Representative N2-fixing bacteria have been isolated from the rhizosphere of rice using the "spermosphere model." They have been characterized using conventional biochemical tests, DNA composition, and DNA–rRNA hybridization studies. In addition to a large number of Enterobacteriaceae, the most commonly encountered isolates belong to the genus Azospirillum and to Pseudomonas paucimobilis, a taxon related to Flavobacterium capsulatum. A nonmotile Azospirillum has been found.

The composition of the N2-fixing microflora of a rice rhizosphere was studied by a new enrichment and isolation procedure: rather than using an artificial C source, the exudates from a germinating seed ("the spermosphere model") are used. In this study, this system gave N2-fixing isolates with a frequency of 65%. Thirty-two of the many isolates obtained have been studied in detail; these were Klebsiella oxytoca, Enterobacter cloacae, Pseudomonas paucimobilis, and Azospirillum. All these bacteria were present at densities higher than 105 per gram of dry soil in the intial rice rhizosphere.