Methyl Red and Voges-Proskauer (MR-VP) Test: Principles and Protocols

The Methyl Red (MR) and Voges-Proskauer (VP) tests are paired biochemical assays used to differentiate members of the Enterobacterales family based on their glucose fermentation pathways. The MR test detects mixed acid fermentation, while the VP test detects the butylene glycol pathway. Together, these tests form the MR-VP pair, a cornerstone of bacterial identification schemes used in clinical, food, environmental, and agricultural microbiology laboratories. This article provides laboratory students, technicians, researchers, and diagnostic professionals with the principles, reagents, step-by-step protocols, quality controls, and interpretation criteria for both tests, with emphasis on practical workflow decisions and record keeping.

At a Glance

The MR and VP tests are performed on the same culture medium, MR-VP broth, which contains peptone, glucose, and phosphate buffer. After incubation, the broth is divided into two portions. One portion receives methyl red indicator to detect mixed acid fermentation. The other portion receives Barritt reagents A and B to detect acetoin production, an intermediate of the butylene glycol pathway. The table below summarizes the key features of both tests.

Purpose and Clinical Relevance of MR-VP Testing

The MR-VP pair distinguishes bacteria that ferment glucose through the mixed acid pathway from those that use the butylene glycol pathway. This distinction is taxonomically significant within the Enterobacterales and is routinely included in identification schemes for Gram-negative bacilli isolated from clinical specimens, food samples, water, and agricultural environments.

In veterinary and agricultural microbiology, MR-VP testing supports the identification of organisms relevant to animal health and food safety. For example, Escherichia coli is a common contaminant of retail meat and is frequently identified through biochemical profiling that includes the MR test. A study of retail meat samples recovered Gram-negative bacteria from all samples tested, with E. coli being the most prevalent organism at 37.4 percent, followed by Klebsiella species at 25.2 percent and Salmonella species at 17.9 percent. Identification in that study relied on culture characteristics, Gram staining, biochemical testing, and API 20E confirmation, which includes MR-VP reactions. The presence of multidrug-resistant organisms in the food chain underscores the need for accurate species identification to support surveillance and antimicrobial stewardship in food animal production.

The MR-VP tests also appear in the characterization of beneficial microorganisms. Studies of plant growth-promoting rhizobacteria and natural fertilizers routinely include methyl red and VP tests in their biochemical screening panels. One investigation of a traditional liquid natural fertilizer used biochemical analysis including catalase, indole synthesis, methyl red, VP test, urease, citrate, and sucrose fermentation to characterize fifteen major bacterial isolates, most of which belonged to the Bacillus and Priestia genera. Similarly, lactic acid bacteria isolated from commercial cheese were preliminarily identified through phenotypic and biochemical tests including Gram staining and carbohydrate fermentation, and the isolates showed a positive reaction to the methyl red test. These examples illustrate that MR-VP testing extends beyond clinical diagnostics into food microbiology, environmental microbiology, and agricultural biotechnology.

Principles of the Methyl Red Test

The methyl red test detects the production of stable acids from glucose fermentation. Certain bacteria, such as E. coli, ferment glucose through the mixed acid pathway, producing large quantities of organic acids including lactic acid, acetic acid, succinic acid, and formic acid. These acids lower the pH of the culture medium to approximately 4.5 or below. The methyl red indicator changes color based on pH. At pH values below 4.4, the indicator appears red. At pH values above 6.2, it appears yellow. The test is read immediately after adding the indicator to the broth culture.

The MR-VP broth is formulated with a low concentration of glucose and a phosphate buffer system. The buffer prevents the medium from becoming too acidic from the small amount of acid produced during the early stages of growth. Organisms that produce large amounts of stable acids overwhelm the buffer and lower the pH sufficiently to produce a red color with methyl red. Organisms that produce lesser amounts of acid or that convert acids to neutral products do not lower the pH enough, and the indicator remains yellow.

The incubation period for the MR test is critical. A 48 hour incubation at 35 to 37 degrees Celsius is standard. Shorter incubation periods may produce false negative results because the organisms have not yet produced sufficient acid to overcome the buffer. Longer incubation periods may produce false positive results because peptone breakdown releases ammonia, which raises the pH and can mask acid production. The timing of the test must be controlled carefully to obtain reliable results.

Principles of the Voges-Proskauer Test

The Voges-Proskauer test detects the production of acetoin, also known as acetyl methyl carbinol, from glucose fermentation. Organisms such as Enterobacter, Klebsiella, and Serratia ferment glucose through the butylene glycol pathway. In this pathway, pyruvate is converted to acetoin, which is then reduced to 2,3-butanediol. The accumulation of acetoin in the culture medium is detected by the Barritt method.

