7O THE TYPES OF MILK BACTERIA. 



probably in most cases an unfortunate one, since they are the 

 organisms that produce the cheese fault known as swelling 

 of cheese. They give rise to prominent odors, such as com- 

 monly found in sour milk. 



In this group are also found quite a number of forms 

 that have been described under distinct names and some of 

 which are undoubtedly distinct species. The well known 

 B. coli communis belongs here, a species frequently present 

 in milk, though commonly not in very great quantity. 

 Closely allied to it, however, is B. aero genes, which in many 

 respects so closely resembles the B. coli as to be difficult to 

 distinguish (37). The two species differ in that B. aero- 

 genes is a stationary rod, while B. coli is motile. To this 

 group belongs also B. acidi lactici of Hueppe. This form 

 has the honor of being the first of the various lactic bac- 

 teria described by modern bacteriological methods, and 

 for a long time was looked upon as the primary lactic bac- 

 teria of the dairy. Here belong also several other forms 

 that have been described by other bacteriologists under dif- 

 ferent names. They are all probably either the same or 

 closely allied varieties and may all be grouped together (58). 

 (See Fig. 17.) 



These organisms also play a very important part in the 

 dairy. They are commonly secondary contaminations, not 

 being found in the milk ducts, except perhaps under excep- 

 tional circumstances. They find their way into the milk 

 from some of the contaminating material that enters the milk 

 during the milking. They are not so frequently concerned 

 in the normal souring of milk as are the bacteria of the first 

 type. Some samples of sour milk have been studied in 

 which the souring has been produced by this type of bac- 

 teria, and some bacteriologists have, therefore, regarded this 

 as the primary cause of milk souring. This may be doubtless 

 true in some individual instances, but in the majority of 

 cases, as has been determined by the study of the last few 



