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Mixed acid fermentation
Some bacteria can ferment glucose to a mixture of the following organic acids: formic acid, acetic acid and lactic acid. This is called mixed acid fermentation and it causes highly decreased pH in the medium. Mixed acid fermentation can, therefore, be detected by addition of the pH indicator methyl red (MR). The test method is sometimes called the MR test.
Some members of the family Enterobacteriaceae have mixed acid fermentation (see the respective bacterial page), which can be used to differentiate these bacteria.