Voges-Proskauer (VP) test is used to identify Gram negative bacteria that produce 2,3-butanediol (acetoin) from the fermentation of glucose. The Voges–Proskauer test determines the capability of some organisms to produce non acidic or neutral end products, such as acetyl methyl carbinol from glucose or sugar fermentation.
VP is a test used to detect acetoin in a bacterial broth culture. The test is performed by adding alpha-naphthol and potassium hydroxide to the Voges-Proskauer broth which has been inoculated with bacteria. In the presence of atmospheric oxygen and 40 % potassium hydroxide, acetoin is converted to diacetyl.
Alpha–naphthol serves as a catalyst to bring out a red complex. Some bacteria utilize glucose producing acetyl methyl carbinol which is oxidized in the presence of alkali to diacetyl. The presence of diacetyl is confirmed by the formation of red colour with arginine, creatine or creatinine.
VP positive bacteria turn the medium in which they are inoculated to brownish-red to pink because they produce acetoin. Bacteria that are VP negative turn the inoculated medium to brownish-green to yellow. Klebsiella pneumoniae and Enterobacter aerogenes are VP positive bacteria.
PROCEDURE FOR VOGES PROSKAUER TEST
- Prepare and sterilize glucose-phosphate as the growth medium and cool to room temperature.
- Inoculate the glucose-phosphate medium with the test bacterium.
- Incubate medium at 37oC overnight.
- Add 5 mg of creatine followed by 5 ml of 40 % NaOH and shake the tube very well.
- Observe tube(s) for a change in colour. Change of the colour of the medium to brownish-red to pink signifies a positive VP test (Figure 1).
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