Genomic insights into Agathobaculum and Butyricicoccus highlight taxonomic refinements and synonymization with proposal of a new genus within the family Oscillospiraceae, Parabutyricicoccus gen. nov

Members of the family Oscillospiraceae are prominent anaerobic bacteria of the gut microbiota, with numerous taxa capable of butyrate production and other metabolically important activities. Despite their growing importance in microbiome research, the taxonomy of several Oscillospiraceae lineages remains unresolved. In particular, the genera Agathobaculum and Butyricicoccus were established largely on the basis of 16S rRNA gene phylogeny, yet accumulating genomic evidence indicates inconsistencies in their current classification. In this study, we performed a comprehensive genome-scale reassessment of all validly published species assigned to these genera using an integrated approach combining core-genome phylogenomic analysis and genome relatedness indices, including average nucleotide identity, average amino acid identity, digital DNA–DNA hybridization and percentage of conserved proteins, complemented by phenotypic and chemotaxonomic comparisons. Genomic similarity metrics revealed that Agathobaculum faecis Liu et al . 2022 and Agathobaculum ammoniilyticum Hitch et al . 2022 exceeded established species delineation thresholds, supporting their synonymization. In addition, Butyricicoccus faecihominis Takada et al . 2016 showed high genomic relatedness to members of Agathobaculum , justifying its transfer to this genus. Conversely, Butyricicoccus porcorum Trachsel et al . 2018 and Butyricicoccus intestinisimiae Li et al . 2022 formed a coherent and well-separated lineage distinct from both genera, warranting recognition of a novel genus within the family Oscillospiraceae , for which the name Parabutyricicoccus gen. nov. is proposed. These taxonomic revisions provide a robust phylogenomic framework for resolving relationships within Oscillospiraceae and improve the accuracy of bacterial classification in gut microbiome studies.