Define Triple Sugar iron - Carbohydrate Utilization Pattern?

Triple sugar iron (TSI) agar is used to observe carbohydrate utilization pattern. The medium contains 1% concentration of each of lactose and sucrose and 0.1% concentration of glucose. Acid base indicator phenol red is also incorporated in the medium to detect the production of acid during carbohydrate fermentation. Presence of acid results in the change of medium colour from orange red to yellow and indicate the carbohydrate fermentation. TSI medium also contains sodium thiosulphate as substrate and ferrous sulphate for detection of H₂S production by certain organisms through reduction of sulphur containing amino acids (cysteine, cystine and methionine) or inorganic sulphur compounds (thiosulphates, sulphates, sulphites). H₂S is a colourless gas commonly called "rotten egg gas" which when react with metal salts (e.g. ferrous sulphate) form visible insoluble black ferrous sulfide precipitates.

TSI medium is inoculated by means of a stab and streak procedure. From the base of the slant, culture is inoculated into the butt by using straight, sterile needle. The slanted surface is streaked. The results are observed within 18-24 hours after incubation otherwise peptone will start degrading after depletion of carbohydrates and produces alkaline reaction. Slants and butts are seen separately for acid production along with gas production (breaks in agar butt).