Try to remember that your Methyl Red & Voges-Proskaur (MR-VP) tests won’t both test positive in almost all organisms, with some very rare exceptions. In fact, a double positive here is a good way to control for errors in your experiment. It’s helpful to remember the organisms in this test do opposite things.
With MR we are looking for organisms capable of performing mixed acid fermentation. These tend to be more stable than most fermentation acid end products and are therefore capable of overcoming the medium’s phosphate buffer, resulting in the medium becoming quite acidic. Methyl red gives a positive result at pH 4.4.
In the VP test we are checking for organisms that recycle their fermentation acids into acetoin and 2,3 butandiol (probably misspelled that). These are neutral substances, so the medium’s pH won’t be very acidic at all. VP reagents turn red in the presence of acetoin and give a positive for this type of organism.
Sorry if I missed anything. Just typed that off the top of my head. Hope this helps.
Forgot to mention the important thing! This is part of the IMViC battery of tests used to differentiate between members of the family Enterobacteriacea and other Gram-negative rods.
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