Deoxyribonuclease abbreviated DNase is an enzyme that catalyzes the hydrolytic cleavage of phosphodiester linkages in the DNA backbone, thereby degrading DNA. Some microorganisms like Staphylococcus species possesses this enzymes hence Dnase test agar is a differential medium that test the ability of Dnase producing microorganisms to hydrolyse DNA.
Principle of Dnase test
As earlier stated,The enzyme Dnase produced by these bacteria and some fungi hydrolyzes DNA.Suspected organisms are inoculated on a growth medium containing DNA (DNase Agar)
- DNase agar contains nutrients necessary for bacteria growth
- DNA, and methyl green act as indicator. Deoxyribonuclease produced by the suspected bacteria break down DNA into smaller fragments.
- When this DNA is broken down by the enzyme Dnase,it no longer binds to the methyl green
- The weak solution of Hydrochloric acid flooded, precipitates unhydrolyzed DNA hence
- A clear halo appears around the areas where the DNase-producing bacteria has grown.
- 02 DNAse agar plate (Test and Control)
- 1 N Hydrochloric acid
- Suspected organism
- Wire Loop
Appearance of Dnase Agar plate
- Light amber after preparation in petri dishes
- Using the sterile wire loop,Inoculate the suspected organism on the DNase agar plate preferably by crossing an X or just making a straight line streak labeling it T as for test
- You can use Staphylococcus epidermidis as your control by inoculating it on another Dnase agar plate.
- Incubate both plates at 35-37 ° C overnight
- Cover the surface of both plates by flooding it with the weak hydrochloric acid solution.Tip off the excess acid.
- If the method of streaking used was a straight line or an X cross,
- Kindly look for a clear halo around the streaked areas within 5 minutes of adding the weak acid.
|Clearing or Halo around streaked lines or colonies||Dnase positive organism|
|No clearing or Halo around streaked lines or colonies||Dnase Negative organism|
|Positive Dnase Control||Staphylococcus aureus|
|Negative Dnase Control||Staphylococcus epidermidis|
Positive Dnase Organism
|Serratia marcescens||positive reaction|
|Staphylococcus aureus||positive reaction|
|Staphylococcus epidermidis||negative reaction|
|Streptococcus pyogenes||positive reaction|
Some resistant Staphylococcus aureus strain (MRSA or Methicillin resistant Staphylococcus aureus) give a negative response to Dnase test while some coagulase negative staphylococci are weakly positive to Dnase test.
Other bacteria like Moraxella,Streptococcus pyogenes,Cryptococcus albidu,C. neoformans and Serratia marcescens frequently gives a positive reaction to Dnase test hence further biochemical test should be done in order to confirm the suspected organism.
- Monica Cheesbrough Part 2,Page 67
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