Rhizomicrobium palustre gen. nov., sp nov., a facultatively anaerobic, fermentative stalked bacterium in the class Alphaproteobacteria isolated from rice plant roots

Facultatively anaerobic bacterial strains (A48(T), RR25 and RR54) were isolated from roots of living rice plants in an irrigated rice-field in Japan. The three strains had identical 16S rRNA gene sequences and showed almost the same phenotypic properties examined. Cells of the strains were Gram-negative, non-spore-forming rods. Reproduction of cells was by binary fission as well as by budding. Cells occurred singly or in pairs arranged angularly. Some cells, including dividing cells, were motile with a single polar flagellum. Cells developed a polar prostheca (stalk) with a holdfast-like structure and the cell with the stalk budded a daughter cell. The strains were chemoorganotrophs and utilized various sugars as growth substrates. The strains fermentatively produced acetate and lactate as well as small amounts of ethanol and H-2 from the substrates. Growth temperature and pH ranges for growth were 15-40 degrees C and pH 5.5-7.3 with optimum growth at 30-35 degrees C and pH 6.8. NaCl concentration range for growth was 0-1.0% (wt/vol) with an optimum at 0% (wt/vol). Catalase and oxidase activities were not detected. The strains reduced Fe(III) to Fe(II) in the presence of glucose, while they did not reduce nitrate, fumarate, malate or sulfate. The major cellular fatty acids of the strains were C-18:1 omega 7, anteiso-C-15:0, iso- C-15:0, C-16:0 and C-18:0. Ubiquinone Q-10 was the major respiratory quinone and the genomic DNA G + C contents were 53.4-55.4 mol%. Phylogenetic analysis based on 16S rRNA gene sequences placed the strains in the class Alpha proteobacteria and the strains formed a novel deep branch in the phylogenetic trees constructed. Based on the differences in 16S rRNA gene sequences and phenotypic properties of the novel strains from those of their relatives, we proposed that the strains be assigned in the novel genus and species as Rhizomicrobium palustre gen. nov., sp. nov. The type strain of the novel species is strain A48(T) (= JCM 14971(T) = DSM 19867(T)).