Mesorhizobium retamae sp. nov., a novel non-nodulating and non-nitrogen-fixing species isolated from the root nodules of Retama raetam sampled in Tunisia

A comprehensive polyphasic taxonomic investigation integrating taxongenomic criteria was conducted on strain IRAMC:0171 T isolated from the root nodules of Retama raetam in Tunisia. This Gram-stain-negative and aerobic bacterium thrived within a temperature range of 5–45 °C, optimal at 28 °C, and tolerated salt concentrations from 0–6 % NaCl, with an optimal range of 0–3 %. It displayed pH tolerance from pH 4 to 10, thriving best at pH 6.8–7.5. Chemotaxonomically, strain IRAMC:0171 T was characterized by diphosphatidylglycerol, phosphatidylglycerol, phosphatidylcholine, and phosphatidylethanolamine as polar lipids. Its predominant fatty acid composition was C 18 : 1 ω 7c (61.2 %), and the primary ubiquinone was Q10 (97 %). Analysis of the 16S rRNA gene of strain IRAMC:0171 T showed 99.08 % similarity to Mesorhizobium waimense ICMP 19557 T , Mesorhizobium amorphae ACCC 19665 T , and Mesorhizobium huakuii IAM 14158. However, digital DNA–DNA hybridization and average nucleotide identity analyses revealed values ranging from 21.1 to 25.2 % and 77.05 to 82.24 %, respectively, signifying significant deviation from established species demarcation thresholds. Phylogenetic studies, encompassing 16S rRNA, whole-genome-based tree reconstruction, and core protein analysis, positioned strain IRAMC:0171 T closest to Mesorhizobium terrae KCTC 72278 T and ‘ Mesorhizobium hungaricum ’ UASWS1009 T , forming together a distinct branch within the genus Mesorhizobium . In consideration of this comprehensive data, we propose strain IRAMC:0171 T (=DSM 112841 T =CECT 30767 T ) as the type strain of a new species named Mesorhizobium retamae sp. nov.