Effect of bile on V. cholerae virulence regulation. A. When V. cholerae is grown in vitro under permissive conditions without bile the ToxR regulon is activated and the major virulence factors CT and TCP are produced. B. In the intestinal lumen where bile concentration is high, UFA present in bile prevents dimerization of ToxT and thus expression of the virulence genes ctxAB and tcpA is reduced. As the bacteria move away from the lumen through the mucus layer to the underlying epithelium, concentration of bile gradually decreases, consequently ctxAB and tcpA expression increases.