Indole test

• Tests for the presence of tryptophanase, 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 indole-positive
  • Specifically can help identify species within a genus:
    • Klebsiella: Klebs‪iella oxytoca is indole-positive whereas Klebsiella pneumoniae and Klebsiella aerogenes are indole-negative
    • Citrobacter: Citrobacter koseri is indole-positive where as Citrobacter freundii is indole-negative
    • Proteus: Proteus vulgaris is indole-positive whereas Proteus mirabilis is indole-negative
  • Notable indole-negative species include Salmonella, Hafnia, Enterobacter
  • Yersinia enterocolitica and some Shigella are indole-variable