: an endocrine keystone species in the mammalian gut.

is a keystone human gut microbial taxonomic group that, while low in abundance, has a disproportionate effect on bile acid and steroid metabolism in the mammalian gut. Numerous studies indicate that the two most studied strains of (i.e., ATCC 35704 and VPI 12708) are important for a myriad of physiological processes in the host. We focus on both historical and current microbiological and molecular biology work on the Hylemon-Björkhem pathway and the steroid-17,20-desmolase pathway that were first discovered in Our most recent analysis now calls into question whether strains currently defined as represent two separate taxonomic groups. Future directions include developing genetic tools to further explore the physiological role bile acid and steroid metabolism by strains of , and the causal role of these pathways in host physiology and disease.