Bile salt biotransformations by human intestinal bacteria

Significance of serum bile acids in small bowel allograft rejection in pigs

OBJECTIVES

To examine changes in individual bile acids in serum in a pig model of allogenic small bowel transplantation.

METHODS

Seventeen pigs were divided into two groups: group 1 (n=10), controls; group 2 (n=7), allotransplantation, nonimmunosuppressed recipients. Both groups received a segmental intestine. Intestinal specimens for histologic studies were obtained at the end of cold flushing (T0) and on postoperative day 8 (T1). Total and individual bile acid concentrations were measured in serum samples taken at T0 and T1.

RESULTS

All animals survived until the end of the study. In group 1, intestinal histology revealed no significant changes between T0 and T1 specimens. In contrast, in group 2, graft histology revealed moderate to severe rejection at T1 in all specimens. In contrast to group 1, serum levels of total bile acids increased significantly in group 2 at T1 compared with baseline (P=0.001). The increase was due to secondary bile acids which were significantly higher at T1 compared with baseline (P=0.0003). In contrast, secondary bile acids increased but not significantly at T1 in group 1 compared with baseline (P=0.056).

CONCLUSION

Our study suggests that total and secondary serum bile acids may be a useful diagnostic tool for the diagnosis of acute intestinal rejection.

Chenodeoxycholate is an inhibitor of Clostridium difficile spore germination

Some cholate derivatives that are normal components of bile can act with glycine to induce the germination of Clostridium difficile spores, but at least one bile component, chenodeoxycholate, does not induce germination. Here we show that chenodeoxycholate inhibits the germination of C. difficile spores in response to cholate and taurocholate.

Characterization of enantiomeric bile acid-induced apoptosis in colon cancer cell lines

Bile acids are steroid detergents that are toxic to mammalian cells at high concentrations; increased exposure to these steroids is pertinent in the pathogenesis of cholestatic disease and colon cancer. Understanding the mechanisms of bile acid toxicity and apoptosis, which could include nonspecific detergent effects and/or specific receptor activation, has potential therapeutic significance. In this report we investigate the ability of synthetic enantiomers of lithocholic acid (ent-LCA), chenodeoxycholic acid (ent-CDCA), and deoxycholic acid (ent-DCA) to induce toxicity and apoptosis in HT-29 and HCT-116 cells. Natural bile acids were found to induce more apoptotic nuclear morphology, cause increased cellular detachment, and lead to greater capase-3 and -9 cleavage compared with enantiomeric bile acids in both cell lines. In contrast, natural and enantiomeric bile acids showed similar effects on cellular proliferation. These data show that bile acid-induced apoptosis in HT-29 and HCT-116 cells is enantiospecific, hence correlated with the absolute configuration of the bile steroid rather than its detergent properties. The mechanism of LCA- and ent-LCA-induced apoptosis was also investigated in HT-29 and HCT-116 cells. These bile acids differentially activate initiator caspases-2 and -8 and induce cleavage of full-length Bid. LCA and ent-LCA mediated apoptosis was inhibited by both pan-caspase and selective caspase-8 inhibitors, whereas a selective caspase-2 inhibitor provided no protection. LCA also induced increased CD95 localization to the plasma membrane and generated increased reactive oxygen species compared with ent-LCA. This suggests that LCA/ent-LCA induce apoptosis enantioselectively through CD95 activation, likely because of increased reactive oxygen species generation, with resulting procaspase-8 cleavage.

Insights into the roles of gut microbes in obesity

Obesity is a major public health issue as it enhances the risk of suffering several chronic diseases of increasing prevalence. Obesity results from an imbalance between energy intake and expenditure, associated with a chronic low-grade inflammation. Gut microbes are considered to contribute to body weight regulation and related disorders by influencing metabolic and immune host functions. The gut microbiota as a whole improves the host's ability to extract and store energy from the diet leading to body weight gain, while specific commensal microbes seem to exert beneficial effects on bile salt, lipoprotein, and cholesterol metabolism. The gut microbiota and some probiotics also regulate immune functions, protecting the host form infections and chronic inflammation. In contrast, dysbiosis and endotoxaemia may be inflammatory factors responsible for developing insulin resistance and body weight gain. In the light of the link between the gut microbiota, metabolism, and immunity, the use of dietary strategies to modulate microbiota composition is likely to be effective in controlling metabolic disorders. Although so far only a few preclinical and clinical trials have demonstrated the effects of specific gut microbes and prebiotics on biological markers of these disorders, the findings indicate that advances in this field could be of value in the struggle against obesity and its associated-metabolic disorders.

