MICROBIOLOGY IMViC series 1) What is the product being detected in a positive in
ID: 73243 • Letter: M
Question
MICROBIOLOGY
IMViC series
1) What is the product being detected in a positive indole test? Where does it come from?
2) What is the chemical composition of Kovac’s reagent?
3) Why does the Kovac’s reagent float atop the broth culture?
4) What metabolic products are being detected in a positive methyl red test? Where do they come from?
5) Predict the expected result for the methyl red test of an organism that cannot ferment glucose. Explain.
6) What is the product being detected in a positive Voges-Proskauer test? Where does it come from?
7) What is the chemical composition of the reagents used for the VP test?
a. Barritt’s A
b. Barritt’s B
8) What is the metabolic ability being detected in a positive citrate test?
Explanation / Answer
1.
Indole is being detected in a positive indole test. Peptone water broth contains tryptophan, which under the action of enzyme tryptophanase is converted to an Indole.
2.
Kovac's reagent is a biochemical reagent consisting of isoamyl alcohol, para-dimethylaminobenzaldehyde and concentrated hydrochloric acid.
3.
Isoamylalcohol in Kovac's reagent allows the complex to float on the top of broth as it is less denser than broth. .
4.
Methyl Red Test is used to detect the production of large amounts of acids like (Acetic, Lactic, and Formic acids) from glucose fermentation.
5.
Methyl red negative: The culture medium remains yellow, as less acid is produced (pH is higher). These large amounts of acid results significant decrease in the pH of the medium below 4.4. This is visualized by using pH indicator, methyl red (p-dimethylaminoaeobenzene-O-carboxylic acid), which is yellow above pH 5.1 and red at pH 4.4.
6.
Voges–Proskauer is a test used to detect acetoin in a bacterial broth culture. The test depends on the digestion of glucose to acetylmethylcarbinol, an intermediate of the 2,3-butanediol fermentation pathway.
7.
Barritt Reagent A (5% a-naphthol in absolute ethanol)
Barritt Reagent B (40% potassium hydroxide)
8.
Citrate test determine the ability of a microorganism to use citrate as its sole carbon source. Bacteria are inoculated on a medium containing sodium citrate and a pH indicator such as bromothymol blue. The medium also contains inorganic ammonium salts, which are utilized as sole source of nitrogen. Use of citrate involves the enzyme citritase, which breaks down citrate to oxaloacetate and acetate. Oxaloacetate is further broken down topyruvate and carbon dioxide (CO2). Production of sodium bicarbonate (NaHCO3) as well as ammonia (NH3) from the use of sodium citrate and ammonium salts results in alkaline pH. This results in a change of the medium’s color from green to blue.
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