Bile Acid Transformation in Gnotobiotic Mice Associated with Human Intestinal Flora
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Bile acids are transformed by intestinal microflora. Secondary bile acids including deoxycholic acid and lithocholic acid appear to show a strong correlation with colorectal cancer. However, the role of intestinal bacteria in the transformation of bile acids <I>in vivo</I> is still not clear. We attempted to evaluate which bacterial groups are responsible for bile acid transformation <I>in vivo</I>. First, we orally inoculated germfree (GF) mice with human intestinal bacteria that have the ability to transform bile acids <I>in vitro</I>, and showed that it is not necessarily true that bacterial transforming ability of bile acids <I>in vitro</I> is reflected <I>in vivo</I>. Then GF mice were orally inoculated with human fecal dilution or various components of human feces. In the cecal contents of ex-GF mice associated with human fecal dilutions of 10<SUP>-2</SUP> or 10<SUP>-6</SUP>, or the anaerobic growth from a dilution of 10<SUP>-6</SUP>, free-form bile acids accounted for more than 80% and DCA for about 20% of total bile acid. When GF mice were associated only with clostridia, free-form bile acids made up less than 40% of total bile acids, but the percentage of secondary bile acids was the same as in the other groups. These results indicate that predominant bacteria -mainly bacteroides in human feces, which are the main bacterial group for deconjugation of bile acids-and clostridia may play an important role in 7α-dehydroxylation of free-form primary bile acids in the intestine. The combination of five strains of bile acid-deconjugating bacteroides and five strains of bile acid 7α-dehydroxylating clostridia isolated from ex-GF mice converted tauro-conjugated cholic acid into DCA both <I>in vitro</I> and in the intestine. This GB mouse model should be useful in studies of bile acid metabolism by human intestinal bacteria <I>in vivo</I>.
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財団法人 日本ビフィズス菌センター | 論文
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