The Barritt method uses two reagents. Reagent A is a solution of alpha-naphthol in ethanol. Reagent B is a solution of potassium hydroxide in water. When these reagents are added to a broth culture containing acetoin, the alpha-naphthol reacts with acetoin in the presence of potassium hydroxide and atmospheric oxygen to produce a pink to red color. The color develops over several minutes and may take up to 30 minutes to reach full intensity. The test should be read within this window because the color can fade or become a copper color upon prolonged exposure to air.

The VP test requires the same 48 hour incubation as the MR test. The broth culture is divided into two portions after incubation, with one portion used for the MR test and the other for the VP test. This division ensures that both tests are performed on the same culture under identical growth conditions, which is essential for accurate interpretation of the MR-VP pair.

MR-VP Broth Composition and Preparation

MR-VP broth is a simple medium containing peptone, glucose, and dipotassium phosphate. The peptone provides nitrogen and amino acids for bacterial growth. The glucose serves as the fermentable carbohydrate. The dipotassium phosphate acts as a buffer to resist pH changes during early growth.

The medium is prepared by dissolving the components in distilled water, adjusting the pH to approximately 6.9, and dispensing into test tubes. The tubes are sterilized by autoclaving at 121 degrees Celsius for 15 minutes. After sterilization, the medium should be allowed to cool before inoculation. The glucose in the medium can caramelize if the medium is overheated or autoclaved for too long, which can affect the test results.

Commercially prepared MR-VP broth is available from multiple manufacturers and is preferred in many laboratories because it provides consistent quality. If the medium is prepared in house, the laboratory should verify each batch with known positive and negative control organisms before use in diagnostic testing. This verification step is part of good laboratory practice and supports reliable test interpretation.

Step-by-Step Protocol for MR-VP Testing

The following protocol describes the standard procedure for performing the MR and VP tests. This protocol assumes the laboratory has a pure culture of the organism to be tested, obtained from an isolation plate.

Inoculation and Incubation

Using a sterile loop, transfer a single well-isolated colony from an 18 to 24 hour culture into a tube of MR-VP broth. The inoculum should be light to avoid excessive turbidity, which can interfere with color interpretation. Alternatively, a suspension of the organism in sterile saline can be used to inoculate the broth with a few drops.

Incubate the inoculated broth at 35 to 37 degrees Celsius for 48 hours. The incubation time is critical for both tests. Do not use a shorter incubation period, as this may produce false negative results. Do not extend the incubation beyond 48 hours, as this may produce false positive MR results due to ammonia production from peptone metabolism.

Division of the Broth Culture

After 48 hours of incubation, remove the tube from the incubator and gently mix the broth. Aseptically transfer approximately half of the broth to a second sterile tube. One tube will be used for the MR test, and the other for the VP test. Both tubes must be handled in the same manner to ensure that the results are comparable.

Methyl Red Test Procedure

Add 2 to 3 drops of methyl red indicator solution to one tube of the broth culture. Mix gently and read immediately. A red color at the surface of the broth indicates a positive result. A yellow color indicates a negative result. An orange color is considered a weak or inconclusive result and may require repeat testing.

Voges-Proskauer Test Procedure

To the second tube of broth culture, add 6 drops of Barritt reagent A (alpha-naphthol solution). Mix gently. Add 2 drops of Barritt reagent B (potassium hydroxide solution). Mix gently and allow the tube to stand at room temperature. Observe for color development at 5 minutes, 15 minutes, and 30 minutes after adding the reagents. A pink to red color indicates a positive result. No color change indicates a negative result. A copper color may appear in some negative reactions and should not be interpreted as positive.

Reading and Recording Results

Record the results for both tests immediately after reading. The MR result is read immediately after adding the indicator. The VP result is read within 30 minutes of adding the reagents. Do not return the tubes to the incubator after adding reagents, as this can affect color development.

Quality Control and Control Organisms

Quality control is essential for reliable MR-VP testing. Each batch of MR-VP broth and each batch of reagents should be tested with known positive and negative control organisms before use in diagnostic testing. The results of quality control testing should be recorded in the laboratory quality control log.

The recommended positive control for the MR test is Escherichia coli, which produces a red color. The recommended negative control for the MR test is Enterobacter cloacae or Klebsiella pneumoniae, which produces a yellow color. The recommended positive control for the VP test is Enterobacter cloacae or Klebsiella pneumoniae, which produces a pink to red color. The recommended negative control for the VP test is E. coli, which produces no color change.

