TRIPLE SUGAR IRON (TSI AGAR)
Triple sugar iron agar (TSI) is a differential medium that contains lactose, sucrose, a small amount of glucose (dextrose), ferrous sulfate, and the pH indicator phenol red. It is used to differentiate enterics based on the ability to reduce sulfur and ferment carbohydrates.
As with the phenol red fermentation broths, if an organism can ferment any of the three sugars present in the medium, the medium will turn yellow. If an organism can only ferment dextrose, the small amount of dextrose in the medium is used by the organism within the first ten hours of incubation. After that time, the reaction that produced acid reverts in the aerobic areas of the slant, and the medium in those areas turns red, indicating alkaline conditions. The anaerobic areas of the slant, such as the butt, will not revert to an alkaline state, and they will remain yellow. This happens with Salmonella and Shigella.
Procedure:
- Touch a well isolated colony with a sterile straight wire.
- Inoculate TSI by first stabbing through the centre of the medium to the bottom of the tube and then streak the surface of the slant.
- Leave the cap loose and incubate the tube at 35 in ambient air for 18 to 24 hours.
- Observe the reaction
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