The rapidity with which bacteria grow is almost beyond comprehension. A single bacillus has been known to increase to four millions in half a day. Pasteur saw a single cell grow to ten millions in twenty-four hours.

The growth of germs is chiefly limited by their food supply and the destructive effects of their own excretions. Left to themselves, even with a sufficient food supply and other favorable conditions, bacteria are likely to die sooner or later killed by their own excretory products.

Bacteria consume food, as do higher plants and animals. Some bacteria require an enormous quantity of food. The energy derived from the food manifests itself as heat, which serves the chlorophyl-lacking plant in place of the heat derived from the sunlight by higher plants.

The energy set free by micro-organisms manifests itself in the heating of fermenting liquids.

The heating of green fodder in a silo and of manure in a hot bed are examples of energy released by the activity of bacteria. Another illustration of a similar sort is found in the bacteria that fix nitrogen in the soil. The fixation of nitrogen is accomplished at the expense of the consumption of a large amount of carbohydrate, not less than two hundred pounds of carbohydrate being required for the fixing of one pound of nitrogen.

Bacteria, like all living organisms, require oxygen. Some bacteria, for example those that produce lactic acid, as in the souring of milk, obtain their oxygen directly from the air (aerobes), whereas other bacteria (anaerobes) grow without the presence of air or oxygen, and may even be destroyed by contact with atmospheric oxygen. These bacteria need oxygen, but they are so constituted that they must obtain their oxygen by breaking up compounds containing oxygen.

Certain bacteria can live and grow either in the presence of free oxygen or excluded from it.

The wonderful activity of microbes in breaking up and destroying organic substances is accomplished by means of diastases or digestive ferments, which they often produce in great quantity. Not only bacteria but yeasts and other living cells behave as ferments when deprived of oxygen.