IMViC Test: Comprehensive Guide for Bacterial Identification

Table of Contents

• Introduction
• Objectives of IMViC Test
• Principle of IMViC Test
• Requirements for IMViC Test
• What is Indole Test?
• What is Methyl Red (MR) Test?
• What is Voges-Proskauer (VP) Test?
• What is Citrate Utilization Test?
• IMViC test result chart of some common bacteria
• Uses of IMViC Test
• Limitations of IMViC Test

Introduction

The IMViC test is a set of four biochemical tests used primarily for the identification and differentiation of bacteria, especially those belonging to the Enterobacteriaceae family. Although applicable to various bacterial types, it is mainly employed to distinguish Gram-negative bacteria. This series is essential for differentiating members of Enterobacteriaceae.

IMViC stands for four biochemical tests, with each letter (except “I”) representing a specific test in the series:

• “I” = Indole Test
• “M” = Methyl Red (MR) Test
• “V” = Voges-Proskauer (VP) Test
• “C” = Citrate Utilization Test (commonly referred to as the Citrate Test)

The lowercase "i" is included for rhyming purposes and does not correspond to a test.

With slight modifications, such as using Sulfide-Indole-Motility (SIM) agar, this series can be expanded to assess six biochemical properties. The SIM medium also tests for bacterial motility and hydrogen sulfide (H2S) production, making it a widely accepted method.

IMViC is one of the most widely used series for preliminary bacterial identification, not only for Enterobacterales but also for some Gram-positive bacteria. It is routinely used in clinical laboratories for teaching and research, as the tests are simple to perform and yield results within 24 to 48 hours, making them ideal for primary screening.

Objectives of IMViC Test

• To examine specific biochemical properties of isolated unknown bacteria, such as indole production, acid production, acetylmethylcarbinol (acetoin) production, and citrate utilization, for their characterization and identification.
• To selectively differentiate and identify members of the Enterobacteriaceae family.

Principle of IMViC Test

The IMViC test is based on differences in metabolic processes and requirements among various bacterial genera and species. The Indole Test and Citrate Utilization Test assess a bacterium's ability to produce specific enzymes and utilize particular nutrients. In contrast, the Methyl Red (MR) Test and Voges-Proskauer (VP) Test detect the end products of bacterial metabolism from specific nutrients. For these tests, bacteria are grown in specific culture media, with the exception of the MR and VP tests, which share the same medium (MR-VP media).

Requirements for IMViC Test

The IMViC test consists of several biochemical tests that require different culture media and reagents. While broths were traditionally used, solid media are now widely recommended for their convenience and ability to assess multiple properties simultaneously.

Requirements for Indole Test:

Culture media:

• Traditionally, tryptophan broth was used for the IMViC test. 
• However, Sulfide-Indole-Motility (SIM) medium is now commonly recommended as it also provides results for hydrogen sulfide (H2S) production and motility. 
• The Motility-Indole-Urea (MIU) medium is another preferred option, as it allows testing for both urease production and motility.

Reagents:

• Kovac’s Indole Reagent, a solution of 4-(dimethylamino)benzaldehyde and hydrochloric acid in n-butanol or amyl alcohol, is commonly used, especially for aerobic organisms. 
• For anaerobic and weak indole-producing organisms, Ehrlich’s Reagent, which contains p-dimethylaminobenzaldehyde and hydrochloric acid in ethyl alcohol, is preferred. 
• Additionally, a 5% solution of p-dimethylaminobenzaldehyde or 1% p-dimethylaminocinnamaldehyde in 10% concentrated HCl is used for the spot indole test.

Requirements for MR Test:

Culture media:

• MR-VP broth

Reagents:

• Methyl red indicator

Requirements for VP Test:

Culture media:

• MR-VP broth

Reagents:

• Barritt’s A solution, also known as VP reagent I, is a 5% α-naphthol solution.
• Barritt’s B solution, or VP reagent II, is a 40% potassium hydroxide (KOH) solution.

