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The Non-Enteric Rods: Indole test

Tryptophan broth is used to identify organisms that are able to metabolize tryptophan to form indole during a 24 hour incubation. Indole production is indicated by the formation of a red ring at the top of the media when Kovac's reagent is added to the tube.

This slide shows typical indole reactions:
The tube on the left is positive (red ring).
The tube on the right is negative (ring is colorless to yellow).