Question

Medical Microbiology Lab Report 36

DNase Test

Name__________________________________ Date _________________

Draw your plate or take photos of your plate before and after the addition of HCl.

Before      After

S. aureus E. aerogenes S. aureus E. aerogenes

Interpretation and Questions:

Which organism secreted DNase?

Why would an organism’s ability to produce this enzyme be clinically significant?

S. aureus is a common component of the biota of the skin and nasal passages. What steps should be taken by the dialysis unit described in the case file to minimize nosocomial infection in its patients?

Answer 1

The yellow coloured streaks in the plates above indicate bacterial growth. DNase agar is a differential medium that tests the ability of an organism to produce deoxyribonuclease or DNase, that hydrolyzes DNA. DNase agar contains high molecular weight DNA in addition to nutrients. Deoxyribonuclease breaks down DNA into smaller fragments. So when an organsim produces DNase, and degrades the DNA in the medium, the area immediately surrounding the growth area of the bacteria appears as a clear halo in an opaque back ground after HCl is flooded onto the medium for about 10 minutes. This is because the HCl precipitates the high mol. weight DNA in the medium and causes the medium to turn cloudy or opaque (easier seen when the plate is kept against a dark background).  DNase negative organisms will not show any halo around the colonies. DNase medium is mainly used in the identification of staphylococci.

S. aureus secreted DNase.

Bacterial DNase production is clinically significant as it can contribute to the virulence or pathogenicity of the bacteria by escaping innate immunity defenses.

To minimise nosocomial infections from dialysis, it is important that the medical peronnel have proper hand hygiene, use masks and adhere to aseptic technique while inserting or removing catheters or changing dressings. An alcohol-based chlorhexidine (>0.5%) solution should be used as the first line skin antiseptic agent during catheter insertions and dressing changes.