Sunday, December 26, 2010



Indole is an aromatic heterocyclic organic compound that can be produced by some bacteria as a degradation product of the amino acid tryptophan. Tryptophanase is the enzyme involved tryptophan degradation resulting indole, pyruvate, and ammonia. Indole test is most used for coliforms identification.

Simplest medium for indole testing is Buffered peptone water (Proteose peptone 10g, NaCl 5g, Disodic phosphate 3.5g, Monopotasic phosphate 1.5g, H
2O ad 1000 ml; pH 7.2), but most used for Enterobacteriaceae identification is MIU medium (Motility Indole Urea).

Inoculate a Buffered peptone water medium or a MIU tube and incubate  at 37°C. After 24 hours of incubation add few drops of Ehrlich-Kovacs reagent. This will combine with the indole forming a red layer on the surface of the medium, if the reaction is positive. Reaction is negative if no red color appear.

Indole should not be tested after 4-5 days of incubation because bacteria may further degrade indole and a false negative result may occur.

1. H. Raducanescu, V.Bica-Popii,1986. Bacteriologie veterinara, Ed. Ceres, Bucuresti.
2. Margaret Barnett, 1992. Microbiology Laboratory Exercises. Wm. C. Brown Publishers.

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