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 S-Q^O- 

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

 nine hundred billions of them would only weigh V of an 

 ounce, f 



Bacteria also vary considerably in shape. They are as a 

 rule classed into three groups: (1) The bacillus or rod-shaped; 

 C2) 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 

 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. t Milk, Its Nature and Composition, by Aikman. 



