should be shadowed forth in the
primary school."[1] This course was advised owing to such knowledge
being of value to those engaged in dairying. As we shall point out at a
later stage, many of the undesirable changes occurring in milk are due
to bacteria, even as the success of the butter and cheese industries
depends on the use and control of the fermentative processes due to
their action. Much of the uncertainty attending the manufacture of
dairy products can only be abolished by the careful application of
some knowledge of the flora of milk. In Denmark and in Scandinavia
the importance of such knowledge is realised and acted upon. America,
too, has not been slow to respond to these needs; but in England
comparatively little has been done in this direction.[2]
Whilst there can be no doubt as to the advantage of a wider
dissemination of the ascertained facts concerning bacteria, it
should be borne in mind that only patient, skilled observation and
experimental research in well-equipped laboratories can advance this
branch of science, or indeed train bacteriologists. The lives of Darwin
and of Pasteur adequately illustrate this truth. Yet it is observable
that States and public bodies are slow to act upon it, and frequently
in the past the most useful and substantial support for the advancement
of science has been forthcoming only from private sources. As the world
learns its intimate relation to science and the interdependence between
its life and scientific truth, it may be expected more heartily to
support science.
BACTERIA
CHAPTER I
_THE BIOLOGY OF BACTERIA_[3]
The first scientist who demonstrated the existence of micro-organisms
was Antony von Leeuwenhoek. He was born at Delft, in Holland, in 1632,
and enthusiastically pursued microscopy with primitive instruments.
He corroborated Harvey's discovery of the circulation of the blood
in the web of a frog's foot; he defined the red blood corpuscles of
vertebrates, the fibres of the lens of the human eye, the scales of the
skin, and the structure of hair. He was neither educated nor trained in
science, but in the leisure time of his occupation as a linen-draper he
learned the art of grinding lenses, in which he became so proficient
that he was able to construct a microscope of greater power than had
been previously manufactured. The compound microscope dates from 1590,
and when Leeuwenhoek was about forty years old, Holland had already
given to the world both microscope and telescope. Robert Hooke did for
England what Hans Janssen had done for Holland, and established the
same conclusion that Leeuwenhoek arrived at independently, viz.,
that a simple globule of glass mounted between two metal plates and
pierced with a minute aperture to allow rays of light to pass was
a contrivance which would magnify more highly than the recognised
microscopes of that day. It was with some such instrument as this that
the first micro-organisms were observed in a drop of water. It was not
until more than a hundred years later that these "animalcules," as
they were termed, were thought to be anything more than accidental to
any fluid or substance containing them. Plenciz, of Vienna, was one
of the first to conceive the idea that decomposition could only take
place in the presence of some of these "animalcules." This was in the
middle of the eighteenth century. Just about a century later, by a
series of important discoveries, it was established beyond dispute that
these micro-organisms had an intimate causal relation to fermentation,
putrefaction, and infectious diseases. Spallanzani, Pasteur, and
Tyndall are the three who more than others contributed to this
discovery. Spallanzani was an Italian, who studied at Bologna, and was
in 1754 appointed to the chair of logic at Reggio. But his inclinations
led him into the realm of natural history. Amongst other things, his
attention was directed to the doctrine of _spontaneous generation_,
which had been propounded by Needham a few years previously. In 1768
Spallanzani became Professor of Natural History at Pavia, and whilst
there he demonstrated that if infusions of vegetable matter were
placed in flasks and hermetically sealed, and then brought to the
boiling point, no living organisms could thereafter be detected, nor
did the vegetable matter decompose. When, however, the flasks were
very slightly cracked, and air gained admittance, then invariably
both organisms and decomposition appeared. Schwann, the founder of
the cell-theory, and Schulze, both showed that if the air gaining
access to the flask were either passed through highly heated tubes
or drawn through strong acid the result was the same as if no air
entered at all, viz., no organisms and no decomposition. The result
of these investigations was that scientific men began to believe that
no form of life arose _de novo_ (_abiogenesis_), but had its source
in previous life (_biogenesis_). It remained to Pasteur and Tyndall
to demonstrate this beyond dispute, and to put to rout the fresh
arguments for spontaneous generation which Pouchet had advanced as late
as 1859. Pasteur collected the floating dust of the air, and found by
means of the microscope many organised particles, which he sowed on
suitable infusions, and thus obtained rich crops of "animalculæ." He
also demonstrated that these organisms existed in different degrees in
different atmospheres, few in the pure air of the Mer de Glace, more
in the air of the plains, most in the air of towns. He further proved
that it was not necessary to insist upon hermetic sealing or cotton
filters to keep these living organisms in the air from gaining access
to a flask of infusion. If the neck of the flask were drawn out into a
long tube and turned downwards, and then a little upwards, even though
the end be left open, no contamination gained access. Hence, if the
infusion were boiled, no putrefaction would occur. The organisms which
fell into the open end of the tube were arrested in the condensation
water in the angle of the tube; but even if that were not so, the
force of gravity acting upon them prevented them from passing up the
long arm of the tube into the neck of the flask. A few years after
Pasteur's first work on this subject Tyndall conceived a precise method
of determining the absence or presence of dust particles in the air by
passing a beam of sunlight through a glass box before and after its
walls had been coated with glycerine. Into the floor of the box were
fixed the mouths of flasks of infusion. These were boiled, after which
they were allowed to cool, and might then be kept for weeks or months
without putrefying or revealing the presence of germ life. Here all the
conditions of the infusions were natural, except that in the air above
them there was no dust.
The sum-total of result arising from all these investigations was
to the effect that no spontaneous generation was possible, that the
atmosphere contained unseen germs of life, that the smallest of
organisms responded to the law of gravitation and adhered to moist
surfaces, and that micro-organisms were in some way or other the cause
of putrefaction.
The final refutation of the hypothesis of spontaneous generation was
followed by an awakened interest in the unseen world of micro-organic
life. Investigations into fermentation and putrefaction followed each
other rapidly, and in 1863 Davaine claimed that Pollender's bacillus
of anthrax, which was found in the blood and body tissues of animals
dead of anthrax, was the cause of that disease. From that time to this
in every department of biology bacteria have been increasingly found
to play an important part. They cause changes in milk, and flavour
butter; they decompose animal matter, yet build up the broken-down
elements into compounds suitable for use in nature's economy; they
assist in the fixation of free nitrogen; they purify sewage; in certain
well-established cases they are the cause of specific disease, and
in many other cases they are the likely cause. No doubt the disposal
of spontaneous generation did much to arouse interest in this branch
of science. Yet it must not be forgotten that the advance of the
microscope and bacteriological method and technique have played a large
share in this development. The sterilisation of culture fluids by heat,
the use of aniline dyes as staining agents, the introduction of solid
culture media (like gelatine and agar), and Koch's "plate" method have
all contributed not a little to the enormous strides of bacteriology.
