Thirteen anaerobic bacteria capable of performing the 7a-dehydroxylation of both cholic acid and chenodeoxycholic acid were isolated from human feces and also from sewage. Ten organisms from heat-treated samples were species of Clostridium identical or closely related to the Clostridium bifermentans-C. sordellii group and consisted of four strains elaborating 7a-dehydroxylase alone and six strains capable of catalyzing both 7a-dehydrogenation and 7a-dehydroxylation. The remaining three organisms, recovered from fresh human feces, were gram-positive, nonflagellated, nonsporeforming, anaerobic rods and comprised two distinct species. Strain HD-17, still unidentified, had both activities, but was unique in that it exclusively 7a-dehydroxylated cholic acid while biotransforming chenodeoxycholic acid, preferably through 7a-dehydrogenation. Two unclassified strains, b-8 and c-25, metabolized both acids through 7a-dehydroxylation and 7adehydrogenation. Except for strains b-8 and c-25, all of the 7a-dehydroxylating bacteria split the conjugated bile acid series, and hydrolases were detected in cellfree filtrates of early stationary-phase broth cultures.

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