Indole is generated by reductive deamination from tryptophan via the intermediate molecule indolepyruvic acid. Tryptophanase catalyzes the deamination reaction, during which the amine (-NH2) group of the tryptophan molecule is removed. Final products of the reaction are indole, pyruvic acid, ammonium (NH4+) and energy.

How do bacteria produce indole?

Escherichia coli produces indole (Figure 37) from tryptophan by tryptophanase and exports it by AcrEF pump. Indole acts as an autoinducer to regulate transcription of tnaAB, astD, and gabT in E. coli. As tnaA encodes tryptophanase, indole signaling enhances the production of indole itself.

How is indole released?

Indole is released when certain gram-negative rods hydrolyze tryptophan. Dimethylaminobenzaldehyde reagent is added which combines with the indole and produces a distinct red color.