The Clostridium-like organisms TO-931 T and HD-17, isolated from human faeces, have high levels of bile acid 7α-dehydroxylating activity. Sequencing of their 16S rDNA demonstrated that they belong to cluster XI of the genus Clostridium and that they represent a new and distinct line of descent. Clostridium bifermentans and Clostridium sordellii in cluster XI also possess bile acid 7α-dehydroxylating activity. DNA-DNA hybridization experiments with the isolates, TO-931 T and HD-17, and C. bifermentans and C. sordellii revealed that the isolates are a single species distinct from C. bifermentans and C. sordellii. On the basis of phylogenetic analysis, using 16S rDNA sequences, and DNA-DNA hybridization analysis, it is concluded that strains TO-931 T and HD-17 are members of a new species of the genus Clostridium, for which the name Clostridium hiranonis is proposed. The type strain is strain TO-931 T (l JCM 10541 T l DSM 13275 T ).

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Methods for isolation of fecal 7α‐dehydroxylating bacteria are presented. A total of 219 strains were isolated from feces of healthy humans, and their ability to 7‐dehydroxylate cholic, chenodeoxycholic, and ursodeoxycholic acids were examined. Of all the isolates, 14 strains were found to be capable of eliminating the hydroxy group at C‐7α and/or C‐7β. All the isolates were strictly anaerobic, Gram‐positive rods. Thirteen isolates were non‐sporeforming bacteria showing certain saccharolytic properties with the production of acid and gas from dextrose, and were catalase‐positive but indole‐, lecithinase‐, urease‐ and oxidase‐negative. Based on the data available at present, it was concluded that they could be regarded as members of the genus Eubacterium. One strain, however was identified as Clostridium sordellii. The isolated strains capable of 7α‐dehydroxylating cholic acid and chenodeoxycholic acid were also able to oxidize the hydroxy group at C‐7α. Nine strains (10, 12, 36S, M‐2, M‐17, M‐18, Y‐98, Y‐1112, and Y‐1113) of the 7α‐dehydroxylating bacteria were confirmed to have 7β‐dehydroxylation ability, but five strains (O‐51, O‐52, O‐71, O‐72, and Y‐67) could not transform ursodeoxycholic acid to lithocholic acid.

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 7a-hydroxy group in both primary bile acids and epimerization of the 3a-hydroxy group in all metabolic bile acids. Extensive epimerization of the 7a-hydroxy group of chenodeoxycholic acid yielding ursodeoxycholic acid was also demonstrated by certain fecal samples. 7a-Dehydrogenase activity was widespread among the fecal isolates (88% of 16 facultative anaerobes and 51% of 59 obligate anaerobes), and 7a-dehydroxylase activity was revealed in one of the isolates, an unidentified gram-positive nonsporeforming anaerobic bacterium. 3a-Epimerization was effected by seven strains assigned to Eubacterium lentum, which were also active for 3a-and 7a-dehydrogenations. No microorganism accounting for 7a-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.Most of the microbial transformations of bile acids in the intestinal tract are reproducible in in vitro cultures offecal microorganisms. The major reactions involved are splitting of conjugated bile acids (deconjugation), removal of the hydroxy group, mainly at C-7 (7a-dehydroxylation), oxidation of hydroxy groups at C-7, C-3, and C-12 (the respective dehydrogenations) and conversion of the hydroxy group at C-3 from a-to ,B-configuration (3a-epimerization) (19,20). With regard to the kinds of microorganisms responsible for the respective reactions, however, only a few systematic studies have been attempted. Midtvedt and Norman (17) investigated the bile acid transforming capacities of 61 laboratory strains of intestinal origin, and Dickinson et al (3) evaluated the activities of 112 isolates from the digestive tract of the rat. They found the deconjugating and dehydrogenating 271