THE RELATION OF MICROORGANISMS TO MILK. 307 



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. lactis acidi is applied. The group undoubtedly 

 includes a large number of organisms, all of which produce, however, a 

 similar change in milk. 



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

 representatives are B. coll comnmnis 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 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 un- 

 doubtedly 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 surrounding 

 production, especially with reference to cleanliness. The bacteria belong- 

 ing 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 enter the 

 milk from the coat of the animal. The cleaner the conditions of pro- 

 duction, the smaller will be the number of these two groups of organisms 

 found in milk. 



The manufacture of the leading type of butter and of all kinds of cheese 

 is dependent on the action of microorganisms, hence dairy manufacturing 

 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 fermentation, and the 

 importance of insuring the presence of desirable organisms, and the ex- 

 clusion of harmful kinds is well recognized. 



The most important properties of organisms employed in the fermen- 

 tation industries are the physiological rather than the cultural or mor- 

 phological, since the quality of the product is dependent on the by- 

 products of the fermentation. Hence in characterizing, the groups of 



