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 i±mol of 02 min-1 mg-' of cell protein) was only one-quarter of that on succinate medium (0.14 ,umol of 02 min-' mg-'). 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 ,umol of 02 min-' mg-') or glucose (0.13 ,umol of 02 min-1 mg-) 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 or 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. Azospirillum spp. are ubiquitous, motile, gram-negative soil microorganisms found in close proximity to the roots of tropical forage grasses, legumes, and grain crops (14). Nitrogen fixation by both species of Azospirillum, A. brasilense and A. lipoferum, in association with sugar cane (4), Panicum maximum (5), rice (17), sorghum (19), corn (16), Cynodon dactylon (10), and others have been detected by acetylene reduction assays. A. brasilense Sp 7, previously described as Spirillum lipoferum, is reported to form colonies and fix nitrogen under microaerophilic conditions when provided with malate, succinate, lactate, or pyruvate (11). However, it cannot utilize glucose as a carbon source for growth or nitrogen fixation. Glycolytic sugars fail to enhance oxygen uptake in both cell-free extracts and intact cells (12). A pathway has also been identified for the metabolism of arabinose to axketoglutarate (9). Crude extracts of A. brasilense Sp 7 were found to exhibit Entner-Doudoroff glycolysis, the anabolic Embden-Meyerhof-Parnas pathway, as well as the amphibolic triosephosphate enzymes (18). In contrast, A. lipoferum forms colonies and fixes nitrogen in semisolid N-free medium containing biotin with either glucose or succinate as its sole carbon source (14). A. brasilense is capable of both assimilatory and dissimilatory nitrate reduction under anaerobic conditions (8). In the absence of oxygen, nitrite accumulates in the medium. Thirty strains of A. brasilense have been identified which can further reduce nitrite to...
