PHYSIOLOGICAL ACTIVITIES.
The physiological activities of motion, reproduction and metabolism
within the cell have been discussed in previous chapters. The
objects in view in the discussion of the “physiological activities”
(sometimes spoken of as “biochemical” activities) of bacteria in
this and subsequent chapters are to familiarize the student to some
extent with the great range of chemical changes brought about by these
minute organisms, to show their usefulness, even their necessity, and
to impress the fact that it is chiefly by a careful study of these
“activities” that individual kinds of bacteria are identified. It
should always be borne in mind that the bacteria, in bringing about
these changes which are so characteristic in many instances, are simply
engaged in their own life struggle, in securing the elements which
they need for growth, in liberating energy for vital processes, or
occasionally in providing conditions which favor their own development
and hinder that of their competitors.
FERMENTATION OF CARBOHYDRATES.
By this is meant the changes which different carbohydrates undergo when
subjected to bacterial action.[11]
These changes are marked chiefly by the production of gas or acid. The
former is called “gaseous fermentation” the latter “acid fermentation.”
The gases commonly produced are carbon dioxide (CO₂) hydrogen and
marsh gas (CH₄). Other gases of the paraffin series may also be formed
as ethane (C₂H₆), acetylene (C₂H₂), etc. CO₂ and H are the ones usually
formed from sugars by the few gas-forming bacteria which produce
disease, though even here some CH₄ is present. The common _Bacterium
coli_ forms all three, though the CH₄ is in smallest quantity.
[Illustration: FIG. 60.--Cylinder to show the formation of gas by
bacteria. The gauge shows 265 pounds. It went beyond 500 pounds.]
[Illustration: FIG. 61.--A burning natural gas well at night. From a
photograph colored.]
In the fermentation of the polysaccharids--starch and especially
cellulose and woody material--large amounts of CH₄ occur, particularly
when the changes are due to anaërobic bacteria. This phenomenon may be
readily observed in sluggish streams, ponds and swamps where vegetable
matter accumulates on the bottom. The bubbles of gas which arise when
the mass is disturbed explode if a lighted match is applied to them.
The author has conducted a number of experiments to demonstrate this
action as follows: Material taken from the bottom of a pond in the
fall after vegetation had died out was packed into a cylinder five
feet long and six inches in diameter, water was added to within about
2 inches of the top. After leaving them open for a few days to permit
all the dissolved oxygen to be used up by the aërobes, the cylinders
were tightly capped and allowed to stand undisturbed. Pressure gauges
reading to 500 lbs. were attached (Fig. 60). At the end of six months
the gauge showed a pressure beyond the limits of the readings on it.
Most of the gas was collected and measured 146 liters. An analysis
of portions collected when about one-half had been allowed to escape
showed the following composition, according to Prof. D. J. Demorest of
the Department of Metallurgy:
CO₂ 18.6 per cent.
CH₄ 76.1 „
H 1.0 „
N 4.3 „
In the author’s opinion natural gas and petroleum have been formed in
this way[12] (Figs. 61 and 62).
[Illustration: FIG. 62.--A “flowing” oil well.]
One of the very few practical uses of the gaseous fermentation of
carbohydrates is in making “salt rising” bread. The “rising” of the
material is due not to yeasts but to the formation of gas by certain
bacteria which are present on the corn meal or flour used in the
process (Fig. 63).
[Illustration: FIG. 63.--A loaf of “salt rising” bread. The porous
structure is due to the gas formed by bacilli and not by yeasts.]
Another is in the formation of the “holes” or “eyes” so characteristic
of Swiss and other types of cheese (Fig. 64).
[Illustration: FIG. 64.--Ohio Swiss cheese. The “eyes” are due to gas
formed by bacteria during the ripening of the cheese.]
A great many organic acids are formed during the “acid fermentation”
of carbohydrates by bacteria. Each kind of bacterium, as a rule, forms
several different acids as well as other substances, though usually one
is produced in much larger amounts, and the kind of fermentation is
named from this acid. One of the commonest of these acids is lactic.
The “lactic acid bacteria” form a very large and important group and
are indispensable in many commercial processes. In the making of butter
the cream is first “ripened,” as is the milk from which many kinds of
cheese are made (Fig. 65). The chief feature of this “ripening” is the
formation of lactic acid from the milk-sugar by the action of bacteria.
A similar change occurs in the popular “Bulgarian fermented milk.” The
reaction is usually represented by the equation:
Milk-sugar. Lactic acid.
