The metabolism of organic acids in Pseudomonas Aeruginosa A.T.C. 9027

The present study has been concerned with the oxidation of the 4-carbon dicarboxylic acids: succinate, fumarate and malate, in the strict aerobe, Pseudomonas aeruginosa A.T.C. 9027. Resting cells harvested from a mineral medium in which any one of these compounds was the sole carbon source, metabolized all three acids immediately and at a constant and rapid rate. This observation has suggested the operation of the enzymatic inter-relationship: Succinate⇌ fumarate⇌malate It is very likely that the oxidation of these substrates follows the familiar tricarboxylic acid cycle, which has already been shown to occur in P. aeruginosa 9027. Questionable quantitative data for the oxidation of the dicarboxylic acids by dried cell preparations has been obtained. These preparations did not require either cytochrome c or pyocyanine as a carrier system for succinate oxidation. The optimum pH range of succinic dehydrogenase in succinate-grown and acetate-grown dried cells lay between pH 7.0 and 8.0, an observation which coincides with values found for other species. The effect of the two metabolic inhibitors, malonate and uranyl nitrate, on the respiration of both resting and dried cells of P. aeruginosa has been investigated. Malonate was not inhibitory in moderately high concentration at pH 7.0, but when the pH was lowered, maximum inhibition was effected by low concentrations. Apparently, permeability functions as a governing factor in malonate inhibition, a suggestion which was borne out in experiments with dried cells, in which the cell membrane has been altered during the drying process. These cells were strongly inhibited at pH 7.0 by low concentrations of malonate. An interesting though complicating feature of the malonate experiments with resting cells has been that P. aeruginosa possesses an adaptive enzyme system for metabolizing the inhibitor. The inhibitory effect of uranyl nitrate on the oxidation of Krebs intermediates or of glucose varied with the substrate - maximum inhibition being obtained with isocitrate oxidation, and none with malate oxidation, while the inhibition of other oxidations fell between these two extremes. The use of this non-specific inhibitor in metabolic studies on whole cells has been discussed and, on the basis of the present and other studies, its use has been precluded.