Isolation and identification of nitrogen-fixing Enterobacter cloacae and Klebsiella planticola associated with rice plants

Abstract

Acid- and gas-producing nitrogen-fixing bacteria associated with rice roots and leaf sheaths were isolated. These isolates along with reference enterobacteria strains were characterized biochemically and serologically. Using selected cultural and biochemical tests, all isolates from rice were identified as Enterobacter cloacae, except two strains which were found similar to Klebsiella pneumoniae. However, further biochemical tests showed that the Klebsiella isolates were K. planticola. Antisera and fluorescent antibodies (FA) were prepared against K. pneumoniae M5a1, K. planticola DWUL2, and E. cloacae EnSs. The results of FA cross-reactions of 28 strains isolated from rice plants and of other type cultures showed a separation into two different serogroups: E. cloacae and K. planticola. However, the cross-reactions in gel immunodiffusion were found to be extensive and widespread. All strains of Enterobacteriaceae tested produced at least one immunodiffusible precipitin band with any one or all antisera. The population of N2-fixing enterobacteria associated with the root and stem of rice fell within the range of 103 and 105 per gram dry weight. The percentage incidence of N2-fixing enterobacteria among the aerobic heterotrophic bacteria in the root and stem was found to be less than 1%.