METHODS FOR MICROSCOPIC STUDY OF BACTERIA 2oo 
acid coagulation of the casein. ^ An acid coagulation can be distin- 
guished from an enzyme (lab or rennin) coagulation; the acid coagu- 
lum will redissolve in alkali, but an enzyme coagulum fails to redissolve 
by merely neutralizing the reaction. 
Some types of bacteria, as B. aerogenes capsulatus, ferment lactose 
energetically, liberating a large amount of gas, and forming butyric 
acid as well. B. coli and allied organisms, which ferment lactose in 
fermentation tubes with the liberation of considerable amounts of 
gas, fail to produce gas from the lactose as it exists in milk. It has 
been shown, however,- that the colon bacillus will liberate gas from 
lactose if the milk is first acted upon by a strongly proteolytic organism, 
as B. mesentericus. The addition of peptone to milk prior to inocula- 
tion also leads to gas production. 
Proteolytic bacteria, which are unable to utilize either the small 
amount of glucose, the lactose or the fats of milk, usually produce 
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 glucose of the milk; 
the residual acid may be associated with the activity of an esterase 
w^iich certain strains of this organism appear to elaborate, as well 
as to the liberation of amino-acids by its soluble proteolytic enzyme. 
Fatty acids are formed by hydrolysis of the glycerides of the cream 
by a soluble esterase, while the metabolic activities of the organism 
appear to be largely directed to the proteins of the medium.^ 
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, 
how^ever, will frequently furnish an explanation for the more involved 
reactions in the highly complex medium, milk. 
' It must be remembered that bacteria grown in litmus milk freciuently fail to cause 
coagulation unless the medium is heated to boiling. 
2 Kendall: Boston Med. and Surg. Jour., 1910, 163, 322. 
3 Kendall, Day and Walker: Jour. Am. Chem. Soc, 1914, 36, 1937. 
