Oxidative Assimilation of Lactic Acid by Escherichia coli.

Abstract

Barker, Geisberger, and Clifton have shown that the oxidation of simple substrates by washed suspensions of actively respiring, non-proliferating cells does not proceed to completion, but instead, a portion of the substrate is apparently assimilated by the cells. For example, the oxidative assimilation of acetate by Escherichia coli may be represented as 2 CH3COOH + 3 O2 = (CH2O) + 3 CO2 + 3 H2O According to this equation three-fourths of the oxygen required for the complete combustion of the acetate is consumed with the production of equivalent amounts of carbon dioxide and water, and at the same time one mol of a substance having the empirical composition of a carbohydrate is formed for each 2 mols of acetate undergoing change. In the presence of appropriate concentrations of sodium azide (NaN3) the assimilatory process appears to be blocked, the oxidation proceeding to completion. These studies have been extended to include the oxidation of lactic acid in well washed suspensions and in actively proliferating cultures of Esch. coli, the rates and extent of oxygen-consumption and of carbon-dioxide production being determined at 30°C by Warburg's technic. The oxidation of lactate (2 ml, 0.008 M, Fig. 1, A) by Esch. coli in a M/15 phosphate buffer of pH 7.1 proceeds at a rapid and quite constant rate until approximately two-thirds of the oxygen required for complete combustion has been consumed. Then the rate of oxidation very rapidly decreases and approaches the level of the substrate-free control. A similar behavior was noted by Cook and Stephenson. In the presence of M/600 or M/400 sodium azide the oxidation proceeds nearer to completion while the theoretical amount (538 μl) of oxygen required for complete combustion of the lactate is consumed in the presence of M/200 azide before the rate of oxidation markedly decreases. These concentrations of azide have little influence on the oxygen-consumption by the substrate-free control, the highest concentration tending to decrease slightly the rate of oxygen-consumption of the blank. Sodium azide, in the concentrations employed, did not appear to exert an appreciably germicidal action under the conditions of these tests. The above results, together with an observed R.Q. of 1.0 suggest that the oxidative assimilation of lactate may be represented as CH3CHOH - COOH + 2 O2 = (CH2O) + 2CO2 + 2 H2O