A New Type of Nitrogen-fixing Bacterium

Abstract

IN the course of an investigation in the Jute Agricultural Research Institute on nitrogen transformation in soils cropped with jute, a nitrogen-fixing bacterium has been isolated by the dilution-plate technique. It shows certain morphological and physiological differences from the known non-symbiotic nitrogen-fixing organisms like Pseudomonas azotocalligans and those belonging to the Azotobacteriaceae¹. The cells at first have a coccus-like appearance and occur singly (measuring 0.75–1.0 × 1.5–2.5µ on an average) or as doublets or triplets, gradually changing to rod shape (Fig. 1). The cells are Gram-negative; they contain fat and are very difficult to suspend in water. The optimum temperature for growth was found to be about 28° C. The organism is aerobic, but can be grown under low oxygen tension in Winogradsky's nitrogen-free glucose–phosphate media. It can grow in acidic media also; even at pH 4.0 growth was obtained. Bromo-cresol purple milk (skimmed) is turned alkaline and a sediment is formed at the bottom with a clear liquid surface after ten days incubation at 30° C.