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Biochemical Test

Indole Test
Biochemical Test
Agar Plates
Coagulase Test
The Catalase Test
Indole Test
Oxidase Test
Citrate Test
Optichin Test
Urease test

Indole Test

Indole is a component of the amino acid tryptophan. Some bacteria have the ability to break down tryptophan for nutritional needs using the enzyme tryptophanase. When tryptophan is broken down, the presence of indole can be detected through the use of Kovacs' reagent. Kovac's reagent, which is yellow, reacts with indole and produces a red color on the surface of the test tube.


Procedure to carry out Indole test:


  • Broth Cultures

o    Inoculate the tryptophan broth with the test organism and incubate at 37C for 24-28h.

o    Add 0.5mL of the Kovac’s reagent and gently agitate

o    Examine the upper layer of liquid


Positive result red colour (occurring within a few seconds)

Negative result yellow colour


  • Spot Test

o    Moisten a piece of filter paper with the spot test reagent and smear a colony on to the surface

o    Examine immediately


Positive result Green/blue colour

Negative result No colour change