Forty-eight Acidobacteriota strains were isolated from soils and sediments in Japan. Among them, six representative strains, designated W79T, W786T, Red222T, Red802T, Red803T, and Red804T, were subjected to the taxonomic classification. These six strains are Gram-stain-negative, non-spore-forming, rod-shaped, and facultative anaerobic bacterium that can reduce ferric iron. Phylogenetic and phylogenomic trees based on 16S rRNA genes and multiple single-copy gene sequences showed that strains Red222T, Red802T, Red803T, and Red804T formed a cluster with the type strains of Geothrix species, but strains W79T and W786T created an independent cluster from any other type strains. The former four strains shared 97.95-99.08% similarities of 16S rRNA gene sequence with the type strains of the genus Geothrix, whereas the latter two strains 94.86-95.49% similarities. The average amino acid identity of strains W79T and W786T were <63 % to any other type strains, which were below the genus delineation thresholds. Moreover, colonies of these two strains were white, while those of the other four isolated strains were reddish-yellow as well as the type strain Geothrix fermentans H-5T. Although the known type strains of Geothrix species have been reported to be non-motile, five strains (W79T, W786T, Red222T, Red803T, and Red804T) except for strain Red802T displayed motility. Furthermore, multiple genomic, phylogenetic, and phenotypic features supported the discrimination between these isolated strains. Based on the study evidence, we propose these six isolates as novel members within the Acidobacteriota/Holophagae/Holophagales/Holophagaceae, comprising two novel species of a novel genus, Mesoterricola silvestris gen. nov., sp. nov., and Mesoterricola sediminis sp. nov., and four novel species of the genus Geothrix: Geothrix oryzae sp. nov., Geothrix edaphica sp. nov., Geothrix rubra sp. nov., and Geothrix limicola sp. nov.
Keywords: Acidobacteriota; Geothrix; Holophagaceae; Mesoterricola; isolation; soils.