Owing to its relation to disease, physicians have entered keenly into
the arena of bacteriological research. Hence, from a variety of causes,
it has come about that the advance has been phenomenal.
We shall now take up a number of points in the biology of bacteria
which call for early attention, and which are mostly the outcome of
comparatively recent work on the subject.
_The Place of Bacteria in Nature._ As we have seen, for a considerable
period of time after their first detection these unicellular organisms
were considered to be members of the animal kingdom. As late as 1838,
when Ehrenberg and Dujardin drew up their classification, bacteria
were placed among the Infusorians. This was in part due to the powers
of motion which these observers detected in bacteria. It is now, of
course, recognised that animals have no monopoly of motion. But what,
after all, are the differences between animals and vegetables so low
down in the scale of life? Chiefly two: there is a difference in
life-history (in structure and development), and there is a difference
in diet. A plant secures its nourishment from much simpler elements
than is the case with animals; for example, it obtains its carbon
from the carbonic acid gas in air and water. This it is able to do,
as regards the carbon, by means of the green colouring matter known
as _chlorophyll_, by the aid of which, with sunlight, carbonic acid
is decomposed in the chlorophyll corpuscles, the oxygen passing back
into the atmosphere, the carbon being stored in the plant in the form
of starch or other organic compound. The supply of carbon in the
chlorophyll-free plants, among which are the bacteria, is obtained
by breaking up different forms of carbohydrates. Besides albumen and
peptone, they use sugar and similar carbohydrates and glycerine as a
source of carbon. Many of them also have the capacity of using organic
matters of complex constitution by converting such into water, carbonic
acid gas, and ammonia. Their hydrogen comes from water, their nitrogen
from the soil, chiefly in the form of nitrates. From the soil, too,
they obtain other necessary salts. Now all these substances are in an
elementary condition, and as such plants can absorb them. Animals, on
the other hand, are only able to utilise compound food products which
have been, so to speak, prepared for them; for example, albuminoids
and proteids. They cannot directly feed upon the elementary substances
forming the diet of vegetables. This distinction, however, did not at
once clear up the difficult matter of the classification of bacteria.
It is true, they possess motion, are free from chlorophyll, and even
feed occasionally upon products of decomposition--three physiological
characters which would ally them to the animal kingdom. Yet by their
structure and capsule of cellulose and by their life-history and mode
of growth they unmistakably proclaim themselves to be of the vegetable
kingdom. In 1853 Cohn arrived at a conclusion to this effect, and since
that date they have become more and more limited in classification and
restricted in definition.
Even yet, however, we are far from a scientific classification for
bacteria. Nor is this matter for surprise. The development in this
branch of biology has been so rapid that it has been impossible
to assimilate the facts collected. The facts themselves by their
remarkable variety have not aided classification. Names which a few
years ago were applied to individual species, like _Bacillus subtilis_,
or _Bacterium termo_, or _Bacillus coli_, are now representative,
not of individuals, but of families and groups of species. Again,
isolated characteristics of certain microbes, such as motility, power
of liquefying gelatine, size, colour, and so forth, which at first
sight might appear as likely to form a basis for classification, are
found to vary not only between similar germs, but in the same germ.
Different physical conditions have so powerful an influence upon these
microscopic cells that their individual characters are constantly
undergoing change. For example, bacteria in old cultures assume a
different size, and often a different shape, from younger members of
precisely the same species; _Bacillus pyocyaneus_ produces a green to
olive colour on gelatine, but a brown colour on potato; the bacillus
of Tetanus is virulently pathogenic, and yet may not act thus unless
in company with certain other micro-organisms. Hence it will at once
appear to the student of bacteriology that, though there is great
need for classification amongst the six or seven hundred species of
microbes, our present knowledge of their life-history is not yet
advanced enough to form more than a provisional arrangement.
We know that bacteria are allied to moulds on the one hand and yeasts
on the other, and that they have no differentiation into root, stem, or
leaf; we know that they are fungi (having no chlorophyll), in which no
sexual reproduction occurs, and that their mode of multiplication is by
division. From such facts as these we may build up a classification as
follows:--
VEGETABLE KINGDOM.
|
+---------------+--------+-------+------------------+
| | | |
Thallophyta. Muscineæ. Pteridophyta. Phanerogamia.
[= The lowest forms
of vegetable life. No
differentiation into
root, stem, or leaf.]
|
Protophyta.
[= No sexual reproduction.]
|
+-------+------------+
| |
Algæ. Fungi.
[= Chlorophyll [= No chlorophyll.]
present.] |
|
+--+--+--+--+--+--+
| | | | | | |
|
Schizomycetes {(1) Coccaceæ[4]--round cells.
[= multiplication by cell {
division or by spores] {(2) Bacteriaceæ--rods and threads.
{
or {(3) Leptotricheæ. }
Bacteria. { } Higher Bacteria.
{(4) Cladotricheæ. }
_Structure and Form._ Having now located micro-organisms in the economy
of nature, we may proceed to describe their subdivisions and form. For
practical convenience rather than academic accuracy, we may accept
the simple division of the family of bacteria into three chief forms,
viz.:--
{ (1) Round cell form--_coccus_.
{
Lower Bacteria { (2) Rod form--_bacillus_.
{
{ (3) Thread form--_spirillum_.
Higher Bacteria--Leptothrix, Streptothrix, Cladothrix, etc.
A classification dependent as this is upon the form alone is not by any
means ideal, for it ignores all the higher and complicated functions of
bacteria, but it is, as we have said, practically convenient.