Requirements for Citrate Utilization Test: 

Culture media:

• Simmon’s Citrate Agar 

Reagent:

• Bromothymol blue, an indicator already incorporated in Simmon's citrate medium, is used in the Citrate Test. 
• A simplified IMViC agar plate, containing modified media for all four IMViC tests, has also been developed, though it is not widely used.

What is Indole Test?

The indole test, part of the IMViC test series, detects a bacterium's ability to produce indole as a metabolic byproduct of tryptophan breakdown. It is represented by the letter “I” in the IMViC acronym.

Principle of Indole Test

Certain bacteria produce an enzyme called tryptophanase, which enables them to metabolize the amino acid tryptophan into indole, pyruvic acid, and ammonia.

When an indole reagent is added to a medium containing a bacterial culture that has produced indole, the indole reacts with the aldehyde in the reagent, forming a distinct color.

• If the reagent contains benzaldehyde, it forms a pink to violet-red quinoidal compound, resulting in a pink to red color ring.
• If the reagent contains cinnamaldehyde, it produces a blue to green compound, creating a green to blue color ring.

Kovac’s indole reagent uses amyl alcohol and benzaldehyde. The amyl alcohol is water-insoluble and forms an oily layer, leading to a cherry-red or pink-red ring at the top.

Bacteria are cultured in a tryptophan-containing medium for 24–48 hours, after which the indole reagent is added to interpret the result. A positive result is indicated by the formation of a pink to violet-red or green to blue color ring, depending on the reagent used. A negative result is shown by the absence of color change or the appearance of a slight yellowish ring at the top.

• Indole Positive Bacteria include Escherichia coli, Klebsiella oxytoca, Vibrio cholerae, Proteus vulgaris, Porphyromonas asaccharolytica, Vibrio spp., Flavobacterium spp., Providencia spp., Enterococcus faecalis, Haemophilus influenzae, Morganella morganii, Aeromonas spp., and Citrobacter koseri.
• Indole Negative Bacteria include Klebsiella pneumoniae, Proteus mirabilis, Salmonella spp., Shigella spp., Citrobacter freundii, Pseudomonas aeruginosa, Bacteroides fragilis, and Staphylococcus aureus.

What is Methyl Red (MR) Test?

The Methyl Red (MR) Test is a biochemical assay used to identify bacteria that produce stable mixed acids as end products of glucose metabolism. It is represented by the letter “M” in the IMViC series.

Principle of MR Test

Certain bacterial species utilize the mixed acid fermentation pathway for glucose metabolism. Through this process, they convert pyruvate into stable mixed acids.

When bacteria that ferment glucose via the mixed acid pathway are cultured in a glucose-containing medium, they produce acids that lower the pH of the medium to 4.4 or below. Adding methyl red indicator to this medium will turn it red, indicating acid production.

After 24 hours of incubation in MR-VP broth, methyl red is added to the medium. A positive result is shown by the development of a red color, while a negative result is indicated by a yellowish color.

• MR Positive Bacteria include Escherichia coli, Salmonella spp., Shigella spp., Citrobacter spp., Proteus spp., Yersinia spp., Edwardsiella spp., and Staphylococcus aureus.
• MR Negative Bacteria include Klebsiella pneumoniae, Enterobacter spp., Hafnia spp., and Serratia marcescens.

What is Voges-Proskauer (VP) Test?

The Voges-Proskauer (VP) Test, part of the IMViC series, is a biochemical assay that detects a bacterium's ability to convert pyruvate into a neutral intermediate product known as acetylmethylcarbinol or acetoin. It is represented by the letter “V” in the IMViC acronym.

Principle of VP Test

During the butanediol fermentation pathway, pyruvate is metabolized into a neutral intermediate product known as acetyl methyl carbinol, commonly referred to as acetoin.

If acetoin is present in the medium, it is readily oxidized to diacetyl in the presence of air and potassium hydroxide (KOH). The diacetyl then reacts with the guanidine component of peptone in the presence of α-naphthol, producing a pink to red color.

