THE RELATION OF MICROORGANISMS TO MILK 379 



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. pyogenes 

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

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

 by different investigators. The most important organism of this group 

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

 edly includes a large nimiber 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. coli communis and Bad. ladis aerogenes. 

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

 named. The most important characteristics of the representatives 

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

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

 that have been studied especially 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 niilk 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 

 and on grain. They are introduced into- the milk with the dirt. The 

 cleaner the conditions of production, the smaller will be the number of 

 these two groups of organisms found in fresh nulk. 



The manufacture of the leading type of butter and of aU kinds of 

 cheese is dependent on the action of microorganisms, hence dairy manu- 

 facturing should be classed as a true fermentation industry. In all 

 such industries one of the factors determining the quality of the product 

 is the type of microorganism employed to produce the desired fermen- 

 tation, and the importance of insuring the presence of desirable organ- 

 isms, and the exclusion of harmful kinds is well recognized. 



