CHEMICAL ENVIRONMENT (CONTINUED).
GENERAL FOOD RELATIONSHIPS. METABOLISM.
The foregoing brief review of the chemical composition of the bacterial
cell illustrates the variety of compounds which necessarily occurs,
but affords no definite clue as to the source of the elements which
enter into these compounds. These elements come from the material
which the organism uses as food. Under this term are included elements
or compounds which serve as building material, either for new cell
substance or to repair waste, or as sources of energy.
An organism which is capable of making use of an element in the free
state is said to be _prototrophic_ for that particular element.
Thus aërobes and facultative anaërobes are prototrophic for O. The
“root-tubercle bacteria” of leguminous and other plants and certain
free living soil organisms are prototrophic for N.[8]
On the other hand, if the element must be secured from compounds, then
the organism is _metatrophic_ in respect to the element in question.
Should the compound be inorganic, the term _autotrophic_ is applied
to the organism and _heterotrophic_ if the compound is organic. It is
very probable that anaërobes, exclusive of a few nitrogen absorbers,
are metatrophic for all the elements they utilize. With the exception
of the anaërobes it seems that all bacteria are _mixotrophic_, that is,
prototrophic for one or two elements and auto- or heterotrophic for the
others.[9]
Those bacteria whose food consists of dead material are spoken of as
_saprophytes_, while those whose natural habitat, without reference to
their food, is in or on other living organisms are called _parasites_.
The _host_ is the organism in or on which the parasite lives.
Parasites may be of several kinds. Those which neither do injury nor
are of benefit to the host are called _non-pathogenic_ parasites or
_commensals_; many of the bacteria in the intestines of man and other
animals are of this class. Those which do injury to the host are
called _pathogenic_ or disease-producing, as the organisms causing the
transmissible diseases of animals and plants.[10] Finally, we have
those parasites which are of benefit to and receive benefit from the
host. These are called _symbionts_ or _symbiotic parasites_ and the
mutual relationship _symbiosis_. Certain of the intestinal bacteria in
man and especially in herbivorous animals are undoubted _symbionts_, as
are also the “root-tubercle bacteria” already mentioned.
It is evident that all parasites that may be cultivated outside
the body are for the time _saprophytic_, hence the terms _strict_
parasites and _facultative_ parasites, which should require no further
explanation.
The changes which the above-mentioned types of food material undergo
in the various anabolic and katabolic processes _within the cell_ are
as yet but very slightly known. Nevertheless there are a number of
reactions brought about by bacteria acting on various food materials,
partly within _but largely without the cell_ which are usually
described as “physiological activities” or “biochemical reactions.”
Some of these changes are to be ascribed to the utilization of certain
of the elements and compounds in these materials as tissue builders,
some as energy-yielding reactions and still others as giving rise to
substances that are of direct benefit to the organism concerned in its
competition with other organisms.
Though all of the twelve elements already mentioned are essential for
the growth of every bacterium, two of them are of especial importance
for the reason that most of the “physiological activities” to be
described in the next chapters are centered around their acquisition
and utilization. These elements are _carbon_ and _nitrogen_. Some few
of the special activities of certain groups have to do with one or the
other of the remaining nine, as will be shown later. But generally
speaking _when a bacterium under natural conditions secures an adequate
supply of carbon and nitrogen, the other elements are readily available
in sufficient amount_.
Carbon is necessary not only because it is an essential constituent
of protoplasm but because its oxidation is the chief source of the
energy necessary for the internal life of the cell, though nitrogen
and sulphur replace it in this function with a few forms. This
latter use of carbon constitutes what may be called its _respiratory
function_. Bacteria like other organisms in their respiration utilize
oxygen and give off carbon dioxide. The amount of the latter given off
from the cell in this way is very small as compared with that which
is frequently produced as an accompaniment of other reactions (see
Fermentation, next chapter). But there is no doubt of its formation and
it has been determined by a few investigators. On account of this use
of carbon, bacteria require relatively large amounts of this element.