After 48 hours of aerobic incubation in MR-VP broth, VP reagents I and II are added. The color change should be observed within 30 minutes. A positive result is indicated by the development of a pink to red color at the top of the broth either immediately or within 30 minutes, but not after one hour. No color change indicates a negative VP test.

• VP Positive Bacteria include Klebsiella spp., Enterobacter spp., Viridans streptococci (excluding S. mitis and S. vestibularis), Proteus mirabilis, Hafnia spp., Serratia spp., and Staphylococcus aureus.
• VP Negative Bacteria include Escherichia spp., Proteus vulgaris, and Citrobacter freundii.

What is Citrate Utilization Test?

Citrate Utilization Test is a biochemical test in the IMViC test series which detects the ability of organisms (bacteria) to utilize citrate as a sole source of energy. It is indicated by the letter “C” of the IMViC. 

Principle of Citrate Utilization Test

Some bacteria can utilize ‘citrate’ as their sole source of carbon. Such bacteria produce citrase enzymes which will break the citrate into oxaloacetic acid and acetic acid. The oxaloacetic acid will then be decarboxylated to produce pyruvate and CO2.

Released CO2 will combine with H2O and excess sodium from sodium citrate to produce alkaline ‘sodium carbonate’.  The sodium carbonate will increase the pH of the medium.

CO2 + H2O + excess sodium from sodium citrate → Na2CO3 (alkaline)

Additionally, the released CO2 will trigger the metabolism of ammonium salts. Utilization of the ammonium salts as a source of nitrogen will cause the production of ammonia (or ammonium hydroxide). 

Ammonium salt → Ammonium hydroxide (alkaline) 

The combined effect of ammonium hydroxide and sodium carbonate will increase the pH of the media above 7.6. This increase in pH will turn the pH indicator bromothymol blue in the medium from deep forest green (at neutral pH) to Prussian blue.

Following the incubation of 24 – 48 hours (up to 4 days for some), bacterial growth and color change in the slant portion is observed. A positive result is indicated by growth and change in color of slant from green to intense blue. A negative result is indicated by no change in the color of the slant.

• Citrate Positive Bacteria: Klebsiella spp., Citrobacter spp., Serratia marcescens, Proteus mirabilis, Enterobacter spp., Salmonella spp. (except Salmonella Typhi and Paratyphi A), Edwardsiella spp., Providencia spp.
• Citrate Variable Bacteria: Proteus vulgaris, V. cholera, V. parahaemolyticus.
• Citrate Negative Bacteria: Escherichia coli, Shigella spp., Salmonella Typhi and Paratyphi A, Yersinia spp., Morganella morganii, Staphylococcus aureus, etc.

IMViC test result chart of some common bacteria

Uses of IMViC Test

• This test is commonly utilized in clinical, research, and teaching laboratories to characterize and identify unknown bacterial isolates at the genus level. 
• Along with the Urease test and Triple Sugar Iron (TSI) test, it helps distinguish and identify members of the Enterobacteriaceae family. 
• Certain tests are also used to differentiate between species within a genus. For instance, the indole test distinguishes between Klebsiella oxytoca and Klebsiella pneumoniae, Citrobacter koseri and Citrobacter freundii, as well as Proteus vulgaris and Proteus mirabilis, among others.

Limitations of IMViC Test

• This method alone is insufficient for fully identifying bacteria at the species level, as additional tests are needed, even when differentiating Enterobacterales. 
• Different genera can yield similar results, leading to ambiguity. Being a culture-based test series, it requires a longer time for completion. Some bacteria may need more than two days of incubation to show a VP reaction, while the citrate test can take over four days of incubation. 
• Additionally, only culturable bacteria can be characterized. The entire process is complex, demanding strong culture skills and various resources. 
• Incorrect incubation times or the use of old or improperly applied reagents can result in false positive or false negative outcomes.