04/12/2025
The indole test is a biochemical test used to identify bacteria based on their ability to produce indole from the amino acid tryptophan. A positive result, indicated by a red or pink color change after adding a reagent like Kovac's reagent, means the organism has the enzyme tryptophanase and can break down tryptophan. It is a component of the IMViC test series used to differentiate members of the family Enterobacteriaceae.
How the test works
Enzyme action: Some bacteria produce the enzyme tryptophanase, which breaks down the amino acid tryptophan into indole, pyruvic acid, and ammonia.
Reagent addition: To detect the presence of indole, a reagent is added after the bacteria have been grown in a tryptophan-rich medium.
Color change: Indole reacts with the reagent, which contains p-dimethylaminobenzaldehyde, to produce a red or pink color, indicating a positive test.
Negative result: If no indole is produced, the reagent remains a pale yellow or shows no significant color change.
How to perform the test
Inoculate: Grow the bacterial culture in a medium that contains tryptophan, such as tryptone broth, for about 24 hours at 37°C.
Add reagent: After incubation, add a few drops (or milliliters, depending on the protocol) of Kovac's reagent to the tube.
Observe: A red or pink color at the top of the tube indicates a positive result. A yellow color or no significant color change indicates a negative result.
Common applications
The test is part of the IMViC (Indole, Methyl red, Voges-Proskauer, and Citrate) biochemical test battery, which helps identify and differentiate bacteria.
It is commonly used to confirm the identification of bacteria like Escherichia coli.
