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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 spp.[1] TCBS Agar is highly selective for the isolation of V. cholerae and V. parahaemolyticus as well as other vibrios. 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 serves as a sulfur source and, in combination with ferric citrate, detects hydrogen sulfide production. Saccharose (sucrose) is included as a fermentable carbohydrate for the metabolism of vibrios. The alkaline pH of the medium enhances the recovery of V. cholerae. Thymol blue and bromothymol blue are included as indicators of pH changes.
Typical colonial morphology on TCBS Agar is as follows:
The non Vibrio but hydrogen sulphide producting organisms produce a black small colonies. This is because, the hydrogen sulphide produced from thiosulphate ( it acts as a source of sulphur besides creating a reduced atmosphere in which Vibrio can grow due to its facultative anaerobic nature) combines with Ferric ions from Ferric Ammonium Citrate to produce Ferric Sulphide which is black in colour.
The TCBS agar is both selective and differential. It is higly selective for Vibrios and differential due to the presence of Sucrose and the dyes. Sucrose fermentation produces acid which converts the colour of bromothymol blue or thymol blue. Two dyes rather than one make the medium produce an array of Yellow, green or blue so that differentiating among various Vibrios is possible.