78 Perspecfives in Microbiology 



frequently postulated dicarboxylic acid cycle in which 

 acetate is converted directly to succinate. Furthermore, 

 most of the earlier results that appeared to disprove the 

 operation of a TCA cycle can now be satisfactorily ex- 

 plained on the basis of permeability barriers in the cell. It 

 seems justifiable to conclude that the TCA cycle is a 

 functional oxidative mechanism in some bacteria, although 

 the existence of additional pathways of terminal respira- 

 tion certainly cannot be excluded. This is essentially the 

 same conclusion that could have been reached ten years 

 ago, but it is now based upon a much larger body of factual 

 information. 



In addition to serving as an oxidative mechanism, the 

 TCA cycle is useful as a device for providing a variety of 

 reactive compounds containing four, five, and six carbon 

 atoms that serve as starting points for the synthesis of 

 essential cellular constituents, such as amino acids. For 

 example, a-ketoglutarate is the immediate precursor of 

 glutamic acid, which can in turn be converted by E. coli 

 via glutamic semialdehyde to proline or via N-acetyl glu- 

 tamic acid and N-acetyl ornithine to arginine, as has been 

 shown by Vogel (27). 



FERMENTATION PRODUCTS 



The anaerobic transformations of pyruvate and acetate 

 have been studied even more extensively than the aerobic 

 reactions. The pathways of formation of most of the com- 

 mon bacterial fermentation products, such as ethanol, 

 acetate, propionate, butyrate, lactate, butanol, and acetone 

 are fairly well understood, although a good deal remains 

 to be done on the enzymatic level. It is also likely that 

 different mechanisms are operative in the formation of 

 some of these compounds in different bacterial species. 

 This is known to be true for acetate, which is formed by 

 several anaerobic bacteria from carbon dioxide by an 

 entirely unknown mechanism. 



