VP Semisolid Medium

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.

Preparation

Suspend 31 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Dispense and sterilize at 121 °C for 15 minutes.

Instructions for use

1. Inoculate and incubate the medium for up to 5 days.
2. Carry out the Voges-Proskauer test (VP):

To 5 mL of the medium inoculated and incubated for up to 5 days, add 0.6 mL of 5% alpha-naphthol in absolute ethanol and 0.2 mL of 40% sodium hydroxide and shake from time to time over a 15-minute period. The tube may be held at room temperature or incubated at 35 ± 2 °C. It is important to add the reagents in sequence. A positive test is indicated by the development of a faint pink to red color. The test should not be read after one hour because negative VP cultures may develop a copper color after that time.

• Clark and Lubs. J. Inf. Dis. 17:160. 1955.
• Ewing. Enterobacteriaceae. USPHS.
• Edwards and Ewing. Identification of Enterobacteriaceae Burgess Publ. Co. Minneapolis, Minn., 1962.
• Voges, O., and B. Proskauer. 1898. Z. Hyg. 28: 20-22.
• Association of Official Analytical Chemists. 1995. Bacteriological analytical manual, 8th ed. AOAC International, Gaithersburg, MD.