Which enzyme does the Indole test detect, and what is a positive result?

Which enzyme does the Indole test detect, and what is a positive result?

• A. Urease; positive result is pink.
• B. Cytochrome c oxidase; positive result is blue.
• C. Tryptophanase; positive result is a red color change after Kovac's reagent.
• D. Lactase; positive result is yellow.

Answer: (C)

Indole test checks whether a bacterium can convert tryptophan into indole using the enzyme tryptophanase. After growth in a tryptophan-containing medium, Kovac's reagent is added; if indole is present, it reacts to form a red (cherry) color on the surface, indicating a positive result. If indole isn’t produced, no red color develops. This distinguishes organisms that have tryptophanase from those that don’t, which is helpful in differentiating certain enteric bacteria.

Indole test checks whether a bacterium can convert tryptophan into indole using the enzyme tryptophanase. After growth in a tryptophan-containing medium, Kovac's reagent is added; if indole is present, it reacts to form a red (cherry) color on the surface, indicating a positive result. If indole isn’t produced, no red color develops. This distinguishes organisms that have tryptophanase from those that don’t, which is helpful in differentiating certain enteric bacteria.