148 THE BACTERIA CONTENT OF MILK 



venient provisional classification of organisms occurring in milk is 

 a fourfold one as follows : — 



(a) Saprophytic bacteria, an indefinite group of unknown func- 

 tion, gaining access to milk from air, water, soil, dirt, dust, etc. 



{b) Fermentation bacteria, which set up the various " diseases " 

 of milk and the specific fermentations, such as lactic, butyric, etc. 



{c) The economic bacteria, which give flavour to butter and 

 cheese, and gain access to milk from various quarters. 



{d) The pathogenic bacteria, some of which, such as the 

 tubercle bacillus, set up slow infective processes ; and others of 

 which, as B. typhosus, are the causal agents in certain epidemic 

 diseases. 



The quality, like the quantity, of bacteria found in milk depends 

 upon a variety of external circumstances. Sometimes only half 

 a dozen species may be found in a milk, whilst at others thirty or 

 more species may be isolated. Several hundred species of bacteria 

 have been described as occurring in milk. The chief of them will 

 be found recorded in a subsequent chapter, and special chapters 

 are devoted to a consideration of the fermentation, economic, and 

 pathogenic bacteria. 



