Methyl Red Test: Use, Principle, Procedure and Result Interpretation

Methyl Red (MR) Test

Methyl Red test is for enteric gram-negative rods, as part of identiﬁcation to species level.

Principle of Methyl Red Test

The methyl red (MR) test uses to determine if an organism is able to produce stable acid end products from glucose fermentation Methyl red indicator (red color below pH 4.4; yellow color at pH 5.8) uses to determine the pH after an enteric gram-negative rod has fermented glucose to completion. All members of the Enterobacteriaceae give a positive methyl red reaction when tested up to 24 h due to conversion of glucose to pyruvic acid by the Embden-Meyerhof pathway. After further incubation (2 to 5 days) those organisms that are MR positive continue to metabolize pyruvic acid to lactic, acetic, and formic acids by the mixed acid pathway and are able to maintain the acid pH (4.4).

Composition of MR-VP broth

Buffered peptone:7.0 g
Glucose:5.0 g
Dipotassium phosphate:5.0 g
Deionized / Distilled water: 1000 ml
Final pH:6.9

Generally dispense approximately 5 ml per tube.
Use enough broth to cover an inverted Durham tube, if it is used.
Dispense 2 ml of MR-VP broth for rapid VP testing and 0.5 ml for rapid MR testing.

Preparation of Methyl red solution

Methyl red solution, 0.02%

Dissolve 0.1 g of methyl red in 300 ml of 95% ethyl alcohol.
Add sufﬁcient distilled water to make 500 ml.
Store at 4 to 8°C in a brown bottle.
The solution is stable for 1 year.

Requirements for Methyl Red Test

MR-VP broth
Test organism
Inoculating wire
Bunsen burner
Incubator
Methyl red indicator
Control strains

Escherichia coli ATCC 25922
Klebsiella pneumoniae ATCC 13883

Procedure of Methyl Red Test

Inoculate test organism and incubate at 35C for at least 48 hours. Note: If the center of one colony is inoculated to a 0.5-ml volume of MR-VP broth, the test can be read at 18 to 24 hours.
Remove approximately 1 ml of the 48-hours broth to a 13-by 100-mm tube. (The remainder should be reserved for testing at 3 to 5 days if necessary.)
Add 3 to 6 drops (or 1 drop to 0.5 ml) of methyl red indicator to the aliquot.
Observe for red color immediately.

Result and Interpretation of Methyl Red Test

The development of a stable red color on the surface of the medium indicates sufficient acid production to lower the pH to 4.4 and constitutes a positive test. Because other organisms may produce smaller quantities of acid from the test substrate, an intermediate orange color between yellow and red may develop. This does not indicate a positive test. The yellow color indicates a negative test.

Result of Quality control strains

Escherichia coli ATCC 25922—MR positive(red),VP negative(no change)
Klebsiella pneumoniae ATCC 13883—MR negative (yellow), VP positive (red)

List of methyl red test positive and negative bacteria

Methyl red test positive bacteria are as follows:

Escherichia coli
Klebsiella ozanae
Klebsiella rhinoscleromatis
Klebsiella ornithiolytica
Citrobacter
Proteus
Yersinia
Edwardsiella
Salmonella

Methyl red test negative bacteria

Klebsiella pneumoniae
Enterobacter species

Limitations of MR Test

For the complete identification of bacteria, further biochemical tests are also used from the pure colony of bacteria.
The methyl red test must not be reported unless the medium has been incubated for a minimum of 48 hours. Tests that are run too early may result in false-positive result interpretation.
If an inoculum is too heavy, bacterial growth may be inhibited that creating invalid test results and hence a light inoculum is preferred.
Incubation periods up to 5 days may be necessary for the methyl red test result interpretation.

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