Intermediary carbon metabolism of Azospirillum brasilense

Azospirillum brasilense Sp 7 grew rapidly in AZO medium containing reduced nitrogen and succinate as an energy source, with a doubling time of 43 min. No growth was measured with glucose as the sole carbon source. In contrast, Azospirillum lipoferum Sp 59b could grow in media containing either succinate or glucose with a doubling time of 69 min and 223 min, respectively. Warburg-Barcroft respirometry showed that the rate of oxygen consumption by A. brasilense Sp 7 on glucose medium (0.034 mumol of O2 min-1 mg-1 of cell protein) was only one-quarter of that on succinate medium (0.14 mumol of O2 min-1 mg-1). Radioisotopic labeling showed that very little glucose was assimilated by A. brasilense Sp 7 as compared to succinate. High respiration rates were measured on A. lipoferum Sp 59b with either succinate (0.15 mumol of O2 min-1 mg-1) or glucose (0.13 mumol of O2 min-1 mg-1) as the sole carbon source. The pattern of CO2 evolution from differentially labeled succinate indicated that A. brasilense Sp 7 had a complete tricarboxylic acid cycle. Assimilation of most of the radioactivity from labeled succinate, pyruvate, and acetate into lipids suggested a strong anabolic metabolism and the presence of an active malic enzyme of phosphoenolpyruvate carboxykinase. The distribution of radioactivity from differentially labeled pyruvate showed that gluconeogenesis competed with pyruvate dehydrogenase. Uptake and incorporation of labeled acetate also indicated the presence of a glyoxylate cycle in A. brasilense Sp 7.