MR-VP Medium is used in the differentiation of enteric Gram-negative bacilli on the basis of the methyl red and acetylmethylcarbinol (Voges-Proskauer) reactions of the Escherichia-Enterobacter group. Casein peptone provides nitrogen, vitamins, minerals and amino acids essential for growth. Dextrose is the fermentable carbohydrate providing carbon and energy. Potassium phosphate acts as a buffer system. In 1915, Clark and Lubs used methyl red as an indicator of acidity in the cultures of the Coli-Enterobacter group. This test is now known as the methyl red test and serves to distinguish between those microorganisms that produce and maintain a high concentration of acid from those that initially produce a small amount of acid and are capable of later attacking those same acids, turning the medium neutral or alkaline, such as Enterobacter species. Voges and Proskauer described in 1898 a fluorescent red coloration that appeared in certain cultures upon adding drops of KOH solution. Later it was supposed that this reaction was due to the oxidation of acetylmethylcarbinol to diacetyl that reacted with the peptone of the medium to give a red color. Enterobacter bacteria oxidize the acetylmethylcarbinol and give a red coloration, in contrast to Escherichia coli that does not.
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