General Characters of Bacteria 109 
which they have all been removed, is said to be 
sterile. The germs found in milk belong to the 
lowest orders of the vegetable kingdom. Most of 
them are included in the bacteria, although many 
yeasts and moulds are frequently found in milk. 
The bacteria.—The bacteria are extremely minute 
bodies consisting of a single cell filled with protoplasm. 
They are of three general forms,— spherical (coceus) ; 
rod-like or cylindrical (bacillus), and curved or wavy 
(spirillum). They reproduce by fission; that is, the 
cell elongates slightly in the direction of its longer 
axis, and a partition is formed across the cell trans- 
versely, and two individuals exist where there was 
but one before. These may break away and form 
separate bacteria, but often they are kept together 
in various ways. Many forms are endowed with 
motion, and all require a liquid or sémi-liquid me- 
dium for growth and development, though many 
may exist for long periods of time in a dry condi- 
tion. Like other plants, in order to grow and de- 
velop, the bacteria must have suitable food. They 
require for their sustenance carbon, hydrogen, oxy- 
gen and nitrogen, together with small amounts 
of mineral matters. Organic compounds are more 
available for food supply than simple inorganic 
salts. Substances like sugar and the various al- 
buminous compounds are admirably suited for their 
food. In ordinary milk nearly all kinds of bacteria 
find an adequate and easily available food supply 
in a medium favorable to their growth, so that not 
only the forms of bacteria ordinarily found in milk, 
