VRBL Agar (Violet Red Bile Lactose Agar) 50 FAQs and 30 MCQs
Master VRBL Agar—the selective medium for detecting coliforms and lactose-fermenting bacteria—with our comprehensive guide! This resource includes:
🔍 50 In-Depth FAQs Covering:
✓ Principle: How VRBL selects for Gram-negative bacteria and differentiates lactose fermenters
✓ Composition: Role of bile salts, crystal violet, and neutral red
✓ Protocol: Step-by-step preparation, inoculation, and incubation
✓ Interpretation: Colony morphology (pink-purple colonies with bile precipitation)
✓ Applications: Water testing, food microbiology (FDA/BAM standards)
✓ Troubleshooting: Overgrowth, weak coloration, false positives/negatives
📝 30 Practice MCQs On:
◼️ Selective agents (bile salts vs. crystal violet)
◼️ Typical colony appearance (E. coli vs. Klebsiella)
◼️ Compliance with ISO 4832 and FDA BAM
◼️ Comparison to MacConkey and EMB agar
🎯 Perfect For:
- Food microbiologists
- Water quality labs
- MLT/MLS students prepping for certification exams
VRBL Agar (Violet Red Bile Lactose Agar) 50 FAQs
What is VRBL Agar used for?
It is a selective medium for isolating, detecting, and enumerating lactose-fermenting bacteria (coliforms) in water, milk, dairy products, and food.
What does VRBL stand for?
Violet Red Bile Lactose Agar.
Is VRBL Agar selective or differential?
It is both selective (inhibits Gram-positive bacteria) and differential (differentiates lactose fermenters from non-fermenters).
What is the principle of VRBL Agar?
It uses bile salts and crystal violet to inhibit Gram-positive bacteria, while lactose fermentation is detected via neutral red (pH indicator).
What is the pH of VRBL Agar?
7.4 ± 0.2 at 25°C.
What are the main components of VRBL Agar?
Peptone, yeast extract, lactose, bile salts, crystal violet, neutral red, sodium chloride, and agar.
Why are bile salts added to VRBL Agar?
To inhibit Gram-positive bacteria.
What is the role of crystal violet in VRBL Agar?
It suppresses Gram-positive bacteria.
Why is lactose included in VRBL Agar?
To differentiate lactose fermenters (coliforms) from non-fermenters.
What is the purpose of neutral red in VRBL Agar?
It acts as a pH indicator, turning red-purple when lactose is fermented (acid production).
Can VRBL Agar be autoclaved?
No, it should not be autoclaved; it must be boiled and cooled to 45°C.
How should VRBL Agar be stored?
Prepared media: 2-8°C (protected from light).
Dehydrated powder: 15-25°C in a dry, sealed container.What is the shelf life of VRBL Agar?
Check the expiration date on the package; discard if clumpy or discolored.
How is VRBL Agar prepared?
Suspend 38.5–40.5 g in 1L distilled water, boil to dissolve, cool to 45°C, and pour into plates.
How long can prepared VRBL Agar be used?
Up to 4 hours after preparation.
What bacteria grow on VRBL Agar?
Coliforms (E. coli, Enterobacter, Klebsiella, Citrobacter) and some other lactose fermenters.
What do lactose-fermenting colonies look like on VRBL Agar?
Purple/red-purple colonies with a halo (acid production).
What do non-lactose fermenters look like on VRBL Agar?
Pale colonies with greenish zones.
Can Pseudomonas aeruginosa grow on VRBL Agar?
Yes, but it forms beige-pink colonies (non-lactose fermenter).
Does Staphylococcus aureus grow on VRBL Agar?
No, it is inhibited by bile salts and crystal violet.
Does Enterococcus faecalis grow on VRBL Agar?
No, it is inhibited.
What temperature is used for incubating VRBL Agar?
30°C, 37°C, or 44°C (depending on target organisms).
How long is VRBL Agar incubated?
Usually 18–24 hours (up to 48 hours for some organisms).
What is the purpose of incubating at 44°C?
