How to Perform a Methyl Red and Voges-Proskauer (MR-VP) Test

The Methyl Red (MR) and Voges-Proskauer (VP) tests are complementary biochemical assays performed sequentially from a single MR-VP broth culture to determine the metabolic pathway used by bacteria for glucose fermentation. The MR test detects the production of stable acid end products (mixed acid fermentation), while the VP test detects the production of acetoin (2,3-butanediol fermentation pathway). These tests are essential components of the IMViC series (Indole, Methyl Red, Voges-Proskauer, Citrate) used for differentiating members of the Enterobacteriaceae family, particularly distinguishing Escherichia coli (MR-positive, VP-negative) from Enterobacter aerogenes and Klebsiella pneumoniae (MR-negative, VP-positive) [2][3][5]. The MR-VP test is routinely performed in teaching laboratories, food microbiology, water quality testing, and veterinary diagnostics under Biosafety Level 1 (BSL-1) conditions using non-pathogenic or low-risk bacterial isolates [6].

At a Glance

Scientific Principle

The MR and VP tests exploit differences in bacterial glucose fermentation pathways. When bacteria ferment glucose, they produce various organic acid end products. The specific pathway used determines the types and amounts of acids or neutral compounds generated.

Mixed Acid Fermentation (MR-Positive)

Organisms such as Escherichia coli and Salmonella spp. use the mixed acid fermentation pathway, producing large quantities of stable organic acids including lactic acid, acetic acid, succinic acid, and formic acid [5]. These acids lower the culture medium pH to approximately 4.0-4.4. The methyl red indicator has a pH range of 4.4 (red) to 6.0 (yellow). When added to a culture with pH ≤ 4.4, the indicator remains red, indicating a positive MR test. The stability of these acid end products is critical—they persist even after extended incubation, which is why the standard 48-hour incubation is recommended to ensure sufficient acid accumulation.

2,3-Butanediol Fermentation (VP-Positive)

Organisms such as Enterobacter aerogenes, Klebsiella pneumoniae, and Serratia marcescens use the 2,3-butanediol pathway. These bacteria produce less acid (pH approximately 5.5-6.0) and generate neutral end products including acetoin (acetylmethylcarbinol), which is an intermediate in 2,3-butanediol synthesis. The VP test detects acetoin through a colorimetric reaction. In the presence of alpha-naphthol (VP reagent A) and potassium hydroxide (VP reagent B), acetoin is oxidized to diacetyl, which reacts with guanidine-containing compounds in the peptone to produce a pink-red complex [4]. This reaction requires oxygen exposure, which is why the tube is shaken vigorously after reagent addition and allowed to stand for 15-30 minutes.

Why These Tests Are Complementary

A single bacterial species will typically be positive for only one of these two tests. The MR and VP tests are inversely correlated for most Enterobacteriaceae: organisms that produce large amounts of acid (MR-positive) do not produce acetoin (VP-negative), and vice versa. However, some organisms may show weak or variable reactions, and a few species (e.g., Vibrio cholerae) can be positive for both tests under specific conditions.

Materials and Instrumentation

MR-VP Broth (Clark and Lubs Medium)

The standard medium contains:

• Peptone: 7.0 g/L (provides nitrogen and carbon sources)
• Dipotassium phosphate (K₂HPO₄): 5.0 g/L (buffering agent)
• Dextrose (glucose): 5.0 g/L (fermentable carbohydrate)
• Distilled water: 1.0 L
• Final pH: 7.0 ± 0.2

The phosphate buffer concentration is critical. Too little buffer allows false-positive MR results from weak acid producers, while too much buffer may mask true acid production. Commercial dehydrated MR-VP broth is available from multiple manufacturers (e.g., BD Difco, Oxoid, HiMedia) and should be prepared according to manufacturer instructions. The medium is dispensed in 5-10 mL volumes into 16 × 150 mm test tubes and sterilized by autoclaving at 121°C for 15 minutes.

Methyl Red Indicator Solution

Prepare 0.1% (w/v) methyl red in 95% ethanol. Dissolve 0.1 g methyl red powder in 100 mL 95% ethanol. Filter sterilize if necessary. Store in a dark glass bottle at 2-8°C. The solution is stable for approximately 6 months. Discard if precipitation or color change occurs.

VP Reagents

VP Reagent A (5% alpha-naphthol): Dissolve 5 g alpha-naphthol in 100 mL absolute ethanol. Prepare fresh or store in a tightly sealed amber bottle at 2-8°C for no more than 2 weeks. Alpha-naphthol is light-sensitive and oxidizes upon exposure to air, leading to false-negative results.