A new, major C27 biliary bile acid in the red-winged tinamou (Rhynchotus rufescens):25R-1beta, 3alpha,7alpha-trihydroxy-5beta-cholestan-27-oic acid

The chemical structures of the three major bile acids present in the gallbladder bile of the Red-winged tinamou (Rhynchotus rufescens), an early evolving, ground-living bird related to ratites, were determined. Bile acids were isolated by preparative reversed-phase HPLC. Two of the compounds were identified as the taurine N-acylamidates of 25R-3alpha,7alpha-dihydroxy-5beta-cholestan-27-oic acid (constituting 22% of biliary bile acids) and 25R-3alpha,7alpha,12alpha-trihydroxy-5beta-cholestan-27-oic acid (constituting 51%). The remaining compound, constituting 21% of biliary bile acids, was an unknown C27 bile acid. Its structure was elucidated by LC/ESI-MS/MS and NMR and shown to be the taurine conjugate of 25R-1beta, 3alpha, 7alpha-trihydroxy-5beta-cholestan-27-oic acid, a C27 trihydroxy bile acid not previously reported. Although C27 bile acids with a 1beta-hydroxyl group have been identified as trace bile acids in the alligator, this is the first report of a major biliary C27 bile acid possessing a 1beta-hydroxyl group.

Vitamin D3 modulates the expression of bile acid regulatory genes and represses inflammation in bile duct-ligated mice

Vitamin D receptor (VDR), a nuclear receptor that regulates calcium homeostasis, has been found to function as a receptor for secondary bile acids. Because the in vivo role of VDR in bile acid metabolism remains unknown, we investigated the effect of VDR activation in a mouse model of cholestasis. We treated mice with 1alpha-hydroxyvitamin D(3) [1alpha(OH)D(3)] after bile duct ligation (BDL) and examined mRNA expression and cytokine levels. 1alpha(OH)D(3) treatment altered the expression of genes involved in bile acid synthesis and transport in the liver, kidney, and intestine but did not decrease bile acid levels in the plasma and liver of BDL mice. 1alpha(OH)D(3) treatment suppressed mRNA expression of proinflammatory cytokines in the liver and strongly decreased the plasma levels of proinflammatory cytokines in BDL mice. These findings indicate that 1alpha(OH)D(3) regulates a network of bile acid metabolic genes and represses proinflammatory cytokine expression in BDL mice. VDR ligands have the potential to prevent the cholestasis-induced inflammatory response.

Intestinal bile acid physiology and pathophysiology

Bile acids (BAs) have a long established role in fat digestion in the intestine by acting as tensioactives, due to their amphipathic characteristics. BAs are reabsorbed very efficiently by the intestinal epithelium and recycled back to the liver via transport mechanisms that have been largely elucidated. The transport and synthesis of BAs are tightly regulated in part by specific plasma membrane receptors and nuclear receptors. In addition to their primary effect, BAs have been claimed to play a role in gastrointestinal cancer, intestinal inflammation and intestinal ionic transport. BAs are not equivalent in any of these biological activities, and structural requirements have been generally identified. In particular, some BAs may be useful for cancer chemoprevention and perhaps in inflammatory bowel disease, although further research is necessary in this field. This review covers the most recent developments in these aspects of BA intestinal biology.

Functional and comparative metagenomic analysis of bile salt hydrolase activity in the human gut microbiome

Bile salt hydrolases (BSHs) catalyze the "gateway" reaction in a wider pathway of bile acid modification by the gut microbiota. Because bile acids function as signaling molecules regulating their own biosynthesis, lipid absorption, cholesterol homeostasis, and local mucosal defenses in the intestine, microbial BSH activity has the potential to greatly influence host physiology. However, the function, distribution, and abundance of BSH enzymes in the gut community are unknown. Here, we show that BSH activity is a conserved microbial adaptation to the human gut environment with a high level of redundancy in this ecosystem. Through metagenomic analyses we identified functional BSH in all major bacterial divisions and archaeal species in the gut and demonstrate that BSH is enriched in the human gut microbiome. Phylogenetic analysis illustrates that selective pressure in the form of conjugated bile acid has driven the evolution of members of the Ntn_CGH-like family of proteins toward BSH activity in gut-associated species. Furthermore, we demonstrate that BSH mediates bile tolerance in vitro and enhances survival in the murine gut in vivo. Overall, we demonstrate the use of function-driven metagenomics to identify functional anchors in complex microbial communities, and dissect the gut microbiome according to activities relevant to survival in the mammalian gastrointestinal tract.

