Methyl red solution

Acid formation during mixed acid fermentation is detected with the methyl red test (MR test).

Mixed acid fermentation produces a lot of acid (pH<4.4), which leads to a deep red colour (positive) after adding the methyl red solution. If no mixed acid fermentation takes place, little acid is present and a yellow/orange colour (negative) results.

The methyl red test is part of the Bunte series for the differentiation of bacteria, especially from the Enterobacteriaceae group.