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Ivy Tech Microbiology Lab Test 2 Practice

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In a TSI slant, which observation indicates hydrogen sulfide production?
Correct Answer:
The presence of a black precipitate.
Explanation:
Hydrogen sulfide production in a Triple Sugar Iron slant is shown by blackening of the medium due to iron sulfide forming (FeS) when H2S reacts with the iron source in the medium. The dye/indicator setup includes ferric or ferrous iron, so any H2S released by the organism will precipitate as a black solid. This distinct black precipitate tells you H2S has been produced, unlike color changes from acid or alkaline conditions (red, green, or yellow) that reflect carbohydrate fermentation and pH shifts rather than H2S formation. Gas production can also occur with fermentation but does not by itself indicate H2S. Hence, the presence of a black precipitate is the observation that signals hydrogen sulfide production.

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