uch as milk with such
material is sure to introduce elements that seriously affect the quality
of the product.

CHAPTER II.

METHODS OF STUDYING BACTERIA.

~Necessity of bacterial masses for study.~ The bacteria are so extremely
small that it is impossible to study individual germs separately without
the aid of first-class microscopes. For this reason, but little advance
was made in the knowledge of these lower forms of plant life, until the
introduction of culture methods, whereby a single organism could be
cultivated and the progeny of this cell increased to such an extent in a
short course of time, that they would be visible to the unaided eye.

This is done by growing the bacteria in masses on various kinds of food
media that are prepared for the purpose, but inasmuch as bacteria are so
universally distributed, it becomes an impossibility to cultivate any
special form, unless the medium in which they are grown is first freed
from all pre-existing forms of germ life. To accomplish this, it is
necessary to subject the nutrient medium to some method of
sterilization, such as heat or filtration, whereby all life is
completely destroyed or eliminated. Such material after it has been
rendered germ-free is kept in sterilized glass tubes and flasks, and is
protected from infection by cotton stoppers.

~Culture media.~ For culture media, many different substances are
employed. In fact, bacteria will grow on almost any organic substance
whether it is solid or fluid, provided the other essential conditions of
growth are furnished. The food substances that are used for culture
purposes are divided into two classes; solids and liquids.

Solid media may be either permanently solid like potatoes, or they may
retain their solid properties only at certain temperatures like gelatin
or agar. The latter two are of utmost importance in bacteriological
research, for their use, which was introduced by Koch, permits the
separation of the different forms that may happen to be in any mixture.
Gelatin is used advantageously because the majority of bacteria present
wider differences due to growth upon this medium than upon any other. It
remains solid at ordinary temperatures, becoming liquid at about 70 deg.
F. Agar, a gelatinous product derived from a Japanese sea-weed, has a much
higher melting point, and can be successfully used, especially with
those organisms whose optimum growth point is above the melting point of
gelatin