VP Semisolid Medium is used in the differentiation of enteric Gram-negative bacilli on the basis of acetylmethylcarbinol (Voges-Proskauer) reactions of the Escherichia-Enterobacter group. Tryptone and Soy peptone provide nitrogen, vitamins, minerals and amino acids essential for growth. Yeast extract is the water-soluble portion of hydrolyzed yeast and is a source of vitamins, particularly of the B-group. Dextrose is the fermentable carbohydrate providing carbon and energy. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Bacteriological agar is the solidifying agent. 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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