One group of bacteria concerned in the spontaneous heating of coal
seems to be able to use free carbon from this material. Another group
is said to be able to oxidize marsh gas, CH₄, and use this as its
source of carbon. The nitrite, nitrate and sulphur bacteria mentioned
later utilize carbon dioxide and carbonates as their carbon supply,
and one kind has been described which uses carbon monoxide. With these
few exceptions bacteria are dependent on _organic compounds_ for their
carbon and cannot use CO₂ as green plants do.
The oxygen requirement is high partly for the same reason that
carbon is, _i.e._, respiration. Oxygen is one of the constituents of
protoplasm, and combined with hydrogen forms water which makes up such
a large part of the living cell. Anaërobic bacteria are dependent on
so-called “molecular respiration” for their energy. That is, through
a shifting or rearrangement of the atoms in the compounds used as
food the oxidation of carbon is brought about. Enzymes are probably
responsible for this action. Carbon dioxide is produced by anaërobes
as well as by aërobes, and frequently in amounts readily collected. A
carbohydrate is usually though not always essential for the growth of
anaërobes and serves them as the best source of energy.
Nitrogen is the characteristic element of living material. Protoplasm
is a chemical substance in unstable equilibrium and nitrogen is
responsible for this instability. No other of the commoner elements
is brought into combination with such difficulty, nor is so readily
liberated when combined (all commercial explosives are nitrogen
compounds). Bacteria, like other forms of protoplasm, require nitrogen.
More marked peculiarities are shown by bacteria with reference to the
sources from which they derive their nitrogen than for carbon. Some can
even combine the free nitrogen of the air and furnish the only natural
means of any importance for this reaction. Some few forms (the nitrite
and nitrate formers, Chapter XI) obtain their energy from the oxidation
of inorganic nitrogen compounds, ammonia and nitrites respectively, and
not from carbon. These latter bacteria use carbon from carbon dioxide
and carbonates. A great many bacteria can secure their nitrogen from
nitrates but some are restricted to organic nitrogen. Many bacteria
obtain their carbon from the same organic compounds from which their
nitrogen is derived.
Sulphur serves mainly as a constituent of protein compounds in the
protoplasmic structure. In some of the _sulphur_ bacteria it is a
source of energy, since either free sulphur or H₂S is oxidized by them.
Some of these bacteria can obtain their carbon from CO₂ or carbonates,
and their nitrogen from nitrates or ammonium salts.
Whether the _iron_ bacteria, belonging to the genus _Crenothrix_ of the
higher, thread bacteria, use this element or its compounds as sources
of energy is still a disputed question. The evidence is largely in
favor of this view.
Free hydrogen has been shown to be oxidized by some forms which obtain
their energy in this way.
Whether there is a special class of _phosphorus_ bacteria remains to be
discovered. That phosphorus is oxidized during the activity of many
bacteria is undoubted, but whether this represents a source of energy
or is the accidental by-product of other activities is undetermined.
Practically nothing is known about the metabolism of the other elements
as such.
From the preceding brief review of the relation of certain bacteria
to some of the elements in the free state and from the further fact
that there is scarcely a known natural organic compound which cannot
be utilized by some kind of bacterium, it is evident that this class
of organisms has a far wider range of adaptability than any other
class, and this adaptability helps to explain their seemingly universal
distribution.
As to the metabolism _within the cell_, no more is known than is the
case with other cells, nor even as much. The materials used for growth
and as sources of energy are taken into the cell, built up into various
compounds some of which have been enumerated and in part broken down
again. Carbon dioxide and water are formed in the latter process. What
other katabolic products occur it is not easy to determine. Certainly
some of the substances mentioned in the next chapters are such products
but it is not always possible to separate those formed _inside_ the
cell from those formed _outside_. Perhaps most of the latter should be
considered true metabolic products. It would seem that on account of
the simplicity of structure of the bacterial cell and of the compounds
which they may use as food they would serve as excellent objects
for the study of the fundamental problems of cell metabolism. Their
minuteness and the nearly impossible task of separating them completely
from the medium in or on which they are grown makes the solution of
these problems one of great difficulty.
