biology

Indole test

Indole test is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Indole test rather than just read about it. In short: The indole test is a biochemical test performed on bacterial species to determine the ability of the organism to convert tryptophan into indole. This division is performed by a chain of a number of different intracellular enzymes, a system generally referred to as "tryptophanase." Biochemistry Indole is generated by reductive deamination from tryptophan via the intermediate molecule indolepyruvic acid.

Key takeaways

Indole test belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Indole test to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Indole test from memory before moving on to harder problems.

Reference excerpt

The indole test is a biochemical test performed on bacterial species to determine the ability of the organism to convert tryptophan into indole. This division is performed by a chain of a number of different intracellular enzymes, 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 the amine (-NH2) group of the tryptophan molecule is removed. Final products of the reaction are indole, pyruvic acid, ammonium (NH4+) and energy. Pyridoxal phosphate is required as a coenzyme.

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, five drops of Kovács reagent (isoamyl alcohol, para-Dimethylaminobenzaldehyde, concentrated hydrochloric acid) are added to the culture broth. A positive result is shown by the presence of a red or reddish-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. The positive red color forms as a result of a series of reactions. The para-Dimethylaminobenzaldehyde reacts with indole present in the medium to form a red rosindole dye. The isoamyl alcohol forms a complex with rosindole dye, which causes it to precipitate. The remaining alcohol and the precipitate then rise to the surface of the medium. A variation on this test using Ehrlich's reagent (using ethyl alcohol in place of isoamyl alcohol, developed by Paul Ehrlich) is used when performing the test on nonfermenters and anaerobes.

Indole-positive bacteria Bacteria that test positive for cleaving indole from tryptophan include: Aeromonas hydrophila, Aeromonas punctata, Bacillus alvei, Edwardsiella sp., Escherichia coli, Flavobacterium sp., Haemophilus influenzae, Klebsiella oxytoca, Proteus sp. (not P. mirabilis and P. penneri), Plesiomonas shigelloides, Pasteurella multocida, Pasteurella pneumotropica, Vibrio sp., and Lactobacillus reuteri.

Indole-negative bacteria Bacteria which give negative results for the indole test include: Actinobacillus spp., Aeromonas salmonicida, Alcaligenes sp., most Bacillus sp., Bordetella sp., Enterobacter sp., most Haemophilus sp., most Klebsiella sp., Neisseria sp., Mannheimia haemolytica, Pasteurella ureae, Proteus mirabilis, P. penneri, Pseudomonas sp., Salmonella sp., Serratia sp., Yersinia sp., and Rhizobium sp. The Indole test is one of the four tests of the IMViC series, which tests for evidence of an enteric bacterium. The other three tests include: the methyl red test [M], the Voges–Proskauer test [V] and the citrate test [C].

References

MacFaddin, Jean F. "Biochemical Tests for Identification of Medical Bacteria." Williams & Wilkins, 1980, pp 173 – 183. Example of typical indole reactions Angen, O; Mutters, R; Caugant, DA; Olsen, JE; Bisgaard, M (1999). "Taxonomic relationships of the [Pasteurella] haemolytica complex as evaluated by DNA-DNA hybridizations and 16S rRNA sequencing with proposal of Mannheimia haemolytica gen. nov., comb. nov., Mannheimia granulomatis comb. nov., Mannheimia glucosida sp. nov., Mannheimia ruminalis sp. nov. and Mannheimia varigena sp. nov". Int J Syst Bacteriol. 49 Pt 1: 67–86. doi:10.1099/00207713-49-1-67. PMID 10028248.

Illustrations

Worked examples

Example 1 — a first encounter with Indole test

Start with the simplest possible case. Write down what Indole test claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Indole test before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Indole test ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Indole test

In research: Indole test appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Indole test in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Indole test is common in secondary-school and first-year university syllabi. It links to neighbouring topics Bacteriology terminology, Microbiology techniques, so understanding it makes those chapters shorter.
In everyday life: Look for Indole test outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Indole test in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Indole test means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Indole test out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Indole test in simple terms?

Why does Indole test matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Indole test?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Indole test.