Triple Sugar Iron Agar (TSI) comes as 500g and is a differential medium used to differentiate enteric Gram-negative Enterobacteria based on 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 faeces. 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 colour changes yellow for acid production and red for alkalinization. Sodium thiosulfate is reduced to hydrogen sulphide, which reacts with the iron salt to give the black iron sulphide. Ferric ammonium citrate is an H2S indicator. Sodium chloride supplies essential electrolytes for transport and osmotic balance and bacteriological agar is the solidifying agent.

The mode of action is like Kligler Iron Agar (Cat. CL-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 colour 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 colour from red to yellow and, usually, a formation of gas, with or without production of hydrogen sulphide in the agar. Precise confirmation may be carried out by the appropriate biochemical and serological tests.