THE KINDS OP MICRO-ORGANISMS IN MILK 341 



ing is the tendency among bacteriologists today to arrange bac- 

 teria in groups, since they are extremely sensitive to environ- 

 mental conditions and respond promptly by varying in one or 

 more of their properties. Owing to their great range in varia- 

 bility, we find the same organism described under different names 

 in some instances. For example. Bacillus aerogenes has been 

 recognized to be identical with Bacillus acidi lactici. Several 

 names have been given to Streptococcus lacticus, as Bacterium 

 lactis acidi, B. gtintheri, Streptococcus hollandicus, etc. The 

 same principle appUes to that group of bacilli now frequently 

 called the Bacillus bulgaricus group. The Bacillus bulgaricus of 

 the Bulgarians is known to be a member of a group of organisms 

 which have the most important properties in common, and differ 

 only in minor points which probably have developed under special 

 environmental conditions. To this group belong the Bacillus 

 lebenis, the bacillus of Boas-Oppler, and others. It is not sur- 

 prising, therefore, that bacteriologists are inclined to reduce the 

 number of "species" rather than to increase it by giving new names 

 to organisms that do not differ materially from well-known types. 



Classification of bacteria in groups is gaining in favor, al- 

 though it must be confessed that it is sometimes difficult to find 

 suitable fundamental properties for this purpose. Morphology, 

 motility, biochemic properties, carbohydrate fermentation, hemol- 

 ysis, and other characteristics have been studied, and authors 

 frequently find that none of these properties are entirely per- 

 manent, but vary sometimes with ease and sometimes with con- 

 siderable difficulty. 



The foregoing discussion applies to the bacteria found in milk 

 with as much force as to bacteria found elsewhere. A sys- 

 tematic arrangement of bacteria found in milk cannot be con- 

 sidered as governing conditions in all cases, but, aside from occa- 

 sional predominance of some rare type, we find that certain bac- 

 teria differ somewhat in their activity and in the quality of their 

 product. For example, butter made in some localities has a bet- 

 ter aroma than butter made in other regions. Still, if the bac- 

 terial flora from milks from two such localities were studied the 

 result would show the presence of similar types in both milks. 

 The aroma of butter is largely due to products of streptococci, and 

 it is a recognized fact that one type of Streptococcus lacticus 

 will produce a superior aroma in butter, while another type — 

 bacteriologically indistinguishable from the first type — will not 

 produce the same aroma. 



Grouping bacteria in milk can manifestly be only tentative, 

 and any attempt at classification must be incomplete and sub- 

 ject to future revision. However, there are some groups that 



