Ralstonia taiwanensis sp. nov., isolated from root nodules of Mimosa species and sputum of a cystic fibrosis patient

Researchers conducted a polyphasic taxonomic study on eight bacterial isolates from root nodules of Mimosa plants in Taiwan and one from a cystic fibrosis patient sputum. Using 16S rDNA sequencing, DNA-DNA hybridization, protein analysis, and biochemical tests, they identified these isolates as a novel Ralstonia species, named Ralstonia taiwanensis sp. nov., with type strain LMG 19424T. The bacteria are Gram-negative, rod-shaped cells that effectively nodulate Mimosa pudica and Mimosa diplotricha. Most notably, this is the first documented β-proteobacterium capable of forming root nodules and fixing atmospheric nitrogen, characteristics previously known only in α-proteobacterial rhizobia. The isolates share ~67 mol% DNA G+C content and show high DNA-DNA binding values (65-100%) among themselves but low binding to other Ralstonia species. The bacteria lack autotrophic growth capability and cannot grow at 42°C, distinguishing them from related species like R. eutropha.

Key findings

• Ralstonia taiwanensis sp. nov. is the first β-proteobacterium capable of root nodule formation and nitrogen fixation, a phenotype previously restricted to α-proteobacterial rhizobia
• Nine isolates from Mimosa root nodules and cystic fibrosis sputum were identified as a single novel Ralstonia species through polyphasic taxonomy including 16S rDNA, protein analysis, and biochemical testing
• R. taiwanensis shows high genetic relatedness within the species (65-100% DNA-DNA hybridization) but only ~45% binding to closest relative R. eutropha, clearly establishing it as a distinct species
• The species is distinguished by absence of autotrophic growth, inability to grow at 42°C, and specific biochemical characteristics compared to other Ralstonia species