Voges Proskauer test or VP test is used to identify some enterobacteria. But some kinds of it are VP negative. The optimal temperature for the VP test is 37 C but some groups show negative results at this temperature.
The Hafina group shows negative results at 37 C, but it gives positive results at 30 C or lower.
Incubation time is 24 to 48 hours but to label an organism as negative VP.
It is suggested that in an organism other than enterobacteria, phosphate interferes with the production so for the testing of these organisms Glucose-Peptone broth (without salt or phosphate ) can be used.
Principle OF Voges Proskauer test (vp test)
The existence of certain enzymes can be used to differentiate the different microorganisms based on the end product of glucose metabolism.
It can be detected by the various color indicator reagents. Acetoin and butanediol fermentation pathway gives pink or red color in the presence of @-naphthol in the relatively alkaline environment.
Voges Proskauer test reagents composition and preparation
VP test media consists of three kinds of reagents. Reagent A and reagent B reagent C.
Reagent A composition and preparation
- @-Naphthol 5 g
- Absolute ethyl alcohol 100 ml
Dissolve the @-naphthol in a small amount of ethyl alcohol and bring the volume up to 100 ml. Alcohol should be colorless. After preparation store it in a brown bottle at 4 C temperature.
Reagent B composition and preparation
- Potassium hydroxide (KOH) 40 g
- Distilled Water 100 ml
First of all, weigh the KOH because it is hygroscopic and turns caustic in moist conditions. Now add distilled water to prevent overheating. Now bring the volume up to 100 ml in a cylinder. Finally, store this reagent in the refrigerator in a polyethylene bottle.
C-Reagent composition and preparation
- Creatine 1 g
- Hydrochloric Acid HCL 100 ml
Voges Proskauer test procedure
Following are the steps in the VP test procedure.
- Take 2 ml glucose phosphate broth in a test tube.
- Inoculate the broth with the test organism. After inoculation, incubate it at 37C for 48 hours.
- NOTE: Take the sample from the pure colony.
- Add 0.6 ml of reagent A and then add reagent B.
- Now gently shake the tube. Make slope of the tube without cover to increase the area of the air-liquid interface.
- Keep the test tube at room temperature for 15 minutes to one hour.
- Now check the tube color after 15 minutes and again after one hour.
Voges Proskauer test result
- The strong red color of the slop is a sign of VP-test positive.
- Colorless or yellow color is for VP-test negative.
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