by the inoculation of pure
cultures into experimental animals, like rabbits, guinea pigs and
white mice.

The methods of the bacteriologist in his laboratory are in their
effect not dissimilar to those which the farmer employs in securing
his crop of pure-bred grain. The laboratory farmer kills the weed
seeds in his culture field by the application of heat. His field,
which is embraced in his culture dish, has been fertilized and
prepared by the addition of certain favorable ingredients. When he
has garnered his crop, he maintains its purity by keeping his
selected seed, the pure culture, free from all contamination. The
dairyman, even though he may not expect to carry on the detailed
operations of the laboratory, will understand the reason for the
directions which he is often required to follow much better if he
knows how the simple operations of the laboratory are carried out.
For a fuller knowledge of these matters, the reader is referred to
the special texts on bacteriology.

CHAPTER III.

CONTAMINATION OF MILK.

=Spoiling of milk.= Materials of animal origin are peculiarly prone to
undergo changes, rendering them unfit for use, and of these, milk is
exceedingly susceptible to such changes. This is due to the fact
that the composition of milk is especially adapted to bacterial
growth, and that the opportunity for entrance of such organisms is
likewise such as to permit of abundant contamination. The
consequence is that milk readily undergoes fermentative changes, due
to the development of one or another type of micro-organism.

=Milk, a suitable bacterial food.= While milk is designed by nature
for the nourishment of mammalian life, it is, curiously enough,
equally well adapted to the growth of these lowest forms of
vegetable life. The nutritive substances required by bacteria are
here sufficiently dilute to make possible rapid growth.

Milk also contains all the necessary chemical substances to make a
suitable bacterial food supply. Of the nitrogenous compounds,
albumen is in a readily assimilable form. Casein, the principal
nitrogenous constituent of milk, exists in an insoluble condition,
and cannot be directly utilized, until it is acted upon by digesting
enzymes. The fat in milk does not readily decompose, and while there
are a few bacteria capable of splitting this substance, the majority
of organisms are unable to utilize it. Milk sugar, on the other
hand, is an excellent food for most speci