50 BACTERIAL FERMENTATIONS 



The formation of the C 5 to C 7 fatty acids involves a 

 condensation between acetyl-SCoA and the thioester of a 

 longer-chain fatty acid. In these condensations the C 2 

 moiety always appears at the carboxyl end of the resulting 

 fatty acid. 57 This result is consistent with a synthetic path 

 involving /?-keto acid derivatives. 



formation of Acetone and Butanol. Acetone and 

 butanol are formed from carbohydrates by CI. acetobutyli- 

 cum and other butyric acid bacteria mainly toward the end 

 of the fermentation when the pH of the medium drops to 

 about 4.0. As we have seen, the accumulation of solvents 

 coincides with a decrease in the concentration of volatile 

 acids. The conversion of acetic acid to acetone and butanol 

 and the conversion of butyric acid to butanol are clearly 

 established. 



Acetone is undoubtedly formed by the decarboxylation 

 of acetoacetate (reaction 27) . Johnson et al. 59 first demon- 



CH3COCH2COOH — > CH3COCH3 + C0 2 (27) 



strated this reaction with growing cultures and cell suspen- 

 sions; later Davies 60 extracted and partially purified the 

 decarboxylase responsible for the reaction and showed that 

 it is most active at pH 5. 



Acetoacetate is almost certainly formed from acetoacetyl- 

 SCoA, although with bacteria only indirect evidence for 

 this is available. In some CI. kluyveri extracts acetoacetate 

 accumulates during the oxidation of butyrate when the 

 normal conversion of acetoacetyl-SCoA to acetyl phosphate 

 is prevented by a lack of either coenzyme A or orthophos- 

 phate. 23 - 36 This can be interpreted to mean that aceto- 

 acetate is formed from its coenzyme A derivative. The 

 nature of the reaction by which acetoacetyl-SCoA is con- 

 verted to acetoacetate in Cl. kluyveri has not been investi- 

 gated in detail but it appears to be an hydrolysis. This is 



