Fig. 4.

Sensing bile salts in vivo. (A) A working model: an intermolecular disulfide bond at C207 is critical for TcpP activity. In the absence of bile salts, an intramolecular disulfide bond forms between C207 and C218. Upon encountering bile salts, isomerization takes place to cause an intermolecular disulfide bond between two subunits of TcpP at C207. (B) In vivo competition assay using an infant mouse model. Six-day-old CD-1 infant mice were inoculated with the mixture of TcpP cysteine mutant and WT at 1:1 ratio with or without cholestyramine. After a 6-h period of colonization, intestinal homogenates were collected, and the ratio of mutant-to-WT bacteria was determined and normalized against input ratios. ***P < 0.001 (Student t test).