[Illustration: VARIOUS FORMS OF BACTERIA
1. Micrococcus
2. Diplococcus
3. Streptococcus
4. Staphylococcus
5. Leuconostoc, showing Arthrospores
6. Merismopedia
7. Sarcina
8. Bacilli
9. Spirillum]
1. _The Coccus._ This is the group of round cells. They vary in size as
regards species, and as regards the conditions, artificial or natural,
under which they have been grown. Some are less than 1/25000 of an inch
in diameter; others are half as large again, if the word large may be
used to describe such minute objects. No regular standard can be laid
down as reliable with regard to their size. Hence the subdivisions
of the cocci are dependent not upon the individual elements so much
as upon the relation of those elements to each other. A simple round
cell of approximately the size already named is termed a _micrococcus_
(μικρος, small). Certain species of micrococci always or almost always
occur in pairs, and such a combination is termed a _diplococcus_. Some
diplococci are united by a thin capsule, which may be made apparent by
special methods of staining; of others no limiting or uniting membrane
can be seen with the ordinary high powers of the microscope.[5] Again,
one frequently finds a species which is exactly described by saying
that two micrococci are in contact with each other, and move and act
as one individual, but otherwise show no alteration; whilst others
are seen which show a flattening of the side of each micrococcus
which is in relation to its partner. Perhaps the diplococci in an even
greater degree than the micrococci respond to external conditions
both as regards size and shape. It must further be borne in mind that
a dividing micrococcus assumes the exact appearance of a diplococcus
during the transition stage of the fission. Hence, with the exception
of several well-marked species of diplococci, this form is somewhat
arbitrary. The third kind of micrococcus is that formed by a number
of elements in a twisted chain, named _streptococcus_ (στρεπτος,
twisted). This form is produced by cells dividing in one axis, and
remaining in contact with each other. It occurs in a number of
different species, or what are supposed by many authorities to be
different species, owing to their different effects. Morphologically
all the streptococci are similar, though a somewhat abortive attempt
was once made to divide them into two groups, according to whether
they were long chains or short. As a matter of fact, the length of
streptococci depends in some cases upon biological properties, in
others upon external treatment or the medium of cultivation which has
been used. Sometimes they occur as straight chains of only half a dozen
elements; at other times they may contain thirty to forty elements,
and twist in various ways, even forming rosaries. The elements, too,
differ not only in size, but in shape, appearing occasionally as
oval cells united to each other at their sides. The fourth form is
constituted by the micrococci being arranged in masses like grapes, the
_staphylococcus_ (σταφυλις, a bunch of grapes). The elements are often
smaller than in the streptococcus, and the name itself describes the
arrangement. There is no matrix and no capsule. This is the commonest
organism found in abscesses, etc. The _sarcina_ is best classified
amongst the cocci, for it is composed of them, in packets of four or
multiples of four, produced by division vertically in two planes. If
the division occurs in one plane, we have as a result small squares
of round cells known as _merismopedia_. In both these conditions it
frequently happens that the contiguous sides of the elements of packets
become faceted or straightened against each other. It may happen, too,
particularly in the _sarcinæ_, that segmentation is not complete,
and that the elements are larger than in any other class of cocci.
They stain very readily. Nearly all the cocci are non-motile, though
Brownian movement may readily be observed.
[Illustration: SARCINA]
2. _The Bacilli._ These consist of rods, having parallel sides and
being longer than they are broad. They differ in every other respect
according to species, but these two characteristics remain to
distinguish them. Many of them are motile, others not. The ends or
poles of a bacillus may be pointed, round, or almost exactly square and
blocked. They all, or nearly all, possess a capsule. Individuals of the
same species may differ greatly, according to whether they have been
naturally or artificially grown, and pleomorphic forms are abundant.
3. _The Spirilla._ This wavy thread group is divisible into a number
of different forms, to which authorities have given special names. It
is sufficient, however, to state that the two common forms are the
non-septate spiral thread (like the _Spirillum Obermeier_ of relapsing
fever), which takes no other form but a lengthened spirillum; and the
spirillum which breaks up into elements or units, each of which appears
comma-shaped (like the cholera bacillus). The degree of curvature in
the spirilla, of course, varies. They are the least important of the
lower bacteria.
The _Higher Bacteria_ group includes more highly organised members
of the Schizomycetes. They possess filaments, which may be branched,
and almost always have septa and a sheath. Perhaps the most marked
difference from the lower bacteria is in their reproduction. In
the higher bacteria we have what is in fact a flower--terminal
fructification by conidia. In this group of vegetables we have the
Beggiatoa, Leptothrix, Cladothrix, and, at the top, the Streptothrix.
It has been demonstrated that _Streptothrix actinomycotica_ and
_Streptothrix maduræ_ are the organismal cause, respectively, of
Actinomycosis and Madura-foot, two diseases which have hitherto been
obscure.
_Pleomorphism._ This term designates an irregular development of
a species. Different media and external conditions bring about in
protoplasm as susceptible as mycoprotein a variety of morphological
phases. These may occur in succession, and represent different stages
in the life-history of a bacterium, or they may be involution forms
resulting from a change of environment, and occurring as "faults" in
the species. In the _Bacillus coli_, _B. typhosus_, bacillus of Plague,
and _B. tuberculosis_ pleomorphism undoubtedly occurs, and is manifest
in the change of shape. This is particularly marked in old cultures
of the last named. The ordinary well-known bacillus may grow out into
threads, with bulbous endings, granular filaments, drumsticks, and
diplococcal forms. Speaking generally, the older the culture, the more
marked is the variation.
_Polymorphism_ is a term used to define the theory which held that
bacteria were one of the intermediate shapes or forms between
something lower and something higher in the vegetable kingdom.
Neither pleomorphism nor polymorphism is fully understood, and many
bacteriologists find shelter from both in the term _involution form_.
What we do know is that the species already named, for example, take on
divers forms when placed under different conditions.
_Composition._ From what we have seen of the diet of micro-organisms,
we shall conclude that in some form or other they contain the elements
nitrogen, carbon, and hydrogen. All three substances are combined
in the _mycoprotein_ or protoplasm of which the body of the microbe
consists. This is generally homogeneous, and there is no sign of a
nucleus. It possesses a fortunate affinity for aniline dyes, and by
this means organisms are stained for the microscope. Besides the
variable quantity of nitrogen present, mycoprotein may also contain
various mineral salts. The uniformity of the cell protoplasm may
be materially affected by disintegration and _segmentation_ due to
degenerative changes. _Vacuoles_ also may appear from a like cause,
which it is necessary to differentiate from spores. Two other signs
of degeneration are the appearance of granules in the body of the
cell protoplasm known as _metachromatic granules_, owing to their
different staining propensities, and the _polar bodies_ which are seen
in some species of bacteria. Surrounding the mass of mycoprotein, we
find in most organisms a capsule or membrane composed, in part at
least, of _cellulose_. This sheath plays a protective part in several
ways. During the adult stage of life it protects the mycoprotein,
and holds it together. At the time of reproduction or degeneration
it not infrequently swells up, and forms a viscous hilum or matrix,
inside which are formed the new sheaths of the younger generation. It
may be rigid, and so maintain the normal shape of the species, or,
on the other hand, flexible, and so adapted to rapid movement of the
individual.
[Illustration: NORMAL AND PLEON-FORMS OF THE BACILLUS OF TUBASH.]
