Introduction
The Methyl Red (MR) Test is an important biochemical test in microbiology that detects the ability of bacteria to perform mixed acid fermentation of glucose, producing stable acidic end-products.
It is one of the IMViC tests (Indole, Methyl Red, Voges-Proskauer, Citrate) used to differentiate Enterobacteriaceae family members, especially Escherichia coli (MR positive) from Enterobacter, Klebsiella (MR negative).
Principle
Some bacteria ferment glucose via the mixed acid pathway, producing stable acids (lactic, acetic, formic acids).
These acids lower the pH of the medium below 4.4.
Methyl red indicator is used:
Red (pH < 4.4) → Positive
Yellow (pH > 6.0) → Negative
Orange (pH 4.4–6.0) → Inconclusive
Procedure
Inoculate the test organism into MR-VP broth (containing glucose).
Incubate at 37°C for 48–72 hours.
Add 5 drops of methyl red indicator after incubation.
Observe the color change immediately.
Interpretation
| Result | Observation | Interpretation |
|---|---|---|
| Positive | Bright red color | Stable acid end-products formed (mixed acid fermentation) |
| Negative | Yellow color | No stable acid formation, glucose metabolized by other pathways |
| Inconclusive | Orange color | Borderline result, repeat test |
Examples of Bacteria
✅ MR Positive Bacteria
Escherichia coli
Proteus spp.
Salmonella spp.
Shigella spp.
❌ MR Negative Bacteria
Enterobacter aerogenes
Klebsiella pneumoniae
Serratia marcescens
Uses of Methyl Red Test
Differentiates E. coli (positive) from Enterobacter/Klebsiella (negative).
Helps in the identification of Enterobacteriaceae.
Part of the IMViC series for Gram-negative bacterial differentiation.
Useful in clinical microbiology for rapid preliminary identification.
Conclusion
The Methyl Red Test is a simple and reliable biochemical test that detects mixed acid fermentation of glucose. It is crucial for differentiating Escherichia coli (positive) from Enterobacter and Klebsiella (negative) and remains an important tool in diagnostic microbiology.
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