At which pH does the methyl red (MR) test become positive?

The methyl red (MR) test is a valuable tool used to determine the ability of certain bacteria to produce and maintain stable acid end-products from glucose fermentation. The test involves a pH indicator that changes color depending on the acidity of the environment. Methyl red gives a positive result at a pH of 4.5 or lower, which indicates that the bacteria have produced enough acid to lower the pH below this threshold. This level of acidity is consistent with the metabolic activity of mixed acid fermenters, which produce a range of organic acids during fermentation.

At pH levels above 4.5, the color of the methyl red indicator transitions from red (indicating acidic conditions) to yellow (indicating neutral to alkaline conditions). Therefore, a positive methyl red test signifies a strong acid production, reflecting the organism's metabolic capability, and differentiates those that engage in acid fermentation from other metabolic pathways that do not produce significant amounts of acid.

Additionally, other choices represent higher pH levels, which are not conducive to a positive methyl red result, as they indicate a reduction in acid production.