Metagenome-assembled genomes from a population-based cohort uncover novel gut species and within-species diversity, revealing prevalent disease associations

Microbiome studies increasingly recognize that species-level profiles can mask critical within-species differences relevant to health and disease. However, our work shows that within-species diversity varies drastically across gut microbes, with some species exhibiting almost as many distinct within-species clusters as recovered genomes, making association studies at the within-species level essentially intractable. To address this, we introduce the genome unit number (GUN), a scalable metric for quantifying within-species structure. Using GUN, we demonstrate that only species with limited within-species diversity, such as <i>Odoribacter splanchnicus</i>, currently allow for robust within-species association testing. These findings emphasize the need to systematically evaluate species structure across the gut microbiome and call for the development of new computational and statistical approaches to enable meaningful within-species analyses in highly diverse species.