Tuesday, June 16, 2009

The Indole Test

The amino acid tryptophan is found in nearly all proteins.

Positive Indole: Bacteria that contain tryptophanase can hydrolyse tryptophan to its metabolic products(indole, pyruvic acid, ammonia)

Tryptophan ----Tryptophanase-----> Indole + Pyruvic Acid + Ammonia

The bacteria use the pyruvic acid and ammonia to satisfy nutritional needs; indole is not used and accumulates in the medium.
This will form a layer of red color on the surface of the medium when added with Kovac’s reagent due to the combination of aldehyde in the Kovac’s reagent and indole.

Indole + added Kovac’s reagent = Red color

Negative Indole: There would be no red color formation on the agar surface when added with Kovac’s solution.

Tryptophan ----No Tryptophanase-----> Tryptophan

Tryptophan + added Kovac’s reagent = no red color