Sierra College Department of Biological Sciences

Microbiology Diagnostic Tests: Triple Sugar Iron (TSI) Medium (Set #1)

The TSI medium can be used to test the ability of bacteria to ferment three sugars (glucose, sucrose and lactose) and to form an end product that reacts with the iron present. In these two tubes the fermentation of glucose and sucrose is not readily determined because of the black precipitate present at the tube bottom (in the butt of the tube).


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1. Both of these tubes contain a black precipitate called what?

2. What metabolic end product is reacting with the iron in the SIM medium to form this black precipitate?

3. What type of organic compound is being degraded by the bacteria producing the black precipitate?

3. Which tube contains organisms that cannot ferment lactose? How do you know?

4. What is the name of the pH indicator in the TSI medium?

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