IMViC Test 50 FAQs and 30 MCQs
The IMViC test is a cornerstone of microbial identification in clinical microbiology, differentiating Enterobacteriaceae through four biochemical assays: Indole, Methyl Red, Voges-Proskauer, and Citrate. This comprehensive guide breaks down each test’s principles, procedures, interpretations, and clinical relevance. With 50 FAQs clarifying concepts and 30 MCQs testing your knowledge, this resource is tailored for lab professionals and medical students preparing for exams or refining diagnostic skills.
The IMViC test (acronym for Indole, Methyl Red, Voges-Proskauer, and Citrate) is a biochemical battery used to distinguish between members of the Enterobacteriaceae family, notably Escherichia coli and Enterobacter spp. Its results are critical for identifying gram-negative pathogens in clinical samples like urine or stool.
Core Components of IMViC
- Indole Test: Detects tryptophan hydrolysis (positive = red layer with Kovac’s reagent).
- Methyl Red (MR): Confirms mixed-acid fermentation (positive = red at pH ≤4.2).
- Voges-Proskauer (VP): Identifies acetoin production (positive = red after Barritt’s reagents).
- Citrate Test: Assesses citrate utilization (positive = blue from bromothymol blue indicator).
Key Interpretation Patterns:
- E. coli: + + – – (Indole+, MR+, VP–, Citrate–)
- Enterobacter: – – + + (Indole–, MR–, VP+, Citrate+)
IMViC Test 50 FAQs
What does IMViC stand for?
It stands for Indole, Methyl Red, Voges-Proskauer, and Citrate tests.
Why is the lowercase “i” in IMViC?
It is added for ease of pronunciation and does not represent any test.
What is the purpose of the IMViC test?
To identify and differentiate bacteria, especially Enterobacteriaceae, based on metabolic properties.
Which bacteria family is primarily identified using IMViC?
Enterobacteriaceae (e.g., E. coli, Klebsiella, Proteus).
Can IMViC tests identify Gram-positive bacteria?
Yes, but they are mainly used for Gram-negative bacteria.
What are the four main tests in the IMViC series?
Indole, Methyl Red (MR), Voges-Proskauer (VP), and Citrate tests.
What additional tests can be included with IMViC?
Motility and H₂S production (using SIM medium).
How long does it take to get IMViC results?
24–48 hours (some VP tests may take longer).
What is the significance of IMViC in clinical labs?
Helps diagnose urinary tract infections (UTIs), gastroenteritis, and other bacterial infections.
What are the limitations of IMViC tests?
Cannot fully identify species, may give ambiguous results, and requires additional confirmatory tests.
What does the Indole test detect?
The ability of bacteria to produce indole from tryptophan using tryptophanase.
Which reagent is used for the Indole test?
Kovac’s reagent (for aerobes) or Ehrlich’s reagent (for anaerobes).
What indicates a positive Indole test?
A red/pink ring after adding Kovac’s reagent.
Which bacteria are Indole-positive?
E. coli, Proteus vulgaris, Klebsiella oxytoca, Vibrio cholerae.
Which bacteria are Indole-negative?
Klebsiella pneumoniae, Salmonella, Shigella, Pseudomonas aeruginosa.
What medium is used for the Indole test?
Tryptophan broth, SIM medium, or MIU medium.
How is the Indole test performed?
Inoculate bacteria in tryptophan broth, incubate, then add Kovac’s reagent.
What enzyme is responsible for indole production?
Tryptophanase.
Can SIM medium test for motility and H₂S along with indole?
Yes, SIM tests for sulfide production, indole, and motility.
What is the principle of the Indole test?
Tryptophan → Indole + Pyruvate + Ammonia → Indole reacts with aldehyde in reagent → Red color.
What does the MR test detect?
Stable acid production from glucose fermentation.
What is the indicator used in the MR test?
Methyl red (turns red at pH ≤ 4.4).
What does a red color in MR test mean?
Positive (mixed acid fermentation).
Which bacteria are MR-positive?
E. coli, Salmonella, Shigella, Proteus.
Which bacteria are MR-negative?
Klebsiella, Enterobacter, Serratia.
What medium is used for the MR test?
MR-VP broth.
How long is incubation for the MR test?
24–48 hours at 37°C.
What is the principle of the MR test?
Glucose → Mixed acids → Low pH → Methyl red turns red.
Can MR and VP tests be done in the same broth?
Yes, both use MR-VP broth but require different reagents.
What is the difference between MR and VP tests?
MR detects acid production, VP detects neutral end products (acetoin).
What does the VP test detect?
Acetoin (neutral end product) from glucose metabolism.
Which reagents are used in the VP test?
Barritt’s A (α-naphthol) and Barritt’s B (KOH).
What indicates a positive VP test?
A red/pink color after 30–60 minutes.
Which bacteria are VP-positive?
Klebsiella, Enterobacter, Serratia, Staphylococcus aureus.
Which bacteria are VP-negative?
E. coli, Salmonella, Shigella.
Why is the VP test important?
Helps differentiate E. coli (MR+ VP-) from Klebsiella (MR- VP+).
How long does the VP test take?
30–60 minutes after adding reagents.
What is the principle of the VP test?
Glucose → Acetoin → Diacetyl → Red complex with guanidine.
Can VP test be false-negative?
Yes, if incubation is too short or reagents are old.
Why must the VP tube be left uncapped?
The reaction requires oxygen for acetoin oxidation.
What does the Citrate test detect?
The ability to use citrate as the sole carbon source.
What medium is used for the Citrate test?
Simmons Citrate Agar (contains bromothymol blue).
What indicates a positive Citrate test?
Blue color (alkaline pH due to citrate breakdown).
