Epimerization of the 7-hydroxy group of bile acids by the combination of two kinds of microorganisms with 7 alpha- and 7 beta-hydroxysteroid dehydrogenase activity, respectively.

An unidentified gram-positive anaeroic organism capable of dehydrogenating the 7 beta-hydroxy group of ursodeoxycholic acid was recovered from human feces. By combined action of this organism with the ordinary 7 alpha-dehydrogenating bacteria, chenodeoxycholic acid and cholic acid alike were readily converted into their respective 7 beta-epimers and the reverse reactions were also carried out. The estimated levels of these 7 alpha- and 7 beta-dehydrogenating organisms among the intestinal microflora give a satisfactory explanation for the frequent appearance of the 7 beta-hydroxylated bile acids in vivo.