MR Test is a quantitative biochemical tests for identification of bacteria by detection of acid production which requires positive organisms to produce strong acid (lactic, formic) from glucose through the mixed acid fermentation pathway.Methyl red is a pH indicator having a range between 6.0(yellow) and 4.4(red). It is used in IMViC test for detection of acid production from glucose fermentation. Since many species of the family of Enterobacteriaceae may produce sufficient quantities of strong acid that can be detected by methyl red indicator during the initial phases of incubation,only those organisms that are able to maintain this low pH after prolonged incubation (48-72 hours) and overcoming the pH buffering system of the medium can be called as methyl red positive.
Principle of Methyl red Test
- The development of stable red colour in surface of the medium indicates sufficient acid production which lower the pH to 4.4 and constitutes positive test.
- MR negative organisms may produce lesser quantities of acid from test substrate, yielding an intermediate orange colour between yellow and red may developed
Composition of Methyl red Test
|Methyl red||0.200 gm|
|Distilled water||40.000 ml|
|Ethyl alcohol||60.000 ml|
Appearance of MR indicator
It has a pinkish red coloured solution.
Methyl red Test Procedure
- Start by adding 1-2 drops of methyl red indicator to a 5 ml of a 24 – 48 hours old culture of the suspected organism
- Observe and interpret the colour change immediately.
Result and interpretation
Interpretation of the results is done immediately after the addition of Methyl red indicator
|Positive Methyl Red Organism||red colouration to the medium|
|Negative Methyl Red Organism||yellow colour|
Culture observation and Methyl Red reaction
These cultural responses is observed after 18-24 hours incubation at temperature 37°C in MR-VP medium followed by Methyl Red test performed by using Methyl red indicator.
|Organism||Methyl Red Reaction|
|Klebsiella pneumoniae||Negative (Yellow colour observed)|
|Enterobacter aerogenes||Negative (Yellow colour observed)|
|Escherichia coli||Positive (Red colour observed)|
- Color Atlas and Textbook of Diagnostic Microbiology,4th edition ,Elmer W. Koneman. Stephen D.Allen. ,William M. Janda., Paul C. Schreckenberger.,Washington C.Winn.