VP Reagent B (40% KOH): Dissolve 40 g potassium hydroxide pellets in 100 mL distilled water. Cool the solution during preparation as heat is generated. Store in a plastic bottle at room temperature. Do not use glass stoppers as KOH etches glass.

Required Equipment

• Inoculating loop or sterile swab
• Bunsen burner or BSL-1 biosafety cabinet
• Incubator set to 35-37°C
• Test tube rack
• Timer
• pH meter or pH test strips (for quality control)
• Vortex mixer (optional, for VP test)
• Personal protective equipment (lab coat, gloves, safety glasses)

Quality Control Strains

• MR-positive, VP-negative: Escherichia coli (ATCC 25922 or equivalent)
• MR-negative, VP-positive: Enterobacter aerogenes (ATCC 13048) or Klebsiella pneumoniae (ATCC 13883)
• MR-negative, VP-negative: Pseudomonas aeruginosa (ATCC 27853) – non-fermenter control

Workflow and Procedure

Step 1: Inoculation of MR-VP Broth

1. Label two MR-VP broth tubes for each test organism: one for the MR test and one for the VP test. Alternatively, a single tube can be used for both tests if sufficient volume (at least 5 mL) is available, as the MR test requires only 1-2 mL and the VP test requires 1-2 mL.
2. Using a sterile inoculating loop, pick a single, well-isolated colony from an 18-24 hour pure culture on non-selective agar (e.g., nutrient agar, tryptic soy agar).
3. Inoculate the broth by emulsifying the colony in the medium. Avoid introducing agar fragments.
4. Incubate tubes at 35-37°C for 48 hours. A 24-hour incubation may be acceptable for rapidly growing organisms, but 48 hours is standard to ensure complete glucose utilization and stable end product accumulation [2][3].

Step 2: Methyl Red Test

1. After 48 hours incubation, remove the MR-VP broth tube from the incubator.
2. Aseptically transfer 1-2 mL of the broth culture to a clean, sterile test tube.
3. Add 2-3 drops of methyl red indicator solution directly to the transferred broth.
4. Gently swirl the tube to mix.
5. Read the result immediately. The color is stable for several minutes but should be read promptly.

Interpretation:

• Positive (MR+): Immediate red color development at the point of reagent addition and throughout the medium. The red color indicates pH ≤ 4.4.
• Negative (MR-): Yellow color indicates pH ≥ 6.0. Orange color is considered negative but may indicate weak acid production (pH approximately 5.0-5.5).
• Weak positive: Red-orange color may occur with some organisms; repeat with fresh culture and confirm with pH measurement.

Step 3: Voges-Proskauer Test

1. From the same 48-hour MR-VP broth culture, aseptically transfer 1-2 mL to a clean, sterile test tube.
2. Add 0.6 mL (approximately 12 drops) of VP reagent A (5% alpha-naphthol).
3. Add 0.2 mL (approximately 4 drops) of VP reagent B (40% KOH).
4. Cap the tube and shake vigorously for 30 seconds to aerate the mixture. Oxygen is essential for the oxidation of acetoin to diacetyl.
5. Allow the tube to stand at room temperature for 15-30 minutes. Do not incubate at 37°C, as elevated temperature may accelerate non-specific color development.
6. Read the result at 15 minutes and again at 30 minutes if negative at 15 minutes.

Interpretation:

• Positive (VP+): Development of a pink-red to rose color, typically appearing first at the top of the liquid (where oxygen exposure is greatest) and diffusing downward. The color may intensify over 30 minutes.
• Negative (VP-): No color change, or development of a yellow, brown, or copper color. A faint pink color that appears only after 30 minutes should be considered negative.
• False positive: A deep red or purple color developing within 1-2 minutes may indicate contamination of reagents or improper storage.

