108 NUTRITION AND METABOLISM. 



the group of gas-producing lactics, B. coli and Bad. aero genes are the 

 main representatives. The fermentation of dextrose by B. coli is very 

 nearly described by the equation 



2 C 6 H 12 O 6 + H 2 O = 2C 3 H 6 O 3 + CH 3 COOH+ C 2 H 5 OH+ 2 CO 2 + 2 H 2 



Dextrose Lactic acid Acetic acid Alcohol 



A certain group of bacteria, including the Bact. aerogenes, the Strept 

 pneumonia, and B. typhosus, cause fermentations which are very similar to 

 the one above. The products depend not only upon the species but also 

 upon the fermented sugar. Grimbert found that B. coli makes acetic 

 acid from all fermentable sugars, but lactic acid only from dextrose, 

 while with all the other sugars, succinic acid is formed instead of lactic 

 acid. B. typhosus gives no succinic acid, but always lactic and acetic 

 acid, often also butyric acid. The mechanism of these fermentations is 

 apparently not as simple as that of the alcoholic and the true lactic fermen- 

 tation. The greatest difficulty for the physiologist is the change of 

 products during the fermentation; at first one, later another product 

 predominates, consequently an attempt to give a certain definite equation 

 of the fermentation is frustrated. It is more convenient merely to enumer- 

 ate all products than to give an equation which may hold true only with 

 a certain stage of growth of a single variety under one set of conditions, 

 without giving any guarantee that other strains under different conditions 

 will give even approximately the same kinds and quantities of products. 



In this group of gas- and acid-forming bacteria, we find a number 

 of organisms of practical importance. They are the organisms of the 

 fermentation of sauerkraut, dill pickles, brine pickles, ensilage. There 

 are further B. coli and a number of pathogenic bacteria, B. typhosus, 

 and the varieties of the Strept. pneumonice. The standard products are 

 acetic acid and alcohol, besides carbon dioxide and hydrogen; lactic 

 acid is usually, succinic acid often, produced. The products depend upon 

 the fermented sugar, the amount of oxygen present, ' temperature, the 

 presence of neutralizing agents and possibly some unknown factors. 



Acetic acid is, besides the lactic acid, the most common acid formed 

 by microorganisms. Formic acid (HCOOH) is produced by a very 

 few bacteria (B. ethaceticus, page 1 1 1) and occasionally by some molds, 

 while acetic acid is a very common product of fermentation. In the 

 decomposition of sugars, it is never the only product; other acids, gases 

 or alcohols are formed besides acetic acid. Pure acetic acid is produced 



