Reduction of nitrate with molecular hydrogen by Proteus vulgaris.

The reduction of nitrate by molecular hydrogen was observed first in Bacterium formicum by Stephenson and Stickland (1931). In this system, the product of the reduction was nitrite, and the theoretical hydrogen uptake occurred only when the endogenous respiration of the cell was suppressed by prior treatment of the cells with toluene (Stickland, 1931). Woods (1938) found that Clostridium perfringens (Clostridium welchii) and certain strains of Escherichia coli reduce nitrate to ammonia with the utilization of four moles of molecular hydrogen. Nitrite and hydroxylamine were shown to be intermediates in this reduction. Lascelles and Stir (1946b) reported that Escherichia coli reduces nitrate to ammonia only in the presence of benzyl viologen as a carrier. In the absence of a carrier, the reduction did not proceed beyond the nitrite stage. This and the action of some inhibitors suggested that different enzymes are responsible for the reduction of nitrate on the one hand and nitrite and hydroxylamine on the other. Cell-free extracts have been obtained which were capable of carrying out these reductions (Back et al., 1946). The reduction of nitrate by molecular hydrogen has been observed also in azotobacter (Phelps and Wilson, 1941), Aerobacter aerogenes (Lascelles and Still, 1944), Micrococcus phos-