BACTERIA COMMONLY FOUND IN COWS' MILK. 99 



cocci which can only multiply in a distinctly alkaline sub- 

 stratum. When the milk once had this reaction due to 

 the activity of the bacteria mentioned, they could develop, 

 which they did to such an extent as to put their benefac- 

 tors to flight. The cream then obtained the disagreeable 

 taste and smell before mentioned as the last stage in the 

 changes of the milk. 



According to my experience the moulds Oidium lactis 

 spoken of, under which name several species seem to hide, 

 often makes the substratum more favorable for various 

 bacteria a fact to which I have referred in an earlier pub- 

 lication.* 



To the third group (indirectly injurious bacteria) some 

 bacteria causing acidity in the milk may also be referred. 

 They more properly belong to the fourth group, how- 

 ever, and will be treated there. Although the bacteria 

 in general require an alkaline or neutral reaction in 

 the substratum, some are found demanding an acid reac- 

 tion. These are, first of all, the butyric-acid bacteria. 

 They thrive only in a strongly acid medium, and are there- 

 fore most frequently found in sour milk, where they strug- 

 gle for a living with the lactic-acid bacteria. These latter 

 are thus their forerunners, which produce a favorable nutri- 

 tive medium for the butyric-acid bacteria, and are then 

 forced out of the way by these. Although the butyric-acid 

 bacteria at first are only in an exceedingly small minority 

 among the micro-organisms of the milk, they multiply rap- 

 idly as soon as the substratum grows too acid for the 

 lactic-acid bacteria, and begin to make their presence felt 



* " Saprofyta mikro-organismer i komjSlk, I," Helsingfors, 

 1890, p. 80; see also Lang and Freudenreich, "Oidium lactis in 

 Milk," Ldw. Jahrb. Schw. 7, p. 229. 



