Cooperative formation of ω-muricholic acid by intestinal microorganisms

Abstract

Three anaerobic bacteria, isolated from the ceca of rats and mice, converted, through a concerted mechanism, β-muricholic acid, the predominant bile acid in germfree rats, into ω-muricholic acid. One isolate was a Eubacterium lentum strain; the second and third isolates were tentatively identified as atypical Fusobacterium sp. strains. The conversion of β-muricholic acid into ω-muricholic acid proceeded in two steps; E. lentum oxidized the 6β-hydroxyl group of β-muricholic acid to a 6-oxo group, which was reduced by either of the two other species to a 6α-hydroxyl group, yielding ω-muricholic acid. This transformation occurred both in vitro and in gnotobiotic rats. Monoassociation of germfree rats with the E. lentum strain gave rise to an unidentified fecal bile acid, probably a derivative of β-muricholic acid having a double bond in the side chain.