Voges-Proskauer Test
MRVP broth contains peptone, glucose, and phosphate buffer. In this broth, some bacteria will ferment the glucose to produce a mixture of fermentation acids (lactic, acetic, and formic acids), while others will produce only acetic acid, and some will not ferment the glucose at all.
The Voges-Proskauer test is an assay for organisms which ferment glucose to form only one fermentation product, usually acetic acid. The acetic acid is not strong enough to overcome the phosphate buffer in the MRVP broth. Instead, the acetic acid is converted to acetylmethylcarbinol, which leads to a pH of approximately 6.2. After incubation of the organism in the MRVP broth, Barritt’s Reagent A (a-napthol) and B (40% KOH) are added. The reagents will react with acetylmethylcarbinol, and a positive reaction will show a maroon band at the top of the broth in the tube which will diffuse over time into the rest of the media.
Tube 1: Negative -- no maroon band formed in tube after addition of Barritt's Reagents A and B
Tube 2: Positive -- maroon band formed after addition of Barritt's Reagents A and B