136 PROVISION OF energy: fermentation 



succinic acid is formed in bacterial fermentation and that is 

 by debydrogenation of acetic acid, 



CH3.COOH CH2.COOH 



+ ^^1 +2H (8) 



CH3.COOH CH2.COOH 



In some cases, but not all, this reaction is easily reversed, and 

 acetic acid itself can be formed by reductive breakdown of 

 succinic acid. These reactions have again all been demon- 

 strated by application of techniques involving heavy-carbon. 



THE FERMENTATION OF PYRUVIC ACID BY 

 AEROBACTER AERO GENES 



Formation of acetylmethylcarbinol, etc. 



Aerohacter aerogenes is an organism very similar in many 

 properties to Esch. coli, but systematically differentiated from 

 the latter by a positive " Voges-Proskauer test " (Table II). 

 This test consists of adding strong alkali to a 24 hours old 

 culture of the organism in glucose-peptone, and a positive 

 reaction is shown by the development of a pink colour near 

 the surface of the medium after 24-48 hours. The colour 

 starts to develop at the surface of the medium and slowly 

 spreads down into the liquid. The chemistry of the colour 

 reaction is complex and is due to a reaction between diacetyl, 

 CHg . CO . CO . CH3, and substances in the medium containing 

 a guanidino-group. To speed up the test it is usual nowadays 

 to add a trace of creatinine to the treated medium when, 

 if positive, the colour develops within a short time. Diacetyl 

 is produced by atmospheric oxidation of acetylmethylcarbinol 

 (acetoin), CH3.CO.CIIOH.CH3, which is a fermentation 

 product formed from glucose by this organism. 



Glucose is fermented by the organism to pyruvic acid, as 

 usual, and Aerohacter aerogenes then attacks pyruvic acid in 

 two ways. 



(a) By the phosphoclastic split to acetic and formic acids 

 in exactly the same way as Esch. coli. 



