THE KINDS OF MICRO-ORGANISMS IN MILK 



In the previous chapter the important sources of bacteria 

 commonly found in milk have been discussed. It is clear that 

 most of these bacteria are saprophytes, and since the bacterial 

 pollution of milk originates from external conditions surrotrnding 

 the dairy it is not surprising that there are certain predomiaant 

 types that are almost universally present in milk. As a rule, 

 therefore, milk undergoes a fermentative process usually termed 

 the "normal" souring of milk, and this process is similar in milks 

 from widely different sources. However, in different locahties and 

 different dairies the kinds of bacteria are not necessarily the 

 same, but since milk is an excellent culture-medium for many 

 bacteria we find that after the lapse of some time distinct phases 

 of fermentation are observable. These phases of fermentation 

 are very similar in milks derived from various locahties, although 

 occasionally there are abnormal products so that the milk is 

 changed in appearance, taste, and odor, and differs materially 

 from the normal product. 



The fact that certain kinds of micro-organisms are found with 

 astonishing persistence in milk has given rise to the term "milk 

 bacteria," a term still adhered to by some authors. However, it 

 is generally conceded that milk is secreted by the mammary glands- 

 in a sterile condition and, therefore, the term "milk bacteria" is 

 misleading and should be abandoned. 



Considerable meritorious work has been carried on in regard 

 to the kinds of bacteria that occur in milk. Conn, Esten, and 

 Stocking have studied about 160 types, and have carefully de- 

 scribed and grouped them. This number includes bacteria iso- 

 lated by the authors from milk in Connecticut. In addition to 

 these forms, cultures were studied that came from different parts 

 of this country and also some from European sources. Besides, 

 descriptions when sufficiently complete were taken from the 

 literature and incorporated in the list. The authors think, 

 therefore, that "while by no means complete, the cultures repre- 

 sent fairly the dairy forms in the parts of the civilized world where 

 bacteria are studied." 



It should be borne in mind in this connection that the term 

 "species" is wholly unsuitable in speaking of bacteria. It is recog- 

 nized that among high forms of life variations occur, and this is 

 true in even larger measure in regard to bacteria. More promis- 



340 



