Triple Sugar Iron Agar (TSI) is a differential medium used to differentiate enteric Gram-negative Enterobacteria on the basis of carbohydrate fermentation and H2S production. It is used as an aid in the identification of pathogenic and saprophytic Enterobacteria isolated from routine bacteriological analysis of material samples such as feces. This medium is used to initiate the identification of Enterobacteria in some FDA schemas. Peptone mixture and the Beef extract provide nitrogen, vitamins, minerals and amino acids essential for growth. Yeast extract is a source of vitamins, particularly of the B-group. TSI contains three carbohydrates (Dextrose, Sucrose and Lactose) as sources of carbon and energy. When these are fermented the acid production is indicated by the Phenol red indicator, being the color changes yellow for acid production and red for alkalinization. Sodium thiosulfate is reduced to hydrogen sulfide, which reacts with the iron salt to give the black iron sulfide. Ferric ammonium citrate is an H2S indicator. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Bacteriological agar is the solidifying agent. The mode of action is similar to Kligler Iron Agar (Cat. 1042) which contains two sugars. The addition of 1% Sucrose in TSI Agar allows differentiating between Proteus and Salmonella. The fermentation of sucrose by Proteus turns the color of the Phenol red indicator in the slant from red to yellow. Dextrose-positive and lactose-negative members of the genus Salmonella all cause a reddening of the slant and acidify the depths of the agar tubes. The presence of salmonellae is provisionally confirmed if in the deep inoculation, but not in the surface culture, there is a change of color from red to yellow and, usually, a formation of gas, with or without production of hydrogen sulfide in the agar. Precise confirmation may be carried out by the appropriate biochemical and serological tests.
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