The IMViC tests are a group of different biochemical tests used in diagnostic clinical microbiology testing to identify an organism in the coliform (Enterobacteriaceae) group. A coliform is a gram negative, aerobic or facultative anaerobic bacilli which produces gas from lactose within 48 hours of incubation. The presence of some coliforms indicate fecal contamination of the sample.
Except for the lowercase letter "i" is added for ease of pronunciation, each of the letters in "IMViC" stands for one of these biochemical tests.
In this test, the organism for identification is inoculated in Peptone Water Broth. The medium contains tryptophan, which under the action of enzyme tryptophanase is converted to an indole molecule, pyruvate and carbon dioxide. The indole is then extracted from the broth by means of xylene. To test the broth for indole production, Kovac's reagent is added after incubation. A positive result is indicated by a pinkish-red layer forming on top of the liquid. Chemical reaction flow is shown.
Tryptophan (Kovac’s) + Tryptophanase → Indole + Xylene (pinkish-red)
Methyl Red--Vogues-Proskauer Test
The chemical reaction flow for Voges-Proskauer:
Citrate Utilization Test
The test uses Simmon's citrate agar to determine the ability of a microorganism to use citrate as its sole carbon source. The agar contains citrate and ammonium ions as nitrogen source and bromothymol blue as an indicator. The citrate agar is green before inoculation, and turns blue as a positive test indicator, meaning citrate is utilized.
The chemical reaction flow for Citrate Utilization:
Simmon’s Citrate (citrate and Ammoium ions) + Bromthymol Blue (Green)→ Blue (Positive result)
Except for the lowercase letter "i" is added for ease of pronunciation, each of the letters in "IMViC" stands for one of these biochemical tests.
- "I" is for Indole test;
- "M" is for Methyl red test;
- "V" is for Voges-Proskauer test, and
- "C" is for Citrate utilization test.
- The lower case "i" is for "in" as the Citrate test requires coliform pure isolate samples to be placed "in Citrate agar".
In this test, the organism for identification is inoculated in Peptone Water Broth. The medium contains tryptophan, which under the action of enzyme tryptophanase is converted to an indole molecule, pyruvate and carbon dioxide. The indole is then extracted from the broth by means of xylene. To test the broth for indole production, Kovac's reagent is added after incubation. A positive result is indicated by a pinkish-red layer forming on top of the liquid. Chemical reaction flow is shown.
Tryptophan (Kovac’s) + Tryptophanase → Indole + Xylene (pinkish-red)
Methyl Red--Vogues-Proskauer Test
These tests both use the same broth for bacterial growth. The broth is called MRVP broth. After growth, the broth is separated into two different tubes, one for the Methyl Red (MR) test and one for the Voges-Proskauer (VP) test.
The Methyl Red test detects production of acids formed during metabolism using mixed acid fermentation pathway using pyruvate as a substrate. Methyl Red, used as a pH indicator is added to one tube and a red color appears at pH lower than 4.2 which indicates a positive test (mixed acid fermentation is used). The solution remaining yellow with pH of 6.2 or above indicates a negative test, meaning the butanediol fermentation is used.
The Methyl Red test detects production of acids formed during metabolism using mixed acid fermentation pathway using pyruvate as a substrate. Methyl Red, used as a pH indicator is added to one tube and a red color appears at pH lower than 4.2 which indicates a positive test (mixed acid fermentation is used). The solution remaining yellow with pH of 6.2 or above indicates a negative test, meaning the butanediol fermentation is used.
The Voges-Proskauer test uses alpha-naphthol and potassium hydroxide to test for the presence of acetylmethylcarbinol or acetoin, an intermediate of the 2,3-butanediol fermentation pathway. After adding both reagents, the tube is shaken vigorously or by vortex mixer then allowed to sit for 5-10 minutes. A pinkish-red color indicates a positive test, meaning the 2,3-butanediol fermentation pathway is used.
The chemical reaction flow for Methyl Red test:
The chemical reaction flow for Methyl Red test:
Pyruvate (Mixed Acid Fermentation) + Methyl Red Indicator → Red (pH4.2)
Pyruvate (Butanediol Fermentation) + Methyl Red Indicator→ Yellow (pH6.2)
Pyruvate (Butanediol Fermentation) + Methyl Red Indicator→ Yellow (pH6.2)
The chemical reaction flow for Voges-Proskauer:
α-napthol + KOH + acetylmethylcarbinol (acetoin)→ (Positive result) pink/red ; (Negative result) yellow
The test uses Simmon's citrate agar to determine the ability of a microorganism to use citrate as its sole carbon source. The agar contains citrate and ammonium ions as nitrogen source and bromothymol blue as an indicator. The citrate agar is green before inoculation, and turns blue as a positive test indicator, meaning citrate is utilized.
The chemical reaction flow for Citrate Utilization:
Simmon’s Citrate (citrate and Ammoium ions) + Bromthymol Blue (Green)→ Blue (Positive result)
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