| Technique | METHYL RED ,(/,)/,( |
| Principle | The methyl red test is used
primarily to distinguish various coliforms. They all
ferment glucose and the pH of the medium drops
quickly.when methyl red is added after overnight
incubation the cultures of all these organisms will be
posit ive. Escherichia coli will continue to ferment
glucose and enough acid may be formed to sterilize the
medium. Klebsiella aerogens cultures decaroxylate and
condense the pyruvic acid to form acetylmethylcarbinol
which causes the pH to rise. when met hyl red is added
the yellow colour indicates a negative reaction. see also
Voges Proskaur The Mr-VP broth (Clark and Luks medium) contains peptones, phosphate and glucose. The methyl red test establishes if the microbe has hydrolyzed glucose to an acid. The culture is incubated for 2-5 days at 37`C and a few drops of methyl-red pH indicator are added. Methyl red indicator is yellow at pH 6.2 or above; and red at pH 4.4 or below. Both Escherchia coli and Aerobacter aerogenes hydrolyze glucose to acids. E. coli produces formic, acetic, lactic and succinic acid (l.59 moles/Mole glucose) and the final pH is 4.5 or lower. When A. aerogenes is grown on glucose it produces less acid more ethanol and butanediol. The final pH is not limiting and when the sugar is exhausted the peptone is attacked and the pH rises to 6.2 which gives a negative methyl red test. see also IMViC tests |
| Cautions | The methyl red test should never be read until the cultures have been incubated for at least 2-3 days. |
| Method | Incubate MR VP medium and incubate for 5 days at 30 C or 2 days at 37 C. Add 2 drops of methyl red solution and examine |
| Results | A distinct red colour is regarded as a positive reaction and a yellow colour as a negative one. |
| Positive control | Escherichia coli red or orange |
| Negative control | Enterobacter cloacae yellow |
| Reagents | |
| Reference |