62 PHYSIOLOGY OF BACTERIA 



only interesting species is the Hydrogenomonas, which 

 requires CO2 for the oxidation of hydrogen, and makes 

 formic acid as intermediate product, according to Kaserer 

 (1906) 



CO2 + 2H2 = HCOH + H2O + 6 cal. 



HCOH + 02 = H2O + CO2 + 132 cal. 



(g) INTERACTION BETWEEN NITROGENOUS AND 

 NON-NITROGENOUS FOOD COMPOUNDS 



We have seen several examples of intermolecular 

 oxido-reductions, i.e. an exchange of hydrogen atoms 

 between molecules of different composition. The Can- 

 nizzaro reaction between methyl glyoxal and acet 

 aldehyde is perhaps the best known case. Since these 

 reactions are essentially caused by the activation of 

 certain hydrogen atoms, it seems probable that many 

 other compounds capable of reduction could act also 

 as hydrogen acceptors, and that the hydrogen acceptors 

 will not be limited to cleavage products of sugar. 



A very simple and instructive case of this type is 

 reported by van Niel (1928) who studied the action of 

 twelve strains of propionic acid bacteria upon lactic 

 acid and found that the products did not correspond to 

 the equation: 



3C3H6O3 = 2C3H6O2 + C2H4O2 + CO2 + H2O 



Lactic acid Propionic Acetic acid 



acid 



The ratio of propionic to acetic acid, instead of being 2:1, 

 was very persistently 1.8:1. Besides, some succinic 

 acid was formed which had to be accounted for. Van 

 Niel could demonstrate that the succinic acid did not 

 come from the lactic acid, but from aspartic acid or 

 some similar amino acid in the rather concentrated 



