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.