- Tests for the presence of trypophanase, which metabolizes tryptophan to indole
- May be done either as a conventional tube indole (overnight incubation) or as a spot indole rapid test
- The tube test requires overnight incubation at 37°C in ambient air, then add the reagent; it turns pink if positive
- The spot test is quick, testing for indole reacting with cinnamaldehyde to give a blue-green colour within 30 seconds
- Helps to differentiate species within the Enterobacteriaciae
- Most strains of Escherichia coli, Proteus vulgaris, Morganella morganii and Providencia are positive
- Specifically can help identify species within a genus:
- Klebsiella species: Klebsiella oxytoca is indole positive whereas Klebsiella pneumoniae and Klebsiella aerogenes are indole negative
- Citrobacter species: Citrobacter koseri is indole positive where as Citrobacter freundii is indole negative
- Proteus species: Proteus vulgaris is indole positive whereas Proteus mirabilis is indole negative
- Notable indole-negative species include Salmonella species, Hafnia species, Enterobacter species
- Yersinia enterocolitica and some Shigella species are indole-variable
