They form important food for other unicellular
organisms which are important links in the chain; they are the agents
of decomposition, by which the complex substances of living things are
reduced to elementary substances and made available for use; without
them plant life would be impossible, for it is by their
instrumentality that material in the soil is so changed as to be
available as plant food; by their action many of the important foods
of man, often those especially delectable, are produced; they are
constantly with us on all the surfaces of the body; masses live on the
intestinal surfaces and the excrement is largely composed of bacteria.
It has been said that life would be impossible without bacteria, for
the accumulation of the carcasses of all animals which have died would
so encumber the earth as to prevent its use; but the folly of such
speculation is shown by the fact that animals would not have been
there without bacteria. It has been shown, however, that the presence
of bacteria in the intestine of the higher animals is not essential
for life. The coldest parts of the ocean are free from those forms
which live in the intestines, and fish and birds inhabiting these
regions have been found free from bacteria; it has also been found
possible to remove small animals from their mother by Caesarian section
and to rear them for a few weeks on sterilized food, showing that
digestion and nutrition may go on without bacteria.

Certain species of bacteria are aerobic, that is, they need free
oxygen for their growth; others are anaerobic and will not grow in the
presence of oxygen. Most of the bacteria which produce disease are
facultative, that is, they grow either with or without oxygen; but
certain of them, as the bacillus of tetanus, are anaerobic. There is,
of course, abundance of oxygen in the blood and tissues, but it is so
combined as to be unavailable for the bacteria. Bacteria may further
be divided into those which are saprophytic or which find favorable
conditions for life outside of the body, and the parasitic. Many are
exclusively parasitic or saprophytic, and many are facultative, both
conditions of living being possible. It has been found possible by
varying in many ways the character of the culture medium and
temperature to grow under artificial conditions outside of the body
most, if not all, of the bacteria which cause disease. Thus, such
bacteria as tubercle bacilli and the influenza bacillus ca