222 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 



(a) Change in Reaction. Milk contains, in addition to protein, two 

 carbohydrates, which play a prominent part in determining the reac- 

 tion of the medium. The principal carbohydrate is lactose, but fresh 

 milk contains in addition, a small amount about 0.08 per cent. of a 

 sugar reacting like dextrose. 1 Changes in the reaction of milk caused 

 by bacterial activity, therefore, may be of several types. An initial 

 acidity followed by an alkaline reaction, as exhibited by the dysentery 

 bacilli and other organisms, is probably due to the initial fermentation 

 of the small amount of dextrose, which results in the formation of 

 acid and then the production of alkaline products from the decom- 

 position of protein when the dextrose is exhausted. These organisms 

 do not ferment lactose. 



A permanent acid reaction is induced either by bacteria which fer- 

 ment lactose, or less commonly, by the decomposition of fat with the 

 liberation of fatty acids. Bacillus typhosus and Bacillus paratyphosus 

 alpha produce a permanently acid reaction in milk, but do not ferment 

 lactose. The exact chemistry of their activity in the medium is not 

 known. An alkaline reaction in milk is usually an indication of proteo- 

 lytic action with the formation of basic products of protein decom- 

 position. 



The accumulation of acid incidental to the fermentation of lactose, 

 as, for example, by B. coli, may be sufficient in amount to cause an 

 acid coagulation of the casein. 2 An acid coagulation can be distin- 

 guished from an enzyme (lab or rennin) coagulation; the acid coagulum 

 will redissolve in alkali, but an enzyme coagulum fails to redissolve 

 by merely neutralizing the reaction. 



Some types of bacteria, as Bacillus aerogenes capsulatus, ferment 

 lactose energetically, liberating a large amount of gas, and forming 

 butyric acid as well. For some unknown reason, Bacillus 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, 3 that the 

 colon bacillus will liberate gas from lactose if the milk is first acted upon 

 by a strongly proteolytic organism, as B. mesentericus. 



Proteolytic bacteria, which are unable to utilize either the small 

 amount of dextrose, the lactose or the fats of milk, usually produce 



1 Theobald Smith, Jour. Boston Soc. Med. Sci., 1897, ii, 236; Jones, Jour. Inf. Dis., 

 1914, xv, 357. 



2 It must be remembered that bacteria grown in litmus milk frequently fail to cause 

 coagulation unless the medium is heated to boiling. 



3 Kendall, Boston Med. and Surg. Jour., 1910, clxiii, 322. 