Functional analysis of four bile salt hydrolase and penicillin acylase family members in Lactobacillus plantarum WCFS1

Bile salts play an important role in the digestion of lipids in vertebrates and are synthesized and conjugated to either glycine or taurine in the liver. Following secretion of bile salts into the small intestine, intestinal microbes are capable of deconjugating the glycine or taurine from the bile salts, using an enzyme called bile salt hydrolase (Bsh). Intestinal lactobacilli are regarded as major contributors to bile salt hydrolysis in vivo. Since the bile salt-hydrolyzing strain Lactobacillus plantarum WCFS1 was predicted to carry four bsh genes (bsh1, bsh2, bsh3, and bsh4), the functionality of these bsh genes was explored using Lactococcus lactis heterologous overexpression and multiple bsh deletion strains. Thus, Bsh1 was shown to be responsible for the majority of Bsh activity in L. plantarum WCFS1. In addition, bsh1 of L. plantarum WCFS1 was shown to be involved in conferring tolerance to specific bile salts (i.e., glycocholic acid). Northern blot analysis established that bsh1, bsh2, bsh3, and bsh4 are all expressed in L. plantarum WCFS1 during the exponential growth phase. Following biodiversity analysis, bsh1 appeared to be the only bsh homologue that was variable among L. plantarum strains; furthermore, the presence of bsh1 correlated with the presence of Bsh activity, suggesting that Bsh1 is commonly responsible for Bsh activity in L. plantarum strains. The fact that bsh2, bsh3, and bsh4 genes appeared to be conserved among L. plantarum strains suggests an important role of these genes in the physiology and lifestyle of the species L. plantarum. Analysis of these additional bsh-like genes in L. plantarum WCFS1 suggests that they might encode penicillin acylase rather than Bsh activity, indicating their implication in the conversion of substrates other than bile acids in the natural habitat.

Bacterial-dependent up-regulation of intestinal bile acid binding protein and transport is FXR-mediated following ileo-cecal resection

BACKGROUND

Bile acid (BA) reclamation following ileo-cecal resection (ICR) may prevent colonic mucosa from chronic injury. In this study, we hypothesized that in a murine model of ICR the remnant colon would upregulate the cellular machinery necessary for BA reclamation and would do so in an FXR- and bacteria-dependent manner.

METHODS

Conventional (WT), conventional FXR knockout (FXR null) and germ-free (GF) mice were randomized to undergo either ICR or sham operation. The ascending colon was harvested for histology and immunohistochemistry and changes in bile acid homeostatic gene expression determined by real-time polymerase chain reaction (RT-PCR) 7 days following surgery.

RESULTS

Following ICR WT mice showed significant increases in the expression of genes regulating bile acid transport including IBABP, Asbt, Ost beta and FGF 15. Increased expression of IBABP and Asbt was confirmed by immunohistochemistry. Induction of bile acid transport genes was absent or attenuated in FXR null and GF mice.

CONCLUSION

Bacterial dependent up regulation of IBABP is FXR mediated in the colon following ICR. Mice lacking microbiota (GF) or FXR are unable to increase the expression of IBABP or FGF 15.

Proton pump inhibitors and the risk of colorectal cancer

INTRODUCTION

Proton pump inhibitor (PPI) use is associated with increased serum gastrin levels and bacterial overgrowth, resulting in more toxic bile salt formation. Concern has risen that these factors may increase the risk of developing colorectal neoplasia.

AIM

To investigate the association between the use of PPIs and the risk of colorectal cancer.

METHODS

A population-based case-control study was conducted within the Dutch Primary Care Information (IPCI) database over the period 1996-2005. Cases with colorectal cancer were matched with up to 20 controls on age, gender, calendar time, and duration of follow-up prior to diagnosis. Cumulative exposure to PPIs was assessed in the 5 yr prior to diagnosis with a 1-yr lag time analysis. We calculated adjusted odds ratios (OR) with 95% confidence intervals (95% CI) using multivariate, conditional logistic regression analysis.

