2{3() BACTERIOLOGICAL CHEMISTRY 



in lactic and succinic acids ; CI. tvelchii : in acetic and 

 lactic acids ; CI. acetobiitylicum : in lactic acid. When- 

 ever succinic acid is formed it contains fixed carbon 

 dioxide, but the other acids do not always contain 

 assimilated carbon dioxide. The amount and rate of 

 production of succinic acid formed by Esch. coli from 

 glucose, galactose or pyruvic acid depends on the quantity 

 of carbon dioxide available. When carbon dioxide is 

 removed by aeration the yield of succinic acid is low, 

 but when the reaction is carried out in presence of carbon 

 dioxide the yield is high. It has been shown, again by 

 the use of " labelled " carbon, that succinic acid can be 

 formed by condensation of two molecules of acetic 

 acid : — 



CH3COOH ('H2.COOH 



+ > I + 2H 



CH3COOH CH2.COOH 



and that the reaction is reversible. Thus acetic acid 

 containing fixed carbon dioxide could arise by the 

 breakdown of succinic acid. 



Formic acid can be derived by direct reduction of 

 carbon dioxide in presence of the enzyme hydrogenlyase : 



OH OH 



I +H2 I 



CO2 + H2O > HO— G = > HO— C— OH > HO— C = + HgO 



I I 



(carbonic acid) H H 



(formaldehyde hydrate) (formic acid) 



Formaldehyde has been isolated from cultures of pro- 

 pionic acid bacteria, by fixation with dimedon, con- 

 firming that carbon dioxide is reduced. 



The precursor of lactic acid containing fixed carbon 

 has been suggested to be a four-carbon dicarboxylic 

 acid (other than succinic acid) formed by condensation 

 of three-carbon and one-carbon molecules ; it is 

 decarboxylated to give lactic acid. 



