How to Identify Staphylococcus Aureus Using DNase Test

Staphylococcus aureus is a gram positive bacteria that has been implicated in several diseases. Some of these diseases include abscesses, furuncles, UTIs bone and joint pains among other ailments. The identification and isolation of S aureus in the medical laboratory is not too difficult as a lot of methods have been developed by scientists. One of such methods for the identification of Staphylococcus aureus is the DNase Test. This Dnase test is used to identify Staphylococcus aureus which produces deoxyribonuclease (DNase) enzymes.

The DNase test is particularly useful when plasma is not available to perform a coagulase test or when the results of a coagulase test are difficult to interpret.

Principle Behind Dnase Test

Deoxyribonuclease hydrolyzes deoxyribonucleic acid (DNA). The test organism is cultured on a medium which contains DNA. After overnight incubation, the colonies are tested for DNase production by flooding the plate with a weak hydrochloric acid solution. The acid precipitates unhydrolyzed DNA. DNase-producing colonies are therefore surrounded by clear areas due to DNA hydrolysis.

Reagents Required For DNase Test

• DNase agar plate          

Up to six organisms may be tested on the same plate

• Hydrochloric acid          1 mol/1 (1N)

Laboratory Identification and Isolation Method

1. Divide a DNase plate into a required number of strips by marking the underside of the plate.
2. Using the sterile loop or swab, spot-inoculate the test and control organisms. Make sure each test area is labeled clearly.
3. Incubate the place at 35 – 37^oC overnight.
4. Cover the surface of the plate with 1 mol/1 hydrochloric acid solution. Tip off the excess acid.
5. Look for clearing around the colonies within 5 minutes of adding the acid, as shown in Plate 7.10.

Results

Clearing around the colonies ……   DNase positive strain

No clearing around the colonies …… DNase negative strain

Note: Some methicillin resistant S. aurens (MRSA) strains give a negative DNase test. Some coagulase negative streptococci are weakly positive. Rarely S. pyogenes, Maraxella and Serratia species may give a positive DNase test.

Controls

Positive DNase control:                 Staphylococcus aureus

Negative DNase control:                Staphylococcus epidermis