Triple Sugar Iron Agar

  • Triple Sugar Iron medium is a differential medium that can distinguish between a number of Gram-negative enteric bacteria based on their physiological ability (or lack thereof) to:

a. metabolize lactose and/or sucrose
b. conduct fermentation to produce acid
c. produce gas during fermentation
d. generate H2S.

  • The medium contains 1.0% each of sucrose and lactose and 0.1% glucose. If only glucose is fermented, acid produced in the butt will turn it yellow, but insufficient acid products are formed to affect the methyl red in the slant. However, if either sucrose or lactose are fermented, sufficient fermentation products will be formed to turn both the butt and the slant yellow. If gas is formed during the fermentation, it will show in the butt either as bubbles or as cracking of the agar. If no fermentation occurs (as for an obligate aerobe), the slant and butt will remain red.The medium also contains ferrous sulfate. If the bacterium forms H2S, this chemical will react with the iron to form ferrous sulfide, which is seen as a black precipitate in the butt (a black butt).
Triple Sugar Iron Agar

Triple Sugar Iron Agar

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