Indole test (tryptophan hydrolysis test): Principle, Procedure, Result

Indole test also called as tryptophan hydrolysis test, due to decomposition of tryptophan (Amino Acid).

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

Indole test (tryptophan hydrolysis test): Principle, Procedure, Result
Indole test (tryptophan hydrolysis test)

What is the procedure of the Indole test?

Following steps are performed.

  • 1. Inoculate the medium by test organisms.
  • 2. Incubate the inoculated media at 37 C for 20 to 48 hours.
  • 3. Add 5 ml Ehrlich reagents.
  • 4. Change in color on the top noticed.
  • 5. 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)

What is the purpose of the Indole test?

The basic purpose of the Indole test is, it is used to differentiate E.coli from other gram-negative rod bacteria.

which amino acid break down into indole ?

Tryptophan which is a type of amino acid, used in the indole test, breaks down into indole.

Indole test (tryptophan hydrolysis test): Principle, Procedure, Result

What enzyme breaks down tryptophan?

Tryptophanase catalyzes the deamination reaction of tryptophan into indole.

Indole test positive bacteria mnemonic

eschrechia coli is indole positive bacteria.

Indole test Negative bacteria

Enterobacter species sp are indole test negative bacteria.

which reagent is added to the sim medium to detect indole production?

Two reagents are used to detect indole production in the medium.

  • 1. Ehrlich reagent
  • 2. Kovac reagent

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