Step 4: Documentation

Record the following information in your laboratory notebook or electronic laboratory information system:

• Date and time of inoculation
• Organism identification and source
• MR-VP broth lot number and expiration date
• Incubation temperature and duration
• MR result (positive/negative) with color description
• VP result (positive/negative) with color description and time of reading
• Control organism results
• Any observations (e.g., turbidity, pellicle formation, gas production)
• Technician initials

Quality Checks and Controls

Daily Quality Control

Run positive and negative control organisms with each batch of tests. Acceptable results:

• E. coli: MR positive (red), VP negative (no color change)
• E. aerogenes or K. pneumoniae: MR negative (yellow), VP positive (pink-red)
• Uninoculated MR-VP broth with methyl red: yellow (pH 7.0)
• Uninoculated MR-VP broth with VP reagents: no color change

Medium Quality Control

Verify each new lot of MR-VP broth:

• Check pH (should be 7.0 ± 0.2)
• Sterility check: incubate one uninoculated tube at 35-37°C for 48 hours; no turbidity should develop
• Performance check with control organisms

Reagent Quality Control

• Methyl red: Add 2-3 drops to pH 4.0 buffer (should turn red) and pH 6.0 buffer (should turn yellow)
• VP reagent A: Should be clear to pale yellow; discard if dark brown or precipitated
• VP reagent B: Should be clear; discard if cloudy or if precipitate forms

Common Pitfalls

• Insufficient incubation time: 24-hour incubation may yield false-negative MR results for slow-growing organisms. Always use 48 hours for definitive results.
• Over-inoculation: Heavy inoculum may cause rapid glucose depletion and early acid production, but the pH may rise again as organisms metabolize acids. Use a single colony.
• Contaminated reagents: Alpha-naphthol solution oxidizes within 2-4 weeks. Prepare fresh monthly and store in amber glass.
• Insufficient aeration for VP test: The VP reaction requires oxygen. Shake the tube vigorously for at least 30 seconds.
• Reading VP too early: Allow full 30 minutes for color development. Some positive reactions develop slowly.

Result Interpretation

MR Test Interpretation

VP Test Interpretation

Typical Results for Common Organisms

Clinical and Environmental Relevance

The MR-VP test is particularly valuable in water quality testing and food microbiology. E. coli (MR+, VP-) is a primary indicator of fecal contamination in water and food samples [3][5]. The presence of MR-positive organisms in dairy products or water sources suggests recent fecal contamination and potential health risks [2][5]. In veterinary medicine, MR-VP testing helps identify pathogens causing mastitis in dairy cows, where E. coli is a common causative agent [2].

Troubleshooting

Limitations

1. Incubation time dependency: The standard 48-hour incubation is critical for accurate MR results. Shorter incubation may yield false-negative MR results, while longer incubation may cause pH reversion as organisms metabolize accumulated acids.

2. Medium composition sensitivity: Variations in peptone source or phosphate buffer concentration between manufacturers can affect test results. Always use the same brand and lot for comparative studies.

3. Reagent stability: Alpha-naphthol solution is particularly unstable and must be prepared fresh or stored under strict conditions. Expired or oxidized reagent is the most common cause of false-negative VP results.

4. Organism variability: Some bacterial species show strain-to-strain variation in MR-VP reactions. For example, Yersinia spp. may be MR-negative or MR-positive depending on the species and growth conditions [1].

5. Mixed cultures: The MR-VP test requires a pure culture. Mixed cultures can produce ambiguous results (e.g., both MR and VP positive) that are uninterpretable.

6. Non-fermenters: Organisms that do not ferment glucose (e.g., Pseudomonas aeruginosa) will be negative for both MR and VP tests. This is a valid result but provides limited diagnostic information.

7. Temperature sensitivity: Incubation at temperatures outside 35-37°C may alter metabolic pathways and produce unreliable results.

8. Subjectivity in color interpretation: The distinction between weak positive and negative can be subjective, particularly for the MR test where orange colors may be interpreted differently by different technicians.

Documentation and Reporting

Laboratory Notebook Entry

Record the following for each MR-VP test:

• Sample information: Organism identification, source, date of isolation
• Medium: MR-VP broth manufacturer, lot number, preparation date
• Reagents: Methyl red and VP reagent lot numbers, preparation dates
• Controls: Positive and negative control results
• Procedure: Incubation temperature and duration, volumes of reagents added
• Results: Color descriptions for both MR and VP tests
• Interpretation: Positive/negative for each test
• Comments: Any unusual observations, troubleshooting actions taken

Reporting Format

For routine laboratory reports, present results as:

• MR: Positive (+) or Negative (-)
• VP: Positive (+) or Negative (-)

Example: E. coli: MR+, VP-

For research publications, include:

• Incubation conditions (time, temperature)
• Reagent sources and concentrations
• Control organism results
• Any modifications to standard protocol

Quality Assurance Records

Maintain records of:

• Medium and reagent lot numbers and expiration dates
• Quality control test results (weekly or with each new lot)
• Technician training and competency assessments
• Instrument calibration (incubator temperature, pH meter)

