Enzymatic Digest of Casein and Enzymatic Digest of Animal Tissue are carbon, nitrogen, and vitamin sources used for general growth requirements in MR-VP Broth. Dextrose is the fermentable carbohydrate. Potassium Phosphate is a buffering agent. Members of Enterobacteriaceae convert glucose to pyruvate by the Embden-Meyerhof pathway. Some bacteria metabolize pyruvate by the mixed acid pathway and produce acidic end products (pH < 4.4), such as lactic, acetic, and formic acids. Other bacteria metabolize pyruvate by the butyleneglycol pathway and produce neutral end products (pH > 6.0), one of which is acetoin (acetylmethylcarbinol). In the MR test the pH indicator, methyl red, detects acidic end products.6 In the VP test, acetoin is oxidized in the presence of oxygen and potassium hydroxide (KOH) to diacetyl, producing a red color.6 The addition of naphthol before KOH enhances the sensitivity of the test.