Proposal for reclassification of the species as nom. nov. and transfer of the genus to the family .

X5-1 is an anaerobic, xylan-fermenting bacterium first isolated from methane-producing cattle manure. Initially identified as , this species was later reclassified as in 2019. Although this reclassification found support through Genome blast Distance Phylogeny analysis which placed X5-1 into the same clade as DSM 24995, it was contradicted by 16S rRNA gene phylogenetic analysis, which associated it with a set of misnamed species later reassigned into the genus . To ascertain its taxonomic position, comparative analyses were performed to re-examine the relationship between X5-1 and all species of the genera and . The ranges of 16S rRNA gene sequence similarity, average amino acid identity, and percentage of conserved protein prediction values were higher between X5-1 and species of the genus than . In addition, X5-1 was found to harbour genes and pathways conserved and exclusive to species within the genus but not . Essentially, in both the 16S rRNA gene phylogenetic tree and the core-genome phylogenomic tree, X5-1 clustered into the same clade as species of the genus , distinct from species of the genus . It is thus clear that X5-1 represents a species within the genus , which we propose to reclassify as nom. nov. Finally, based on the results from the phylogenetic and comparative analyses, the genus was transferred to the family .