Bradyrhizobium subterraneum sp. nov., a symbiotic nitrogen-fixing bacterium from root nodules of groundnuts

Seven strains of symbiotic bacteria from root nodules of local races of Bambara groundnut (Vigna subterranea) and peanuts (Arachis hypogaea) grown on subsistence farmers' fields in the Kavango region, Namibia, were previously characterized and identified as a novel group within the genus Bradyrhizobium. To corroborate their taxonomic status, these strains were further characterized using a polyphasic approach. All strains possessed identical 16S rRNA gene sequences with Bradyrhizobium yuanmingense CCBAU 10071T being the most closely related type strain in the 16S rRNA gene phylogenetic analysis, and Bradyrhizobium daqingense CCBAU 15774T in the ITS sequence analysis. Phylogenetic analysis of concatenated glnII-recA-rpoB-dnaK placed the strains in a highly supported lineage distinct from named species of the genus Bradyrhizobium, most closely related to Bradyrhizobium yuanmingense CCBAU 10071T. The species status was validated by results of DNA–DNA hybridization. Phylogenetic analysis of nifH genes placed the novel strains in a group with nifH of ‘Bradyrhizobium arachidis’ CCBAU 051107 that also nodulates peanuts. The combination of phenotypic characteristics from several tests including carbon source utilization and antibiotic resistance could be used to differentiate representative strains from recognized species of the genus Bradyrhizobium. Novel strain 58 2-1T induced effective nodules on V. subterranea, Vigna unguiculata and A. hypogaea, and some strains on Lablab purpureus. Based on the data presented, we conclude that our strains represent a novel species for which the name Bradyrhizobium subterraneum sp. nov. is proposed, with 58 2-1T [ = DSM 100298T = LMG 28792T = NTCCM0016T (Windhoek)] as the type strain. The DNA G+C content of strain 58 2-1T was 64.7 mol% (T m).