100 BACTERIOLOGY. 



It has been indicated above that acetic acid fermenta- 

 tion is not a specific process, due to a single organism. Sev- 

 eral bacteria have already been mentioned as being capable 

 of producing acetic acid. In these cases the acid is made 

 out of alcohol. In some instances, it can apparently be 

 made direct from sugar. This is true of the pheumococcus. 

 Moreover, it is a frequent product in the anaerobic putre- 

 faction of •proteins. The common intestinal bacteria, 

 such as B. lactis aerogenes and B. coli communis, pro- 

 duce acetic acid together with formic and lactic acids 

 and other products. These acids, when formed in the 

 intestines of infants, are unquestionably intense irritants, 

 and may, therefore, be considered as factors in the pro- 

 duction of infant diarrhea. Consequently, the summer 

 complaint of infants is essentially, an abnormal fermenta- 

 tion in the intestines. 



Lactic acid fermentation. — An unusually large number of 

 bacteria are capable of producing lactic acid, as the cMef 

 product, in the decomposition of sugar and other carbohy- 

 drates. As in other fermentations various products are 

 present, though in small amounts. Carbonic acid is usually 

 present, accompanied by traces of alcohol, acetone, formic 

 and acetic acids. 



Owing to the wide distribution of lactic acid bac- 

 teria this type of fermentation will be frequently met 

 with. Thus, in the souring of milk, of sugar-beet juice, 

 in pickles, sauer-kraut, ensilage, etc., lactic acid is a 

 characteristic product. It plays an important part, more- 

 over, in certain affections of the mouth, stomach and intes- 

 tines. 



A temperature of 30-35° is most favorable to lactic 

 acid fermentation. The latter ceases when about 0.75 per 

 cent, of lactic acid has formed. The material usually 

 acted upon is glucose, or lactose, but lactic acid may be 

 formed from various other compounds. The transformation 



