GENERAL CONDITIONS FOR GROWTH.
OCCURRENCE.
Bacteria are probably the most widely distributed of living organisms.
They are found practically everywhere on the surface of the earth.
Likewise in all surface waters, in streams, lakes and the sea. They
occur in the air immediately above the surface, since they are carried
up mechanically by air currents. They cannot fly of themselves. There
is no reason to believe that any increase in numbers occurs to an
appreciable extent in the air. The upper air, for example, on high
mountains, is nearly free from them. So also is the air over midocean,
and in high latitudes. As a rule, the greater the amount of dust in
the air, the more numerous are the bacteria. Hence they are found more
abundantly in the air in cities and towns than in the open country.
The soil is especially rich in numbers in the upper few feet, but they
diminish rapidly below and almost disappear at depths of about six
feet unless the soil is very porous and open, when they may be carried
farther down. Hence the waters from deep wells and springs are usually
devoid of these organisms. In the sea they occur at all levels and have
been found in bottom ooze dredged from depths of several miles. It is
perhaps needless to add that they are found on the bodies and in the
alimentary tract of human beings and animals; on clothing, utensils;
in dwellings, stables, outhouses, etc. From one-fourth to one-half of
the dry weight of the feces of animals and men is due to the bacteria
present. The urine is practically free from them in health.
While bacteria are thus found nearly everywhere, it is an entirely
mistaken idea to suppose that all are injurious to man. As a matter
of fact, those which are dangerous are relatively few and are for the
most part found only in close association with man. Most bacteria are
harmless and the vast majority are beneficial or even essential to
man’s existence on the earth. These facts must be constantly borne in
mind, and it is hoped that the pages which follow will make them clear.
In order that any organism may thrive there are a number of general
environmental conditions which must be fulfilled. These conditions
vary more or less for each kind of organism. Bacteria are no exception
to this general rule. These conditions may be conveniently considered
under the general heads of _moisture_; _temperature_; _light_; _oxygen
supply_; _osmotic pressure_; _action of electricity_; of _Röntgen_
and _radium rays_; _pressure_; _mechanical vibration_; and _chemical
environment_, including the _reaction of the medium_, _the effect
of injurious chemicals_, and especially the _food requirements of
bacteria_. For each of these conditions there is a _maximum_, meaning
the greatest amount of the given condition which the organism can
withstand, a _minimum_, or the least amount, and an _optimum_ or that
amount which is most favorable for development. Further, there might be
distinguished a maximum for _mere existence_ and a lower maximum for
_development_; also a minimum for _mere existence_ and a higher minimum
for _development_. These maxima, minima, and optima for bacteria have
been determined with exactness for only a very few of the general
conditions and for comparatively few kinds.
MOISTURE.
The _maximum_ moisture is absolutely pure water, and no organism can
thrive in this alone owing to the factor of too low osmotic pressure
and to the further factor of absence of food material. There are many
bacteria which thrive in water containing only traces of mineral salts
and a large class whose natural habitat is surface water. These “water
bacteria” are of great benefit in the purification of streams. They are
as a class harmless to men and animals. Some of the disease-producing
bacteria like _Bacterium typhosum_ (of typhoid fever) and _Vibrio
choleræ_ (of Asiatic cholera) were undoubtedly originally water
bacteria, and it is rather striking that in these diseases conditions
are induced in the intestine (diarrheas) which simulate the original
watery environment. The _minimum_ moisture condition is absolute
dryness, and no organism can even exist, not to say develop, in such
a condition since water is an essential constituent of living matter.
Some bacteria and especially most spores may live when dried in the
air or by artificial means for months and even years, while some are
destroyed in a few hours or days when dried (typhoid, cholera, etc.).
The optimum amount of moisture has not been determined with any great
accuracy and certainly a rather wide range in percentage of water is
permissible with many, though a liquid medium is usually most favorable
for artificial growth. The “water bacteria” have been mentioned. In the
soil a water content of 5 to 15 per cent. seems to be most suitable for
many of the organisms which aid in plant growth. In animals and man the
organisms infecting the intestinal tract prefer a high percentage of
moisture as a rule, especially those causing disease here. Those found
on the surface of the body (pus cocci) need a less amount of water,
while those invading the tissues (tuberculosis, black-leg, etc.) seem
to be intermediate in this respect. In artificial culture media a water
content of less than 30 per cent. inhibits the growth of most bacteria.
As a general rule those bacteria which require the largest percentage
of water are most susceptible to its loss and are most readily killed
by drying. The typhoid and cholera organisms die in a few hours when
dried, while pus cocci and tubercle bacilli live much longer.
TEMPERATURE.
The temperature conditions for bacterial existence and growth have been
determined more accurately than any of the other general conditions.
