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Mesorhizobium australicum sp. nov. and Mesorhizobium opportunistum sp. nov., isolated from Biserrula pelecinus L. in Australia

¹ Centre for Rhizobium Studies, Murdoch University, Murdoch, Western Australia 6160, Australia
² Laboratory of Microbiology, Faculty of Sciences, Ghent University, B-9000 Ghent, Belgium

Correspondence
Kemanthi G. Nandasena
kemanthi{at}murdoch.edu.au

Biserrula pelecinus L. is a pasture legume that was introduced to Australia from the Mediterranean basin in 1993. Although the native rhizobial population could not nodulate B. pelecinus at the time of its introduction, recent research has shown the emergence of a diversity of strains (novel isolates) that are able to do so. Three novel isolates, WSM2073^T, WSM2074 and WSM2076, had nearly identical 16S rRNA gene sequences, and clustered separately with all recognized species of the genus Mesorhizobium. Conversely, the novel isolate WSM2075^T had >23 nt mismatches with the above three isolates. All four novel isolates shared 97–99 % 16S rRNA gene sequence similarity with the type strains of all recognized Mesorhizobium species. However, strains WSM2073^T, WSM2074 and WSM2076 showed <95.2 % dnaK gene sequence similarity to the type strains of recognized Mesorhizobium species, and <92.9 % to WSM2075^T (which also shared <95.5 % dnaK gene sequence similarity to the type strains of recognized Mesorhizobium species). Results for GSII gene sequencing were consistent with those for the dnaK gene. The fatty acid profiles of the novel isolates were diagnostic of root-nodule bacteria, but did not match those of recognized bacterial species. Strain WSM2075^T had a significantly different fatty acid profile from the other three isolates. The above results indicated that strains WSM2073^T, WSM2074 and WSM2076 represent the same species. Strain WSM2073^T showed <45 % DNA–DNA relatedness and WSM2075^T <50 % DNA–DNA relatedness with the type strains of recognized Mesorhizobium species; these two novel isolates shared 59 % DNA–DNA relatedness. Collectively, these data indicate that strains WSM2073^T, WSM2074 and WSM2076, and strain WSM2075^T belong to two novel species of the genus Mesorhizobium, for which the names Mesorhizobium australicum sp. nov. and Mesorhizobium opportunistum sp. nov. are proposed, respectively. The type strain of Mesorhizobium australicum sp. nov. is WSM2073^T (=LMG 24608^T=HAMBI 3006^T) and the type strain of Mesorhizobium opportunistum sp. nov. is WSM2075^T (=LMG 24607^T=HAMBI 3007^T).

Neighbour-joining phylogenetic trees based on dnaK and GSII gene sequences showing the positions of the novel isolates WSM2073^T, WSM2074, WSM2075^T and WSM2076, Mesorhizobium ciceri bv. biserrulae isolated from the Mediterranean basin and other root-nodule bacteria in the Alphaproteobacteria, together with tables detailing differential phenotypic characteristics of the novel species described here and recognized species of the genus Mesorhizobium, and levels of 16S rRNA gene sequence similarity between strains WSM2073^T and WSM2075^T and type strains of Mesorhizobium species are available as supplementary material with the online version of this paper.