32 PRACTICAL DAIRY BACTERIOLOGY 



some of the cultures found in a sample of milk will sour and 

 curdle it quickly, while others do not, only indicates that we 

 are dealing with varieties of the same bacterium, and not with 

 different species. 



To this type belong the largest number of bacteria known 

 to cause the common souring of milk. Most of the butter starters 

 (see page 203) are cultures of this class and the same is true 

 of most of the cheese starters. Several different types of lactic 

 acid bacteria have been described by Freudenreich as associated 

 with the ripening of Swiss cheese. These, as he studied them, 

 showed considerable difference in their action on cheese. Some 

 appeared to benefit the cheese, while others did not. They 

 showed other differences also, but most of them belong to this 

 same general type of bacteria. This emphasizes the fact that 

 the Bad. lactis acidi represents a group of varieties, and not a 

 single organism. If the question is asked whether it represents 

 a single species, or a collection of species, the answer is, that 

 no one knows what is meant by the word species among bac- 

 teria, and hence, this question cannot be answered. It cer- 

 tainly represents a collection of forms related to each other, 

 but showing some noticeable variations. 1 



The type of milk curdling produced by this organism . is 

 quite easily recognized. The milk becomes strongly acid, and 

 turns into a hard curd, without any trace of gas bubbles, and 

 without the separation of any whey. (Fig. 18, .) The milk 

 simply becomes one smooth, solid curd, with a clean, sharp, 

 sour taste, and no odor. This type of curdling has been recog- 

 nized as a desirable one by the dairyman, since it is most favor- 

 able for dairy processes, and is consistent with the production 

 of the best grades of butter and cheese. This organism grows 

 readily at temperatures from 60 to 100, growing more rapid- 

 ly at higher temperatures. At a temperature of about 70 it 



iMtiller. Cent. f. Bact., II., xvii., p. 468. 

 Lohnis. Cent. f. Bact., II., xviii., p. 97, 1907. 