C₁₂H₂₂O₁₁ + H₂O + (bacteria) = 4C₃H₆O₃
It is not probable that the change occurs quantitatively as indicated,
because a number of other substances are also formed. Some of these
are acetic and succinic acids and alcohol. Another industrial use of
this acid fermentation is in the preparation of “sauer kraut.” These
bacteria are chiefly anaërobic and grow best in a relatively high salt
concentration. They occur naturally on the cabbage leaves.
[Illustration: FIG. 65.--A cream ripener. In this apparatus cream is
“ripened,” _i.e._, undergoes lactic acid fermentation, preparatory to
making it into butter.]
In the formation of ensilage (Fig. 66) the lactic acid bacteria play
a very important part, as they do also in “sour mash” distilling, and
in many kinds of natural “pickling.” In fact, whenever green vegetable
material “sours” spontaneously, lactic acid bacteria are always present
and account for a large part of the acid. This property of lactic acid
formation is also taken advantage of in the preparation of lactic acid
on a commercial scale in at least one plant in this country.
[Illustration: FIG. 66.--Filling a silo on the University farm.]
Acetic acid is another common product of acid fermentation. However, in
vinegar making the acetic acid is not formed directly from the sugar in
the fruit juice by bacteria. The sugar is first converted into alcohol
by yeasts, then the alcohol is _oxidized_ to acid by the bacteria (Fig.
67). The reaction may be represented as follows:
Dextrose. Ethyl alcohol. Acetic acid.
C₆H₁₂O₆ = 2C₂H₅OH + 2CO₂
C₂H₅OH + O₂ + (bacteria) = CH₃COOH + H₂O.
Butyric acid is generally produced where fermentation of carbohydrates
occurs under _anaërobic_ conditions. Some of the “strong” odor of
certain kinds of cheese is due to this acid which is formed partly
from the milk-sugar remaining in the cheese. Most of it under these
conditions comes from the proteins of the cheese and especially from
the fat (see page 101).
As has been indicated alcohol is a common accompaniment of most acid
fermentations, as are the esters of acids other than the chief product.
Bacteria are not used in a commercial way to produce alcohol, however,
as the yield is too small. There are some few bacteria in which the
amount of alcohol is prominent enough to call the process an “alcoholic
fermentation” rather than an acid one. In brewing and distilling
industries, _yeasts_ are used to make the alcohol, though molds replace
them in some countries (“sake” and “arrak” from rice).
[Illustration: FIG. 67.--A vinegar ripener. The tank shown opened at
the side is filled with a special type of beech shavings which thus
provide a very large surface. The apple juice which has been previously
fermented with yeast, which converts the sugar into alcohol, is allowed
to trickle through the openings at the top over the shavings. The
acetic acid bacteria on the shavings rapidly oxidize the alcohol to
acetic acid. The vinegar is drawn off below.]
Under ordinary conditions the carbohydrate is never completely
fermented, since the accumulation of the product--acid--stops
the reaction. If the acid is neutralized by the addition of an
alkali--calcium or magnesium carbonate is best--then the sugar
may all be split up. Where such fermentation occurs under natural
conditions, the products are further split up, partly by molds and
partly by acid-destroying bacteria into simpler acids and eventually
to carbon dioxide and water, so that the end-products of the complete
fermentation of carbohydrate material in nature are carbon dioxide,
hydrogen, marsh gas, and water.
In all of these fermentations the bacteria are utilizing the _carbon_
both as building material and for oxidation and the fermentations
are incidental to this use. As a rule, the acid-forming bacteria can
withstand a higher concentration of acid than the other bacteria that
would utilize the same material, and in a short time crowd out their
competitors or inhibit their growth, and thus have better conditions
for their own existence, though finally their growth is also checked by
the acid.
SPLITTING OF FATS.
The _splitting of fats_ into glycerin and the particular acid or
acids involved may be brought about by bacteria. An illustration
is the development of rancidity in butter at times and the
“strong” odor of animal fats on long keeping and of many kinds of
cheese--“limburger”--in this country. Generally speaking, however, fats
are not vigorously attacked, as is illustrated by the difficulties due
to accumulation of fats in certain types of sewage-disposal works. The
chemical change is represented by the equation:
Fat. Glycerin. Fatty acid.
C₃H₅(C{_n_}H₂{_n_}₋₁O₂)₃ + 3 H₂O = C₃H₅(OH)₃ + 3 (C{_n_}H₂{_n_}O₂).