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Mixed acid fermentation

General

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.

Method

MR-test

Methyl red test, where a positive reaction has occured in the right tube. (Image: SLU/SVA.)

  1. Suspend a bacterial colony from a pure culture in the MR/VP medium.
  2. Incubate at 30°C during 3 days or at 37°C during 48 h.
  3. Add 2-3 drops of a solution of methyl red.
  • Positive test result: red colour change
  • Negative test result: no colour change.

Use

Some members of the family Enterobacteriaceae have mixed acid fermentation (see the respective bacterial page), which can be used to differentiate these bacteria.

Updated: 2017-12-08.


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Swedish University of Agricultural Sciences