What is the indicator used in the methyl red test?

The indicator used in the methyl red test is methyl red itself. This test is specifically designed to determine whether an organism produces enough acid from glucose fermentation to lower the pH below 4.4. Methyl red changes color in response to pH, turning red in acidic conditions (pH below 4.4) and remaining yellow in neutral to alkaline conditions (pH above 6.2).

In this context, methyl red is an effective pH indicator for differentiating between fermentative and non-fermentative microorganisms, particularly in Escherichia coli versus Enterobacter species. The specificity of methyl red in this application makes it a crucial component of the test, allowing microbiologists to interpret the results accurately based on the observed color change.

Other indicators listed, such as phenol red, bromthymol blue, and litmus, all have their respective applications in microbiological testing and pH measurement; however, they are not utilized in the methyl red test.