FERMENTS IN MILK. 45 
while the plants that we see in every-day life are aggregations 
of cells. Some bacteria are motile, while others are not. 
Size and Shape of Bacteria.—In size, bacteria are the smallest 
organisms that exist, so far as known. The size varies con- 
siderably. Russell * gives the average diameter as xp455 
of an inch. They are so inconceivably small and light that 
nine hundred billions of them would only weigh =‘; of an 
ounce. t 
Bacteria also vary considerably in shape. They are as a 
rule classed into three groups: (1) The bacillus or rod-shaped; 
(2) The coccus or ball-shaped; (8) The spirillum or spiral- 
shaped (like a corkscrew). Some types of bacteria are clas- 
sified according to the way in which they adhere to each other. 
For instance, when two cocci occur together and form a pair, 
they are called diplococci, when bacteria occur in chains, 
they are called streptococci, when bacteria appear in bunches 
they are called staphylococci, etc. 
FavorABLE CoNDITIONS FOR BacrertaL GROWTH. 
Food.—Bacteria are like other plants in nature,—they need 
food for their existence. However, they require their food in 
solution. Nitrogen, carbon, oxygen, and mineral matter are 
essentials for bacteria. These substances are furnished in 
abundance in milk from casein, albumen, milk-sugar, and the 
mineral salts. Butter-fat in milk is said to be of little value 
as a food for bacteria. 
Some bacteria prefer a substance having an acid reaction in 
which to grow; others thrive best in an alkaline medium. 
Most bacteria, however, prefer a neutral or slightly alkaline 
substance. Darkness is essential to some bacteria, and is 
preferred by the majority of the different species. Bright 
sunlight is a very effective germicide. It is fatal to all species, 
so far as known. Some germs require air for their growth. 
These are called aerobic. Others again grow only in the 
* Dairy Bacteriology. ft Milk, Its Nature and Composition, by Aikman. 
