Dehydroxylation of cholic acid at C 12 and epimerization at C 5 and C 7 by Bacteroides species

Abstract

Freshly isolated cultures (2060) of human intestinal bacteria of the predominant flora, among them 1029 strains of saccharolytic Bacteroides species, were tested for cholic acid transformation. Eight Bacteroides strains reduced cholate to chenodeoxycholate, while 73 strains dehydroxylated at C 7, producing deoxycholate. Concurrent oxidation of hydroxyl groups, mainly at C 7, was seen with many strains. No strain was able to dehydroxylate simultaneously at C 7 and C 12. One isolate, identified as a mixed culture of Bacteroides fragilis and B. uniformis, epimerized cholic acid at C 5 and simultaneously epimerized, oxidized and dehydroxylated at C 7. The following transformation products were identified: 3α,7α,12α-trihydroxy-5α-cholanoic acid, 3α,7β,12α-trihydroxy-5β-cholanoic acid (ursocholic acid), 3α,12α-dihydroxy-7-keto-5β-cholanoic acid, 3α,12α-dihydroxy-5α-cholanoic acid and a 3α,12α-dihydroxy-5α-cholenoic acid. Dehydroxylating and epimerizing abilities were detected when fresh isolates were tested first for cholate transformation. They were no longer recognizable after some serial transfers. Dehydroxylation at C 12 of cholate could not be demonstrated with mixed fecal cultures. The possible intermediate, however, 3α,7α-dihydroxy-5β-chol-11-enoate, was abundantly hydrogenated by stool suspensions.