What is this medium used for? When a microbe is given glucose, the resulting products of its metabolism can be many. This medium is used to determine whether the products fall into either the mixed acids fermentation products category or are the so-called neutral fermentation products. Mixed acids fermentation results in accumulation of a variety of acids and a significant drop in the pH of the medium. Neutral fermentation can generate a product called 2,3-butanediol.
How is mixed acids fermentation determined? If the microbe performs mixed acids fermentation, the medium will acquire an acidic pH. Addition of a pH indicator which changes color below pH 4 indicates presence of the extreme acidity associated with mixed acids fermentation. The reagent added to determine pH is methyl red, and so the test is called the methyl red (MR) test.
How is 2,3-butanediol production determined? 2,3-butanediol is the end product of a long fermentation pathway, but it is not easily detected. So, the test detects one of the pathway intermediates instead: acetylmethylcarbinol (acetoin). The production of 2,3-butanediol is thus indirectly detected when the pathway intermediate acetoin reacts with reagents to turn red. The test is named for its discoverers, as the Voges-Proskauer (VP) test.
What is the content of this medium? MRVP broth is a nutrient medium with 0.5% glucose added.
How are these tests performed? For information on how to determine mixed acids fermentation, refer to the methyl red test. For more information on how to determine 2,3-butanediol production, refer to the Voges-Proskauer test.