ACID PRODUCTION 85 



being of considerable economic value is often used advantageously 

 to distinguish closely related species, notably in the groups of para- 

 typhoid and dysenteria bacilli. In addition to the acid produced 

 by the fermentation of carbohydrates, many bacteria hydrolyze 

 proteins with the formation of an acid reaction. Formic acid is 

 the simplest organic acid which can be formed. It is produced 

 by R. typhosus, the causative agent of typhoid fever. B. typhosus 

 does not form gas, but B. coli does; and in this latter case it may 

 result from the decomposition of the formic acid. 



HCOOH = H 2 + CO 2 . 



Acetic acid is one of the most important acids formed by bacteria. 

 Bact. aceti and Bact. pasteurianum are two of the more important 

 acetic acid-forming bacteria. They occur in fermenting fruit juices 

 and convert the alcohol into acetic acid. 



CH 3 CH 2 OH + Oa CHsCOOH + H 2 O. 



Many other species also possess the power of transforming 

 alcohol and other substances containing the characteristic radical 

 CH 2 CH20H into acetic acid. This fermentation, however, can 

 take place only within certain limits of concentration, and even then 

 there must be available nitrogen in the form of proteoses, peptones 

 or amino-acids, and mineral elements, especially phosphorus in the 

 form of a phosphate. Acetic acid is produced on the commercial 

 scale by a number of processes. Two of the best are the Orleans 

 and the Quick, or German, methods. 



Lactic Acid. This product is formed by a great number of bac- 

 teria. The chief species, however, is the Streptococcus lacticus 

 which produces only a scanty grow r th on agar, but an excellent 

 growth in milk, bringing about a solid curdling in a few days. The 

 lactose of the milk is first inverted forming two hexoses dextrose 

 and galactose. 



Ci 2 H 22 On + H 2 O = CeHy-Oe + C 6 Hi 2 O 6 

 Lactose. Dextrose. Galactose. 



The hexose in turn is decomposed yielding two molecules of lactic- 

 acid. 



e 2C 3 H 6 O3 



Dextrose. Lactic acid. 



In actual experience the reaction occurring is not as simple as 

 written in the equation, but there are other products formed. For 

 instance, B. coli ferments glucose with the formation of alcohol, 

 carbon dioxid, hydrogen, lactic acid, succinic acid, and other 