Which bacteria are Citrate-positive?
Klebsiella, Enterobacter, Citrobacter, Serratia.
Which bacteria are Citrate-negative?
E. coli, Shigella, Salmonella Typhi.
What enzyme is involved in citrate utilization?
Citrase (breaks citrate into oxaloacetate and acetate).
Why does the medium turn blue in a positive test?
Due to alkaline byproducts (Na₂CO₃ & NH₄OH) raising pH.
How long is incubation for the Citrate test?
24–48 hours (some require up to 4 days).
Can Proteus vulgaris be Citrate-positive?
Rarely (11–25% positive), most are negative.
What is the principle of the Citrate test?
Citrate → Oxaloacetate + Acetate → CO₂ + Na₂CO₃ → Alkaline pH → Blue color.
IMViC Test 30 MCQs
- What does IMViC stand for?
a) Indole, Motility, Voges-Proskauer, Citrate
b) Indole, Methyl Red, Voges-Proskauer, Citrate ✔
c) Indole, MRVP, Voges, Citrate
d) Indole, MacConkey, VP, Citrate - The lowercase “i” in IMViC is used for:
a) Indicating an additional test
b) Ease of pronunciation ✔
c) Representing indole production
d) No specific reason - IMViC tests are primarily used to identify:
a) Gram-positive cocci
b) Gram-negative enteric bacteria (Enterobacteriaceae) ✔
c) Anaerobic bacteria
d) Acid-fast bacilli - Which of the following is NOT part of the IMViC series?
a) Indole test
b) Methyl Red test
c) Oxidase test ✔
d) Citrate test - The IMViC test helps differentiate between:
a) E. coli and Klebsiella ✔
b) Staphylococcus and Streptococcus
c) Mycobacterium and Pseudomonas
d) None of the above
- The Indole test detects the breakdown of which amino acid?
a) Lysine
b) Tryptophan ✔
c) Methionine
d) Cysteine - Which reagent is used for the Indole test?
a) Kovac’s reagent ✔
b) Methyl red
c) Barritt’s reagent
d) Bromothymol blue - A positive Indole test is indicated by:
a) Yellow color
b) Red/pink ring ✔
c) Blue color
d) Black precipitate - Which bacteria is Indole-positive?
a) Escherichia coli ✔
b) Klebsiella pneumoniae
c) Pseudomonas aeruginosa
d) Salmonella Typhi - Which medium is commonly used for the Indole test?
a) Blood agar
b) SIM agar ✔
c) MacConkey agar
d) Nutrient agar
- The MR test detects:
a) Mixed acid fermentation ✔
b) Butanediol fermentation
c) Citrate utilization
d) Indole production - A positive MR test shows which color?
a) Blue
b) Red ✔
c) Green
d) Yellow - Which bacteria is MR-positive?
a) E. coli ✔
b) Klebsiella pneumoniae
c) Enterobacter aerogenes
d) Serratia marcescens - The MR test uses which pH indicator?
a) Bromothymol blue
b) Methyl red ✔
c) Phenol red
d) Litmus - The MR test is performed in:
a) MR-VP broth ✔
b) SIM medium
c) Citrate agar
d) Tryptone broth
- The VP test detects the production of:
a) Acetoin ✔
b) Indole
c) Hydrogen sulfide
d) Citrate - Which reagents are used in the VP test?
a) Barritt’s A (α-naphthol) and Barritt’s B (KOH) ✔
b) Kovac’s reagent
c) Methyl red
d) Ferric chloride - A positive VP test shows:
a) Blue color
b) Red/pink color ✔
c) Yellow color
d) Black precipitate - Which bacteria is VP-positive?
a) Klebsiella pneumoniae ✔
b) E. coli
c) Salmonella Typhi
d) Shigella dysenteriae - The VP test requires the tube to be:
a) Left uncapped for oxygenation ✔
b) Sealed tightly
c) Covered with aluminum foil
d) None of the above
- The Citrate test detects the ability to use citrate as a:
a) Nitrogen source
b) Carbon source ✔
c) Energy source
d) Sulfur source - Which medium is used for the Citrate test?
a) Simmons Citrate Agar ✔
b) MR-VP broth
c) SIM medium
d) TSI agar - A positive Citrate test shows:
a) Red color
b) Blue color ✔
c) Yellow color
d) Green color - Which bacteria is Citrate-positive?
a) Klebsiella pneumoniae ✔
b) E. coli
c) Shigella flexneri
d) Salmonella Typhi - The pH indicator in Simmons Citrate Agar is:
a) Bromothymol blue ✔
b) Methyl red
c) Phenol red
d) Neutral red
- Which IMViC result is typical for E. coli?
a) + + – – ✔
b) – – + +
c) + – + –
d) – + – + - Which IMViC result is typical for Klebsiella pneumoniae?
a) + + – –
b) – – + + ✔
c) + – + –
d) – + – + - A bacterium is MR+ and VP–. It is most likely:
a) E. coli ✔
b) Klebsiella
c) Enterobacter
d) Serratia - A bacterium is Citrate+ and Indole–. It is most likely:
a) Klebsiella pneumoniae ✔
b) E. coli
c) Proteus vulgaris
d) Shigella - Which test helps differentiate E. coli from Enterobacter?
a) VP test ✔
b) Oxidase test
c) Catalase test
d) Urease test
Preparing for Exams & Clinical Practice
- For Students: Use MCQs to self-test. Focus on memorizing patterns (e.g., Klebsiella: – – + +).
- For Lab Professionals: FAQs address real-world issues (e.g., handling equivocal results, QC strains).
Key Takeaways
- IMViC remains vital despite automation due to its cost-effectiveness.
- Always correlate results with colony morphology, gram stain, and patient history.
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