Here, then, we have the major parts in the constitution of a
bacillus--its body, mycoprotein; its capsule, cellulose. But,
further than this, there are a number of additional distinctive
characteristics as regards the contents inside the capsule which
call for mention. _Sulphur_ occurs in the Beggiatoa which thrive
in sulphur springs. _Starch_ is commoner still. _Iron_ as oxide
or other combination is found in several species. Many are highly
coloured, though these are generally the "innocent" bacteria, in
contradistinction to the disease-producing. A pigment has been found
which is designated _bacterio-purpurin_. According to Zopf, the
colouring agents of bacteria are the same as, or closely allied to, the
colouring matters occurring widely in nature. Migula holds that most
of the bacterial pigments are non-nitrogenous bodies. There are a very
large number of chromogenic bacteria, some of which produce exceedingly
brilliant colours. Among some of the commoner forms possessing this
character are _Bacillus et micrococcus violaceus_ (violet); _B. et M.
aurantiacus_ (orange); _B. et M. luteus_ (yellow); _M. roseus_ (pink);
many of the _Sarcinæ_; _B. aureus_ (golden-yellow); _B. fluorescens
liquefaciens et non-liquefaciens_ (green); _B. pyocyaneus_ (green); _B.
prodigiosus_ (blood-red).
_Motility._ When a drop of water containing bacteria is placed upon
a slide, a clean cover glass superimposed, and the specimen examined
under an oil immersion lens, various rapid movements will generally
be observed. These are of four kinds: (1) A dancing stationary
motion known as _Brownian movement_. This is molecular, and depends
in some degree upon heat and the medium of the moving particles. It
is non-progressive, and is well known in gamboge particles. (2) An
_undulatory_ serpentine movement, with apparently little advance being
made. (3) A _rotatory_ movement, which in some water bacilli is very
marked, and consists of spinning round, with sometimes considerable
velocity, and maintained for some seconds or even minutes. (4) A
_progressive_ darting movement, by which the bacillus passes over some
considerable distance.
The conditions affecting the motion of bacteria are but partly
understood. Heating the slide or medium accelerates all movement.
A fresh supply of oxygen, or indeed the addition of some nutrient
substance, like broth, will have the same effect. There are also the
somewhat mysterious powers by which cells possess inherent attraction
or repulsion for other cells, known as _positive_ and _negative
chemiotaxis_. These powers have been observed in bacteria by Pfeiffer
and Ali-Cohen.
[Illustration: BACILLI, SHOWING FLAGELLA]
The essential condition in the motile bacilli is the presence of
_flagella_.[6] These cilia, or hairy processes, project from the
sides or from the ends of the rod, and are freely motile and elastic.
Sometimes only one or two terminal flagella are present; in other
cases, like the bacillus of typhoid fever, five to twenty may occur all
round the body of the bacillus, varying in length and size, sometimes
being of greater length even than the bacillus itself. It is not yet
established as to whether these vibratile cilia are prolongations of
capsule only, or whether they contain something of the body
protoplasm. Migula holds the former view, and states that the position
of flagella is constant enough for diagnostic purposes. They are but
rarely recognisable except by means of special staining methods.
_Micrococcus agilis_ (Ali-Cohen) is the only coccus which has flagella
and active motion.
_Modes of Reproduction._ Budding, division, and spore formation are the
three chief ways in which Schizomycetes and Saccharomycetes (yeasts)
reproduce their kind. _Budding_ occurs in some kinds of yeast, and
would be classified by some authorities under spore formation, but in
practice it is so obviously a "budding" that it may be so classified.
The capsule of a large or mother cell shows a slight protrusion
outwards which is gradually enlarged into a daughter yeast and later
on becomes constricted at the neck. Eventually it separates as an
individual. The protoplasm of spores of yeasts differs, as Hansen has
pointed out, according to their conditions of culture.
_Division_, or fission, is the commonest method of reproduction. It
occurs transversely. A small indentation occurs in the capsule, which
appears to make its way slowly through the whole body of the bacillus
or micrococcus until the two parts are separate, and each contained in
its own capsule. It has been pointed out already that in the incomplete
division of micrococci we observe a stage precisely similar to a
diplococcus. So also in the division of bacilli an appearance occurs
described as a diplobacillus.
Simple fission requires but a short period of time to be complete.
Hence multiplication is very rapid, for within half an hour a new
adult individual can be produced. It has been estimated that at this
rate one bacillus will in twenty-four hours produce 17,000,000 similar
individuals; or, expressed in another way, Cohn calculated that in
three days, under favourable circumstances, this rate of increase would
form a mass of living organisms weighing 7300 tons, and numbering
about 4772 billions. Favourable conditions do not occur, fortunately,
to allow of such increase, which, of course, can only be roughly
estimated. But the above figures illustrate the enormous fertility of
micro-organic life. When we remember that in some species it requires
10,000 or 15,000 fully grown bacilli placed end to end to stretch
the length of an inch, we see also how exceedingly small are the
individuals composing these unseen hosts.
_Spore formation_ may result in the production of germinating cells
inside the capsule of the bacillus, _endospores_, or of modified
individuals, _arthrospores_. The body of a bacillus, in which
sporulation is about to occur, loses its homogeneous character
and becomes granular, owing to the appearance of globules in the
protoplasm. In the course of three or four hours the globule enlarges
to fill the diameter of the rod, and assumes a more concentrated
condition than the parent cell. At its maturity, and before its
rupture of the bacillary capsule, a spore is observed to be bright and
shining, oval and regular in shape, with concentrated contents, and
frequently causing a local expansion of the bacillus. In a number of
rods lying endwise, these local swellings produce a beaded or varicose
appearance, even simulating a streptococcus. In the meantime the rod
itself has become slightly broader and pale. Eventually it breaks down
by segmentation or by swelling up into a gelatinous mass. The spore
now escapes and commences its individual existence. Under favourable
circumstances it will germinate. The tough capsule gives way at one
point, generally at one of the poles, and the spore sprouts like a
seed. In the space of about one hour's time the oval refractile cell
has become a new bacillus. One spore produces by germination one
bacillus. Spores never multiply by fission, nor reproduce themselves.
Hueppe has stated that there are certain organisms (like leuconostoc,
and some streptococci) which reproduce by the method of _arthrospores_.
Defined shortly, this is simply an enlargement of one or more cell
elements in the chain which thus takes on the function of maternity.
On either side of the large coccus may be seen the smaller ones, which
it is supposed have contributed of their protoplasm to form a mother
cell. An arthrospore is said to be larger, more refractile, and more
resistant than an ordinary endospore. Many bacteriologists of repute
have declined hitherto to definitely accept arthrospore formation as a
proved fact.
[Illustration: VARIOUS FORMS OF SPORE FORMATION AND FLAGELLA
A. Stages in formation of spore and its after development. B. Spirillum
with terminal flagella.]
