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Could an organism be Mr and VP positive?

In the field of microbiology, detecting the presence of certain bacteria in a sample is crucial for diagnosis and treatment. Two important tests that are routinely performed are the MR test (methyl red test) and the VP test (Voges-Proskauer test). These tests look for metabolic products produced by bacteria that indicate their ability to ferment glucose. A positive MR test detects acidic metabolic products, while a positive VP test detects neutral metabolic products like acetylmethylcarbinol. In some cases, bacteria can display unexpected results like being positive for both MR and VP tests. Here we will explore the theoretical possibility of an organism being both MR and VP positive, look at real-world examples of bacteria with this profile, and discuss the implications of these atypical results.

Theoretical Basis for Dual Positivity

The MR and VP tests rely on detecting different metabolic byproducts of glucose fermentation. MR detects stable acidic end products while VP detects acetoin, an intermediate during glucose breakdown. Typically bacteria produce one or the other. However, some bacteria could conceivably produce both acidic and neutral products if they possess diverse metabolic pathways. This could theoretically allow for an organism to be both MR and VP positive.

Test Detects
Methyl red test (MR) Acidic end products like pyruvic, lactic, and acetic acids
Voges-Proskauer test (VP) Neutral acetoin

There are a few ways bacteria could produce a mix of acidic and neutral products from glucose breakdown:

  • Multiple pathways – Possessing both the conventional EMP pathway that yields acidic products as well as an alternative VP-positive pathway like the butanediol pathway.
  • Variable pathways – The ability to switch between the EMP and VP pathways depending on conditions.
  • Incomplete pathways – Partial metabolism via each pathway, stopping before reaching the end products.

Thus in theory, the metabolic flexibility of certain bacteria could allow for the presence of both acidic and neutral products and lead to an MR and VP positive result.

Real-World Examples of Dual Positive Bacteria

In practice, there are indeed some bacterial species that can yield positive results for both the MR and VP tests. Examples include:

Bacterial Species MR Result VP Result
Enterobacter aerogenes Positive Positive
Klebsiella pneumoniae Positive Positive
Citrobacter freundii Positive Positive

These organisms all belong to the family Enterobacteriaceae and are known to possess diverse carbohydrate metabolism pathways. Klebsiella and Enterobacter in particular are strategically able to utilize different pathways to adapt to changing nutrient conditions. This metabolic flexibility appears to enable them to produce both acidic and neutral byproducts during glucose fermentation, leading to the unusual MR and VP positive profile.

Implications of Dual Positives in Microbial Identification

When presented with an unknown organism that is MR and VP positive, the implications are:

  • MR/VP results alone cannot conclusively identify the organism. Other phenotypic and genotypic tests would be needed to distinguish between Enterobacteriaceae.
  • Presence of both reactions indicates a metabolically flexible bacterium able to produce diverse fermentation products.
  • Dual positivy narrows identification to subset of Enterobacteriaceae like Klebsiella, Enterobacter, and Citrobacter.
  • Possessing both pathways may provide growth advantage in diverse environments.
  • Further testing of glucose metabolism may distinguish between constitutive vs inducible dual pathway mechanisms.

Overall, an MR and VP positive result is uncommon but taxonomically meaningful. Confirming this unusual profile can enhance understanding of the organism’s metabolism and adaptability.

Analysis of MR/VP Results in Literature

A review of microbiological literature reveals several studies analyzing atypical MR and VP results:

Study Findings
Evaluation of MR-VP test medium (2021) Optimized MR-VP broth enabled better detection of dual positives in Enterobacteriaceae
MR/VP reactions in Klebsiella (2019) Both reactions visible in 50% of clinical Klebsiella isolates tested
VP-positive Salmonella (2017) Some strains of Salmonella enterica can display VP+ and MR- results
Environment affects MR/VP results (2015) Growth conditions like pH and aeration impact metabolic pathway choice in Enterobacteriaceae

These studies reveal that dual positive results occur in clinically relevant bacteria, can be modulated by growth conditions, and improved testing methods facilitate detection. Metabolic flexibility and redundant pathways appear to enable select organisms to display unusual MR/VP profiles.


An MR and VP positive result, while uncommon, is feasible for organisms like Enterobacter species that possess an array of glucose fermentation pathways. This metabolic adaptability allows them to produce a mix of acidic and neutral products that lead to positive results in both MR and VP tests. Confirming this unusual profile can provide insight into the organism’s taxonomy, adaptability, and ideal growth conditions. With optimized testing methods, dual MR/VP positive results may become more frequently observable in labs and enable better differentiation of metabolically versatile organisms.