Lab method
Indole test
Cherry-red ring = E. coli (indole positive)
IN-dohl
High-yield clue
A cherry-red ring after Kovac's reagent marks indole-positive Escherichia coli, a core Enterobacterales differentiation clue.
Overview
A biochemical concept detecting tryptophanase, the enzyme that breaks tryptophan into indole; Kovac's reagent then forms a cherry-red ring in positives.
Classification
- Enzyme-detection concept
- Part of IMViC panel
- Kovac's reagent color readout
Lab & identification clues
- Cherry-red ring = indole positive (E. coli)
- Klebsiella and Enterobacter are usually indole negative
- Spot indole variant for rapid reads
- The I in the IMViC panel
Associations
- E. coli vs Klebsiella/Enterobacter differentiation
- IMViC panel study framing
- Enteric Gram-negative identification
Commonly confused with
- Methyl red / Voges-Proskauer test
- Citrate utilization test
Frequently asked questions
What is Indole test?
A biochemical concept detecting tryptophanase, the enzyme that breaks tryptophan into indole; Kovac's reagent then forms a cherry-red ring in positives.
What is the high-yield clue for Indole test?
A cherry-red ring after Kovac's reagent marks indole-positive Escherichia coli, a core Enterobacterales differentiation clue.
What is Indole test associated with?
Indole test is associated with E. coli vs Klebsiella/Enterobacter differentiation, IMViC panel study framing, and Enteric Gram-negative identification.
What is Indole test commonly confused with?
Indole test is commonly confused with Methyl red / Voges-Proskauer test and Citrate utilization test.
Original concept summary for coursework. Sources checked: OpenStax Microbiology 2e and NCBI Bookshelf Medical Microbiology; reviewed 2026-06. Describes vocabulary and interpretation concepts only; not a lab protocol and not for handling specimens or identifying patient isolates.