It is important to note that spore formation in bacteria must not
be considered as a method of multiplication. The general rule
is undoubtedly that one bacillus produces one spore, and one
spore germinates into one bacillus. It is a reproduction, not a
multiplication. Indeed, the whole process is of the nature of a
resting stage, and is due (_a_) to the arrival of the adult bacillus
at its biological zenith, or (_b_) to the conditions in which it
finds itself being unfavourable to its highest vegetative growth,
and so it endeavours to perpetuate its species. Most authorities are
probably of the latter opinion, though there is not a little evidence
for the former. Exactly what conditions are favourable to sporulation
is not known. Nutriment has probably an intimate effect upon it. The
temperature must not be below 16° C., nor much above 40° C. Oxygen, as
we have seen, is favourable, if not necessary, to many species, which
will in cultivation in broth rise to the surface and lodge in the
pellicle to form their seeds. Moisture, too, is considered a necessity.
The position and size of the spore are of considerable use in
differential diagnosis. The terminal spore of _Bacillus tetani_ is
well known. It is rarely seen at both ends of the bacillus, and
hence when poised only at one end causes the "drumstick" appearance.
In the bacillus of Quarter Evil the spore is generally towards one
end of the rod rather than in the middle; in Malignant Œdema the
bacillus in the blood grows out into long threads, and when such a
thread sporulates the spore is also near one end. The latter further
illustrates the fact that in some species the spore is of greater
diameter than the mother cell, and hence dilates the bacillary capsule.
The spores of anthrax are typical oval endospores. When free in the
field of the microscope, spores must be distinguished from fat cells,
micrococci, starch cells, some kinds of ova, yeast cells, and other
like objects. Spores are detected frequently by their resistance to
ordinary stains and the necessity of colouring them by special staining
methods. When, however, a spore has taken on the desired colour, it
retains it with tenacity. In addition to their shape, size, thickened
capsule, and staining characteristics, spores also resist desiccation
and heat in a much higher degree than bacilli not bearing spores. Roux
and some other eminent bacteriologists suggest that bacteria should be
classified according to their method of spore formation.
THE INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH OF BACTERIA
_Nutritive Medium._ In the very earliest days of the study of
micro-organisms it was observed that they mostly congregate where there
is pabulum for their nourishment. The reason why fluids such as milk,
and dead animal matter such as a carcass, and living tissues such as
a man's body contain so many microbes is because each of these three
media is favourable to their growth. Milk affords almost an ideal
food and environment for microbes. Its temperature and constitution
frequently meet their requirements. Dead animal matter, too, yields
a rich diet for some species (saprophytes). In the living tissues
bacteria obtain not only nutriment, but a favourable temperature
and moisture. Outside the human body it has been the endeavour of
bacteriologists to provide media as like the above as possible, and
containing many of the same elements of food. Thus the life-history
may be carried on outside the body and under observation. By means of
cover-glass preparations for the microscope we are able to study the
form, size, motility, flagella, spore formation, and peculiarities
of staining, all of which characters aid us in determining to what
species the organism under examination belongs. By means of artificial
nutrient media we may further learn the characters of the organism in
"pure culture,"[7] its favourable temperature, its power or otherwise
of liquefaction, the curdling milk, or of gas production, its behaviour
towards oxygen, its power of producing indol, pigment, and chemical
bodies, as well as its thermal death point and resistance to light
and disinfectants. It is well known that under artificial cultivation
an organism may be greatly modified in its morphology and physiology,
and yet its conformity to type remains much more marked than any
degeneration which may occur.
The basis of many of these artificial media is _broth_. This is made
from good lean beef, free from fat and gristle, which is finely
minced up and extracted in sterilised water (one pound of lean beef
to every 1000 cc. of water). It is then filtered and sterilised. It
will be understood that such an extract is acid. To provide _peptone
beef-broth_, ten grains of peptone and five grains of common salt
are added to every litre of acid beef-broth. It is rendered slightly
alkaline by the addition of sodium carbonate, and is filtered and
sterilised. _Glycerine-broth_ indicates that 6 to 8 per cent. of
glycerine has been added after filtration, _glucose-broth_ 1 or 2 per
cent. of grape-sugar. This latter is used for anaërobic organisms. The
use of broth as a culture medium is of great value. It is undoubtedly
our best fluid medium, and in it may not only be kept pure cultures of
bacteria which it is desired to retain for a length of time, but in
it also emulsions and mixtures may be placed preparatory to further
operations. _Gelatine_ is broth solidified by the addition of 100 grams
of best French gelatine to the litre. Its advantage is twofold: it is
transparent, and it allows manifestation of the power of liquefaction.
When we speak of a liquefying organism we mean a germ having the power
of producing a peptonising ferment which can at the temperature of the
room break down solid gelatine into a liquid. _Grape-sugar gelatine_ is
made like grape-sugar broth. _Agar_ was introduced as a medium which
would not melt at 25° C., like gelatine, but remain solid at blood-heat
(37·5° C.; 98·5° F.). It is a seaweed generally obtained in dried
strips from the Japanese market. Ten to fifteen grams are added to
every litre of peptone-broth. Filtration is slow and often difficult,
and the result not as transparent as desirable. The former difficulty
is avoided by filtering in the Koch's steamer or with a hot-water
filter, the latter by the addition of the white of an egg. Glycerine
and grape-sugar may be added as elsewhere. _Blood agar_ is ordinary
agar with fresh sterile blood smeared over its surface. _Blood serum_
is drawn from a jar of coagulated horse-blood, in which the serum has
risen to the top. This is collected in sterilised tubes and coagulated
in a special apparatus (the serum inspissator). _Potato_ is prepared
by scraping ordinary potatoes, washing in corrosive sublimate, and
sterilising. They may then be cut into various shapes convenient for
cultivation. Upon any of these forms of solid media the characteristic
growth of the organism can be observed. Of the nutrient elements
required, nitrogen is obtained from albumens and proteids, carbon
from milk-sugar, cane-sugar, or the splitting up of proteids; salts
(particularly phosphates and salts of potassium) are readily obtainable
from those incorporated in the media; and the water which is required
is obtainable from the moisture of the media.
[Illustration: POTATO IN A ROUX TUBE PREPARED FOR CULTIVATION
Glycerine is placed in the bulb of the tube]
There are two common forms of test-tube culture, viz.: on the surface
and in the depth of the medium. In the former the medium is sloped,
and the inoculating needle is drawn along its surface; in the latter
the needle is thrust vertically downwards into the depth of the solid
medium. Plate cultures and anaërobic cultures will be described at a
later stage. When the medium has been inoculated the culture is placed
at a temperature which will be favourable. Two standards of temperature
are in use in bacteriological laboratories. The one is called _room
temperature_, and varies from 18° C.-20° C.; the other is _blood-heat_,
and varies from 35° C.-38° C. It is true, some species will grow below
18° C., and others above 38° C. The pathogenic (disease-producing)
bacteria thrive best at 37° C., and the non-pathogenic at the ordinary
temperature of the room. The different degrees of temperature are
regulated by means of incubators. For the low temperatures gelatine
is chosen; as a medium for the higher temperatures agar.
