ll bacteria, as a result of their growth in
food substances, form more or less characteristic compounds that are
known as _by-products_. The changes brought about are those of
decomposition and are collectively known as _fermentations_; they
are characterized by the production of a large amount of by-products
as the result of the development of a relatively small amount of
cell life. The souring of milk, the rotting of eggs, the spoiling of
meats, the making of vinegar from cider are examples of
fermentations caused by different bacteria.

If the substances decomposed contain but little sugar, as do animal
tissues, the conditions are favorable for the growth of the
putrefactive bacteria, and foul-smelling gases are formed. When
sugars are present, as in milk, the environmental conditions are
most favorable for the acid-forming bacteria that do not as a rule
produce offensive odors.

Many of the bacteria form substances known as enzymes which are able
to produce certain decomposition changes in the absence of the
living cells, and it is by virtue of these enzymes that the
organisms are able to break down such enormous quantities of
organic matter. Most of these enzymes react toward heat, cold,
and chemical poisons in a manner quite similar to the living
cells. In one respect, they are readily differentiated, and that
is, that practically all of them are capable of producing their
characteristic chemical transformations under conditions where the
activity of the cell is wholly suspended as in a saturated ether or
chloroform atmosphere. The production of enzymes is not confined to
bacteria, but they are found throughout the animal and plant world,
especially in those processes that are concerned in digestion.
Rennet, used in cheese making, is an example of an animal enzyme.

=Distribution of bacteria.= As bacteria possess greater powers of
resistance than almost any other form of life, they are found very
widely distributed over the surface of the earth. In soil they are
abundant, because of the fact that all of the conditions necessary
for growth are here best satisfied. They are, however, distributed
with reference to the layers of the soil; the soil proper, i.e.,
that turned over by the plow, is extremely rich in them on account
of the abundance of organic matter. But at the depth of a few feet
they decrease rapidly in numbers, and in the deeper layers, from six
to ten feet, or more, they are normally not present, b