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 bring