When all of the environmental conditions necessary for the best
development of a given bacterium are fulfilled, it will then develop
to the limit of its capacity. This development is characterized
essentially by its reproduction, which occurs by transverse division.
The rate of this division varies much with the kind even under good
conditions. The most rapid rate so far observed is a division in
eighteen minutes. A great many reproduce every half-hour and this may
be taken as a good average rate. If such division could proceed without
interruption, a little calculation will show that in about sixty-five
hours a mass as large as the earth would be produced.
Starting with 1 coccus, 1µ in diameter,
its volume = 0.0000000000005 cc.
1/2 hour = 2
1 hour = 4
2 hours = 16
4 hours = 256
5 hours = 1024 = 10³+
15 hours = 1,000,000,000 = 10⁹ = 0.5 cc.
35 hours = 10²¹+ = 500.0 cu.m.
About 65 hours = 2 × 10⁴²+ = 5 × 10²⁰ cu.m. = a mass as large
as the earth.
Such a rate of increase evidently cannot be kept up long on account of
many limiting factors, chief of which is the food supply.
The foregoing calculation is based on the assumption that the organism
divides in one plane only. If it divides in 2 or 3 planes, the rate is
much faster, as is shown by the following formulæ, which indicate the
theoretical rate of division:
S = number of bacteria after a given number of divisions.
a = number at the beginning, and n = number of divisions.
1 plane division S = 2_ⁿ_a
2 „ „ S = 2²_ⁿ_a
3 „ „ S = 2³_ⁿ_a
With two-plane or three-plane division, assuming that each organism
attains full size, as was assumed in the first calculation, the “mass
as large as the earth” would be attained in about thirty-two and
twenty-two hours respectively.
This extraordinary rate of increase explains in large measure why
bacteria are able to bring about such great chemical changes in so
short a time as is seen in the rapid “spoiling” of food materials,
especially liquids. The reactions brought about by bacteria on
substances which are soluble and diffusible are essentially “surface
reactions.” The material diffuses into the cell over its entire surface
with little hindrance. The bacteria are usually distributed throughout
the medium, so that there is very intimate contact in all parts of
the mass which favors rapid chemical action. The following calculation
illustrates this:
The volume of a coccus 1µ in diameter is 0.5236 × 10⁻¹³ cc.
The surface of a coccus 1µ in diameter is π × 10⁻⁸ sq. cm.
It is not uncommon to find in milk on the point of souring
1,000,000,000 bacteria per cc.
Assuming these to be cocci of 1µ diameter the volume of these bacteria
in a liter is only 0.05 cc. or in the liter there would be 19999 parts
of milk and only 1 part bacteria. The surface area of these bacteria is
3141.6 sq. cm. With this large surface exposed, it is not strange that
the change from “on the point of souring” to “sour” occurs within an
hour or less.
Although large numbers of bacteria can and do cause great chemical
changes the amount of material actually utilized for maintenance of
the cell is very slight, infinitesimal almost, and yet is fairly
comparable to that required for man, as is illustrated by the following
computations:
E. Kohn has shown that certain water bacteria grew well in water to
which there was added per liter 0.000002 mg. dextrose, 0.00000007
mg. (NH₄)₂SO₄ and 0.0000000007 mg. (NH₄)₂HPO₄. The bacteria numbered
about 1000 per cc. Taking the specific gravity at 1 (a little too
low) the mass of the bacteria in the liter was about 0.001 mg. Hence
the bacteria used 0.002 of their weight of carbohydrate and 0.00007
of ammonium sulphate. A 150-pound (75-kilo) man can live on 375 g.
of sugar (0.005 of his weight) and 52.5 g. of protein (0.0007 of his
weight). From these figures it can be calculated that the man utilizes
about two and a half times as much carbohydrate and about seven times
as much nitrogen as the bacterium, relatively speaking.