Quality control organisms should be maintained as stock cultures and subcultured onto appropriate media before use. The control organisms should be tested in parallel with the test organisms, using the same lot of medium and reagents. If the control organisms do not produce the expected results, the test results are invalid and the testing should be repeated with fresh medium and reagents.

The World Health Organization Laboratory Quality Management System Handbook emphasizes the importance of quality control in laboratory testing. The handbook provides guidance on establishing quality control programs, documenting test procedures, and maintaining records. Laboratories performing MR-VP testing should follow these principles to ensure the reliability of their results.

Interpretation of Results

The MR and VP results are interpreted together as a pair. The four possible combinations are MR positive VP negative, MR negative VP positive, MR positive VP positive, and MR negative VP negative.

The MR positive VP negative pattern is characteristic of E. coli and other organisms that use the mixed acid fermentation pathway. These organisms produce large amounts of stable acids from glucose and do not produce acetoin. This pattern is also observed in Salmonella, Shigella, and Proteus species. A study of Proteus vulgaris identification reported that the organism was positive for indole, motility, methyl red, and urease, and was unable to utilize citrate for carbon and energy. This pattern illustrates the use of MR testing in a broader biochemical identification scheme.

The MR negative VP positive pattern is characteristic of Enterobacter, Klebsiella, Serratia, and other organisms that use the butylene glycol pathway. These organisms produce acetoin and do not produce sufficient stable acids to lower the pH below the methyl red threshold.

The MR positive VP positive pattern is uncommon but can occur in some organisms. This pattern may indicate that the organism produces both mixed acids and acetoin, or it may indicate a problem with the test procedure. If this pattern is observed, the testing should be repeated with fresh medium and reagents.

The MR negative VP negative pattern is also uncommon among the Enterobacterales. This pattern may indicate that the organism does not ferment glucose, or it may indicate a problem with the test procedure, such as failure of the organism to grow in the broth. If this pattern is observed, the testing should be repeated and the purity of the culture should be verified.

Common Failure Patterns and Troubleshooting

Several common problems can produce unreliable MR-VP results. Recognizing these problems and taking corrective action is essential for accurate testing.

False Negative Methyl Red Results

A false negative MR result occurs when a mixed acid fermenter produces a yellow color instead of red. The most common cause is insufficient incubation time. If the broth is tested before 48 hours, the organism may not have produced enough acid to overcome the buffer. The test should be repeated with a full 48 hour incubation.

Another cause of false negative MR results is excessive glucose in the medium. If the medium contains too much glucose, the organism may consume all the glucose and then begin metabolizing peptone, producing ammonia and raising the pH. This problem is avoided by using a properly prepared medium with the correct glucose concentration.

False Positive Methyl Red Results

A false positive MR result occurs when a non mixed acid fermenter produces a red color. This can happen if the broth is incubated too long, allowing ammonia production from peptone metabolism to be masked by other factors. It can also happen if the methyl red indicator is added in excess, which can produce a red color even at higher pH values. The test should be repeated with fresh medium and the correct amount of indicator.

False Negative Voges-Proskauer Results

A false negative VP result occurs when an acetoin producer produces no color change. The most common cause is insufficient reagent addition or failure to mix the reagents thoroughly. The alpha-naphthol and potassium hydroxide must be mixed well to allow the reaction to proceed. The test should be repeated with fresh reagents and careful attention to the mixing steps.

Another cause of false negative VP results is reading the test too early. The color develops over 30 minutes, and reading before full development can produce a false negative. The test should be observed at multiple time points up to 30 minutes.

False Positive Voges-Proskauer Results

A false positive VP result occurs when a non acetoin producer produces a pink or red color. This can happen if the reagents are contaminated or if the alpha-naphthol solution has deteriorated. The alpha-naphthol solution should be stored in a dark bottle and protected from light. If the reagent has turned brown, it should be discarded and replaced.

Contamination and Purity Issues

Contamination of the broth culture with another organism can produce confusing results. The purity of the culture should be verified by streaking a sample of the broth onto a non selective agar plate and examining the growth after incubation. If more than one colony type is present, the test should be repeated with a pure culture.

Biosafety and Laboratory Safety Considerations

MR-VP testing involves the culture of bacteria, some of which may be pathogenic. The World Health Organization Laboratory Biosafety Manual provides guidance on the safe handling of microorganisms in the laboratory. Laboratories performing MR-VP testing should follow these guidelines to protect laboratory workers and the environment.

All work with bacterial cultures should be performed in a biological safety cabinet if the organism is known or suspected to be pathogenic. The cabinet should be certified and maintained according to the manufacturer's instructions. Personal protective equipment, including a laboratory coat, gloves, and eye protection, should be worn at all times when handling cultures.

