Enterobacteriaceae Practice Test

The methyl red (MR) test is designed to detect acid production by certain bacteria during glucose fermentation. It utilizes a pH indicator—methyl red—that changes color depending on the acidity of the environment. The MR test becomes positive at a pH level of 4.5 or below, which is indicative of a strong acid production in the fermentation process.

When glucose is fermented, some bacteria produce organic acids, and this shift in pH to a more acidic level is what the MR test detects. At pH 4.5 or lower, methyl red turns red, signaling a positive result for acid production. This is crucial for differentiating between members of the Enterobacteriaceae family, particularly those that produce stable acidic end products.

In contrast, at pH levels above 4.5, such as 6.0, 6.5, or 7.0, the methyl red indicator shifts to yellow, indicating a neutral to mildly alkaline environment and a negative result. Thus, the correct answer reflects the necessary conditions for the methyl red test to show a positive result.