The maximum for existence must be placed at or near 100° since it is
known that all bacteria including spores may be killed by boiling in
time. Nevertheless, certain forms have been reported as thriving in hot
springs where the water temperature was 93°. This is the highest known
temperature for development. The minimum for existence lies at or near
the absolute zero (-273°) since certain organisms have been subjected
to the temperature produced by the sudden evaporation of liquid
hydrogen (-256° to -265°) and have remained alive. Whether they could
withstand such temperatures indefinitely is not known. The minimum for
development is near the freezing-point of water, since reproduction
by division has been observed in the water from melting sea-ice at
a temperature of -1.5°. Thus bacteria as a class have a range for
existence of about 373° (-273° to +100°) and for development of 94.5°
(-1.5° to +93°) certainly much wider ranges than any other group of
organisms.[5]
The optimum temperature for development varies within rather wide
limits for different organisms. In general it may be stated that the
optimum temperature is approximately that of the natural habitat of
the organism, though there are exceptions. The optimum of the “hot
spring” bacteria just mentioned is apparently that of the springs (93°
in this case). Many soil organisms are known whose optimum is near
70° (a temperature rarely, if ever, attained in the soil), _but only
when grown in air or oxygen_; but is very much lower when grown in the
_absence of oxygen_. Many other soil organisms exhibit very little
difference in rate or amount of growth when grown at temperatures which
may vary as much as 10° or 15°, apparently an adaptation to their
normal environment. The disease-producing organisms show much narrower
limits for growth, especially those which are difficult to cultivate
outside the body. For example, the bacterium of tuberculosis in man
scarcely develops beyond the limits of 2° or 3° from the normal body
temperature of man (37°), while the bacterium of tuberculosis in birds
grows best at 41° to 45°, the normal for birds, and the bacterium of
so-called tuberculosis of cold-blooded animals at 14° to 18°.
Those bacteria whose optimum temperature is above 40° are sometimes
spoken of as the “_thermophil_” bacteria. The fixing of the “thermal
death-point” that is, the minimum temperature at which the bacteria
are killed is a matter of great practical importance in many ways
and numerous determinations of this have been made with a great many
organisms and by different observers. The factors which enter into such
determinations are so many and so varied that unless all the conditions
of the experiment are given together with the time of application,
the mere statements are worthless. It may be stated that all _young,
actively growing_ (non-spore-containing) _disease-producing bacteria,
when exposed in watery liquids and in small quantities are killed at
a temperature of 60° within half an hour_. It is evident, that this
fact has very little practical application, since the conditions stated
are rarely, if ever, fulfilled except in laboratory experiments. (See
Sterilization and Pasteurization, Chapter XIII.)
LIGHT.
Speaking generally, it can be said that light is destructive to
bacteria. Many growing forms are killed in a few hours when properly
exposed to direct sunlight and die out in several days in the diffuse
daylight of a well-lighted room. Even spores are destroyed in a
similar manner, though the exposure must be considerably longer.
Certain bacteria which produce colors may grow in the light, since
the pigments protect them. Some few kinds, like the sulphur bacteria,
which contain a purplish-red pigment that serves them to break up
H₂S, need light for their growth. Since disease-producing bacteria
are all injuriously affected by light, the advantage of well-lighted
habitations both for men and animals is obvious.
OXYGEN SUPPLY.
Oxygen is one of the constituents of protoplasm and is therefore
necessary for all organisms. This does not mean that all organisms
must obtain their supply from _free oxygen_, however, as animals and
plants generally do. This fact is well illustrated by the differences
among bacteria in this respect. Some bacteria _require free oxygen_ for
their growth and are therefore called _aërobic_ bacteria or _aërobes_
(sometimes _strict aërobes_, though the adjective is unnecessary).
Others _cannot grow in the presence of free oxygen_ and are therefore
named _anaërobic bacteria_ or _anaërobes_ (strict is unnecessary).
There are still other kinds which may grow either in the presence of
free oxygen or in its absence, hence the term _facultative anaërobes_
(usually) is applied to them. The distinction between _facultative
aërobe_ and _facultative anaërobe_ might be made. The former means
those which grow best in the absence of free oxygen, though capable of
growing in its presence, while the latter term means those which grow
best in the presence of free oxygen, but are capable of growing in its
absence. The amount of oxygen in the atmosphere in which an organism
grows may be conveniently expressed in terms of the oxygen pressure,
_i.e._, in millimeters of mercury. It is evident that the maximum,
minimum and optimum oxygen pressures for anaërobic bacteria are the
same, namely, 0 mm. Hg. This is true only for natural conditions,
since a number of anaërobic organisms have been gradually accustomed
to increasing amounts of O, so that by this process of training they
finally grew in ordinary air, that is, at an oxygen pressure of about
150 mm. Hg. (Normal air pressure is 760 mm. Hg. and oxygen makes up
one-fifth of the air.) The minimum O pressure for facultative anaërobes
is also 0 mm. Hg. Some experiments have been made to determine the
limits for aërobes, but on a few organisms only, so that no general
conclusions can be drawn from them. To illustrate: _Bacillus subtilis_
(a common “hay bacillus”) will grow at 10 mm. Hg. pressure but not at 5
mm. Hg. It will also grow in compressed oxygen at a pressure of three
atmospheres (2280 mm. Hg.), but not at four atmospheres (3040 mm. Hg.),
though it is not destroyed.
