428 FURTHER EVOLUTION 



agree with the scheme which has been given, but recently 

 M. Gibbs and R. D. DeMoss®^ have shown that the initial 

 stages of its metabolism deviate from the scheme in some 

 details. As soon as hexose-6-phosphate is formed it is de- 

 hydrogenated anaerobically to form 6-phosphogluconic acid. 

 This is then decarboxylated to give a pentose. The pentose 

 is broken down by the disruption of a carbon-carbon bond 

 to form alcohol and phosphoglyceraldehyde- 3 -phosphate 

 which is converted to alcohol in the same way as in ordinary 

 alcoholic fermentation by yeast. 



In a group of typical obligately anaerobic bacteria, the 

 Clostridia, which can carry out butyric and acetone-butyl 

 alcohol fermentation, this takes place by essentially the same 

 method of glycolytic transformation of sugars as is found in 

 alcoholic fermentation. For example, the experiments of B. 

 Rosenfeld and E. Simon®^ showed that phosphoenol pyruvic 

 acid is formed during the process of acetone-butyl alcohol 

 fermentation. Pyruvic acid seems to be a necessary inter- 

 mediate product in other forms of butyric acid fermentation 

 but its further transformation in other bacteria of this group 

 gives rise to a whole range of different organic substances : 

 butyric acid, butyl alcohol, wopropyl alcohol, acetone, ethyl 

 alcohol, acetic acid, formic acid, hydrogen and carbon 

 dioxide. 



For example, Clostridium acetobutylicum ferments glucose 

 with the formation of butyl alcohol, acetone, ethyl alcohol and 

 hydrogen. Another organism, CI. sac char obutyricum, forms 

 butyric and acetic acids, carbon dioxide and hydrogen. 

 Zymosarcina maxima forms butyric, acetic and lactic acids, 

 carbon dioxide and hydrogen.®* 



The work of H. G. Wood and his colleagues,®^ and of 

 H. A. Barker,®® has established that the 4-carbon compounds 

 which are produced during various types of butyric acid 

 fermentation are formed by the condensation of active 

 residues of acetic acid, in the form of acetyl-coenzyme A, 

 with the formation of acetoacetic acid and its subsequent 

 reduction to butyric acid. Acetyl-coenzyme A is either 

 formed directly from acetic acid or from pyruvic acid by 

 anaerobic dehydrogenation and decarboxylation. 



Acetone is formed by the decarboxylation of acetoacetic 



