258 BACTERIOLOGICAL CHEMISTRY 



tliroiigJi fiirtlier cyck\^ of pliuy])horylation, carl)Ox\iation 

 and reduction, or by condensation with similar molecules, 

 in a manner analogous to the enzymatic synthesis of 

 starch or glycogen from glucose- 1 -phosphate. 



In heterotrophic systems the acceptor of carbon 

 dioxide is probably a phosphorylated Cg compound, as 

 has been shown for the propionic acid bacteria. These 

 organisms, which occur in dairy products (particularly in 

 Emmenthaler cheese), silage, soil and similar situations, 

 ferment carbohydrates with production, mainly, of 

 propionic acid, acetic acid and carbon dioxide, with 

 smaller amounts of lactic and succinic acids and, some- 

 times, acetoin. When grown on a glycerol medium 

 containing " labelled," that is C^^, sodium bicarbonate, 

 Propionibacterium pentosacemn utilises carbon dioxide 

 which is found almost entirely in the carboxyl groups of 

 the succinic acid formed. The succinic acid formed and 

 the carbon dioxide taken up are very nearly in equi- 

 molecular proportion ; in the absence of carbonate 

 practically no succinic acid is formed. The small amounts 

 of " labelled " carbon found in the other products of 

 fermentation, propionic and acetic acids and propyl 

 alcohol, are probably derived by side reactions involving 

 succinic acid. Wood and Werkman suggested that the 

 carbon dioxide condensed with pyruvic acid to give 

 oxalacetic acid : — 



CH3CO.COOH + CO2 > COOH.CHo.CO.COOH 



Succinic acid then arises via mahc and fumaric acids : — 



COOH COOH COOH COOH 



CO + 2H CHOH — H2O CH + 2H CHo 



I > I > II ^ I 



CH2 CH, CH CHo 



I I " I I 



COOH COOH COOH COOH 



(oxalacetic acid) (malic acid) (fumaric acid) (succinic acid) 