The MR-VP broth cultures should be handled as potentially infectious material. After the tests are completed, the tubes should be decontaminated by autoclaving before disposal. The methyl red indicator and Barritt reagents should be handled with care. Alpha-naphthol is an irritant and should be handled with gloves. Potassium hydroxide is corrosive and should be handled with care to avoid skin contact.

Spills of bacterial cultures should be contained and decontaminated immediately. The spill area should be cleaned with an appropriate disinfectant, and the spill should be reported according to the laboratory's safety procedures. Laboratory workers should be trained in the safe handling of cultures and reagents before performing MR-VP testing.

Records and Documentation

Accurate records are essential for reliable MR-VP testing. The laboratory should maintain a log of all tests performed, including the date, the organism tested, the source of the organism, the lot numbers of the medium and reagents, the control organisms used, and the results obtained. This information supports the interpretation of results and provides a basis for troubleshooting if problems arise.

The World Health Organization Laboratory Quality Management System Handbook emphasizes the importance of documentation in laboratory testing. The handbook recommends that laboratories establish standard operating procedures for all tests, maintain records of quality control testing, and document any deviations from the standard procedures. These practices support the reliability and traceability of laboratory results.

The results of MR-VP testing should be recorded in a format that is clear and unambiguous. The MR result should be recorded as positive, negative, or weak. The VP result should be recorded as positive, negative, or weak. The results should be interpreted together as a pair, and the interpretation should be recorded in the laboratory report.

Limitations of MR-VP Testing

The MR-VP tests are useful for differentiating members of the Enterobacterales, but they have limitations. The tests are phenotypic assays that depend on the expression of metabolic pathways under specific culture conditions. Variations in the medium, incubation time, temperature, and inoculum size can affect the results. The tests should be performed according to a standardized protocol to minimize variability.

The MR-VP tests do not provide species level identification on their own. They must be used in combination with other biochemical tests, such as indole production, citrate utilization, urease production, and carbohydrate fermentation, to identify an organism to the species level. The tests are most useful when included in a comprehensive biochemical identification scheme.

Some organisms produce weak or variable reactions in the MR-VP tests. For example, some strains of E. coli may produce a weak MR reaction, and some strains of Enterobacter may produce a weak VP reaction. These weak reactions should be interpreted with caution and may require repeat testing or additional tests to confirm the identification.

Molecular methods, such as 16S rRNA gene sequencing, provide more definitive identification than phenotypic tests. A study of bacteria isolated from a natural fertilizer used 16S rRNA gene sequencing to confirm the identification of isolates that were initially characterized by biochemical tests. The study found that most isolates belonged to the Bacillus and Priestia genera. Molecular methods can resolve ambiguous phenotypic results and provide a more accurate identification.

Professional Escalation Criteria

Laboratory personnel should escalate results to a supervisor or a more experienced colleague in certain situations. These situations include unexpected results that do not match the expected pattern for the organism, results that are inconsistent with other biochemical tests, and results that are difficult to interpret due to weak reactions or contamination.

If the MR-VP results suggest an organism that is clinically significant or that has public health implications, the results should be reported to the appropriate authority. For example, the isolation of Salmonella or E. coli from a food sample may require notification of the relevant food safety authority. The laboratory should have procedures in place for reporting notifiable organisms.

If the MR-VP results are ambiguous or if the organism cannot be identified with confidence, the laboratory should consider sending the isolate to a reference laboratory for confirmation. Reference laboratories have access to additional tests and expertise that can resolve difficult identifications.

Applications in Veterinary and Agricultural Microbiology

MR-VP testing has direct applications in veterinary and agricultural microbiology. The identification of bacteria from animal specimens, food products, and environmental samples often relies on biochemical testing that includes the MR-VP pair.

In food safety, the identification of Gram-negative bacteria from retail meat is important for understanding the prevalence of antimicrobial resistant organisms in the food chain. A study of retail meat samples identified E. coli as the most prevalent organism, followed by Klebsiella, Salmonella, and Enterobacter species. The identification of these organisms relied on biochemical testing, which included the MR-VP reactions. The study highlighted the potential role of retail meat as a source of antimicrobial resistant bacteria and underscored the need for strengthened surveillance and antimicrobial stewardship in food animal production.

In agricultural microbiology, the characterization of beneficial microorganisms often includes MR-VP testing. A study of plant growth promoting rhizobacteria associated with chilli used morphological, biochemical, enzymatic, and molecular approaches to characterize twenty three isolates. The biochemical characterization included tests for catalase, nitrate reduction, and carbohydrate utilization. The study identified Lysinibacillus macroides and Lysinibacillus fusiformis as promising biofertilizer candidates for chilli cultivation.

