Yeast Extract, Enzymatic Digest of Casein, and Enzymatic Digest of Animal Tissue provides the nitrogen, vitamins, and amino acids in TCBS Agar. Sodium Citrate, Sodium Thiosulfate, Sodium Cholate, and Oxbile are selective agents, providing an alkaline pH to inhibit Gram-positive organisms and suppress coliforms. An increased pH is used to enhance growth of Vibrio cholerae, because this organism is sensitive to acid environments. Sucrose is the fermentable carbohydrate. Sodium Chloride stimulates organism growth and maintains osmotic balance of the medium. Sodium Thiosulfate is also a sulfur source, and acts with Ferric Citrate as an indicator to detect hydrogen sulfide production. Bromthymol Blue and Thymol Blue are pH indicators. Agar is the solidifying agent.