Cat. no. Z91 Voges-Proskauer Reagent A 15ml
Cat. no. Z92 Voges-Proskauer Reagent B 15ml


Hardy Diagnostics Voges-Proskauer Reagents are recommended for use in determining the Voges-Proskauer (VP) reaction of bacteria.


Certain bacteria produce stable acid end products when cultivated in specific media. After fermentation of glucose, particular enteric bacteria metabolize pyruvic acid to acetylmethyl carbinol. This end product reacts with alpha-naphthol in the presence of 40% potassium hydroxide to produce a red color complex.


Ingredients per liter:*

Voges-Proskauer Reagent A:
Alpha-Naphthol, 5% 50.0gm
Absolute Ethanol 1000.0ml

Voges-Proskauer Reagent B:
Potassium Hydroxide 400.0gm
Deionized Water 1000.0ml

* Adjusted and/or supplemented to meet performance criteria.


Upon receipt store at 2-30ºC. Products should not be used if there are any signs of contamination, deterioration, or if the expiration date has passed. Do not expose to excessive heat or moisture.


Sterilize all biohazard waste before disposal.

Warning! Voges-Proskauer Reagent A can cause irritation.

Warning! Voges-Proskauer Reagent B is extremely caustic; it will cause burns.

Avoid contact with skin. If contact occurs, rinse thoroughly with copious amounts of water.


Specimen Collection: This product is not intended for primary isolation of patient specimens. It should be used in conjunction with other biochemical tests to identify cultures of isolated organisms.

Method of Use: Inoculate MR-VP (Cat. no. K37) medium with the test organism. Incubate 24 to 48 hours at 35ºC. Aseptically transfer 1ml of the incubated broth to a clean test tube. Add 15 drops of Voges-Proskauer Reagent A followed by 5 drops of Voges-Proskauer Reagent B. Shake gently to aerate. Examine for the appearance of a red color within 20 minutes. If the 24 hour test is negative, repeat the test with a 48 hour culture of the test organism. If equivocal results are obtained, repeat the test with cultures incubated for 5 days at 25-30 degrees C.

If VP Reagents are to be used with commercial identification tests, such as API®, Microgen™ GnA+B-ID System, EnteroPluri-Test, consult the manufacturer's literature.


Positive reactions are denoted by the development of a red color within 20 minutes. Negative reactions remain yellow to amber. Development of a copper color should be considered negative. Hold negatives for an additional 45 minutes since the maximum color development occurs 1 hour after the reagents are added. Do not read the test after more than 1 hour of incubation.


The Voges-Proskauer test may be used in the identification of gram-negative bacteria. Additional biochemical testing using pure culture is recommended for complete identification.

Prolonged incubation (greater than 72 hours at 35ºC.) of certain VP-positive strains of Enterobacteriaceae may result in false-negative reactions due to the breakdown of acetylmethyl carbiol.

The order of the addition of reagents is extremely important. Reversal of the order may result in weak positive reactions or false-negative reactions.


Standard microbiological supplies and equipment such as loops, needles, incubators, and incinerators, etc., as well as biochemical and serological reagents, are not provided.


Test Organisms Reaction
Enterobacter cloacae
ATCC® 23355
Positive VP; red color change
Escherichia coli
ATCC® 25922
Negative VP; no color change

User Quality Control

It is recommended that each new lot or shipment of reagent be tested with known positive and negative controls.(3,4)

Physical Appearance

Negative VP Test

Esherichia coli (ATCC® 25922) grown in MR-VP Broth (Cat. no. K37). Incubated aerobically for 24 hours at 35ºC. A 1mL aliquot was removed and 0.6mL of alpha-naphthol (Cat. no. Z91) and 0.2mL of 40% KOH (Cat. no. Z92) was added to the tube. No development of a pink-red color within 10-60 minutes was indicative of a negative VP reaction.

Positive VP Test

Enterobacter cloacae (ATCC® 23355) grown in MR-VP Broth (Cat. no. K37). Incubated aerobically for 24 hours at 35ºC. A 1mL aliquot was removed and 0.6mL of alpha-naphthol (Cat. no. Z91) and 0.2mL of 40% KOH (Cat. no. Z92) was added to the tube. Development of a pink-red color within 10-60 minutes was indicative of a positive VP reaction.


1. Versalovic, J., et al. Manual of Clinical Microbiology. American Society for Microbiology, Washington, D.C.

2. Tille, P., et al. Bailey and Scott's Diagnostic Microbiology, C.V. Mosby Company, St. Louis, MO.

3. Anderson, N.L., et al. Cumitech 3B; Quality Systems in the Clinical Microbiology Laboratory, Coordinating ed., A.S. Weissfeld. American Society for Microbiology, Washington, D.C.

4. Isenberg, H.D. Clinical Microbiology Procedures Handbook, Vol. I, II & III. American Society for Microbiology, Washington, D.C.

5. MacFaddin, J.F. Biochemical Tests for Identification of Medical Bacteria,, Lipincott Williams & Wilkins, Philadelphia, PA.

API is a registered trademark of bioMeriuex, France.
ATCC is a registered trademark of the American Type Culture Collection.
Microgen™ GnA+B-ID System is a trademark or Microgen® Bioproducts, U.K.