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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)