[Illustration: STAPHYLOCOCCUS PYOGENES AUREUS
× 1000]
[Illustration: INCUBATOR
(Temperature of blood-heat, registered by thermometer, and regulated by
thermo-regulator)]
_Moisture_ has been shown to have a favourable effect upon the growth
of microbes. Drying will of itself kill many species (_e. g._, the
spirillum of cholera), and, other things being equal, the moister a
medium is, the better will be the growth upon it. Thus it is that the
growth in broth is always more luxuriant than that on solid media. Yet
the growth of _Bacillus subtilis_ and other species is an exception to
this rule, for they prefer a dry medium.
[Illustration: CULTURE MEDIA READY FOR INOCULATION]
_Temperature._ Most bacteria grow well at room temperature, but they
will grow more luxuriantly and speedily at blood-heat. The optimum
temperature is generally that of the natural habitat of the organism.
In exceptional cases growth will occur as low as 5° C. or as high as
70° C. Indeed, some have been cooled to-20° C. and-30° C., and yet
retained their vitality,[8] whereas some few can grow at 60-70° C.
These latter are termed _thermophilic_ bacteria. The average thermal
death-point is at or about 50° C.
[Illustration: INOCULATING NEEDLES
Plantinum wire fused into glass handles]
_Light_ acts as an inhibitory or even germicidal agent. This fact
was first established by Downes and Blunt in a memoir to the Royal
Society in 1877. They found by exposing cultures to different degrees
of sunlight that thus the growth of the culture was partially or
entirely prevented, being most damaged by the direct rays of the sun,
although diffuse daylight acted prejudicially. Further, these same
investigators proved that of the rays of the spectrum which acted
inimically the blue and violet rays acted most bactericidally, next
to the blue being the red and orange-red rays. The action of light,
they explain, is due to the gradual oxidation which is induced by the
sun's rays in the presence of oxygen. Duclaux, who worked at this
question at a later date, concluded that the degree of resistance to
the bactericidal influence of light which some bacteria possess might
be due to difference in species, difference in culture media, and
difference in the degrees of intensity of light. Tyndall tested the
growth of organisms in flasks exposed to air and light on the Alps, and
found that sunlight inhibited the growth temporarily. A large number
of experimenters in Europe and England have worked at this fascinating
subject since 1877, and though many of their results appear
contradictory, we may be satisfied to adopt the following conclusions
respecting the matter:
(1) Sunlight has a deleterious effect upon bacteria, and to a less
extent on their spores.
(2) This inimical effect can be produced by light irrespective of rise
in temperature.
(3) The ultra-violet rays are the most bactericidal, and the infra-red
the least so, which indicates that the phenomenon is due to chemical
action.
(4) The presence of oxygen and moisture greatly increases this action.
(5) The sunlight acts prejudicially upon the culture medium, and
thereby complicates the investigation and after-growth.
(6) The time occupied in the bactericidal action depends upon the heat
of the sun and the intrinsic vitality of the organism.
(7) With regard to the action of light upon pathogenic organisms, some
results have recently been obtained with _Bacillus typhosus_. Janowski
maintains that direct sunlight exerts a distinctly depressing effect on
typhoid bacilli. At present more cannot be said than that sunlight and
fresh air are two of the most powerful agents we possess with which to
combat pathogenic germs.
[Illustration: PASTEUR'S LARGE INCUBATOR FOR CULTIVATION AT ROOM
TEMPERATURE]
A very simple method of demonstrating the influence of light is to grow
a pure culture in a favourable medium, either in a test-tube or upon
a glass plate, and then cover the whole with black paper or cloth. A
little window may then be cut in the protective covering, and the whole
exposed to the light. Where it reaches in direct rays it will be found
that little or no growth has occurred; where, on the other hand, the
culture has been in the dark, abundant growth occurs. In diffuse light
the growth is merely somewhat inhibited. It has been found that the
electric light has but little action upon bacteria, though that which
it has is similar to sunlight. Recent experiments with the Röntgen rays
have given negative results.
In 1890 Koch stated that tubercle bacilli were killed after an
exposure to direct sunlight of from a few minutes to several hours.
The influence of diffuse light would obviously be much less. Professor
Marshall Ward has experimented with the resistant spores of _Bacillus
anthracis_ by growing these on agar plates and exposing to sunlight.
From two to six hours' exposure had a germicidal effect.
It should be remembered that several species of sea-water bacteria
themselves possess powers of phosphorescence. Pflüger was the first to
point out that it was such organisms which provided the phosphorescence
upon decomposing wood or decaying fish. To what this light is due,
whether capsule, or protoplasm, or chemical product, is not yet known.
The only facts at present established are to the effect that certain
kinds of media and pabulum favour or deter phosphorescence.
_Desiccation._ A later opportunity will occur for consideration of
the effect of drying upon bacteria. Here it is only necessary to say
that, other things being equal, drying diminishes virulence and lessens
growth.
_Oxygen._ Pasteur was the first to lay emphasis upon the effect which
free air had upon micro-organisms. He classified them according to
whether they grew in air, _aërobic_, or whether they flourished most
without it, _anaërobic_. Some have the faculty of growing with or
without the presence of oxygen, and are designated as _facultative_
aërobes or anaërobes. As regards the cultivation of anaërobic germs, it
is only necessary to say here that hydrogen, nitrogen, or carbonic acid
gas may be used in place of oxygen, or they may be grown in a medium
containing some substance which will absorb the oxygen.
_Modes of Bacterial Action._ In considering the specific action of
micro-organisms, it is desirable, in the first place, to remember
the two great functional divisions of saprophyte and parasite. A
_saprophyte_ is an organism that obtains its nutrition from dead
organic matter. Its services, of whatever nature, lie outside the
tissues of living animals. Its life is spent apart from a "host." A
_parasite_, on the other hand, lives always at the expense of some
other organism which is its host, in which it lives and upon which it
lives. There is a third or intermediate group, known as "facultative,"
owing to their ability to act as parasites or saprophytes, as the
exigencies of their life-history may demand.
[Illustration: METHOD OF PRODUCING HYDROGEN BY KIPP'S APPARATUS FOR
CULTIVATION OF ANAËROBES (See page 139)]
[Illustration: ANAËROBIC CULTURE
(Buckner's Tube) with Pyrogallic Solution in Bulb.]
