Mesorhizobium

Abstract Me.so.rhi.zo'bi.um. Gr. adj. mesos middle; M.L. neut. n. Rhizobium bacterial generic name; M.L. neut. n. Mesorhizobium the meso ‐growing rhizobium, referring to the growth rate intermediate between those of the genera Rhizobium and Bradyrhizobium . Rods 0.4–0.9 × 1.2–3.0 μm. Commonly pleomorphic under adverse growth condition or in the root nodules as bacteroids . Usually contain granules of poly‐β‐hydroxybutyrate, which are refractile by phase‐contrast microscopy. Nonsporeforming. Gram negative. Motile by one polar or subpolar flagellum or by peritrichous flagella. Aerobic, possessing a respiratory type of metabolism with oxygen as the terminal electron acceptor. Optimal temperature 25–30°C. Optimal pH 6–8. Growth in yeast extract–mannitol agar (YMA) 1 produces colonies that are circular, convex, semitranslucent, raised, and mucilaginous, 2–4 mm diameter within 5–6 d for some species or 1 mm after a 7‐d incubation for other species. The generation times of Mesorhizobium strains range from 4–15 h . Chemoorganotrophic, utilizing a wide range of carbohydrates and salts of organic acids as carbon sources without gas production. Cellulose and starch are not utilized. Produces an acidic reaction in YMA . Ammonium salts, nitrates, urea, and most amino acids are utilized as nitrogen sources. Peptone is poorly utilized. Mesorhizobium strains are only weakly proteolytic, but can produce a slow digestion in litmus milk; some strains form a clear serum zone. Some strains require thiamin, nicotinamide, and riboflavin for growth. The organisms are characteristically able to invade the root hairs of a wide range of temperate, subtropical, and tropical leguminous plants, inciting production of root nodules where the bacteria reduce atmospheric nitrogen into a combined form available for the host plants . All strains exhibit host specificity. The mol % G + C of the DNA is : 59–64. Type species : Mesorhizobium loti (Jarvis, Pankhurst, and Patel 1982) Jarvis, van Berkum, Chen, Nour, Fernandez, Cleyet‐Marel and Gillis 1997, 898 ( Rhizobium loti Jarvis, Pankhurst, and Patel 1982, 378.) Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Mesorhizobium is: correct name (last update, February 2025) * . LPSN classification: Bacteria / Pseudomonadati / Pseudomonadota / Alphaproteobacteria / Hyphomicrobiales / Phyllobacteriaceae / Mesorhizobium The genus Mesorhizobium can also be recovered in the Genome Taxonomy Database (GTDB) as g__Mesorhizobium (version v220) ** . GTDB classification: d__Bacteria / p__Pseudomonadota / c__Alphaproteobacteria / o__Rhizobiales / f__Rhizobiaceae / g__Mesorhizobium * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776