The Voges-Proskauer (VP) Test: Principle, Procedure, and Results
The Voges-Proskauer (VP) test is a foundational biochemical assay in microbiology, primarily utilized for the differentiation of bacterial species within the family Enterobacteriaceae and other groups like Actinobacteria. Developed by German bacteriologists Daniel Wilhelm Otto Voges and Bernhard Proskauer in 1898, the test detects the ability of an organism to ferment glucose and produce a neutral, non-acidic end product known as acetylmethylcarbinol, or acetoin. The VP test is not performed in isolation; it is one of the four essential components of the IMViC series of tests, which also includes the Indole, Methyl Red, and Citrate utilization tests. Historically, the VP test, alongside the Methyl Red (MR) test, was crucial for distinguishing between organisms that use the mixed acid fermentation pathway (typically MR positive, VP negative) and those that use the butanediol fermentation pathway (typically MR negative, VP positive), highlighting the inverse relationship often found between the two fermentation routes for glucose metabolism. This test remains indispensable in clinical and research settings for accurate bacterial identification and classification.
Principle of the VP Test: The Butanediol Pathway
The biochemical foundation of the Voges-Proskauer reaction lies in the butylene glycol pathway, also known as the butanediol fermentation pathway. Following the initial breakdown of glucose via glycolysis, which produces the key intermediate pyruvic acid, some bacteria further metabolize the pyruvate through this specific pathway, bypassing the strong acid production characteristic of mixed acid fermentation. In this multi-step process, two molecules of pyruvate are condensed to form the intermediate compound, acetylmethylcarbinol (acetoin), along with the release of carbon dioxide. The acetoin can then be further reduced by the enzyme butanediol dehydrogenase, utilizing NADH, to the final neutral end product, 2,3-butanediol. However, the VP test itself is not designed to detect the final product, 2,3-butanediol, directly. Instead, it is a test for the intermediate, acetoin, which accumulates in the culture medium.
The key to the positive reaction is the addition of the test reagents, which trigger a crucial secondary chemical reaction. When the reagents are added, the acetoin present in the broth is chemically oxidized to a highly reactive diketone compound called diacetyl. This oxidation step requires a strong alkali, typically Potassium Hydroxide (KOH), and the presence of atmospheric oxygen, which is incorporated by vigorous shaking of the tube. Once formed, the diacetyl then reacts with guanidine-containing compounds, which are integral components of the peptone found in the broth medium. This final condensation reaction forms a pinkish-red or cherry-red colored polymer. The alpha-naphthol reagent, a critical addition suggested by Barritt in 1936, functions as a color intensifier and a catalyst for the oxidation, dramatically improving the test’s sensitivity and speed. The visual appearance of this red color complex indicates a positive VP reaction and confirms the organism’s ability to utilize the butylene glycol fermentation pathway.
Media and Reagents Used
The VP test is universally performed using a specialized liquid culture medium known as Methyl Red-Voges-Proskauer (MR-VP) broth, also referred to as glucose-phosphate broth. The composition of this medium is specifically formulated to support the growth of Enterobacteriaceae and simultaneously facilitate the differential testing. It contains three main components: buffered peptone, which supplies the necessary nutrients (including the crucial guanidine compounds) for bacterial growth; a low concentration of glucose (dextrose) as the sole fermentable carbohydrate; and a phosphate buffer (dipotassium phosphate) to maintain a near-neutral pH initially, allowing for the stable accumulation of acetoin. The MR-VP broth is one medium that supports two different tests; after the culture is grown, it is split into two aliquots—one for the Methyl Red test and one for the Voges-Proskauer test.
Two primary reagents, collectively known as Barritt’s Reagents, are added sequentially to the VP aliquot to achieve the chemical conversion and color development. The first reagent is Alpha-naphthol (Barritt’s Reagent A), typically prepared as a 5% solution in 95% ethyl alcohol. Alpha-naphthol is not an original part of the 1898 method but acts as a powerful catalyst and color enhancer for the final complex formation. The second reagent is Potassium Hydroxide (KOH) (Barritt’s Reagent B), usually a 40% aqueous solution. The strong alkali is essential for both the chemical oxidation of acetoin to diacetyl and for creating the highly alkaline environment necessary for the subsequent condensation reaction with the peptone constituents. Due to its caustic nature, KOH must be handled with appropriate safety precautions. The correct order of addition—alpha-naphthol followed by potassium hydroxide—is critical for preventing weak or false-negative results.
