THE RELATION OF MICROORGANISMS TO MILK 445 



ucts upon the other groups of bacteria, the acid-forming types tend to 

 predominate in milk and the specific change which they produce, the 

 souring, is of such common occurrence that it is often looked upon as 

 something inherent in milk. 



GROUPS OF ACID-FORMING BACTERIA.*- -The acid-forming bacteria 

 that are constantly present in milk represent many kinds which differ in 

 morphology, in cultural characteristics, and in their products of fermen- 

 tation. They may be divided into four groups that vary greatly as far 

 as their importance in the handling of milk is concerned. If milk is pro- 

 duced under clean conditions and is kept at temperatures ranging from 

 15 to 35, the acid fermentation will be almost wholly due to a group of 

 bacteria closely allied to one of the pathogenic forms, Strept. pyo genes 

 (Rosenbach). To representatives of this group, which is of the great- 

 est importance in all phases of dairying, have been given various names 

 by different investigator?. The most important organism of this group 

 is one to which the name Bad. lactis acidi is applied. The group undoubt- 

 edly includes a large number of organisms, all of which produce, how- 

 ever, a similar change in milk. 



Second in importance is a group of organisms, of which the best 

 known representatives are B. coll communis and Bact. lactis aerogenes. 

 A large number of organisms of this group have been described and 

 named. The most important characteristics of the representatives 

 mentioned will, however, suffice to characterize the group. A third 

 group is represented by Bact. bulgaricum and the rod-shaped organisms 

 that were first studied in detail by de Freudenreich. A fourth group 

 includes many acid-forming cocci, some of which exhibit proteolytic 

 properties while others do not. Organisms of the third and fourth 

 groups exert little or no effect in the normal acid fermentation of milk, 

 although they are constantly present in varying numbers, as can be 

 demonstrated by appropriate means, and are of importance in certain 

 phases of dairy manufacturing. 



In any sample of milk the relative number of bacteria belonging to 

 each of the first two groups is dependent upon the conditions surround- 

 ing production, especially with reference to cleanliness. The bacteria 

 belonging to the first group come largely from the milk utensils and are 

 also found in the dust of the barn and on the coat of the animal. The 

 source of the second group is largely the fecal matter that gains entrance 

 to the milk, although they are also found in the upper layers of the soil 



* Prepared by E. G. Hastings. 



