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 -g-^-^-^-^ 

 of an inch. They are so inconceivably small and light that 

 nine hundred bilUons of them would only weigh jV 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; (3) 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 Bacterial 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 

 es.'^entials 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 alkahne medium. 

 Most bacteria, however, prefer a neutral or slightly alkahne 

 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 BacteriologJ^ f Milk, Its Nature and Composition, by Aikman. 



