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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.



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.


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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Wild bee on a black chokeberry flower.

It has recently been shown that phage therapy can be used to treat and cure hives of honey bees, which are suffering from American foulbrood. Read more about this here or on our blog.

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Swedish University of Agricultural Sciences