Indole Test: Do's & Don'ts

Indole is one of the metabolic degradation products of the amino acid tryptophan. Indole production is an important characteristic in the identification of many species of microorganism, particularly distinguishing Escherichia coli from other members of Enterobacteriaceae family. This bacteria has the tryptophanase enzyme, which can hydrolyze and deaminate tryptophan while producing indole, pyruvic acid, and ammonia.

The indole test is based on the formation of a red complex (at the interface of the reagent and broth) when indole reacts with the aldehyde group present in Kovac reagent or Ehrlich reagent. 

Rapid spot test is an additional alternate technique. Filter paper strips that have been treated with reagent are utilized for screening of prompt indole producers.

When doing the indole test and interpreting the result, microbiologists should keep a few dos and don'ts in mind. Let's look at these key recommendations.