Three strains of a butyrate-producing bacterium were isolated from the rumen contents of grazing sheep and cows. The strains were anaerobic, with Gram-positive cell walls, straight-to-slightly-curved, rod-shaped, non-spore-forming and single flagellate. C_{14 : 1}, C_{14 : 0}, C_{16 : 0} and C_{16 : 1} were the predominant fatty acids. The cell-wall peptidoglycan type was A1γ. The DNA G+C content varied from 41.4 to 42.2 mol%. 16S rRNA gene sequence similarities between the isolates and Eubacterium rectale, Roseburia hominis and Roseburia intestinalis were found to be 96, 95 and 95 %, respectively. The phylogenetic tree showed that the strains constituted a different taxon, separate from other taxa with validly published names and forming a cluster with strains of Eubacterium rectale. On the basis of phenotypic, chemotaxonomic and phylogenetic results (16S RNA, dnaK, groEL, atpA genes), the isolates are considered to represent a novel species of a new genus of the family Lachnospiraceae, for which the name Agathobacter ruminis gen. nov., sp. nov. is proposed (type strain JK623^T = DSM 29029^T = LMG 28559^T). We also propose the transfer of Eubacterium rectale to the new genus as Agathobacter rectalis gen. nov., comb nov. This new genus represents saccharoclastic, chemo-organotrophic and obligatory anaerobic, non-spore-forming rods with Gram-positive membrane. The main fermentation products on peptone yeast glucose (PYG) medium were butyrate, acetate, hydrogen and lactate. The type species of the genus is Agathobacter rectalis gen. nov., comb nov. (Prévot, 1938) with type strain ATCC 33656^T ( = JCM 17463^T).

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