Methyl Red Test
MRVP broth contains peptone, glucose, and phosphate buffer. In this broth, some bacteria will ferment the glucose to produce a mixture of fermentation acids (lactic, acetic, and formic acids), while others will produce only acetic acid, and some will not ferment the glucose at all.
The Methyl Red test assays for organisms which ferment glucose to form a mixture of fermentation acids. These acids are strong enough to overcome the phosphate buffer in the MRVP broth and lower the pH of the broth to below 4.4, which causes the broth to turn a red color upon addition of the pH indicator methyl red.
Tube 1: Negative
Tube 2: Positive