MR-VP Broth (Methyl Red-Voges Proskauer Medium/Broth, also known as Buffered Peptone-Glucose Broth) is used for the differentiation of bacteria by means of the methyl red and Voges-Proskauer reactions.
SUMMARY AND EXPLANATION
Voges and Proskauer, in the latter part of the 19th century, reported the initial observations regarding the production of a red color after the addition of potassium hydroxide to specific culture media in which various organisms had grown.
Clark and Lubs, in 1915, found that the addition of methyl red to cultures of Escherichia coli resulted in a red color due to the high acidity produced during the fermentation of dextrose. The smaller amount of acid produced by Klebsiella pneumoniae and Enterobacter aerogenes is converted to acetoin resulting in an alkaline reaction (negative methyl red test).
In the Voges-Proskauer test, Reagent A (5% [w/v] alphanaphthol in absolute alcohol) contains a catalyst enhancing the formation of specific metabolic products that form a red complex upon the addition of Reagent B (40% [w/v] potassium hydroxide in purified water).
MR-VP Medium/Broth was developed to enable both the MR and the VP tests to be performed in the same medium, although in different tubes or on aliquots from the same tube.
Methyl red-positive organisms produce high levels of acid during fermentation of dextrose, overcome the phosphate buffer system and produce a red color upon the addition of the methyl red pH indicator.
In the Voges-Proskauer test, the red color produced by the addition of potassium hydroxide to cultures of certain microbial species is due to the ability of the organisms to produce a neutral end product, acetoin (acetylmethylcarbinol), from the fermentation of dextrose. The acetoin is oxidized in the presence of oxygen and alkali to produce a red color. This is a positive Voges-Proskauer reaction.