IMVIC Tests and Microbial Biochemistry

IMViC Testing Overview

  • IMViC tests comprise four specific biochemical tests: Indole, Methyl Red (MR), Voges-Proskauer (VP), and Citrate.
  • Primarily used to differentiate members of the Enterobacteriaceae family from other Gram-negative rods.
  • Enterics characteristics:
    • Gram-negative
    • Bacilli
    • Facultative anaerobes
    • Glucose fermenters
  • Includes:
    • Normal flora (e.g., Escherichia coli, Proteus, Klebsiella, Enterobacter)
    • Pathogens (e.g., Salmonella, Shigella)
    • Coliforms (e.g., Escherichia coli, Enterobacter aerogenes) are lactose fermenters.

1. SIM Test: Sulfur, Indole, Motility

Sulfur Reduction

  • Medium: SIM containing thiosulfate and iron.
  • Two enzyme systems:
    • Cysteine desulfurase:
    • Hydrolyzes cysteine to produce pyruvate, ammonia, and H2S.
    • H2S reacts with iron forming black precipitate (ferric sulfide).
    • Thiosulfate reductase:
    • Reduces sulfate to H2S in anaerobic respiration.
  • Both systems generate H2S gas leading to black precipitate.

Indole Production

  • Tryptophan hydrolyzation by tryptophanase in SIM medium produces pyruvate, ammonia, and indole.
  • Post-incubation: add 3 drops of Kovac’s reagent:
    • Positive: Red ring signifies indole presence.

Motility

  • Lower agar concentration (0.4% vs. 1.5%) in SIM allows for bacterial movement.
  • Inoculation method: stabbing with a needle.
  • Observations:
    • Motile: Fuzzy growth diffusing from stab line.
    • Non-motile: No diffusion, clear tube except along the stab line.

2. TTC Motility Agar

  • Semisolid medium with 0.4% agar containing triphenyltetrazolium chloride (TTC).
  • TTC as an electron acceptor:
    • Oxidized (colorless, soluble) vs. Reduced (red precipitate).
  • Inoculation:
    • Motile: Red growth radiating from stab line.
    • Non-motile: Red growth not diffusing from stab line.

3. Methyl Red (MR) Test

  • Medium: MR-VP which contains glucose and potassium phosphate.
  • Pacifies between acidic fermentation products and neutral products:
    • Acidic: Mixed acid fermentation (e.g., lactic acid, acetic acid).
    • Test organisms: MR positive include Escherichia coli, Shigella sp., Salmonella sp.

Procedure for MR Test

  • Add 5 drops of Methyl Red pH indicator:
    • Red: pH < 5 (Positive: Acid produced).
    • Yellow: pH > 6 (Negative: No acid).

4. Voges-Proskauer (VP) Test

  • Introduces means to identify organisms with neutral products from glucose fermentation (e.g., 2,3-butanediol, acetoin).
  • Distinguishes VP positive organisms (e.g., Enterobacter sp., Serratia sp., Klebsiella aerogenes) from negative ones (e.g., Escherichia coli).

Procedure for VP Test

  • Add reagents A & B; reactions lead to:
    • Red color: Indicates presence of acetoin (Positive).
    • Yellow color: Indicates absence (Negative).

5. Citrate Utilization Test

  • Media is specifically designed to differentiate organisms based on their ability to use citrate as the sole carbon source.
  • Example media: Simmons Citrate.
  • The presence of citrate permease allows for citrate transport into the cell, facilitating citrate fermentation.
  • Ammonium ion (NH4+) as the nitrogen source enables growth that increases pH.

Growth Indication

  • Bromothymol blue pH indicator used:
    • Green: pH 7 (Neutral, no growth).
    • Blue: pH > 7.6 (Alkaline, positive growth indicating citrate metabolism).

Summary of IMViC Tests

  • Importance of distinguishing Enterobacteriaceae members.
  • Application in clinical diagnostics for identifying pathogens and commensals.