To select for thermotolerant coliforms (E. coli).
Can VRBL Agar detect fecal coliforms?
Yes, by incubating at 44°C.
What is the pour plate method for VRBL Agar?
Mix sample with melted VRBL Agar (47°C) and pour into plates.
What is the spread plate method for VRBL Agar?
Spread sample on solidified VRBL Agar.
How are coliform colonies counted on VRBL Agar?
Count purple/red colonies (≥0.5 mm) with halos.
What is the acceptable colony count range on VRBL Agar?
10–150 colonies per plate for accurate enumeration.
What confirms a positive coliform result on VRBL Agar?
Further testing in Brilliant Green Lactose Bile Broth (BGLB).
What samples can VRBL Agar test?
Water, milk, dairy products, food, and environmental swabs.
Is VRBL Agar used for clinical diagnosis?
No, it is for food/water testing, not human diagnosis.
What are the limitations of VRBL Agar?
Some stressed coliforms may not grow; Fecal Coliform Agar (FCA) may be better.
Can VRBL Agar detect all coliforms?
No, some strains may grow poorly or not at all.
What QC organisms are used for VRBL Agar?
E. coli (growth), Enterococcus faecalis (inhibition), Pseudomonas aeruginosa (non-fermenter).
What precautions should be taken when using VRBL Agar?
Wear gloves/lab coat, avoid contamination, and dispose of waste properly.
How should used VRBL plates be discarded?
Follow local biohazard waste regulations.
Is VRBL Agar similar to MacConkey Agar?
Yes, but VRBL is more selective (higher bile salts & crystal violet).
What standards approve VRBL Agar?
ISO 4832, APHA, and EN ISO 11133.
Can VRBL Agar be used for RODAC plates?
Yes, for surface hygiene testing.
Why are there no colonies on VRBL Agar?
Sample may lack coliforms, or medium was overheated.
Why are colonies too small to count?
Incubation time may be insufficient or dilution too high.
Why is there excessive growth of non-target bacteria?
Medium may have been improperly prepared or stored.
Why is the agar color fading?
Exposure to light or expired medium.
Where can I buy VRBL Agar?
From microbiological suppliers (e.g., ready-to-use plates, tubes, or powder).
What are common packaging sizes for VRBL Agar?
500 g powder, 100 ml bottles, 20–100 plates.
Can VRBL Agar detect Salmonella?
Some Salmonella (e.g., S. arizonae) may grow, but it’s not the primary target.
Does VRBL Agar support anaerobic growth?
Yes, anaerobic incubation can improve selectivity.
What is the difference between VRBL and VRB (Violet Red Bile) Agar?
VRBL contains lactose, while VRB may not (used for different tests).
Can VRBL Agar be used for yeast and mold testing?
No, it is specific for bacteria (coliforms).
VRBL Agar (Violet Red Bile Lactose Agar) 30 MCQs
- What is the primary purpose of VRBL Agar?
a) Isolate fungi
b) Detect lactose-fermenting bacteria (coliforms
c) Culture viruses
d) Identify Gram-positive cocci - VRBL Agar is both selective and differential. What makes it selective?
a) Lactose fermentation
b) Bile salts and crystal violet
c) Neutral red indicator
d) Peptone content - Which pH indicator is used in VRBL Agar?
a) Phenol red
b) Bromothymol blue
c) Neutral red
d) Methyl red - What is the final pH of VRBL Agar?
a) 6.2 ± 0.2
b) 7.4 ± 0.2
c) 8.0 ± 0.2
d) 5.4 ± 0.2 - Which sugar is fermented by coliforms in VRBL Agar?
a) Glucose
b) Sucrose
c) Lactose
d) Mannitol
- Which component inhibits Gram-positive bacteria in VRBL Agar?
a) Yeast extract
b) Bile salts
c) Sodium chloride
d) Agar - Why should VRBL Agar NOT be autoclaved?
a) It becomes too solid
b) Heat degrades bile salts and crystal violet
c) It changes color
d) It loses lactose - How should prepared VRBL Agar be stored?
