Detailed Study Notes on IMViC Tests

IMViC Battery of Tests

• IMViC refers to a series of biochemical tests used to differentiate among members of the Enterobacteriaceae family.

• The tests involved are:

  • Indole production (Tryptone broth)

  • Methyl Red and Voges Proskauer (MRVP) broths

  • Citrate slant (citrate test)

• These tests are useful particularly for differentiating Gram-negative short rod enteric lactose fermenters.

Members of Enterobacteriaceae

• Characteristics:

  • Gram-negative

  • Short rods

  • Examples include:

  • E. coli

  • Enterobacter

  • Proteus

• Fermentation:

  • Generally, these bacteria ferment glucose.

  • Not all members of the family ferment lactose.

• Metabolic traits:

  • Facultative anaerobes: can grow in both oxygen-rich and oxygen-poor environments.

  • They can reduce nitrate and produce catalase.

Indole Production via Tryptone Broths

• Indole Test Procedure:

  • Reaction: Tryptophan + Water → (tryptophanase) → Pyruvic Acid + Indole

  • After inoculating the tryptone broth, shake it, and add Kovac’s reagent to the stab. Do not shake after adding the reagent.

• Observation:

  • Look for a red layer on top of the broth:

  • If red appears, it indicates that Kovac’s reagent is binding to indole, confirming the presence of tryptophanase.

  • Testing Serratia may yield a different result.

• Conclusion:

  • Both E. coli and Proteus are positive for indole production.

  • Serratia is negative; the red color observed after the addition of Kovac’s may be due to pigments dissolved by Kovac’s reagent.

MRVP - Methyl Red vs Voges Proskauer

• Rationale for 2 Broths:

  • Allows for either inoculation of one broth split into two for testing different things or inoculation of two broths simultaneously for convenience.

• Composition:

  • The MRVP broth consists of buffered peptone, glucose (with protein), minus phenol red, and plus a phosphate buffer.

• The primary purpose is to assess the microbe’s ability to ferment glucose through two different pathways:

  • Mixed acid fermentation pathway

  • 2,3-butanediol fermentation pathway

• Microbes typically select one pathway for glucose fermentation and not both.

Methyl Red Test (MR)

• Purpose:

  • To investigate the microbe's ability to produce mixed acids via the mixed acid fermentation pathway.

• Procedure:

  • Grow MRVP broth for at least 72 hours.

  • Shake the broth thoroughly before adding methyl red reagent (15 drops).

• Indicator Characteristics:

  • Methyl Red: a pH indicator.

  • If the pH is approximately 4 or lower, it turns red;

  • A pH above 6 turns yellow;

  • Between 4 and 6 gives an orange color (neither positive nor negative).

• Results Interpretation:

  • Red indicates a positive result for mixed acids.

  • E. coli and Proteus both test positive for mixed acids.

Voges Proskauer Test (VP)

• Overview:

  • The Voges Proskauer test is designed to identify bacteria that produce neutral end products from glucose fermentation.

• Focus:

  • The primary product of interest is acetoin (not to be confused with 2,3-butanediol, which comes after).

• Procedure:

  • Add Alpha-naphthol and KOH in a 3:1 ratio to the culture tubes.

• Results Interpretation:

  • A red color indicates a positive reaction for acetoin production.

  • Enterobacter and Serratia should both test positive for the VP test since they are negative for the MR test.

Citrate Utilization Test

• Purpose:

  • To assess the microbe’s ability to utilize citrate as its sole carbon source.

• Color Indicators:

  • Positive result is indicated by a blue color.

  • Negative result is signified by a green color.

• Mechanism:

  • If the microbe can utilize citrate, it must also use nitrogen in the form of ammonium salts, cleaving it and producing ammonia.

• pH Indicator:

  • Bromthymol blue: turns green at pH levels below 7 and becomes Prussian blue when pH exceeds 7.