METHODS FOR THE MICROSCOPIC STUDY OF BACTERIA 223 



an alkaline reaction which may be a simple alkalinity without obvious 

 change in the appearance of the medium (as, for example, B. alkali- 

 genes) or a deep peptonization of the casein, as illustrated by B. 

 pyocyaneus. B. mesentericus peptonizes casein energetically, but 

 the reaction of the medium is persistently acid. The initial acidity 

 is probably due to the formation of acid from the dextrose of the 

 milk; the residual acid may be associated with the activity of an 

 esterase which certain strains of this organism appear to elaborate. 

 Fatty acids are formed by hydrolysis of the glycerides of the cream 

 by the soluble esterase, while the metabolic activities of the organism 

 appear to be largely directed to the proteins of the medium. 1 



It is apparent, therefore, that the chemical and physical changes 

 induced in milk incidental to bacterial development in the medium 

 are, or may be, complex in their origin. A knowledge of the proteo- 

 lytic and fermentative activities of bacteria in the simpler media, 

 however, will frequently furnish an explanation for the more involved 

 reactions in the highly complex medium, milk. 



1 Kendall, Day and Walker, Jour. Am. Chera. Soc., 1914, xxxvi, 1937. 



