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)
The basic purpose of the Indole test is, it is used to differentiate E.coli from other gram-negative rod bacteria.
Tryptophan which is a type of amino acid, used in the indole test, breaks down into indole.
Tryptophanase catalyzes the deamination reaction of tryptophan into indole.
eschrechia coli is indole positive bacteria.
Enterobacter species sp are indole test negative bacteria.
Two reagents are used to detect indole production in the medium.
1. Ehrlich reagent
2. Kovac reagent