What is the primary function of the methyl red (MR) test?

The primary function of the methyl red (MR) test is to measure acid production. This test is particularly useful for differentiating between bacterial species based on their ability to ferment glucose and produce acids as a byproduct. When bacteria metabolize glucose, some produces stable acids that lower the pH of the medium.

The methyl red indicator, which is added to the test medium, turns red at a pH of 4.4 or lower, indicating a high concentration of acid. If the medium remains yellow, it suggests that the bacteria did not produce significant acid or that any produced was not stable. Thus, a positive MR test (red color) demonstrates that the organism is capable of mixed acid fermentation, which is a characteristic of certain groups of Enterobacteriaceae.

In contrast, the other options focus on different metabolic pathways that are not the primary purpose of the MR test. For example, glucose fermentation is certainly part of what is assessed in the MR test, but it is specifically the production of stable acids that is directly measured. Indole production is determined by a separate test, and protein catabolism does not directly relate to the MR test's focus on acid production during fermentation.