Biosafety Considerations

The MR-VP test is routinely performed at Biosafety Level 1 (BSL-1) when using non-pathogenic or low-risk bacterial isolates such as E. coli K-12 strains, Enterobacter aerogenes, or environmental isolates [6]. Standard microbiological practices apply:

• Wear a lab coat, gloves, and safety glasses
• Perform all work on a disinfected bench surface or in a biosafety cabinet
• Decontaminate work surfaces before and after procedures with 10% bleach or 70% ethanol
• Autoclave all contaminated materials before disposal
• Wash hands thoroughly after handling cultures
• Do not eat, drink, or apply cosmetics in the laboratory

For clinical specimens or isolates suspected to contain pathogens (e.g., Salmonella spp., Shigella spp., Yersinia pestis), work at BSL-2 with appropriate containment [6]. Follow institutional biosafety committee guidelines and any applicable NIH Guidelines for recombinant or synthetic nucleic acid work if genetically modified organisms are used [7].

Waste Disposal

• All MR-VP broth cultures must be autoclaved at 121°C for 30 minutes before disposal
• Used pipettes and tubes should be placed in biohazard waste containers
• Reagent bottles should be disposed of according to institutional chemical waste guidelines

Frequently Asked Questions

1. Can I use a single MR-VP broth tube for both tests?

Yes, provided the broth volume is at least 5 mL. Transfer 1-2 mL to a clean tube for the MR test and another 1-2 mL for the VP test. Do not add reagents directly to the original culture tube if you plan to use it for additional tests or subculturing. The remaining broth can be used for repeat testing if needed.

2. Why is the VP test read at 15-30 minutes instead of immediately?

The VP reaction requires time for the oxidation of acetoin to diacetyl and subsequent color complex formation. Reading too early may miss slow-developing positive reactions. However, waiting longer than 30 minutes may allow non-specific color development from reagent oxidation, leading to false-positive interpretations. The 15-30 minute window balances sensitivity and specificity.

3. What does it mean if both MR and VP tests are negative?

Both tests negative indicates that the organism does not produce sufficient acid to lower the pH below 6.0 (MR-negative) and does not produce acetoin (VP-negative). This is typical of non-fermenting organisms such as Pseudomonas aeruginosa or organisms that use alternative metabolic pathways. It may also indicate that the organism did not grow well in MR-VP broth or that the medium was not properly prepared.

4. Can the MR-VP test differentiate between E. coli and Salmonella?

No, both E. coli and Salmonella spp. are typically MR-positive and VP-negative. The MR-VP test alone cannot distinguish between these organisms. Additional biochemical tests such as indole production (indole-positive for E. coli, negative for Salmonella), citrate utilization (citrate-negative for E. coli, positive for Salmonella), and urease production are required for definitive identification. The MR-VP test is most useful when combined with other IMViC components [2][3][5].

References and Further Reading

1. Investigation of Antibiotic Susceptibility Patterns in Bacteria Isolated During a Male Sheep Castration Surgery – Zahra A, Neli Z, Mohaddeseh B. (2025). Reports MR and VP negative results for Yersinia spp., demonstrating variable MR-VP reactions among enteric bacteria.

2. Antibiotic resistance of Escherichia coli isolated from dairy cow milk associated with subclinical mastitis – Praja RN, et al. (2025). Uses IMViC tests including MR and VP for identification of E. coli from bovine mastitis cases.

3. Isolation and Identification of Pathogenic Bacteria Present in Commercially Important Fish, Shellfish, Water, and Soil Samples of Kaptai Lake – Chakma S, et al. (2025). Employs biochemical tests including MR-VP for enteric pathogen identification from environmental samples.

4. Evaluation of the antimicrobial properties of potential probiotic lactic acid bacteria from raw and fermented cow milk – Kolech A, et al. (2026). Describes biochemical characterization of LAB isolates, relevant to understanding fermentation pathways.

5. Prevalence and antimicrobial drug resistance of gram-negative bacteria in dairy feed and water – Arbab S, et al. (2025). Uses IMViC tests including MR-VP for identification of E. coli and Salmonella from dairy farm samples.

6. Biosafety in Microbiological and Biomedical Laboratories (BMBL), 6th Edition – CDC and NIH (2020). Authoritative guidelines for biosafety levels and laboratory practices.

7. NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules – National Institutes of Health. Institutional framework for biosafety in research settings.

8. NCBI Bookshelf: Molecular Biology and Laboratory Methods – National Center for Biotechnology Information. Comprehensive collection of biomedical methods references.

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