Bile acids are a two-way signaling currency between host + gut microbiota. Primary bile acids (cholic acid, chenodeoxycholic acid) synthesized in the liver from cholesterol → conjugated with taurine/glycine → secreted into bile → small intestine. Gut bacteria modify them in two steps: (1) deconjugation — bile salt hydrolases (BSH; Bacteroides, Bifidobacterium, Lactobacillus, Clostridium have broad BSH expression) cleave taurine/glycine. (2) 7α-dehydroxylation — performed only by a small Clostridium cluster (XIVa, including C. scindens, C. hylemonae) converting primary → secondary bile acids (CDCA → litho-CA; CA → deoxycholic-CA). Secondary BA differ profoundly in receptor activity (FXR / TGR5, cross-link fxr tgr5 bile acid receptor) — LCA + DCA are potent TGR5 agonists; tauro-β-muricholic acid (rodents) is a natural FXR antagonist. Enterohepatic recirculation: ~95% reabsorption in terminal ileum via ASBT → portal blood → liver → re-secretion. Disease relevance: dysbiosis (antibiotic-disrupted, IBD, MASH) → altered primary:secondary BA ratio + signaling. C. difficile susceptibility — antibiotic loss of 7α-dehydroxylating bacteria allows C. diff germination (deoxycholic acid normally suppresses germination). FXR-FGF15/19: secondary BA → ileal release of FGF15 (mouse) / FGF19 (human) → hepatic FGFR4 → ↓CYP7A1 → BA synthesis feedback. Therapeutics: UDCA (cytoprotective hydrophilic BA — PBC); colesevelam + cholestyramine (BA sequestrants → ↑synthesis + glycemic + lipid effects); rifaximin (non-absorbed gut antibiotic — IBS-D + HE); FXR agonists (obeticholic acid PBC). Cross-links: bile acid synthesis (upstream), fxr tgr5 bile acid receptor (host receptors), microbiome scfa butyrate (parallel microbial signaling).
Organ Systems
digestive
endocrine
immune-hematologic
Pathway Steps
liver hepatocyte → primary bile acids (cholic + chenodeoxycholic — Tau/Gly conjugated) — via CYP7A1 + downstream enzymes; bile secretion via BSEP. Hepatocytes synthesize the primary bile acids (cholic and chenodeoxycholic acid) from cholesterol and conjugate them to taurine or glycine for solubility before secreting them into bile. This host-made, conjugated pool is the starting material the microbiome then transforms — the first half of a host-microbe metabolic dialogue.
small intestine + bacterial BSH → deconjugation → free primary bile acids — via Bacteroides + Bifidobacterium + Lactobacillus + Clostridium broad BSH activity. In the intestine, bacterial bile-salt hydrolases (BSH) deconjugate bile acids, removing taurine/glycine to yield free primary bile acids. BSH is widespread among gut bacteria (a probiotic-relevant activity); deconjugation is the gateway reaction that enables all further microbial bile-acid modification.
free primary BA + Clostridium XIVa → secondary BA (DCA from CA, LCA from CDCA) — via 7α-dehydroxylation — narrow taxa; antibiotic-sensitive. Specific gut bacteria (Clostridium cluster XIVa) 7α-dehydroxylate free primary bile acids into secondary bile acids — deoxycholic acid (from cholic) and lithocholic acid (from chenodeoxycholic). Only a few taxa do this, so the secondary-bile-acid pool is a sensitive readout of microbiome composition.
secondary BA (DCA, LCA) → TGR5 + FXR activation profile shift — via distinct pharmacology from primary BA → cross-link fxr_tgr5_bile_acid_receptor. Secondary bile acids have a different receptor-activation profile: DCA and LCA are strong TGR5 agonists and modulate FXR differently than the primary acids. So microbial transformation does not just degrade bile acids — it retunes the signal the host’s FXR/TGR5 receptors receive, shaping metabolism and immunity.
ileal BA + FXR → FGF15 / FGF19 release — via enteroendocrine → portal → hepatic FGFR4 → ↓CYP7A1 (feedback). In the ileum, bile acids activate enterocyte FXR to release the hormone FGF15 (mouse)/FGF19 (human), which feeds back to the liver to repress bile-acid synthesis. Because microbial deconjugation changes which bile acids reach the ileum, the microbiome indirectly tunes this feedback loop.
antibiotic-disrupted microbiota → loss of 2° BA → C. difficile germination — via normally DCA suppresses C. diff spore germination; CDI risk rises. Antibiotics that deplete the 7α-dehydroxylating bacteria collapse the secondary-bile-acid pool — and secondary bile acids normally suppress Clostridioides difficile spore germination and outgrowth. Their loss is a key reason antibiotics predispose to C. difficile infection, reversible by microbiota transplant.
Known Modulators
rifaximin (inhibitor) — gut microbiota composition; paradoxically often ↑beneficial taxa. IBS-D + hepatic encephalopathy + SIBO