DNase Test

Figure 1: DNase Test as given by Staphyloccocus aureus. Picture Credit: Samjhana Kapali

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The DNase test, also known as the DNA hydrolysis test, is a biochemical test used to determine the ability of an organism to hydrolyze DNA using the enzyme DNase. This test is often performed in microbiology laboratories to differentiate between bacteria based on their ability to degrade DNA.

Principle:

The DNase test is based on the ability of certain bacteria to produce the enzyme DNase, which hydrolyzes DNA into smaller fragments. The test utilizes a nutrient agar medium supplemented with DNA as the substrate. If the organism produces DNase, it will degrade the DNA in the medium, resulting in a clear zone around the bacterial growth.

Procedure:

1. Inoculate a DNase test agar plate with the organism to be tested by streaking a single line of the organism down the center of the plate.
2. Incubate the plate at the appropriate temperature for the organism being tested (usually 37°C for 18-24 hours).
3. After incubation, flood the surface of the agar with a solution of methyl green or toluidine blue, which binds to the DNA and makes it visible.
4. After a few minutes, examine the plate for the presence of a clear zone around the bacterial growth. A clear zone indicates DNase activity, as the DNA in the medium has been degraded.
5. If there is no clear zone, the organism is DNase-negative. If a clear zone is present, the organism is DNase-positive.

Interpretation:

• DNase-positive organisms produce a clear zone around the bacterial growth due to the hydrolysis of DNA in the medium.
• DNase-negative organisms do not produce a clear zone, and the DNA in the medium remains intact.

Examples:

• Staphylococcus aureus is a common DNase-positive bacterium and produces a clear zone on the DNase test agar.
• Escherichia coli is a DNase-negative bacterium and does not produce a clear zone on the DNase test agar.

The DNase test is useful for differentiating between bacteria and is particularly important in the identification of Staphylococcus species, where DNase production is a characteristic feature.