Active cholesterol metabolites are produced by interspecies biosynthetic pathways.

<p>Bile acids are the main metabolites of cholesterol (CHOL). Primary bile acids (blue ovals; cholic acid (CA) and chenodeoxycholic acid (CDCA)) are produced by endogenous enzymes in the liver and are modified by bacteria of the genus <i>Clostridia</i> colonizing the gut to form secondary bile acids (yellow ovals; lithocholic acid (LCA) and deoxycholic acid <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0025482#pone.0025482-Houten1" target="_blank">[18]</a>). Arrows broken by double lines represent multiple enzymatic steps and only the genes encoding the rate limiting enzymes are listed. The bacterial operon baiABCDEFGHI encodes eight enzymes and a bile acid transporter that together form the pathway for synthesis of secondary bile acids. The amino acids glycine and taurine are conjugated to primary and secondary bile acids in the liver by the host encoded bile acid-CoA amino acid transferase (BAAT) to form tauro- and glycolithocholic acid (TLCA, GLCA) and tauro- and glycodeoxycholic acid (TDCA GDCA). Gut bacteria of genus <i>Lactobacillus</i> catalyze the conversion of cholesterol metabolites to coprostanol (COPR) and can limit the intestinal absorption of cholesterol.