Methyl red test

Methyl Red Test

The Methyl Red Test is a biochemical test performed to identify bacteria that produce stable acid end products by fermenting glucose. The test is part of the IMViC series—a group of tests including Indole test, Methyl Red test, Voges-Proskauer test, and Citrate test—used for differentiating Enterobacteriaceae, especially members of the genera Escherichia and Enterobacter.

Procedure

The test involves the addition of a pH indicator, methyl red, to a culture broth that has been incubated with a specific bacterium. Methyl red is a pH indicator that turns red in a pH environment of 4.4 or lower, indicating a positive test, and yellow at a pH of 6.2 or higher, indicating a negative test. A positive Methyl Red Test indicates that the organism is capable of performing mixed acid fermentation when supplied glucose.

Interpretation

• Positive result: A red color indicates the presence of stable acid end products, primarily lactic, acetic, and formic acids, showing that the bacterium ferments glucose with the production of these acids.
• Negative result: A yellow color indicates the absence of these stable acid end products, suggesting the bacterium either does not ferment glucose or it metabolizes it through pathways that do not result in acid accumulation.

Applications

The Methyl Red Test is widely used in microbiology to distinguish between different members of the Enterobacteriaceae family. For example, Escherichia coli typically tests positive, indicating it ferments glucose with the production of stable acids, while Enterobacter aerogenes and Klebsiella pneumoniae typically test negative, indicating they ferment glucose without producing stable acids.

Limitations

The accuracy of the Methyl Red Test can be affected by the age of the culture, as older cultures may have consumed the initial glucose supply and started metabolizing other components, leading to a false negative. Additionally, the test is specific to members of the Enterobacteriaceae family and may not provide reliable results for other bacteria.

Methyl red test Resources
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