Procedure and Protocol
The standard conventional procedure for the Voges-Proskauer test is straightforward and follows a sequence of inoculation, incubation, reagent addition, and observation. First, a tube of sterile MR-VP broth is lightly inoculated with a pure culture of the test organism, often a fresh 18-to-24-hour colony. Second, the inoculated tube is incubated aerobically at a temperature of 37°C. The standard incubation period is 24 to 48 hours, although some protocols recommend retesting after 48 hours and up to five days to maximize the detection of all positive strains. Third, after incubation, a small volume (usually 1.0 to 2.0 mL) of the culture broth is aseptically transferred to a clean, separate tube for the test. The two reagents are then added to this aliquot in the mandatory sequence: first, a specified number of drops of alpha-naphthol (Reagent A) followed by a specified number of drops of potassium hydroxide (Reagent B).
Fourth, immediately after reagent addition, the tube is gently but vigorously shaken for several minutes. This shaking is not just for mixing; it is a critical step for maximizing aeration, which introduces the necessary atmospheric oxygen required to catalyze the oxidation of acetoin to diacetyl. The tube is then allowed to stand undisturbed in a test tube rack at room temperature for final observation. The formation of a positive color is a time-dependent reaction; therefore, the tube must be continuously observed for up to one hour, with most positive results appearing rapidly within the first 10 to 30 minutes. Rapid modifications of the VP test exist, such as the addition of creatine, which can shorten the required observation time significantly, allowing for quicker results in high-throughput diagnostic laboratories.
Results and Interpretation
Interpretation of the Voges-Proskauer test is based solely on the color change observed on the surface layer of the medium. A positive Voges-Proskauer test is confirmed by the development of a distinct pink-red or cherry-red color on the top, or air-broth interface, of the medium. This color is concentrated at the surface because the key oxidation reaction of acetoin to diacetyl is oxygen-dependent. The reaction must be read within a specific time window, typically 10 to 60 minutes after reagent addition; if the characteristic red color appears within this period, the organism is VP-positive, indicating it produces acetoin as a product of glucose fermentation.
A negative VP test is indicated by the culture medium remaining yellow, amber, or colorless in the presence of the reagents. This result signifies that the organism fermented glucose via the mixed acid pathway or another route that did not result in the accumulation of a detectable amount of the neutral intermediate, acetoin. An important cautionary note in interpretation is the appearance of a copper or rust-like color, which may sometimes form due to a non-specific reaction between the KOH and alpha-naphthol reagents alone. This copper color should always be interpreted as a negative result, and the test should not be read after more than one hour to prevent potential false-positive interpretations caused by this background reaction.
Significance and Clinical Application
The VP test holds considerable significance in clinical and public health microbiology for the accurate identification of bacteria, especially within the context of the IMViC battery of tests. It provides essential metabolic data that allows for the clear differentiation of major groups of Gram-negative bacilli, which is crucial for diagnosis and treatment of infections. The test fundamentally separates the butanediol fermenters (VP-positive) from the mixed acid fermenters (VP-negative). Classic examples of VP-positive organisms, which typically show an inverse MR-negative result, include clinically important species from the genera *Enterobacter* (e.g., *E. aerogenes* and *E. cloacae*), *Klebsiella* (e.g., *K. pneumoniae*), *Hafnia alvei*, and *Serratia marcescens*. In contrast, classic VP-negative organisms are generally MR-positive, such as *Escherichia coli*, *Salmonella*, *Shigella*, and *Proteus*. The ability of the VP test to reliably distinguish between these pathogens makes it a cornerstone of the traditional identification flowchart for the Enterobacteriaceae family.
Furthermore, the test’s application extends beyond enteric bacteria to characterize other groups, such as *Staphylococcus* species (generally VP-positive) from *Micrococcus* species (VP-negative). The data generated by the VP test, combined with the other IMViC results, forms a highly specific metabolic fingerprint that is used globally to classify, track, and manage potential bacterial contamination in food, water, and clinical specimens.