Indole test

Principle

Tryptophanase converts tryptophan to indole, which forms a red compound with Kovács (or Ehrlich) reagent.

Procedure

• Tube: grow the organism in tryptone broth for 24–48 h; add a few drops of Kovács reagent.
• Spot: rub a colony on filter paper wetted with DMACA reagent.

Results

• Positive: red ring (tube) or blue-green (spot) = e.g. E. coli, Proteus vulgaris, Klebsiella oxytoca
• Negative: yellow = e.g. Klebsiella pneumoniae, Enterobacter, Proteus mirabilis