Answer to Question #11763 in Microbiology for Marcy Randall

Question #11763
Sugar fermentation tests are positive when the pH turns acidic. How are media for these tests modified to allow production of acids to be detected?
Expert's answer
In this case the media will be modified with indicatir adding.You can use for instance the MR test which employs the addition of the pH indicator methyl red, to determine whether an organism has produced high levels of acid during glucose fermentation.

Bacteria that use the mixed acid fermentation pathway produce high levels of organic acid—such high levels of acid causes the methyl red to turn red in color. If the amount of acid production is relatively lower, then the medium remains yellow or turns slightly orange after the addition of methyl red. If the media remains red this is a positive (+) reaction for the MR test if the medium remains yellow or turns slightly orange, this is a negative reaction for the MR test.

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