RESULTS

Within the source population of 457,024 persons, we identified 595 colorectal cancer cases. The odds of colorectal cancer were not increased among patients ever using PPIs compared with patients who never used PPIs (OR 0.85, 95% CI 0.63-1.16). Also, the use of PPIs for >365 days was not associated with a greater risk of colorectal cancer (OR 0.79, 95% CI 0.44-1.41) compared with nonusers. The odds of colorectal cancer in neither the right nor the left hemicolon were significantly increased in patients using PPIs.

CONCLUSION

The present study indicates no association between PPI use and the risk of colorectal cancer. Larger numbers of long-term PPI users are needed to confirm the absence of a risk-increasing effect of long-term PPI exposure.

Bile salts and glycine as cogerminants for Clostridium difficile spores

Spore formation by Clostridium difficile is a significant obstacle to overcoming hospital-acquired C. difficile-associated disease. Spores are resistant to heat, radiation, chemicals, and antibiotics, making a contaminated environment difficult to clean. To cause disease, however, spores must germinate and grow out as vegetative cells. The germination of C. difficile spores has not been examined in detail. In an effort to understand the germination of C. difficile spores, we characterized the response of C. difficile spores to bile. We found that cholate derivatives and the amino acid glycine act as cogerminants. Deoxycholate, a metabolite of cholate produced by the normal intestinal flora, also induced germination of C. difficile spores but prevented the growth of vegetative C. difficile. A model of resistance to C. difficile colonization mediated by the normal bacterial flora is proposed.

Review article: The function and regulation of proteins involved in bile salt biosynthesis and transport

BACKGROUND

Bile salts are produced and secreted by the liver and are required for intestinal absorption of fatty food components and excretion of endobiotics and xenobiotics. They are reabsorbed in the terminal ileum and transported back to the liver via the portal tract. Dedicated bile salt transporters in hepatocytes and enterocytes are responsible for the unidirectional transport of bile salts in the enterohepatic cycle.

AIM

To give an overview of the function and regulations of proteins involved in bile salt synthesis and transport.

METHODS

Data presented are obtained from PubMed-accessible literature combined with our own recent research.

RESULT

Hepatocytes and enterocytes contain unique bile salt importers (sodium-taurocholate cotransporting polypeptide and apical sodium-dependent bile acid transporter, respectively) and exporters (bile salt export pump and organic solute transporter alpha-beta, respectively). Enzymes involved in bile salt biosynthesis reside in different subcellular locations, including the endoplasmic reticulum, mitochondria, cytosol and peroxisomes. Defective expression or function of the transporters or enzymes may lead to cholastasis. The bile salt-activated transcription factor Farnesoid X receptor controls expression of genes involved in bile salt biosynthesis and transport.

CONCLUSIONS

Detailed knowledge is available about the enzymes and transporters involved in bile salt homeostasis and how their defective function is associated with cholestasis. In contrast, the process of intracellular bile salt transport is largely unexplored.

Clostridium scindens baiCD and baiH genes encode stereo-specific 7alpha/7beta-hydroxy-3-oxo-delta4-cholenoic acid oxidoreductases

Secondary bile acids, formed by intestinal bacteria, are suggested to play a significant role in cancers of the gastrointestinal tract in humans. Bile acid 7alpha/beta-dehydroxylation is carried out by a few species of intestinal clostridia which harbor a multi-gene bile acid inducible (bai) operon. Several genes encoding enzymes in this pathway have been cloned and characterized. However, no gene product(s) has yet been assigned to the production of 3-oxo-Delta4-cholenoic acid intermediates of cholic acid (CA), chenodeoxycholic acid (CDCA) or ursodeoxycholic acid (UDCA). We previously reported that the baiH gene encodes an NADH:flavin oxidoreductase (NADH:FOR); however, the role of this protein in bile acid 7-dehydroxylation is unclear. Homology searches and secondary structural alignments suggest this protein to be similar to flavoproteins which reduce alpha/beta-unsaturated carbonyl compounds. The baiH gene product was expressed in Escherichia coli, purified and discovered to be a stereo-specific NAD(H)-dependent 7beta-hydroxy-3-oxo-Delta4-cholenoic acid oxidoreductase. Additionally, high sequence similarity between the baiH and baiCD gene products suggests the baiCD gene may encode a 3-oxo-Delta4-cholenoic acid oxidoreductase specific for CDCA and CA. We tested this hypothesis using cell extracts prepared from E. coli overexpressing the baiCD gene and discovered that it encodes a stereo-specific NAD(H)-dependent 7alpha-hydroxy-3-oxo-Delta4-cholenoic acid oxidoreductase.

