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Spot indole test. A. Application of spot indole reagent; B. Application of indole positive bacterium; C. Application of indole negative bacterium. - Click on the image to enlarge it.
Bacteria, which express the enzyme tryptophanase can hydrolyze the amino acid tryptophan to indole, pyruvic acid and ammonia. Presence of indole can be shown by means of Kovác's reagent or by spot indole test. In the spot test indole reacts with p-Dimethylaminocinnamaldehyde to produce a blue to blue-green product. Kovác's reagent contains p-dimethylaminobenzaldehyde, which forms a red complex with indole.
Method, spot indole test
Method, Kovac´s reagent
Confirmation of suspected E. coli-strains. Typing (species determination) of Brachyspira spp. in combination with other tests. Kovac's indole reagent is more sensitive than the indole spot reagent, but it is not recommended for use with anaerobic bacteria. The indole spot reagen is suitable for both aerobic and anaerobe use.