ACETONE-BUTANOL FERMENTATION 145 



further metabolic changes before giving rise to acetone and 

 butyl alcohol. 



Acetone is produced by the decarboxylation of acetoacetic 

 acid: 



Acetoacetic 



CH3.CO.CH2.COOH -> CH3.CO.CH3 + CO2, 



decarboxylase 



and the acetoacetic decarboxylase, which has been obtained 

 in a cell-free condition from the organism, is formed within 

 the cells only when the environmental pH has fallen to a low 

 value — the growth conditions stimulating its formation being 

 similar to those stimulating the formation of the amino-acid 

 decarboxylases in other bacteria. Accumulation of acetoacetic 

 acid in the medium can be demonstrated by means of a colour 

 reaction, and the presence of the acid can be shown towards 

 the end of the growth of the culture when acetone formation 

 is taking place. There is little doubt but that acetic acid is 

 reduced to acetone through acetoacetic acid, but it is not 

 known how acetoacetic acid is formed froni acetic acid. Recent 

 investigations, using isotopic-C compounds, suggest that acetic 

 acid is also the precursor of butyric acid and butyl alcohol. 



Iso-propyl alcohol, CH3 . CHOH . CH3, is formed by the 

 fermentation of CI. butylicum, an organism closely related to 

 CI. acetohutylicum. Isotopic-C studies again indicate that the 

 iso-propyl alcohol is formed by reduction of acetone, which is 

 formed by the organism in the same way as by CI. aceto- 

 hutylicum, but the acetone in this case is reduced to the 

 corresponding alcohol so rapidly that it does not accumulate 

 and appear as a fermentation end-product. 



Fatty Acid Sjmthesis 



The synthesis of butyric acid has been investigated in detail 

 by Barker and his colleagues for the case of CI. Muyverii. This 

 is a strict anaerobe, isolated from mud, which cannot utilise 

 glucose but requires ethyl alcohol and a fatty acid such as 

 acetate for growth. It obtains its energy by metabolism of 

 these substances resulting in the synthesis of higher fatty acids ; 



