The IMViC tests involve a set of tests which is used to differentiate bacteria from one another. For example, to differentiate organisms such as Klebsiella, Entezrobacter, and Escherichia coli.
There are mainly four individual test- the Indole test, methyl red test, Voges-Proskauer reaction and the citrate test.
Indole test
The purpose of the Indole test is to differentiate species of the family Enterobacteriaceae by testing the bacteria’s ability to produce Indole from amino acid tryptophan. When indole is produced, it reacts with Kovac’s reactant to create a cherry red-coloured layer.
Procedure:
1. Add 5 drops of Kovac’s reactant to each test tube of Trypticase soy broth culture.
2. If a cherry red-coloured layer is form, it shows that an Indole positive bacteria is present (eg. E.coli) as the bacteria produces an enzyme which cleaves tryptophan to produce indole and other products.
The Methyl Red and Voges-Proskauer Tests
The two tests are used to help differentiate bacteria species of the family Enterobacteriaceae. The methyl red test tests for acid end products from glucose fermentation whereas the Voges-Proskauer tests for acetoin production from glucose fermentation.
Procedure for methyl red test:
1. Add 4-5 drops of the indicator, methyl red to a test tube containing the bacteria culture.
2. If a red precipitate is formed, it is a positive test where glucose is being fermented to form acid, hence causing the pH to drop and form a red coloration. On the other hand, if a yellow precipitate is formed, it is a negative test.
Left- Negative test; Right – Positive test
Procedure for Voges-Proskauer Test:
1. Add 5 drops of the alpha-naphthol solution to a test tube containing the bacteria culture.
2. Add 5 drops of potassium hydroxide-creatine solution to the test tube and shake well for 1 minute.
3. If a red precipitate is formed, it is a positive test. Otherwise, there is no colour change which signifies a negative test.
[Positive test => acetoin produced; Negative test => acetoin not being produced]
Citrate test
This test helps to differentiate species of the family Enterobacteriaceae. It is selective for bacteria which is able to consume citrate as the only source of carbon and ammonium as its sole nitrogen source.
Procedure:
1. Inoculate bacteria onto the slant agar containing Simmon's citrate media.
Left- Positive; Right- Negative
When it is a positive test, the green Simmon’s Citrate Agar (pH of 6.9) will turn blue as the bacteria metabolizes citrate to produce an acidic product.