a) Frozen at -20°C
b) At 2-8°C, protected from light
c) At room temperature
d) In direct sunlight - What is the correct amount of VRBL Agar powder for 1L of water?
a) 20.5 g
b) 38.5–40.5 g
c) 50.0 g
d) 10.0 g - How long can prepared VRBL Agar be used after preparation?
a) 24 hours
b) 1 week
c) 4 hours
d) 30 minutes
- What color are lactose-fermenting colonies on VRBL Agar?
a) Blue
b) Purple/red with a halo
c) Yellow
d) Green - Which bacterium grows as pink-purple colonies with a halo on VRBL Agar?
a) Escherichia coli
b) Staphylococcus aureus
c) Pseudomonas aeruginosa
d) Enterococcus faecalis - Which organism is inhibited on VRBL Agar?
a) Klebsiella pneumoniae
b) Enterobacter aerogenes
c) Staphylococcus aureus
d) Citrobacter freundii - What do non-lactose fermenters look like on VRBL Agar?
a) Purple colonies
b) Pale colonies with greenish zones
c) Black colonies
d) Shiny metallic colonies - At what temperature should VRBL Agar be incubated for thermotolerant coliforms?
a) 25°C
b) 37°C
c) 44°C
d) 55°C
- VRBL Agar is commonly used to test:
a) Blood samples
b) Milk and dairy products
c) Soil samples
d) Viral cultures - Which standard method uses VRBL Agar for coliform enumeration?
a) ISO 4832
b) USP <61>
c) CLSI M100
d) FDA BAM - What is a limitation of VRBL Agar?
a) Supports fungal growth
b) May not recover stressed coliforms
c) Turns black with E. coli
d) Requires anaerobic incubation - Which alternative medium can be used for stressed coliforms?
a) MacConkey Agar
b) Fecal Coliform Agar (FCA)
c) Blood Agar
d) Sabouraud Dextrose Agar - VRBL Agar is NOT suitable for detecting:
a) Escherichia coli
b) Yeasts and molds
c) Enterobacter species
d) Klebsiella species
- What is the role of crystal violet in VRBL Agar?
a) Nutrient source
b) Inhibits Gram-positive bacteria
c) Enhances lactose fermentation
d) Acts as a pH indicator - Which organism produces beige-pink colonies on VRBL Agar?
a) Escherichia coli
b) Pseudomonas aeruginosa
c) Enterococcus faecalis
d) Salmonella Typhimurium - How does bile salt selectivity work in VRBL Agar?
a) Promotes Gram-positive growth
b) Inhibits Gram-negative bacteria
c) Disrupts Gram-positive cell membranes
d) Enhances sporulation - What confirms a true coliform in VRBL Agar testing?
a) Gas production in BGLB broth
b) Hemolysis on blood agar
c) Catalase test
d) Oxidase test - Why is incubation at 44°C used in VRBL testing?
a) To kill all bacteria
b) To select for thermotolerant coliforms (E. coli)
c) To enhance fungal growth
d) To speed up Gram-positive growth
- If VRBL Agar turns yellow after preparation, what might be wrong?
a) Contamination
b) Overheating during prep
c) Incorrect pH
d) All of the above - What should be done if VRBL powder appears clumpy?
a) Autoclave it
b) Discard it (likely moisture exposure)
c) Use it normally
d) Freeze it - Which safety precaution is necessary when handling VRBL Agar?
a) Wear gloves and lab coat
b) Use in open air
c) Store in sunlight
d) No precautions needed - How should used VRBL plates be disposed of?
a) Regular trash
b) Biohazard waste (autoclave/incineration)
c) Down the sink
d) Freezing - What does poor growth on VRBL Agar indicate?
a) Too much lactose
b) Old or improperly prepared medium
c) Incubation at wrong temperature
d) Both b and c
- “VRBL agar preparation for coliforms”
- “How to interpret pink colonies on VRBL agar”
- “VRBL vs. MacConkey for E. coli detection”
- “FDA BAM method for dairy testing”
