Transformation of Bile Acids by Mixed Microbial Cultures from Human Feces and Bile Acid Transforming Activities of Isolated Bacterial Strains

Microbial transformation of cholic acid and chenodeoxycholic acid by anaerobic mixed cultures of human fecal microorganisms was investigated, and the results were examined in relation to the bile acid transforming activities of 75 bacterial strains isolated from the same fecal cultures. The reactions involved in the mixed cultures were dehydrogenation and dehydroxylation of the 7α‐hydroxy group in both primary bile acids and epimerization of the 3α‐hydroxy group in all metabolic bile acids. Extensive epimerization of the 7α‐hydroxy group of chenodeoxycholic acid yielding ursodeoxycholic acid was also demonstrated by certain fecal samples. 7α‐Dehydrogenase activity was widespread among the fecal isolates (88% of 16 facultative anaerobes and 51% of 59 obligate anaerobes), and 7α‐dehydroxylase activity was revealed in one of the isolates, an unidentified gram‐positive nonsporeforming anaerobic bacterium. 3α‐Epimerization was effected by seven strains assigned to Eubacterium lentum, which were also active for 3α‐ and 7α‐dehydrogenations. No microorganism accounting for 7α‐epimerization was recovered among the isolates. Splitting of conjugated bile acid was demonstrated by the majority of obligate anaerobes but the activity was rare among facultative anaerobes.