Faecalibacterium harmsenii sp. nov., an abundant but previously overlooked Faecalibacterium in the human gut.

Faecalibacterium is one of the most abundant anaerobes in the human colon. At the genus level, this bacterium shows a strong positive association with human health. Expanding collections of isolates and metagenome-assembled genomes have revealed its species diversity, yet species-level functions remain so far underexplored. Here, we describe a novel species, Faecalibacterium harmsenii. In addition, we reclassify another isolate as a member of the recently reported Faecalibacterium langellae species. Despite close genomic relatedness, these isolates exhibit distinct physiological and biochemical traits, including differences in carbohydrate utilization, stress tolerance, enzymatic activity, Gram-staining and fatty acid composition. Our present comparative genomics analyses further uncover extensive functional diversity and plasticity across type strains, with F. harmsenii being distinguished by an expanded carbohydrate gene repertoire and reduced defense systems, mobile genetic elements and antibiotic resistance genes. Extending to the species, we identify species-specific ecological niches across hosts and differential sensitivities to human diseases, highlighting certain species as reliable biomarkers of gut health. Together, these findings refine our understanding of Faecalibacterium diversity and provide a framework for its use in microbiome-based diagnostics and therapeutic development.