Triple Sugar Iron Agar is used (TSI Agar)for confirmation of gram negative enteric bacilli on basis of dextrose, lactose and sucrose fermentation and H2S production or confirmation of gram negative enteric bacilli on basis of dextrose, lactose and sucrose fermentation and H2S production. It is recommended by BIS committee under specification IS:5887 (Part I, III and V) 1976, reaffirmed 2005. (TSI Agar)Triple Sugar Iron Agar was originally proposed by Sulkin and Willet and modified by Hajna for identifying Enterobacteriaceae. ISO Committee and BIS has recommended a slight modification for the identification of Salmonellae. BIS has recommended the medium for detection of Escherichia coli and Vibrios. (TSI Agar)Triple Sugar Iron Agar is recommended for confirmation of gram negative bacilli on basis of dextrose, lactose and sucrose fermentation and H2S production. The ingredients included in the medium such as peptone, yeast extract, meat extract provide the nitrogen, carbon, and vitamins required for organism growth. Triple sugar Iron Agar Consists of three carbohydrates-dextrose, Lactose and Sucrose. When the carbohydrate are fermented, acid production is detected by the phenol red pH indicator. Sodium thiosulphate is reduced to hydrogen sulphide, and hydrogen sulphide reacts with an iron salt yielding the typical black iron sulphide. Ferric ammonium citrate is the hydrogen sulphide (H2S) indicator. Sodium chloride maintains the osmotic balance of the medium. Agar is used as a solidifying agent.
Peptone, yeast extract and beef extract provide nitrogenous compounds, sulphur, trace elements and vitamin B complex etc. Sodium chloride maintains osmotic equilibrium. Lactose, sucrose and dextrose monohydrate are the fermentable carbohydrates. Sodium thiosulphate and ferric or ferrous ions make H2S indicator system. Sodium thiosulphate is also an inactivator of halogen and can minimize its toxicity in the testing sample, if any during microbial limit tests. Phenol red is the pH indicator. The dextrose concentration in TSI Agar is one-tenth the concentration of lactose and sucrose which serves to distinguish dextrose-only fermenting organisms from those which ferment lactose and/or sucrose. The small amount of acid produced in the slant during dextrose fermentation oxidizes rapidly, causing the slant to revert to alkaline (red) while the yellow acid reaction is maintained in the butt due to the absence of oxygen. If the lactose or sucrose is also fermented, sufficient acid is produced to retain the yellow color on the slant and in the butt. Ferric ammonium citrate and sodium thiosulfate are indicators of H2S production which form a black precipitate (ferrous sulfate) in the butt of the tube. Gas production is indicated by bubbles, splitting of the agar, or displacement of the agar in the tube.