The saprophytic organisms are, generally speaking, those which
contribute most to the benefit of man, and the parasitic the reverse,
though this statement is only approximately true. In their relation
to the processes of fermentation, decomposition, nitrification, etc.,
we shall see how great and invaluable is the work which saprophytic
microbes perform. Their result depends, in nearly all cases, upon the
organic chemical constitution of the substances upon which they are
exerting their action, as well as upon the varieties of bacteria
themselves. Nor must it be understood that the action of saprophytes is
wholly that of breaking down and decomposition. As a matter of fact,
some of their work is, as we shall see, of a constructive nature; but,
of whichever kind it is, the result depends upon the organism and its
environment. This, too, may be said of the pathogenic species, all
of which are in a greater or less degree parasitic. It is well known
how various are the constitutions of man, how the bodies of some
persons are more resistant than those of others, and how the invading
microbe will find different receptions according to the constitution
and idiosyncrasy of the body which it attacks. Indeed, even after
invasion the infectivity of the special disease, whatever it happens
to be, will be materially modified by the tissues. When we come to
turn to the micro-organisms which are pathogenic parasites we shall
further have to keep clear in our minds that their action is double
and complex, and not single or simple. In the first place, we have
an infection of the body due to the bacteria themselves. It may be a
general and widespread infection, as in anthrax, where the bacilli
pass, in the blood or lymph current, to each and every part of the
body; or it may be a comparatively local one, as in diphtheria, where
the invader remains localised at the site of entrance. But, be that as
it may, the micro-organisms themselves, by their own bodily presence,
set up changes and perform functions which may have far-reaching
effects. It is obvious that the wider the distribution the wider is
the area of tissue change, and _vice versâ_. Yet there is something
of far greater importance than the mere presence of bacteria in human
or animal tissues; for the secondary action of disease-producing
germs--and possibly it is present in all bacteria--is due to their
poisonous products, or _toxins_, as they have been termed. These may
be of the nature of ferments, and they become diffused throughout the
body, whether the bacteria themselves occur locally or generally. They
may bring about very slight and even imperceptible changes during the
course of the disease, or they may kill the patient in a few hours.
Latterly bacteriologists have come to understand that it is not so much
the presence of organisms which is injurious to man and other animals,
as it is their products which cause the mischief; and the amount of
toxic product bears no known proportion to the degree of invasion by
the bacteria. The various and widely differing modes of action in
bacteria are therefore dependent upon these three elements: the tissues
or medium, the bacteria, and the products of the bacteria; and in all
organismal processes these three elements act and react upon each other.
A word may be said here respecting the much-discussed question of
_species_ in bacteria. A species may be defined as "a group of
individuals which, however many characters they share with other
individuals, agree in presenting one or more characters of a peculiar
and hereditary kind with some certain degree of distinctness."[9] Now,
as regards bacteria, there is no doubt that separate species occur
and tend to remain as separate species. It is true, there are many
variations, due in large measure to the medium in which the organisms
are growing,--variations of age, adaptation, nutrition, etc.,--yet
the different species tend to remain distinct. Involution forms occur
frequently, and degeneration invariably modifies the normal appearance.
But because of the occurrence of these morphological and even
pathological differences it must not be argued that the demarcation of
species is wholly arbitrary.
_Means of Sterilisation._ As this term occurs frequently in even a book
of this untechnical nature, and as it is expressive of an idea which
must always be present to the mind of the bacteriologist, it may be
desirable to make some passing allusion to it.
Chemical substances, perfect filtration, and heat are the three means
at our command in order to secure germ-free conditions of apparatus or
medium. The first two, though theoretically admissible, are practically
seldom used, the former of the two because the addition of chemical
substances annuls or modifies the operation, the latter of the two on
account of the great practical difficulties in securing perfection.
Hence in the investigation involved in bacteriological research heat
is the common sterilising agent. A temperature of 70° C. (158° F.)
will kill all bacilli; even 58° C. will kill most kinds. Boiling at
100° C. (212° F.) for three minutes will kill anthrax spores, and
boiling for thirty to sixty minutes will kill all bacilli and all
spores. This difference in the _thermal death-point_ between bacilli
and their spores enables the operator to obtain what are called
"pure cultures" of a desired bacillus from its spores which may be
present. For example, if a culture contains spores of anthrax and is
contaminated with micrococci, heating to 70° C. (158° F.) will kill
all the micrococci, but will not affect the spores of anthrax, which
can then grow into a pure culture of anthrax bacilli. _Fractional or
discontinuous sterilisation_ depends on the principle of heating to
the sterilising point for bacilli (say 70°C.) on one day, which will
kill the bacilli, but leave the spores uninjured. But by the following
day the spores will have germinated into bacilli, and a second heating
to 70°C. will kill them before they in their turn have had time to
sporulate. Thus the whole will be sterilised, though at a temperature
below boiling.
Successful sterilisation, therefore, depends upon killing both bacteria
and their spores, and nothing short of that can be considered as
sterilisation. The following methods are those generally used in the
laboratory. For dry heat (which is never so injurious to organisms
as moist heat)[10]: (_a_) _the Bunsen burner_, in the flame of which
platinum needles, etc., are sterilised; (_b_) _hot-air chamber_, in
which flasks and test-tubes are heated to a temperature of 150-170°
C. for half an hour. For moist heat: (_c_) _boiling_, for knives and
instruments; (_d_) _Koch's steam steriliser_, by means of which a crate
is slung in a metal cylinder, at the bottom of which the water is
boiled; (_e_) _the autoclave_, which is the most rapid and effective
of all the methods. This is in reality a Koch steriliser, but with
apparatus for obtaining high pressure. The last two (_d_, _e_) are
used for sterilising the nutriment media upon which bacteria are
cultivated outside the body. Blood serum would, however, coagulate at a
temperature over 60° C. (124° F.), and hence a special steriliser has
been designed to carry out fractional sterilisation daily for a week at
about 55° C.-58° C.
[Illustration: KOCH'S STEAM STERILISER]
_The Association of Organisms._ At a later stage we shall have an
opportunity of discussing symbiosis and allied conditions. Here it is
only necessary to draw attention to a fact that is rapidly becoming of
the first importance in bacteriology. When species were first isolated
in pure culture it was found that they behaved somewhat differently
under differing circumstances. This modification in function has been
attributed to differences of environment and physical conditions.
Whilst it is true that such external conditions must have a marked
effect upon such sensitive units of protoplasm as bacteria, it has
recently been proved that one great reason why modification occurs in
pure artificial cultures is that the species has been isolated from
amongst its colleagues and doomed to a separate existence. One of
the most abstruse problems in the immediate future of the science of
bacteriology is to learn what intrinsic characters there are in species
or individuals which act as a basis for the association of organisms
for a specific purpose. Some bacteria appear to be unable to perform
their regular function without the aid of others. An example of such
association is well illustrated in the case of tetanus, for it has been
shown that if the bacilli and spores of tetanus alone obtain entrance
to a wound the disease may not follow the same course as when with the
specific organism the lactic-acid bacillus or the common organisms of
suppuration or putrefaction also gain entrance. There is here evidently
something gained by association. Again, the virulence of other bacteria
is also increased by means of association. The _Bacillus coli_ is an
example, for, in conjunction with other organisms, this bacillus,
although normally present in health in the alimentary canal, is able to
set up acute intestinal irritation, and various changes in the body
of an inflammatory nature. It is not yet possible to say in what way
or to what degree the association of bacteria influences their _rôle_.
