Indole test principle and procedure microbiology

The indole test is a biochemical test used primarily in microbiology to determine an organism’s ability to produce indole from the amino acid tryptophan. It is particularly useful for identifying certain species of bacteria.

Principle of indole test

when some bacteria react with Tryptophan (amino acid), hydrolysis it and change it o Indole compound. This Indole compound is visualized by the Ehrlich reagent. This compound changes the agar color into PINK.

Note: If we differentiating gram-negative rods especially e. coli, we use Kovac Reagent in the place of Ehrlich reagent. Kovacs reagent gives RED color instead of PINK

Procedure of the Indole test

Following steps are performed.

• Inoculate the medium by test organisms.
• Incubate the inoculated media at 37 C for 20 to 48 hours.
• Add 5 ml Ehrlich reagents.
• Observe the change in color on the top noticed.
• Pink or red color is the indication of a positive test result.

Indole test reagents

Following reagents are used in the indole test

• a. NaCl (sodium chloride)
• b. Distil water
• c. Peptone water (Containing Tryptophan)
• d. Ehrlich reagent
• e. Kovacs reagents (In case of Gram-negative bacteria rod differentiation).
• f. Amy Alcohol
• g. P-dimethyl-amino benzaldehyde
• h. Concentrated HCL (hydrochloric acid)