Microscopic appearance of ordinary milk, showing fat globules and bacteria which cause the souring of milk.
Relation to Fermentation.—They may incidentally, as a result of this process of decay, continue the process of fermentation begun by the yeasts. In making vinegar the yeasts first make alcohol (see page [135]) which the bacteria change to acetic acid. The lactic acid bacteria, which sour milk, changing the milk sugar to an acid, grow very rapidly in a warm temperature; hence milk which is cooled immediately and kept cool or which is pasteurized and kept in a cool place will not sour readily. Why? These same lactic acid bacteria may be useful when they sour the milk for the cheese maker.
Other Useful Bacteria.—Certain bacteria give flavor to cheese and butter, while still other bacteria aid in the "curing" of tobacco, in the production of the dye indigo, in the preparation of certain fibers of plants for the market, as hemp, flax, etc., in the rotting of animal matter from the skeletons of sponges, and in the process of tanning hides to make leather.
A field of alfalfa, a plant which harbors the nitrogen-fixing bacteria.
Nitrogen-fixing Bacteria.—Still other bacteria, as we have seen before, "change over" nitrogen in organic material in the soil and even the free nitrogen of the air so that it can be used by plants in the form of a compound of nitrogen. The bacteria living in tubercles on the roots of clover, beans, peas, etc., have the power of thus "fixing" the free nitrogen in the air found between particles of soil. This fact is made use of by farmers who rotate their crops, growing first a crop of clover or other plants having root tubercles, which produce the bacteria, then plowing these in and planting another crop, as wheat or corn, on the same area. The latter plants, making use of the nitrogen compounds there, produce a larger crop than when grown in ground containing less nitrogenous material.
Bacteria cause Disease.—The most harmful bacteria are those which cause diseases of plants and animals. Certain diseases of plants—blights, rots, and wilts—are of bacterial nature. These do much annual damage to fruits and other parts of growing plants useful to man as food. But by far the most important are the bacteria which cause disease in man. They accomplish this by becoming parasites in the human body. Millions upon millions of bacteria exist in the human body at all times—in the mouth, on the teeth, in the blood, and especially in the lower part of the food tube. Some in the food tube are believed to be useful, some harmless, and some harmful; others in the mouth cause decay of the teeth, while a few kinds, if present in the body, may cause disease.
Tubercles on the roots of the soy bean. They contain the nitrogen-fixing bacteria. (Fletcher's Soils.) Copyright by Doubleday, Page and Company.
It is known that bacteria, like other living things, feed and give off organic waste from their own bodies. This waste, called a toxin, is poison to the host on which the bacteria live, and it is usually the production of this toxin that causes the symptoms of disease. Some forms, however, break down tissues and plug up the small blood vessels, thus causing disease.