Taxonomic reclassification and expansion of the family Selenomonadaceae

The family Selenomonadaceae, first introduced in 2015, comprises a group of Gram-stain-negative, anaerobic, non-spore-forming, rod-shaped bacteria distributed across diverse hosts and environmental niches. At present, this family includes ten genera, some of whose species overlap on the phylogenetic tree; a systematic re-evaluation of their taxonomic placement is therefore warranted. In this study, using a combination of phylogenetic, phenotypic and comparative genomic approaches, we propose the description of Selenomonas oralis sp. nov. (ATCC 35185^T=DSM 20758^T=VPI D19B-28^T), as well as the reclassification of 16 Selenomonas (sub)species, Megamonas rupellensis and Propionispira raffinosivorans, with the latter two designated as later heterotypic synonyms of existing species. The reclassification of the Selenomonas (sub)species is accompanied by the introduction of five novel genera, namely, Dongibacter gen. nov., Pseudoselenomonas gen. nov., Schleiferibacter gen. nov., Wylensekiella gen. nov. and Candidatus Dighea gen. nov., to accommodate eight misplaced (sub)species, while the remaining eight species are reclassified into the genus Centipeda. As a result, the family Selenomonadaceae is now expanded to 15 genera, with 40 species and 2 proposed later heterotypic synonyms. Additionally, we establish Selenomonadaceae-specific thresholds for 16S rRNA gene sequence similarity, average amino acid identity and percentage of conserved proteins to guide future genus-level delineation within this family.