The methyl red (MR) test is a common biochemical procedure used in microbiology laboratories. It helps identify bacteria by revealing their metabolic capabilities, specifically how they ferment glucose.
The Methyl Red Test’s Core Principle
The fundamental principle of the methyl red test involves assessing a bacterium’s ability to perform mixed acid fermentation of glucose. During this process, certain bacteria metabolize glucose, converting it to pyruvic acid. They then further process pyruvic acid through a mixed acid pathway, yielding stable acidic end products. These products typically include lactic, acetic, formic, and succinic acids. Their accumulation in the culture medium significantly lowers the pH.
Interpreting a Positive Methyl Red Result
A positive methyl red test is indicated by a red color change in the culture medium. This distinct red color signifies that the organism has produced and maintained a stable, high concentration of acidic end products from glucose fermentation. The red color appears when the pH of the medium is at or below 4.4. The methyl red indicator is red at pH 4.4 or less, yellow at pH 5.8 or higher, with an orange color indicating an intermediate result.
The persistence of this low pH is a key factor, differentiating it from other fermentation pathways where initial acid production might occur but is later neutralized by other metabolic byproducts. Some bacteria produce neutral end products, like acetoin and butanediol, which prevent the pH from dropping to the acidic range. A negative result appears yellow or yellowish-orange, indicating a higher pH, generally above 4.4, often around 6.0 or higher.
Utilizing the Test for Bacterial Identification
The methyl red test is important in microbiology for identifying bacterial species. It is frequently employed as part of a battery of tests, such as the IMViC tests, to differentiate bacterial types, particularly within the Enterobacteriaceae family, which includes common intestinal bacteria. The test helps distinguish species based on their distinct glucose fermentation patterns.
For example, Escherichia coli is methyl red positive (red color) because it conducts mixed acid fermentation, resulting in a stable low pH. In contrast, bacteria like Klebsiella pneumoniae and Enterobacter aerogenes are methyl red negative (yellow or orange color). These organisms ferment glucose via a different pathway, yielding more neutral end products and preventing the necessary pH drop. This metabolic difference aids their identification and differentiation in diagnostic settings.