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Principle
- Triple sugar iron agar is designed to differentiate among organisms based on the differences in carbohydrate fermentation patterns and hydrogen sulfide production. Carbohydrate fermentation is indicated by the production of gas and a change in the color of the pH indicator from red to yellow.
- To facilitate the observation of carbohydrate utilization patterns, TSI Agar contains three fermentative sugars, lactose, and sucrose in 1% concentrations and glucose in 0.1% concentration. Due to the building of acid during fermentation, the pH falls.
- The acid-base indicator phenol red is incorporated for detecting carbohydrate fermentation that is indicated by the change in color of the carbohydrate medium from orange-red to yellow in the presence of acids. In the case of oxidative decarboxylation of peptone, alkaline products are built and the pH rises.
- This is indicated by the change in color of the medium from orange-red to deep red. Sodium thiosulfate and ferrous ammonium sulfate present in the medium detects the production of hydrogen sulfide which is indicated by the black color in the butt of the tube.
- To facilitate the detection of organisms that only ferment glucose, the glucose concentration is one-tenth the concentration of lactose or sucrose. The meager amount of acid production in the slant of the tube during glucose fermentation oxidizes rapidly, causing the medium to remain orange-red or revert to an alkaline pH.
- In contrast, the acid reaction (yellow) is maintained in the butt of the tube since it is under lower oxygen tension. After depletion of the limited glucose, organisms able to do so will begin to utilize lactose or sucrose. To enhance the alkaline condition of the slant, free exchange of air must be permitted by closing the tube cap loosely.
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