Principles:

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.