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Indole production

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indole production test microbiology tryptophanase bacteria

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https://microbenotes.com/indole-test-objective-principle-med…

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Here is a complete overview of Indole Production as a microbiological concept:

Indole Production Test

Definition

The indole production test is a biochemical test used to detect the ability of certain bacteria to split the amino acid tryptophan into indole, pyruvate, and ammonia, using a group of enzymes collectively called tryptophanase.
It is part of the classic IMViC battery of tests (Indole, Methyl Red, Voges-Proskauer, Citrate), used to differentiate members of the family Enterobacteriaceae.

Biochemical Reaction

Tryptophan is broken down in a multi-step process:
  1. Tryptophanase cleaves tryptophan at the indole ring.
  2. Products: Indole + Pyruvate + NH3
L-Tryptophan  →(tryptophanase)→  Indole + Pyruvic acid + NH3
Indole accumulates in the growth medium and is then detected by a reagent.

Principle of Detection

Two main reagents are used to detect indole:
ReagentCompositionUse
Kovac's reagentp-dimethylaminobenzaldehyde in isoamyl alcohol + HClStandard aerobes
Ehrlich's reagentp-dimethylaminobenzaldehyde in ethyl alcohol + HClNon-fermenters and anaerobes
Indole reacts with p-dimethylaminobenzaldehyde (the active component in both reagents) to form a cherry-red/pink-red colored compound (rosindole dye) at the interface of the reagent layer and broth - this is a positive result.
A yellow or orange ring (or no color change) = negative result.

Media Used

  • Tryptone broth (1% tryptone) - most common; rich in tryptophan
  • Blood agar plate (BAP) or Chocolate agar (CHOC) - for rapid testing from primary culture
  • SIM medium (Sulfide-Indole-Motility) - can simultaneously test for H2S, indole, and motility

Procedure

  1. Inoculate tryptone broth with the test organism.
  2. Incubate at 35-37°C for 24-48 hours (up to 7 days for slow growers).
  3. Add 5 drops of Kovac's reagent to the culture.
  4. Do NOT shake after adding reagent.
  5. Observe for color change at the reagent-broth interface within 1-2 minutes.

Interpretation

ResultColorMeaning
PositiveCherry-red / pink-red ringIndole produced; tryptophanase present
NegativeYellow / orange / no changeNo indole; tryptophanase absent

Organisms and Their Results

Indole Positive (+)

  • Escherichia coli (most strains - consistently +)
  • Proteus vulgaris
  • Morganella morganii
  • Klebsiella oxytoca
  • Citrobacter koseri (C. diversus)
  • Providencia rettgeri
  • Most Plesiomonas spp.
  • Many anaerobes (e.g., Bacteroides fragilis)

Indole Negative (-)

  • Klebsiella pneumoniae
  • Enterobacter spp.
  • Proteus mirabilis (key distinguishing feature from P. vulgaris)
  • Citrobacter freundii
  • Salmonella spp.
  • Shigella spp.
  • Serratia spp.

Clinical and Diagnostic Significance

  • Primary use: Differentiating E. coli (+) from Enterobacter (-) and Klebsiella pneumoniae (-) - all lactose fermenters.
  • Proteus differentiation: P. vulgaris (+) vs. P. mirabilis (-).
  • Klebsiella differentiation: K. oxytoca (+) vs. K. pneumoniae (-).
  • Citrobacter differentiation: C. koseri (+) vs. C. freundii (-).
  • Part of the IMViC pattern for coliforms:
    • E. coli = + + - - (I, MR, VP, Citrate)
    • Enterobacter aerogenes = - - + +

Types of Indole Tests

  1. Tube indole test - Standard method, most sensitive; uses xylene extraction for enhanced sensitivity.
  2. Spot/rapid indole test - Fast (minutes), from colony on filter paper or directly from plate; used in labs for rapid screening.
  3. Xylene extraction method - Most sensitive; xylene concentrates indole before adding Kovac's reagent.

Gut Microbiome Context

Beyond the diagnostic test, bacteria in the intestinal microbiome that express tryptophanase (such as E. coli) produce indole from dietary tryptophan in the gut. This intestinal indole is then absorbed, converted to indoxyl in the liver, and subsequently sulfated to indoxyl sulfate - a known uremic toxin that accumulates in chronic kidney disease (CKD) and contributes to kidney and cardiovascular damage (Brenner and Rector's The Kidney).

Limitations

  • A negative rapid (spot) test does not rule out positivity on the full tube test.
  • False negatives can occur if the medium lacks sufficient tryptophan.
  • Xylene substitutes (used in some labs) are less sensitive than xylene itself.
  • For fastidious organisms (e.g., Capnocytophaga hominis), a heavy inoculum is needed.
  • Some organisms may require longer incubation (72 hours or more).
Sources: Jawetz, Melnick & Adelberg's Medical Microbiology, 28th Ed.; Microbe Notes - Indole Test; Brenner and Rector's The Kidney.
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