The indole produced yields a red complex with para-dimethylaminobenzaldehyde under the given conditions.
[1] This was invented by the Hungarian physician Nicholas Kovács and was published in 1928.
This reagent is used in the confirmation of E. coli and many other pathogenic microorganisms.
Kovacs, N. (1928): Eine vereinfachte Methode zum Nachweis der Indolbildung durch Bakterien.
Zeitschrift für Immunitätsforschung und Experimentelle Therapie, 55, 311.