Thiosulfate-Citrate-Bile-Salt Sucrose Agar, or TCBS Agar, is a selective culture medium used in microbiology laboratories to isolate Vibrio species. TCBS Agar is highly selective for the isolation of V. cholerae and V. parahaemolyticus as well as other Vibrio species. TCBS Agar contains high concentrations of sodium thiosulfate and sodium citrate to inhibit the growth of Enterobacteriaceae. Inhibition of Gram-positive bacteria is achieved by the incorporation of beef gall, a natural substance containing a mixture of bile salts and sodium cholate, a pure bile salt. Sodium thiosulfate also serves as a source of sulfur and its presence, in combination with ferric citrate, makes it easy to detect the production of hydrogen sulfide. Sucrose (sucrose) is included as a fermentable carbohydrate for the metabolism of Vibrio species. The alkaline pH of the medium improves the recovery of V. cholerae and inhibits the growth of others. Thymol blue and bromothymol blue are included as indicators of pH changes.