Figure 1.

The influence of bile acid (BA) metabolism on the gut microbiome and host immunity. (A) Primary BAs are produced from cholesterol in the liver, conjugated to glycine or taurine, and secreted into the gut lumen. In the intestine, primary BAs undergo un-conjugation by the microbiota, followed by further rounds of modifications (oxidation and epimerization, dehydroxylation, esterification, or desulfation), to yield secondary BAs. Conjugated, unconjugated, and secondary BAs can all shape the microbial composition in the intestine (dashed arrows). Specific examples include unconjugated and taurine or glycine-conjugated UDCA promoting the growth of A municiphila, LCA derived from the conversion of CDCA by C scindens and promoting the growth of E faecium while inhibiting the growth of C difficile, DCA, derived from the conversion of CA by C scindens and inhibiting the growth of C difficile and finally, isoDCA, derived from the conversion of DCA by E lenta, and promoting the growth of Bacteroides (see also main text). (B) Schematic overview of the G-protein coupled (yellow) and nuclear receptors (blue) specific for bile acids (greater-than signs denote higher affinities). The panel of BARs expressed in immune cells was extracted from the Human Protein Atlas, using the HPA (26) and Monaco datasets (27). Immunomodulatory effects of receptor signaling by at least one natural ligand is noted (question marks denote receptor signaling with the influence on immunity that remains to be identified).