[Metabolism of a psychrophilic bacterium from fresh water]

Psychophilic microorganisms, able to grow at 0-1 degrees C, were isolated from water obtained from the Paraná River at Rosario. One of the strains, R-12, was identified as Pseudomonas fluorescens according to the description in Bergey's Manual of Determinative Bacteriology (8th Ed., 1974). The microorganism was able to grow in liquid minimal medium with glucose, acetate, glutamate or casein hydrolysate as sole carbon source, at 20 degrees C. The enzymes of the Entner-Doudoroff pathway were induced in cells grown on glucose. The Krebs cycle was apparently operative in all cases; the lower levels of citrate synthase and isocitrate dehydrogenase were found in glucose-grown cells. Isocitrate lyase was present at a high concentration, and malate synthase considerably increased, in acetate-grown cells, thus suggesting the operation of the glyoxylate cycle. When cells were grown on glucose the anaplerotic function was probably fulfilled by pyruvate carboxylase, although phosphoenolpyruvate carboxylase was also present. The gluconeogenic enzyme phosphoenolpyruvate carboxykinase was repressed by glucose; malic enzyme was repressed by acetate. The regulatory patterns shown by citrate synthase and pyruvate carboxylase were similar to those described for the enzymes from other Pseudomonas. Whole cells of the R-12 strain were able to decarboxylate the aminoacids serine, aspartate and glutamate. Aspartate aminotransferase was present at high levels in aminoacid-grown cells, thus suggesting a catabolic role, whereas glutamate dehydrogenase had increased levels in glucose - or acetate-grown cells, suggesting that it fulfilled a mainly biosynthetic role.