Diagnostic Test for Bacterial Identification: Indole Test

The indole test is used to determine the ability of an organism to spilt amino acid tryptophan to form the compound indole. Tryptophan is hydrolysed by tryptophanase to produce three possible end products – one of which is indole.

Indole production is detected by Kovac's or Ehrlich's  reagent which contains 4 (p)-dimethylamino benzaldehyde, this reacts with indole to produce a red coloured compound.
Indole test is a commonly used biochemical test (eg in IMVIC test, SIM test etc). Indole test helps to differentiate Enterobacteriaceae and other genera.

Two methods are in used;

1. a spot indole test, which detects rapid indole producing organisms and
2. a conventional tube method requiring  overnight incubation, which identifies weak indole producing organisms.

Methods
a. Inoculate the tryptophan broth with broth culture or emulsify isolated colony of the test organism in tryptophan broth.
b. Incubate at  37°C for 24-28 hours in ambient air.
c. Add 0.5 ml of Kovac's reagent to the broth culture.

Photo Courtesy: Karen M. Kiser

Expected results:
Positive: Pink colored rink after addition of appropriate reagent
Negative: No color change even after the addition of appropriate reagent. e.g. Klebsiella Pneumoniae

Indole positive organisms: Most strains of E.coli, P. vulgaris, M. morganii and providenica are indole positive.

Point to remember: Indole test can also aid in species differentiation. Klebs‪iella Oxytoca, Citrobacter Koseri, and Proteus Vulgaris are indole positive, where are K. Pneumoniae, Citrobacter freundii and Proteus mirabilis are indole negative.