Indole Production
DESCRIPTION
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.
MATERIALS
Simplest medium for indole testing is Buffered peptone water (Proteose peptone 10g,
NaCl 5g, Disodic phosphate 3.5g, Monopotasic phosphate 1.5g, H2O ad 1000 ml; pH
7.2), but most used for Enterobacteriaceae identification is Motility Indole Urea
medium (MIU).

Kovacs reagent composition:
Amyl alcohol (may be replaced by n-butanol or ethanol) ......................................... 750 ml,
p-Dimethylaminobenzaldehyde ..................................................................................... 50.0 g,
Hydrochloric acid (conc.) ................................................................................................ 250 ml.
REFERENCES
1. Helgomar Raducanescu, Valeria Bica-Popii,1986. Bacteriologie veterinara, Ed. Ceres, Bucuresti.
2. Margaret Barnett, 1992. Microbiology Laboratory Exercises. Wm. C. Brown Publishers.
3. Standards Unit, Microbiology Services, PHE. UK Standards for Microbiology Investigations. Identification of Clostridium species.
Bacteriology – Identification, ID 8, Issue no: 4, Issue date: 12.01.15.
Indole positive result: red ring on
MIU medium surface (middle tube)
PROCEDURE
Inoculate a Buffered peptone water medium or a MIU tube and incubate at 37°C. After 24 hours of incubation add few drops of Kovacs
reagent. This will combine with the indole forming a pink-red compound. Reaction is positive if a red layer appears on the surface of
the medium. Reaction is negative if red color does not appear.

NOTES
Indole should not be tested after 4-5 days of incubation because bacteria may further degrade indole and a false negative result may
occur.
Do not use peptone media containing glucose. A decrease in pH towards acid production may produce a false negative result or the
reaction may be weak.
Anaerobes, particularly Clostridium species, form indole but can rapidly break it down as it is produced; therefore, false negative
reactions may occur.
Antibiogram
Encyclopedia
Culture media
Biochemical tests
Stainings
Images
Movies
Articles
Identification
Software
(c) Costin Stoica
R E G N U M
PROKARYOTAE
Previous page
Back
TSI, MIU & Simmons citrate