64 THE TYPES OF MILK BACTERIA. 



also be produced by rennet, and souring and curdling may 

 be produced by certain yeasts or certain species of molds 

 (39). But in the great majority of cases it is caused by 

 bacteria. This change in the milk is the one which produces 

 the greatest amount of trouble to the dairyman, and milk- 

 men in general look upon it as the one troublesome fermen- 

 tation in milk .which they desire to avoid. Hence the lactic 

 bacteria must be regarded as the dairy bacteria par excel- 

 lence, and ever since the beginning of the study of bacteria 

 of milk they have been looked upon as the chief dairy organ- 

 isms. In a sense this is true, but in another sense it is quite 

 false. Although the lactic bacteria are the predominant 

 dairy organisms, this does not mean that they are more 

 abundant in the dairy than any other kind of bacteria, nor 

 does it mean that, of the bacteria that get into the milk by 

 the ordinary contamination in the milking, the majority are 

 lactic bacteria. Indeed, as we shall see presently, the reverse 

 is the case, and the larger proportion of the bacteria in fresh 

 milk belong to other types than the lactic bacteria. In say- 

 ing that they are primary dairy bacteria we mean simply that 

 this group of organisms finds milk more favorable to its 

 rapid growth than any of the other miscellaneous bacteria 

 which contaminate the milk and that it causes an almost 

 universal souring. 



The Lactic Bacteria. These include a large number of 

 varieties, over one hundred different types of bacteria hav- 

 ing been described as producing lactic acid. Although many 

 of these have nothing to do with dairy problems, a large 

 number have been found in milk capable of producing lactic 

 acid, and these may, in general, be regarded as normal milk 

 bacteria. This large number of species would doubt- 

 less be very much reduced if the organisms could be com- 

 pared together, for the same species has been described sev- 

 eral times under different names by different investigators. 

 This confusion has been increased by the fact that the same 