Parodko has determined the oxygen limits for five common organisms as
follows:
Minimum
Maximum. Vol. Mm.
In atmospheres. Mm. Hg. per cent. Hg.
_Bacterium 1.94 to 2.51 1474 to 1908 0.00016 = 0.0012
fluorescens_
_Sarcina lutea_ 2.51 to 3.18 1908 to 2417 0.00015 = 0.0011
_Proteus vulgaris_ 3.63 to 4.35 2749 to 3306 0 0
_Bacterium coli_ 4.09 to 4.84 3108 to 3478 0 0
_Erythrobacillus 5.45 to 6.32 3152 to 4800 0 0
prodigiosus_
These few instances do not disclose any general principles which may
be applied either for the growth or for the distinction of aërobes or
facultative anaërobes.
It has been shown that compressed oxygen will kill some bacteria but
this method of destroying them has little or no practical value. Oxygen
in the form of ozone, O₃, is rapidly destructive to bacteria, and this
fact is applied practically in the purification of water supplies for
certain cities where the ozone is generated by electricity obtained
cheaply from water power. The same is true of oxygen in the “nascent
state” as illustrated by the use of hypochlorites for the same purpose.
It was stated (p. 74) that certain thermophil bacteria in the soil have
an optimum temperature for growth _in the air_ which is much higher
than is ever reached in their natural habitat and that they grow at a
moderate temperature under _anaërobic_ conditions. It has been shown
that if these organisms are grown with aërobes or facultative anaërobes
they thrive at ordinary room temperature. These latter organisms by
using up the oxygen apparently keep the tension low, and this explains
how such organisms grow in the soil.[6]
OSMOTIC PRESSURE.
Like all living cells bacteria are very susceptible to changes in
the density of the surrounding medium. If placed in a medium less
concentrated than their own protoplasm water is absorbed and they
“swell up”; while if placed in a denser medium, water is given off and
they shrink (plasmoptysis or plasmolysis). Should these differences
be marked or the transition be sudden, the cell walls may even burst
and the organisms be destroyed. If the differences are not too great
or if the transition is made gradually, the organisms may not be
destroyed, but will either cease to grow and slowly die out, or will
show very much retarded growth, or will produce abnormal cell forms.
This is illustrated in the laboratory in attempting to grow bacteria on
food material which has dried out. A practical application of osmotic
effects is in the use of a high percentage of sugar in preserving
fruits, etc., and in the salting of meats. Neither the cane-sugar nor
the common salt themselves injure the bacteria chemically, but by the
high concentration prevent their development. In drying material in
order to preserve it there are two factors involved: first, the loss of
water necessary for growth and second, the increased osmotic pressure.
In a medium of greater density diffusion of water is outward from the
cell and this will continue until an equilibrium is established between
cell contents and medium. Food for the organism _must be in solution
and enter the cell by diffusion_. Therefore, growth ceases in a medium
too dense, since water to carry food in solution does not enter the
cell.
ELECTRICITY.
Careful experimenters have shown that the electric current, either
direct or alternating, has no direct destructive effect on bacteria.
In a liquid medium the organisms may be attracted to or repelled
from one or the other pole or may arrange themselves in definite
ways between the poles (galvanotaxis), but are not injured. However,
electricity through the _secondary_ effects produced, may be used to
destroy bacteria. If the passage of the electric current _increases the
temperature_ of the medium sufficiently, the bacteria will be killed,
or if _injurious chemical substances_ are formed (ozone, chlorine,
acids, bases, etc.), the same result will follow (see Ozone, pp. 77 and
157).
RADIATIONS.
Röntgen or _x_-rays and radium emanations when properly applied to
bacteria will destroy them. The practical use of these agents for
the direct destruction of bacteria in diseases of man or animals is
restricted to those cases where they may be applied directly to the
diseased area, since they are just as injurious to the animal cell
as they are to the bacteria, and even more so. Their skilful use as
_stimuli to the body cells_ to enable them to resist and overcome
bacteria and other injurious organisms or cell growths is an entirely
different function and will not be considered here.
PRESSURE.
Hydrostatic pressure up to about 10,000 pounds per square inch is
without appreciable effect on bacteria as has been shown by several
experimenters and also by finding living bacteria in the ooze dredged
from the bottom of the ocean at depths of several miles.
Pressures from 10,000 to 100,000 pounds show variable effects. Some
bacteria are readily killed and others, even non-spore formers,
are only slightly affected. The time factor is important in this
connection. The presence of acids, even CO₂, or organic acids, results
in the destruction of most non-spore formers.
MECHANICAL VIBRATION.
Vibrations transmitted to bacteria in a liquid may be injurious to them
under certain circumstances. Some of the larger forms like _Bacillus
subtilis_ may be completely destroyed by shaking in a rapidly moving
shaking machine in a few hours. Bacteria in liquids placed on portions
of machinery where only a slight trembling is felt, have been found
to be killed after several days. Reinke has shown that the passing of
strong sound waves through bacterial growths markedly inhibits their
development.