Isolation and chemical synthesis of a major, novel biliary bile acid in the common wombat (Vombatus ursinus): 15alpha-hydroxylithocholic acid

The major bile acids present in the gallbladder bile of the common Australian wombat (Vombatus ursinus) were isolated by preparative HPLC and identified by NMR as the taurine N-acylamidates of chenodeoxycholic acid (CDCA) and 15alpha-hydroxylithocholic acid (3alpha,15alpha-dihydroxy-5beta-cholan-24-oic acid). Taurine-conjugated CDCA constituted 78% of biliary bile acids, and (taurine-conjugated) 15alpha-hydroxylithocholic acid constituted 11%. Proof of structure of the latter compound was obtained by its synthesis from CDCA via a Delta14 intermediate. The synthesis of its C-15 epimer, 15beta-hydroxylithocholic acid (3alpha,15beta-dihydroxy-5beta-cholan-24-oic acid), is also reported. The taurine conjugate of 15alpha-hydroxylithocholic acid was synthesized and shown to have chromatographic and spectroscopic properties identical to those of the compound isolated from bile. It is likely that 15alpha-hydroxylithocholic acid is synthesized in the wombat hepatocyte by 15alpha-hydroxylation of lithocholic acid that was formed by bacterial 7alpha-dehydroxylation of CDCA in the distal intestine. Thus, the wombat appears to use 15alpha-hydroxylation as a novel detoxification mechanism for lithocholic acid.

Biochemical and genetic investigation of initial reactions in aerobic degradation of the bile acid cholate in Pseudomonas sp. strain Chol1

Bile acids are surface-active steroid compounds with toxic effects for bacteria. Recently, the isolation and characterization of a bacterium, Pseudomonas sp. strain Chol1, growing with bile acids as the carbon and energy source was reported. In this study, initial reactions of the aerobic degradation pathway for the bile acid cholate were investigated on the biochemical and genetic level in strain Chol1. These reactions comprised A-ring oxidation, activation with coenzyme A (CoA), and beta-oxidation of the acyl side chain with the C(19)-steroid dihydroxyandrostadienedione as the end product. A-ring oxidizing enzyme activities leading to Delta(1,4)-3-ketocholyl-CoA were detected in cell extracts and confirmed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Cholate activation with CoA was demonstrated in cell extracts and confirmed with a chemically synthesized standard by LC-MS/MS. A transposon mutant with a block in oxidation of the acyl side chain accumulated a steroid compound in culture supernatants which was identified as 7alpha,12alpha-dihydroxy-3-oxopregna-1,4-diene-20-carboxylate (DHOPDC) by nuclear magnetic resonance spectroscopy. The interrupted gene was identified as encoding a putative acyl-CoA-dehydrogenase (ACAD). DHOPDC activation with CoA in cell extracts of strain Chol1 was detected by LC-MS/MS. The growth defect of the transposon mutant could be complemented by the wild-type ACAD gene located on the plasmid pBBR1MCS-5. Based on these results, the initiating reactions of the cholate degradation pathway leading from cholate to dihydroxyandrostadienedione could be reconstructed. In addition, the first bacterial gene encoding an enzyme for a specific reaction step in side chain degradation of steroid compounds was identified, and it showed a high degree of similarity to genes in other steroid-degrading bacteria.

Human cecal bile acids: concentration and spectrum

To obtain information on the concentration and spectrum of bile acids in human cecal content, samples were obtained from 19 persons who had died an unnatural death from causes such as trauma, homicide, suicide, or drug overdose. Bile acid concentration was measured via an enzymatic assay for 3alpha-hydroxy bile acids; bile acid classes were determined by electrospray ionization mass spectrometry and individual bile acids by gas chromatography mass spectrometry and liquid chromatography mass spectrometry. The 3alpha-hydroxy bile acid concentration (mumol bile acid/ml cecal content) was 0.4 +/- 0.2 mM (mean +/- SD); the total 3-hydroxy bile acid concentration was 0.6 +/- 0.3 mM. The aqueous concentration of bile acids (supernatant after centrifugation) was identical, indicating that most bile acids were in solution. By liquid chromatography mass spectrometry, bile acids were mostly in unconjugated form (90 +/- 9%, mean +/- SD); sulfated, nonamidated bile acids were 7 +/- 5%, and nonsulfated amidated bile acids (glycine or taurine conjugates) were 3 +/- 7%. By gas chromatography mass spectrometry, 10 bile acids were identified: deoxycholic (34 +/- 16%), lithocholic (26 +/- 10%), and ursodeoxycholic (6 +/- 9), as well as their primary bile acid precursors cholic (6 +/- 9%) and chenodeoxycholic acid (7 +/- 8%). In addition, 3beta-hydroxy derivatives of some or all of these bile acids were present and averaged 27 +/- 18% of total bile acids, indicating that 3beta-hydroxy bile acids are normal constituents of cecal content. In the human cecum, deconjugation and dehydroxylation of bile acids are nearly complete, resulting in most bile acids being in unconjugated form at submicellar and subsecretory concentrations.

