DNase Test Agar- Principle, Procedure and Result Interpretation
Objectives of DNase Test Agar (DNA Hydrolysis)
This test is used to differentiate organisms based on the production of deoxyribonuclease. It is used to distinguish Serratia spp (positive) from Enterobacter spp., Staphylococcus aureus (positive) from other species, and Moraxella catarrhalis (positive) from Neisseria sp.
Principle of DNase Test Agar (DNA Hydrolysis)
The test is used to determine the ability of an organism to hydrolyze DNA. The medium is pale green because of the DNA–methyl green complex. If the organism growing on the medium hydrolyses DNA, the green color fades and the colony is surrounded by a colorless zone.
Media: Pancreatic digest of casein (10 g), yeast extract (10 g), deoxyribonucleic acid (2 g), NaCl (5 g), agar (15 g), methyl green (0.5 g), pH 7.5.
Procedure of DNase Test Agar (DNA Hydrolysis)
- Inoculate the DNase agar with the organism to be tested and streak for isolation.
- Incubate aerobically at 35°-37°C for 13 to 24 hours.
Result Interpretation of DNase Test Agar (DNA Hydrolysis)
Positive: When DNA is hydrolyzed, methyl green is released and combines with highly polymerized DNA at a pH of 7.5, turning the medium colorless around the test organism.
Negative: If no degradation of DNA occurs, the medium remains green.
Limitations of DNase Test Agar (DNA Hydrolysis)
Agar must be inoculated with a suspension of a young broth culture (4 hours old) or an 18- to 24-hour colony in 1-2 mL of saline.
Positive: Staphylococcus aureus (ATCC25923)
Negative: Escherichia coli (ATCC25922)