Glucose and Mannitol by B. coli communis. 



481 



Glucose, CgTI^Og. 



Lactic acid, C 3 H 6 3 . 



Formic acid, C0 2 -H 2 . 



Intermediate substance B. 



Alcohol, C.,H 6 0. 



Acetic acid, C 2 H 4 3 . 



The intermediate substance A is unknown, but is postulated to account for 

 the formation of lactic acid in such a way that the enzyme which produces 

 lactic acid from glucose may also produce lactic acid from mannitol. The 

 substance is probably related to pyruvic aldehyde. 



The intermediate substance B from which it is suggested that alcohol and 

 acetic acid are derived is probably acetaldehyde. This view is supported 

 by the evidence that acetaldehyde may be detected among the products of 

 decomposition of glucose by B. coli communis (Grey, 1913). 



Two molecules of acetaldehyde might undergo the Cannizzaro reaction 

 (Parnas, 1910) with the production of alcohol and acetic acid, thus 



If this were the main change in the case of glucose, it would account for 

 the production of alcohol and acetic acid in approximately eqttimolecular 

 proportions. 



Again, acetaldehyde might be reduced to alcohol as postulated by 

 Kostytscheff (1912) for alcoholic fermentation by yeast or directly oxidised. 



In the case of mannitol this reduction might be of great importance. It is 

 represented by the dotted lines in the scheme. And since, in this case 

 the whole, or nearly the whole, of the hydrogen formed, along with the 

 intermediate substance A, would be available for this purpose, the result 

 would be the production of alcohol in large excess over that of acetic 

 acid, which is actually observed. 



While, however, the decomposition of mannitol and glucose may thus be 

 represented as occurring along the same general Hues, it is clear that some 

 essential difference must exist between the mechanisms of the two reactions, 

 or they would not be so differently affected by the process of selection on 

 chloroacetate agar. 



The simplest supposition is that this difference affects the production of 

 formic acid and intermediate substance B, for artificially selected organisms 

 produce these substances from glucose in greatly diminished amount, whereas 

 from mannitol their production is not seriously altered. 



"While the exact nature of the difference in the two mechanisms must still 



2CH 3 -CHO + H 2 = CH 3 COOH + CH 3 CH 2 OH. 



