Acetic acid oxidation by Escherichia coli and Aerobacter aerogenes.

Evidence against the occurrence of the Krebs oxidation cycle in bacterial respiration has been steadily accumulating. Escherichia coli and many other bacteria do not readily metabolize the three tricarboxylic acids. Aerobacter aerogenes will not readily attack citrate when measured manometrically unless the organism is grown in the presence of this acid as the sole source of carbon. Recently Lenti (1946) was able to show inhibition of succinic acid oxidation in E. coli by malonate, but the oxidation of pyruvate was not affected. Karlsson and Barker (1948) obtained evidence against the tricarboxylic acid cycle in Azotobacter agilis. There is, therefore, little support for the assumption that the cycle occurs in those bacteria whose intermediary metabolism has been studied in detail. On the other hand, many bacteria, including E. coli and A. aerogenes, oxidize succinate, fumarate, and malate, and reduce anaerobically oxalacetate to succinate, and this suggests that the Szent-Gy6rgyi system, which constitutes an integral part of the Krebs cycle, is operative in microorganisms. By the use of arsenious oxide and cyclohexanol it has been possible to show that glucose or pyruvate is oxidized aerobically as far as acetic acid without the mediation of the C4 dicarboxylic acids as catalytic hydrogen carriers. In other words, the possibility of pyruvic acid initially condensing with oxalacetate to form procitric acid or some other C7 intermediate in its oxidation scheme is ruled out. By applying the principle of "simultaneous adaptation" (Stanier, 1947), it was observed that acetate that arises from the breakdown of either glucose or pyruvic acid is further oxidized to CO2 and water without the mediation of cis-aconitate or a-ketoglutarate. These results may in turn be used as evidence against the occurrence of the tricarboxylic acid cycle in the organisms under consideration.