Studies of Sulfur Reduction of Taurine and Taurine-Conjugated Bile Acids by Bile acid 7α-Dehydroxylating Bacteria
Bile acids are C24 steroids that are derived in the liver from cholesterol and secreted into the intestinal lumen to aid in emulsification of dietary lipids and lipid-soluble vitamins. The indigenous intestinal microflora modify bile acids, producing up to 20 unique bile acid metabolites. The 7α-dehydroxylation of the bile acids is the most physiologically important bile acid biotransformation. All known intestinal bacteria capable of bile acid 7α-dehydroxylation are anaerobic, gram-positive rods of the genera Clostridium and Eubcicterium. Bile acid 7α-dehydroxylating bacteria often contain bile salt hydrolase, which hydrolyzes the peptide bond in taurine-conjugated bile acids to yield a free bile acid and taurine. Taurine is an organosulfonate containing a sulfite moiety. There have been no published reports indicating whether 7α-dehydroxylating bacteria can utilize taurine. Given that taurine and taurine-conjugated bile acids are found at great concentrations in the intestine, the ability to utilize the compound would confer a competitive advantage to these bacteria. In this study, the ability of 7α-dehydroxylating bacteria to dissimilate taurine and taurine-conjugated bile acids produce hydrogen sulfide was investigated. First, hydrogen sulfide produced by bile acid 7α-dehydroxylating bacteria cultured in tryptic soy broth and semi-defined media from taurine and taurine-conjugated bile acids was qualitatively detected by inclusion of ferric ammonium citrate in the media. The results obtained from trials utilizing anaerobic tryptic soy broth and from semi-defined medium were not consistent, suggesting that qualitative determination of sulfide by inclusion of ferric ammonium citrate is inconclusive. Then hydrogen sulfide produced by bile acid 7α-dehydroxylating bacteria cultured in modified semi-defined medium (not containing a reducing agent) over time in the presence or absence of taurine-conjugated bile acids was quantified using the methylene blue method. Sulfide concentrations in medium cultured with two different strains of bile acid 7α-dehydroxylating bacteria, Eubcicterium sp. 12708 and Clostridium sp. HD-17, in presence of 100 |j.M or 5 mM sulfonates were not significantly higher than those in the absence of sulfonate. In addition, the highest sulfide concentration determined from medium cultured with two different strains of bile acid 7α-dehydroxylating bacteria for a period of five days was not above backgroud level. These data demonstrated that these two bile acid 7α-dehydroxylating bacteria, Eubcicterium sp. 12708 and Clostridium sp. HD-17, are not capable of desulfonating taurine and taurineconjugated bile acids to produce hydrogen sulfide under the conditions tested.
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