The Fundamentals of Bacteriology

CHAPTER IX.
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.

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:

In the author’s opinion natural gas and petroleum have been formed in this way12 (Figs. 61 and 62).

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).

Another is in the formation of the “holes” or “eyes” so characteristic of Swiss and other types of cheese (Fig. 64).

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:

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.

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.

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:

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).

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: