Oxidation of Citric Acid by Coli-aërogenes Bacteria

Species of Aërobacter and Citrobacter utilize citric acid as a sole source of carbon in contrast to Escherichia. Previous work 1 has established the nature of the anaërobic dissimilation of citrate by Aërobacter. The present communication reports the results of an investigation of the aërobic breakdown of citric acid by Aërobacter indologenes and Citrobacter freundii.

The course of citrate-oxidation was followed by serial analysis of media containing 0.1 M citrate as a sole carbon source. After inoculation, the media were kept saturated with air and the gaseous products removed continuously by forced circulation of the gas in a macrorespirometer. Results are presented in Table I.

During the first 39 hours only 4 mM O₂ were consumed by A. indologenes although the medium was saturated with air. By comparison with the anaërobic dissimilation shown in the bottom line of the table, the products during the initial stages of aërobic citrate breakdown appear to be those of a normal fermentation. The second and third analyses demonstrate that the action of oxygen is on acetic, succinic, and formic acids rather than citric acid itself. It is noteworthy that hydrogen was produced throughout the experiment even when its only apparent source was the aërobic dissimilation of acetic or succinic acid.

The experiment with C. freundii shows that in contrast to A. indologenes the former attacks citrate less vigorously but oxidizes the products of citric acid more completely. Hydrogen is not produced. Accumulation and subsequent oxidation of acetic and succinic acids is apparent as with Aërobacter.

Carbon recoveries were consistently lower in aërobic than in anaërobic experiments. The undetermined carbon was found to be non-volatile and non-ether-soluble. The properties of the unidentified material resemble those of complex carbohydrates, suggesting an oxidative assimilation of a part of the organic acids.