102 BACTERIOLOGY. 



the stomach. In some instances, acetic, or butyric acid 

 may predominate in the stomach contents, and, at other 

 times considerable gas may be given off. The production 

 of lactic and acetic acids in the intestines has already been 

 referred to, in connection with infant diarrhea. 



The most common illustration of lactic acid fermenta- 

 tion is met with in the souring of milk. This secretion, as 

 elaborated in the gland, is strictly sterile, and if col- 

 lected under suitable precautions it will keep indefinitely. 

 In ordinary milking an enormous number of bacteria are 

 introduced into the milk, from the air and through unclean 

 hands, containers, etc. Given a suitable temperature, these 

 organisms rapidly multiply and convert the milk-sugar into 

 lactic acid. As a result, the reaction of the milk, which was 

 neutral or alkaline in the beginning, becomes acid. Inas- 

 much as the casein is insoluble in the presence of an acid, 

 it forms a precipitate which mechanically drags down the 

 fat. The curdling of milk may result from the action of a 

 rennet ferment, but, as a rule, it is due directly to the acid 

 reaction which develops in the milk. 



It is evident that some bacteria will coagulate milk, 

 whereas others will not. This fact is made use of, at times, 

 in differentiating bacteria. Thus, milk is coagulated by 

 the colon bacillus but is not altered by the typhpid bacillus. 



In eastern Europe and in Asia, the milk of the cow, ass, 

 or camel is subjected to an alcoholic, as well as lactic 

 acid fermentation. The resultant beverage is known as 

 kephyr or koumiss. The fermentation is induced by adding 

 to the milk the so-called kephir grains, which consist 

 essentially of bacteria and yeasts. 



The lactic acid bacteria favor the development, in the 

 body, of anaerobic bacteria, such as the tetanus bacillus. 

 Microbic associations of this kind play an important part 

 in many diseases. 



It may be well to briefly allude in this connection to the 

 so-called ripening of cheese. Bacteria are to a very large 



