Bacteria are propagated in a very simple manner. The parent cell divides into two; these two into two others, and so on. The rapidity with which these organisms multiply under favorable conditions, makes them, in some cases, most dangerous enemies. It has been calculated that if all of the organisms survived, one bacterium would lead to the production of several billions of others in twenty-four hours.

395. The Struggle of Bacteria for Existence. Like all kinds of living things, many species of bacteria are destroyed if exposed to boiling water or steam, but seem able to endure prolonged cold, far below the freezing-point. Thus ice from ponds and rivers may contain numerous germs which resume their activity when the ice is melted. Typhoid fever germs have been known to take an active and vigorous growth after they have been kept for weeks exposed in ice to a temperature below zero.

The bacteria of consumption (bacillus tuberculosis) may retain their vitality for months, and then the dried expectoration of the invalids may become a source of danger to those who inhale air laden with such impurities (sec. 220 and Fig. 94).

Like other living organisms, bacteria need warmth, moisture, and some chemical compound which answers for food, in order to maintain the phenomena of life. Some species grow only in contact with air, others need no more oxygen than they can obtain in the fluid or semi-fluid which they inhabit.

396. Importance of Bacteria in Nature. We might well ask why the myriads of bacteria do not devastate the earth with their marvelous rapidity of propagation. So indeed they might, were it not for the winds, rains, melting snow and ice which scatter them far and wide, and destroy them.

Again, as in countless other species of living organisms, bacteria are subject to the relentless law which allows only the fittest to survive. The bacteria of higher and more complex types devour those of a lower type. Myriads perish in the digestive tract of man and other animals. The excreta of some species of bacteria act as poison to destroy other species.

It is true from the strictest scientific point of view that all living things literally return to the dust whence they came. While living they borrow a few elementary substances and arrange them in new combinations, by aid of the energy given them by the sun, and after a time die and leave behind all they had borrowed both of energy and matter.

Countless myriads of bacteria are silently at work changing dead animal and vegetable matter into useful substances. In brief, bacteria prepare food for all the rest of the world. Were they all destroyed, life upon the earth would be impossible, for the elements necessary to maintain it would be embalmed in the bodies of the dead.

397. Action of Bacteria. In certain well-known processes bacteria have the power of bringing about decomposition of various kinds. Thus a highly organized fungus, like the yeast plant, growing in the presence of sugar, has the power of breaking down this complex body into simpler ones, viz., alcohol and carbon dioxid.