Intended Use
DNase Test Agar, is a media used for the detection of deoxyribonuclease activity to aid in the identification of bacteria isolated from clinical specimens.

Summary and Explanation
The DNase test is used to detect the degradation of deoxyribonucleic acid (DNA).
The test is useful for differentiating Serratia from Enterobacter, Staphylococcus aureus from coagulase-negative staphylococci, and Moraxella catarrhalis from Neisseria species.
In 1957, Jeffries et al. described a rapid agar plate method for demonstrating DNase activity of microorganisms.

This procedure utilized a semi-synthetic medium with nucleic acid solution incorporated in the medium. Enzymatic activity is detected by flooding the plate with 1 N hydrochloric acid (HCl).
A clear zone surrounding growth indicates a positive reaction. DNase Test Agar is based on a medium developed by DiSalvo to adapt the rapid plate method for staphylococci.
Rather than using semi-synthetic medium, DiSalvo incorporated DNA into Trypticase™ Soy Agar and subsequently reported a correlation between coagulase production and DNase activity.

Principles of the Procedure
Peptones provide amino acids and other complex nitrogenous substances to support bacterial growth. Sodium chloride maintains osmotic equilibrium. DNA is the substrate for DNase activity. DNase is an extracellular enzyme that breaks the DNA down into subunits composed of nucleotides.
The depolymerization of the DNA may be detected by flooding the surface of the medium with 1 N HCl and observing for clear zones in the medium surrounding growth. In the absence of DNase activity, the reagent reacts with the intact nucleic acid, resulting in the formation of a cloudy precipitate.

Difco™ DNase Test Agar
 - Dehydrated Appearance: Light beige, free-flowing, homogeneous.
 - Solution: 4.2% solution, soluble in purified water upon boiling. Solution is light to medium amber, very slightly to slightly opalescent, may have a slight precipitate.
 - Prepared Appearance: Light to medium amber, slightly opalescent, may have a slight precipitate.
 - Reaction of 4.2%
 - Solution at 25°C: pH 7.3 ± 0.2

Difco™ DNase Test Agar
Approximate Formula* Per Liter
 - Tryptose : 20.0 g
 - Deoxyribonucleic Acid : 2.0 g
 - Sodium Chloride : 5.0 g
 - Agar : 15.0 g