Thiosulfate-citrate-bile salts-sucrose agar

Thiosulfate-citrate-bile salts-sucrose agar, or TCBS agar, is a type of selective agar culture plate that is 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 ox gall, which is a naturally occurring substance containing a mixture of bile salts and sodium cholate, a pure bile salt. Sodium thiosulfate also serves as a sulfur source and its presence, in combination with ferric citrate, allows for the easy detection of hydrogen sulfide production. Saccharose (sucrose) is included as a fermentable carbohydrate for metabolism by Vibrio species. The alkaline pH of the medium enhances the recovery of V. cholerae and inhibits the growth of others. Thymol blue and bromothymol blue are included as indicators of pH changes.

Bacteria that are not Vibrio but produce hydrogen sulfide grow as small black colonies. This is because the hydrogen sulphide produced from thiosulphate, which acts as a source of sulphur and creates a reduced oxygen tension in which Vibrio can grow due to its facultative anaerobic nature, combines with ferric ions from ferric citrate to produce ferric sulphide, which is black.

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