able of infecting milk through
the development of disease in the animal are parasites in the strict
sense of the term. Most disease-producing species, as diphtheria or
typhoid fever, while parasitic in man lead a saprophytic method of life
so far as their relation to milk is concerned.

Bacteria require for their growth, nitrogen, hydrogen, carbon, oxygen,
together with a limited amount of mineral matter. The nitrogen and
carbon are most available in the form of organic compounds, such as
albuminous material. Carbon in the form of carbohydrates, as sugar or
starch, is most readily attacked by bacteria.

Inasmuch as the bacteria are plant-cells, they must imbibe their food
from material in solution. They are capable of living on solid
substances, but in such cases, the food elements must be rendered
soluble, before they can be appropriated. If nutritive liquids are too
highly concentrated, as in the case of syrups and condensed milk,
bacteria cannot grow therein, although all the necessary ingredients may
be present. Generally, bacteria prefer a neutral or slightly alkaline
medium, rather than one of acid reaction; but there are numerous
exceptions to this general rule, especially among the bacteria found in
milk.

~Temperature.~ Growth of bacteria can only occur within certain
temperature limits, the extremes of which are designated as the
_minimum_ and _maximum_. Below and above these respective limits, life
may be retained in the cell for a time, but actual cell-multiplication
is stopped. Somewhere between these two cardinal temperature points, and
generally nearer the maximum limit is the most favorable temperature for
growth, known as the _optimum_. The temperature zone of most dairy
bacteria in which growth occurs ranges from 40 deg.-45 deg. F. to somewhat
above blood-heat, 105 deg.-110 deg. F., the optimum being from 80 deg.-95
deg. F. Many parasitic species, because of their adaptation to the bodies
of warm-blooded animals, generally have a narrower range, and a higher
optimum, usually approximating the blood heat (98 deg.-99 deg. F). The
broader growth limits of bacteria in comparison with other kinds of life
explain why these organisms are so widely distributed in nature.

~Air supply.~ Most bacteria require as do the green plants and animal
life, the free oxygen of the air for their respiration. These are called
_aerobic_. Some species, however, and some yeasts as well possess the
peculiar property of taki