In animal health, the identification of bacteria from clinical specimens supports the diagnosis and treatment of infections. The MR-VP tests are part of the standard biochemical identification scheme for Gram-negative bacilli isolated from veterinary specimens. Accurate identification supports appropriate antimicrobial therapy and infection control measures.

Frequently Asked Questions

What is the difference between the methyl red and Voges-Proskauer tests?

The methyl red test detects the production of stable acids from glucose fermentation through the mixed acid pathway. The Voges-Proskauer test detects the production of acetoin from glucose fermentation through the butylene glycol pathway. Both tests are performed on the same MR-VP broth culture after 48 hours of incubation. The methyl red test uses a pH indicator that turns red in acidic conditions. The Voges-Proskauer test uses alpha-naphthol and potassium hydroxide to detect acetoin, producing a pink to red color.

Why are the methyl red and Voges-Proskauer tests performed together?

The MR and VP tests are performed together because they differentiate organisms based on alternative glucose fermentation pathways. Organisms that use the mixed acid pathway are MR positive and VP negative. Organisms that use the butylene glycol pathway are MR negative and VP positive. The paired results provide more diagnostic information than either test alone. The tests are performed on the same broth culture to ensure that both results reflect the same growth conditions.

What is the recommended incubation time for MR-VP testing?

The recommended incubation time is 48 hours at 35 to 37 degrees Celsius. The incubation time is critical for both tests. A shorter incubation may produce false negative MR results because the organism has not produced enough acid to overcome the buffer. A longer incubation may produce false positive MR results because ammonia production from peptone metabolism raises the pH. The incubation time should be controlled carefully to obtain reliable results.

How is the methyl red test read?

The methyl red test is read immediately after adding 2 to 3 drops of methyl red indicator to the broth culture. A red color at the surface of the broth indicates a positive result. A yellow color indicates a negative result. An orange color is considered weak or inconclusive and may require repeat testing. The test should be read immediately because the color can change upon prolonged exposure to air.

How is the Voges-Proskauer test read?

The Voges-Proskauer test is read after adding Barritt reagent A (alpha-naphthol) and reagent B (potassium hydroxide) to the broth culture. The tube is mixed gently and allowed to stand at room temperature. The color develops over 30 minutes. A pink to red color indicates a positive result. No color change indicates a negative result. A copper color may appear in some negative reactions and should not be interpreted as positive.

What control organisms should be used for MR-VP testing?

The recommended positive control for the methyl red test is Escherichia coli. The recommended negative control for the methyl red test is Enterobacter cloacae or Klebsiella pneumoniae. The recommended positive control for the Voges-Proskauer test is Enterobacter cloacae or Klebsiella pneumoniae. The recommended negative control for the Voges-Proskauer test is E. coli. Control organisms should be tested in parallel with test organisms using the same lot of medium and reagents.

What are the limitations of MR-VP testing?

The MR-VP tests are phenotypic assays that depend on the expression of metabolic pathways under specific culture conditions. Variations in the medium, incubation time, temperature, and inoculum size can affect the results. The tests do not provide species level identification on their own and must be used in combination with other biochemical tests. Some organisms produce weak or variable reactions that require repeat testing or additional tests to confirm the identification. Molecular methods, such as 16S rRNA gene sequencing, provide more definitive identification than phenotypic tests.

When should MR-VP results be escalated to a supervisor?

MR-VP results should be escalated to a supervisor when the results are unexpected, inconsistent with other biochemical tests, or difficult to interpret due to weak reactions or contamination. Results that suggest a clinically significant organism or an organism with public health implications should be reported to the appropriate authority. If the organism cannot be identified with confidence, the isolate should be sent to a reference laboratory for confirmation.

Related Diagnostic Guides

• Methyl Red and Voges-Proskauer Tests: Principles and Interpretation
• How to Perform a Methyl Red and Voges-Proskauer (MR-VP) Test
• BCA Assay Protocol: Principles and Step-by-Step Instructions
• Nitrate Reduction Test: Principle, Reagents, and Interpretation
• Starch Hydrolysis Test: Principle, Protocol, and Interpretation

References and Further Reading

• Laboratory Quality Management System Handbook. World Health Organization.
• Laboratory Biosafety Manual. World Health Organization.
• Assay Guidance Manual. National Center for Advancing Translational Sciences.
• Bioanalytical Method Validation Guidance. U.S. Food and Drug Administration.
• NCBI Literature Resources. National Center for Biotechnology Information.
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This article is educational and does not replace validated laboratory procedures, institutional biosafety review, manufacturer instructions, or professional interpretation.