- Indole test
The indole test is a biochemical test performed on bacterial species to determine the ability of the organism to split
indole from theamino acid tryptophan . This division is performed by a chain of a number of different intracellularenzyme s, a system generally referred to as "tryptophanase."Biochemistry
Indole is generated by reductive
deamination from tryptophan via the intermediate molecule indolepyruvic acid. Tryptophanase catalyzes the deamination reaction, during which theamine (-NH2) group of the tryptophan molecule is removed. Final products of the reaction are indole,pyruvic acid ,ammonia (NH3) and energy. Pyridoxal phosphate is required as acoenzyme .Performing a Test
Like many biochemical tests on bacteria, results of an indole test are indicated by a change in color following a reaction with an added reagent.
Pure bacterial culture must be grown in sterile tryptophan or peptone broth for 24-48 hours before performing the test. Following incubation, add 5 drops of Kovac's reagent (
isoamyl alcohol , "p"-Dimethylaminobenzaldehyde, concentratedhydrochloric acid ) to the culture broth.A variation on this test using Ehrlich's reagent (using
ethyl alcohol in place of isoamyl alcohol, developed byPaul Ehrlich ) is used when performing the test on nonfermenters andanaerobe s.A positive result is shown by the presence of a red or red-violet color in the surface alcohol layer of the broth. A negative result appears yellow. A variable result can also occur, showing an orange color as a result. This is due to the presence of
skatole , also known as methyl indole or methylated indole, another possible product of tryptophan degradation.Indole-Positive Bacteria
Bacteria that test positive for cleaving indole from tryptophan include: "Aeromonas hydrophilia", "Aeromonas punctata", "Bacillus alvei", most "
Citrobacter " spp., "Edwardsiella" spp., "Escherichia coli ", "Flavobacterium" spp., "Haemophilus influenzae ", most "Proteus" spp. (not "P. mirabilis"), "Plesiomonas shigelloides ", "Pasturella multocida", "Pasturella pneumotropica", and "Vibrio " spp.Indole-Negative Bacteria
Bacteria which give negative results for the indole test include: "Actinobacillus" spp., "Aeromonas salmonicida", "
Alcaligenes " spp., most "Bacillus " spp., "Bordtella" spp., "Enterobacter " spp., most "Haemophilus " spp., most "Klebsiella " spp., "Neisseria " spp., "Pasturella haemolytica", "Pasturella ureae", "Proteus mirabilis ", "Pseudomonas " spp., "Salmonella " spp., "Serratia " spp., "Yersinia " spp.References
* MacFaddin, Jean F. "Biochemical Tests for Indentification of Medical Bacteria." Williams & Wilkins, 1980, pp 173 - 183.
* [http://www.bact.wisc.edu/microtextbook/index.php?module=Book&func=displayarticle&art_id=119 Example of typical indole reactions]
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