MR-VP

This media contains only Peptone(protein source), Glucose(Carbon source), and Sodium Phosphate (buffer). This media forms the basis for two biochemical tests, Methyl Red and Voges-Proskauer. The two tests look for evidence of the final fate of glucose from the bacteria. The two tests cannot be performed in the same tube, and so samples are taken from the main tube into two disposable tubes for the reactions. The Methyl Red component identifies the presence of mixed acid byproducts from glucose. It is the simpler of the two tests, requiring only a few drops of methyl red added to a sample from the reaction tube. The absense of mixed acid products will cause the die to turn yellow (a negative result), if the dye remains red, however, this is a positive result.

Voges-Proskauer gets a little more in depth. This test only targets a single glucose product, Acetoin. Two reagents are required for this to become a visible trait in the media. Alpha-Napthol (a double benzene ring with a hydroxyl group) and Potassium Hydroxide catylize Acetoin into Diacetyl. Diacetyl will react with Guanidine found in the peptone to produce a red coloration indicative of a positive test. No red color is a negative result.

Gram positives tend to react variably to this test, sometimes getting both sections positive or negative. The majority of Gram negatives, especially Gram Negative rods will only ever be positive for one and negative for the other test. The enterics are good examples of this.