A top-down systems biology view of microbiome-mammalian metabolic interactions in a mouse model

Symbiotic gut microorganisms (microbiome) interact closely with the mammalian host's metabolism and are important determinants of human health. Here, we decipher the complex metabolic effects of microbial manipulation, by comparing germfree mice colonized by a human baby flora (HBF) or a normal flora to conventional mice. We perform parallel microbiological profiling, metabolic profiling by (1)H nuclear magnetic resonance of liver, plasma, urine and ileal flushes, and targeted profiling of bile acids by ultra performance liquid chromatography-mass spectrometry and short-chain fatty acids in cecum by GC-FID. Top-down multivariate analysis of metabolic profiles reveals a significant association of specific metabotypes with the resident microbiome. We derive a transgenomic graph model showing that HBF flora has a remarkably simple microbiome/metabolome correlation network, impacting directly on the host's ability to metabolize lipids: HBF mice present higher ileal concentrations of tauro-conjugated bile acids, reduced plasma levels of lipoproteins but higher hepatic triglyceride content associated with depletion of glutathione. These data indicate that the microbiome modulates absorption, storage and the energy harvest from the diet at the systems level.

Atherogenic diet causes lethal ileo-ceco-colitis in cyclooxygenase-2 deficient mice

Cyclooxygenases (COX) regulate a variety of inflammatory diseases, including inflammatory bowel disease (IBD). While the pathological effects of COX-1 inhibition by NSAIDs on intestinal ulceration are well established, the role of COX-2 on intestinal inflammation remains under investigation. In this paper, we report a protective role for COX-2 against diet-mediated intestinal inflammation in mice. COX-2(-/-) mice fed an atherogenic diet or diet containing cholate, but not chow or fat alone, had a high mortality whereas COX-1(-/-) mice and wild-type mice were unaffected by the dietary changes. Histological analysis identified the cause of death in COX-2(-/-) mice due to severe intestinal inflammation that was surprisingly limited to the ileo-ceco-colic junction. COX-2 expression is induced in the cecum of wild-type mice fed an atherogenic diet. Our findings show that COX-2 plays an anti-inflammatory role at the ileo-ceco-colic junction in mice, and the pathology of diet-mediated intestinal inflammation in COX-2(-/-) mice offers an excellent model system to elucidate the molecular mechanisms of intestinal inflammation.

Deoxycholic acid formation in gnotobiotic mice associated with human intestinal bacteria

In humans and animals, intestinal flora is indispensable for bile acid transformation. The goal of our study was to establish gnotobiotic mice with intestinal bacteria of human origin in order to examine the role of intestinal bacteria in the transformation of bile acids in vivo using the technique of gnotobiology. Eight strains of bile acid-deconjugating bacteria were isolated from ex-germ-free mice inoculated with a human fecal dilution of 10(-6), and five strains of 7alpha-dehydroxylating bacteria were isolated from the intestine of limited human flora mice inoculated only with clostridia. The results of biochemical tests and 16S rDNA sequence analysis showed that seven out of eight bile acid-deconjugating strains belong to a bacteroides cluster (Bacteroides vulgatus, B. distasonis, and B. uniformis), and one strain had high similarity with Bilophila wadsworthia. All five strains that converted cholic acid to deoxycholic acid had greatest similarity with Clostridium hylemonae. A combination of 10 isolated strains converted taurocholic acid into deoxycholic acid both in vitro and in the mouse intestine. These results indicate that the predominant bacteria, mainly Bacteroides, in human feces comprise one of the main bacterial groups for the deconjugation of bile acids, and clostridia may play an important role in 7aplha-dehydroxylation of free-form primary bile acids in the intestine although these strains are not predominant. The gnotobiotic mouse with bacteria of human origin could be a useful model in studies of bile acid metabolism by human intestinal bacteria in vivo.