Methyl red test (MR) is used to identify the production of acid during the fermentation of glucose and the maintenance of pH below 4.5 in an old culture.
Five drops of 0.04% solution of methyl red are added to the culture in glucose phosphate medium which had been incubated at 50 C for five days, mixed well and read at once. Red colour is positive while yellow color signifies a negative test.
Principle of MR Test
As discussed above, MR test is used for identification of special bacterial character. i.e. production of acid while fermenting the sugar.
When these bacteria ferment glucose and produce acid from it, this acid formation lower the pH of medium. Methyl red pH indicator confirm presence of acid by changing its color from yellow to pink (or red). Change in color clearly shows that bacteria present in culture is utilizing glucose and producing acid from it.
This property of bacteria help us to group these bacteria in a special call, away from non acid producing bacteria.
- Dextrose broth
- coli form (entrobacteriaceae family member)
- test tubes
Composition of Dextrose Broth
- Tryptone 10 gm/L
- Peptone 3 gm/L
- Dextrose (Glucose) 5 gm/L
- Sodium Chloride 5 gm/L
- pH: 7.2
Procedure of Methyl Red (MR) Test
- Prepare the dextrose broth culture media.
- Dispense the media in test tubes and autoclave it.
- Autoclave the culture media in test tubes at 121 C at 15 lbs pressure for 15 minutes.
- Once culture media is sterilized, inoculate the test microbes in test tube media.
- Incubate the test tubes of inoculated media in incubator for 24 hours.
- Take out the culture media, added with test microbe, from incubator and add 5 drops of methyl red pH indicator into the incubated tube culture.
- Mix the methyl red pH indicator well by moving test tube of culture media, in circular motion.
Observation of Methyl Red (MR) Test
Observe the incubated test tube for change in color. The media could be red or yellow in color.
Result of Methyl Red (MR) Test
If the media remains yellow in color, this means test is negative. And microorganism under test does not produce acid by utilizing glucose.
However, if the color changes to Red, after adding methyl red pH indicator, it shows microorganism under test produce acid by sugar fermentation.
If the color change occur after adding Methyl Red pH indicator shows +ve test while no change in color show -ve MR test.