Herbaspirillum lusitanum sp. nov., a novel nitrogen-fixing bacterium associated with root nodules of Phaseolus vulgaris

Researchers isolated six bacterial strains from root nodules of Phaseolus vulgaris (common bean) plants grown in Portuguese soil. Through comprehensive molecular and phenotypic characterization, including 16S rRNA gene sequencing, DNA-DNA hybridization, and biochemical testing, the isolates were identified as a novel species within the genus Herbaspirillum, a group of nitrogen-fixing bacteria. The bacteria are Gram-negative, curved rods with polar flagella that grow at temperatures between 20-35°C. Phylogenetic analysis confirmed the isolates belong to the beta-Proteobacteria and showed 97.9% similarity to Herbaspirillum frisingense. DNA-DNA hybridization revealed only 10-29% relatedness to known Herbaspirillum species, confirming a distinct species. The isolates possess the nifD gene and demonstrated nitrogen-fixing capability through pellicle formation on nitrogen-free medium. Infectivity studies confirmed strain P6-12T successfully colonizes common bean roots. This represents the first confirmed association of a Herbaspirillum species with a legume plant, expanding the known host range of this genus beyond gramineous plants. The authors propose the name Herbaspirillum lusitanum sp. nov., with type strain P6-12T.

Key findings

• Herbaspirillum lusitanum sp. nov. is a novel nitrogen-fixing bacterium species isolated from Phaseolus vulgaris root nodules in Portugal
• This is the first confirmed association of a Herbaspirillum species with a legume plant, as previous reports were limited to gramineous plants
• The isolates show 97.9% 16S rRNA similarity to H. frisingense but only 10-29% DNA-DNA relatedness to known Herbaspirillum species, confirming distinct species status
• All six isolates possess the nifD gene and demonstrated dinitrogen-fixation ability through microaerophilic growth on nitrogen-free medium