vi] THE BACTERIA OF MILK 79 



stopped ; but, as a rule, by cooling to 7 C. (45 Fahr.) 

 the same end will be effected. 



Among other conditions which regulate the de- 

 velopment of micro-organic life are the reaction and 

 nature of the liquid or other medium in which they 

 exist, the absence or presence of the oxygen of the 

 air, and the absence or presence of light. With 

 regard to the reaction of the liquid, it is a noteworthy 

 fact that some bacteria only develop in the presence 

 of a neutral or alkaline reaction, while others can 

 develop in an acid reaction. Most bacteria require 

 the former condition. An example of the latter kind 

 are those producing lactic acid the bacteria which 

 induce the souring of milk. Acid reaction, we may 

 add, is especially favourable for the development of 

 moulds. 



But one of the most important factors in regulat- 

 ing bacterial development is the nature of the 

 nutritive medium in which they find themselves. 

 Like higher organisms, they have their preference in 

 the matter of food. They require oxygen, carbon, 

 water, and mineral salts. Most of them also, how- 

 ever, require nitrogenous diet. It is hardly necessary 

 to observe that, of all media, milk is the best, and 

 hence it is that it becomes the happy hunting-ground 

 of so many of them. 



Their behaviour in the presence of oxygen serves 

 to divide bacteria into two great classes. Some 



