Methyl Red Test is widely used to test the pH of microbial culture media caused by the production of acid from glucose fermentation.
Methyl Red test and Voges Proskauer are used to differentiate different members of Enterobacteriaceae family.
Introduction:
In IMViC test, the M stands for Methyl Red test. It tests the ability of microbes to utilize glucose and produce acids as the product. This test is used to distinguish and characterize the bacteria based on their metabolic pathways. The Biochemical study of Enterobacteriaceae family states that the different members follow mixed acid pathway, as these pathways are anaerobic, they are also called as mixed acid fermentation.
The Enterobacteriaceae family members utilizes glucose and produces mixture of acid such as lactate, acetate, succinate, format, ethanol and gas like H2 and CO2. The enzymes present in the bacteria dictate the formation of these end products. Different members of this family produce these mixed acids in different proportion and hence it can be used as parameter for characterization of bacteria.
Principle:
When bacteria ferment glucose and produce mixed acids it causes decline in pH. The Methyl Red pH indicator indicates the change in pH caused by bacterial glucose fermentation. The pH indicator is red in color at acidic pH and yellow in alkaline pH. If the media turns from yellow to red color, it means the test is positive.
Requirements:
- Dextrose broth
| Sr. No. | Ingredients | Quantity |
| 1 | Tryptone | 10 gm/L |
| 2 | Peptone | 3gm/L |
| 3 | Dextrose (Glucose) | 5gm/L |
| 4 | Sodium chloride | 5gm/L |
| 5 | pH | 7.2±0.2 |
- Coliform (Enterobacteriaceae family member)
- Test tubes
- Alcohol
Procedure of Methyl Red (MR) Test:
- Prepare the Dextrose broth according to the mentioned composition
- Dispense the media in test tubes.
- Autoclave the dispensed media at 121° C, at 15 lbs pressure for 15 minutes
- Once the media is sterile, inoculate the test organism in the media.
- Incubate the inoculated media in incubator for 24 hours.
- Add 5 drops of methyl red pH indicator in the incubated tube culture.
- Move the test tube in circular motion in order to mix the methyl red with media uniformly.
Observation of the Methyl Red (MR)Test:
The change in color is observed. The media could be red or yellow in color.
Result of the Methyl Red (MR) Test: If the media remains yellow in color, it means the inoculated bacteria does not produce acids, and if media turns red in color then it means acids are produced.
Conclusion:
The yellow color of media means the test organism is (MR) test negative and red color means the test organism is (MR) test positive.
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