Indole test

• 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 E.coli, P. vulgaris, M. morganii and Providenica
  • Specifically can help identify species within a genus:
    • Klebsiella species: Klebs‪iella oxytoca is indole positive whereas Klebsiella pneumoniae is 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