Define the Methyl Red Test?

As you have read in previous section, all enteric bacteria utilize glucose to obtain energy. Depending upon the end products formed, all enteric bacteria can be categorized into two major groups. One group produces large amount of acid, i.e., formic, acetic, lactic and succinic acid from glucose, which is stabilized and maintained in the culture while other group produces low amount of acid which soon get converted to neutral product, acetoin. The end products formed depend upon the enzymatic pathways present in the organism. These two groups can be differentiated by MR-VP test, the two tests performed simultaneously on same MR-VP medium.

Generally the results are opposite, i.e., if organism is giving MR test positive, VP test is negative and vice-versa. MR-VP test is of value in separating E. coli and Enterobacter aerogenes in particular. Both are coliforms and are identical except in certain physiological characteristics. In MR test, methyl red is used as pH indicator. If the methyl red remains red on addition into the culture medium, it indicates the production of large amount of acid (pH 4.0) and MR test is positive for the organism. However, if methyl red turns yellow, it indicates that high pH (pH 6.0) is produced because of conversion of organic acids to non-acidic end components, i.e., ethanol and acetoin (acetyl methyl carbinol). It indicates negative MR.