That is a problem for the future. But whilst we have examples of this
association in streptococcus and the bacillus of diphtheria, _B. coli_
and yeasts, tetanus and putrefactive bacteria, _Diplococcus pneumoniæ_
and streptococcus, and association amongst the various suppurative
organisms, we cannot doubt that there is an explanation to be found
here of many hitherto unsolved results of bacterial action. This is
the place in which mention should also be made of higher organisms
associated for a specific purpose with bacteria. There is some evidence
to support the belief that some of the Leptotricheæ (Crenothrix,
Beggiatoa, Leptothrix, etc.) and the Cladotricheæ (Cladothrix) perform
a preliminary disintegration of organic matter before the decomposing
bacteria commence their labours. This occurs apparently in the
self-purification of rivers, as well as in polluted soils.
_Antagonism of Bacteria._ Study of the life-history of many of the
water bacteria will reveal the fact that they can live and multiply
under conditions which would at once prove fatal to other species. Some
of these water organisms can indeed increase and multiply in distilled
water, whereas it is known that other species cannot even live in
distilled water, owing to the lack of pabulum. Thus we see that what is
favourable for one species may be the reverse for another.
Further, we shall have opportunity of observing, when considering
the bacteriology of water and sewage, that there is in these media
in nature a keen struggle for the survival of the fittest bacteria
for each special medium. In a carcass it is the same. If saprophytic
bacteria are present with pathogenic, there is a struggle for the
survival of the latter. Now whilst this is in part due to a competition
owing to a limited food supply and an unlimited population, as occurs
in other spheres, it is also due in part to the inimical influence of
the chemical products of the one species upon the life of the bacteria
of the other species. Moreover, in one culture medium, as Cast has
pointed out, two species will often not grow. When Pasteur found that
exposure to air attenuated his cultures, he pointed out that it was not
the air _per se_ that hindered his growth, but it was the introduction
of other species which competed with the original. The growth of the
spirillum of cholera is opposed by _Bacillus pyogenes fœtidus_.
_B. anthracis_ is, in the body, opposed by either _B. pyocyaneus_ or
_Streptococcus erysipelatis_, and yet it is aided in its growth by _B.
prodigiosus_. _B. aceti_ is, under certain circumstances, antagonistic
to _B. coli communis_.
In several of the most recent of the admirable reports of Sir Richard
Thorne issued from the Medical Department of the Local Government
Board, we have the record of a series of experiments performed by Dr.
Klein into this question of the antagonism of microbes. From this
work it is clearly demonstrated that whatever opposition one species
affords to another it is able to exercise by means of its poisonous
properties. These are of two kinds. There is, as is now widely known,
the poisonous product named the _toxin_, into which we shall have
to inquire more in detail at a later stage. There is also in many
species, as Dr. Klein has pointed out, a poisonous constituent or
constituents included in the body protoplasm of the bacillus, and which
he therefore terms the _intracellular poison_. Now, whilst the former
is different in every species, the latter may be a property common to
several species. Hence those having a similar intracellular poison are
antagonistic to each other, each member of such a group being unable to
live in an environment of its own intracellular poison. Further, it has
been suggested that there are organisms possessing only one poisonous
property, namely, their toxin--for example, the bacilli of tetanus and
diphtheria--whilst there are other species, as above, possessing a
double poisonous property, an intracellular poison and a toxin. In this
latter class would be included the bacilli of Anthrax and Tubercle.
Reference has been made to the associated work of higher vegetable life
and bacteria. The converse is also true. Just as we have bacterial
diseases affecting man and animals, so also plant life has its
bacterial diseases. Wakker, Prillieux, Erwin Smith, and others have
investigated the pathogenic conditions of plants due to bacteria, and
though this branch of the science is in its very early stages, many
facts have been learned. _Hyacinth disease_ is due to a flagellated
bacillus. _The wilt of cucumbers and pumpkins_ is a common disease in
some districts of the world, and may cause widespread injury. It is
caused by a white microbe which fills the water-ducts. Wilting vines
are full of the same sticky germs. Desiccation and sunlight have a
strongly prejudicial effect upon these organisms. Bacterial _brown-rot_
of potatoes and tomatoes is another plant disease probably due to a
bacillus. The bacillus passes down the interior of the stem into the
tubers, and brown-rots them from within. There is another form of
brown-rot which affects cabbages. It blackens the veins of the leaves,
and a woody ring which is formed in the stem causes the leaves to
fall off. This also is due to a micro-organism, which gains entrance
through the water-pores of the leaf, and subsequently passes into the
vessels of the plants. It multiplies by simple fission, and possesses a
flagellum.
There can be no doubt that these complex biological properties of
association and antagonism, as well as the parasitic growth of bacteria
upon higher vegetables, are as yet little understood, and we may be
glad that any light is being shed upon them. In the biological study
of soil bacteria in particular may we expect in the future to find
examples of association, even as already there are signs that this
is so in certain pathogenic conditions. In the alimentary canal, on
the other hand, and in conditions where organic matter is greatly
predominating, we may expect to see further light on the subject of
antagonism.
_Attenuation of Virulence or Function._ It was pointed out by some of
the pioneer bacteriologists that the function of bacteria suffered
under certain circumstances a marked diminution in power. Later workers
found that such a change might be artificially produced. Pasteur
introduced the first method, which was the simple one of allowing
cultures to grow old before sub-culturing. Obviously a pure culture
cannot last for ever. To maintain the species in characteristic
condition it is necessary frequently to sub-culture upon fresh
media. If this simple operation be postponed as long as possible
consistent with vitality, and then performed, it will be found that
the sub-culture is _attenuated_, _i. e._, weakened. Another mode is
to raise the pure culture to a temperature approaching its thermal
death point. A third way of securing the same end is to place it under
disadvantageous external circumstances, for example a too alkaline or
too acid medium. A fourth, but rarely necessary, method is to pass
it through the tissues of an insusceptible animal. Thus we see that,
whilst the favourable conditions which we have considered afford full
scope for the growth and performance of functions of bacteria, we are
able by a partial withdrawal of these, short of that ending fatally, to
modify the character and strength of bacteria. In future chapters we
shall have opportunity of observing what can be done in this direction.