PROCEDURE
Inoculate VP broth tube with one drop or loopful of fresh inoculum.
Incubate at 37 ºC, 24-48 hours  or until turbid growth is observed.
Add reagent A (KOH 40%) and reagent B (alpha naphthol 6 g in 100 ml ethanol).
Reaction is positive if a red color is seen within 30 minutes. Reaction requires oxygen
presence and it only appears on the surface of the medium unless the tube  is
vigorously shaken.

Werkman published an improvement of VP technique, recommending the utilization
of the ferric chloride which hastens the oxidation of the acetylmethylcarbinol to diacetyl
which reacts with peptone in an alkaline solution to produce a copper coloration.
V-P reaction: negative (left) and positive (right)
Voges-Proskauer Test
DESCRIPTION
Test checks for the presence of acetoin (acetyl methyl carbinol), butylene glycol and finally diacetyl resulted from bacterial
decomposition of carbon compounds (glucose). Diacetyl in the presence of strong KOH solution reacts with proteins to give a pink
coloration.
MEDIUM COMPOSITION
Peptone ................................................................................5g,
Glucose ................................................................................5g,
K
2HPO4 .................................................................................2g,
Distilled water ...........................................................1000 ml.
NOTES
For streptococcal identification weak reactions (pink or rust colors) are interpreted as
positive.
Admin notes:
1. A KOH 60% solution seems to produce a more intense color reaction (see
description paragraph
); admin note.
2. The ethanol may produce the precipitation of medium components leading to a
false-negative result. Do not add reagent B before reagent A
; admin note.
REFERENCES:
1. H. Raducanescu, V.Bica-Popii,1986. Bacteriologie veterinara, Ed. Ceres, Bucuresti.
2. J. F. Gracey, D. S. Collins, Robert J. Huey: Meat hygiene. 1999; 375-376.
3. Margaret Barnett, 1992. Microbiology Laboratory Exercises. Wm. C. Brown Publishers.
4. Werkman C.H.: An Improved Technic For The Voges-Proskauer Test. J Bacteriol. 1930; 20(2): 121–125.
5. Murray, P.R., Baron, E. J., Jorgensen, J.J., Pfaller, M.A., and Yolken, R.H. Manual of Clinical Microbiology, 8th ed. ASM Press:
Washington, DC, 2003.
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