OF 



UNIVERSITY ) 

 J 



TYPfcrf OF BACTERIA FOUND IN MILK 25 



Varieties of Bacteria Which Sour Milk. The bacteria which 

 sour milk are known, in general, as lactic acid bacteria. There 

 are many varieties of them, however, differing more or less 

 from each other. A very large number of different acid bac- 

 teria have been separated from milk by bacteriologists, and have 

 been described. Exactly how many of them there are it is im- 

 poxsible to say. The reason for this impossibility is the fact 

 that up to the present time we do not know what constitutes 

 a type or a species of bacteria. We know that large numbers 

 of lactic bacteria have been found which differ from each other 

 in certain small points. Whether these differences are enough 

 to separate them into different types or species, or whether they 

 are only slight variations which appear in one and the same 

 organism, is not yet known. We do not know, moreover, to 

 what extent these slight differences may come and go under 

 different conditions. For example, the various types of this 

 class of organisms show great variation in the rapidity with 

 which they can curdle milk. Some of them curdle it in a 

 very few hours where the temperature is warm, others require 

 several days for producing the same effect, while still others 

 will not curdle the milk under any circumstances. Whether 

 these should be recognized as different varieties, or simply as 

 modified forms of the same type has been a subject of dispute. 

 It is certain that this power of curdling milk may be increased 

 in the same organisms by proper treatment in the laboratory, 

 and the general opinion, to-day, is that some bacteria which 

 curdle milk and others which sour it without curdling, cannot 

 be regarded as separate species, but only as modifications of 

 the same. Similarly in regard to other slight differences, our 

 bacteriologists are not certain as to how wide variations may be 

 found in the same organism. For this reason it is impossible to 

 state how many species of lactic acid bacteria there are. Al- 

 though this is true, it has become evident in the last few years 

 that the lactic acid bacteria separate themselves into two some- 



