Principle
- The test organism is inoculated into tryptone broth, a rich source of the amino acid tryptophan.
- Indole positive bacteria such as Escherichia coli produce tryptophanase, an enzyme that cleaves tryptophan, producing indole and other products.
- When Kovac’s reagent (p-dimethylamino benzaldehyde) is added to a broth with indole in it, a dark pink colour develops.
- The indole test must be read by 48 hours of incubation because the indole can be further degraded if prolonged incubation occurs. The acidic pH produced by Escherichia coli limits its growth.
Indole Test
This Test Used for:
- This test is performed to help differentiate species of the family Enterobacteriaceae.
- It tests for the bacteria species’ ability to produce indole. Bacteria use an enzyme, tryptophanase to break down the amino acid, tryptophan, which makes by-products, of which, indole is one.
Media and Reagents Used:
- Tryptone broth contains tryptophan. Kovac’s reagent—contains hydrochloric acid, dimethylaminobenzaldehyde, and amyl alcohol—yellow in color.
- Reading Results: Kovac’s reagent reacts with indole and creates a red color at the top part of the test tube.
