FIG. 1.

(A) Chemical structure of bile acids. Primary bile acids are synthesized in the liver from cholesterol and are conjugated with either glycine or taurine prior to secretion. The carboxyl group of the bile acid and the amino group of the amino acid are linked by an amide bond. (B) Reaction catalyzed by BSH enzymes. BSHs cleave the peptide linkage of bile acids, which results in removal of the amino acid group from the steroid core. The resulting unconjugated bile acids precipitate at low pH. (C) Detection of BSH activity (as described in reference 18). L. plantarum, which was grown overnight in MRS broth, was streaked onto MRS (Difco) agar (A) or MRS agar supplemented with 0.2% (wt/vol) glycodeoxycholic acid (GDCA; Sigma) (B) and incubated anaerobically for 48 h. The white precipitates around colonies and the clearing of the medium are indicative of BSH activity (see reference 18).