What is the indicator in the methyl red test?

The indicator used in the methyl red test is methyl red itself. This test is primarily employed to determine if an organism can perform mixed acid fermentation, which produces significant amounts of acid resulting in a low pH. Methyl red is a pH indicator that changes color based on the acidity of the solution; it appears red at a pH of 4.4 or lower and yellow at a pH of 6.2 or higher. Therefore, if the test culture is acidic enough to lower the pH below the threshold, the solution will remain red, indicating a positive result for mixed acid fermentation.

The other indicators listed do not serve this purpose in the methyl red test. Bromthymol blue, for instance, changes color in a different pH range and is not suitable for detecting the specific outcome of the methyl red test. Similarly, phenol red and bromcresol purple, while they are also pH indicators, do not provide the same acid detection specific to the metabolic pathways that the methyl red test assesses.