DNASE AGARo:p>

 

Code: CB1/1028

 

Formula g/l

 

Tryptone	20.0
Deoxyribonucleic acid (DNA)	2.0
Sodium chloride	5.0
Agar	15.0

     

Final pH: 7.3 ± 0.2

 

Description

 

DNase agar provides a convenient means of identifying potentially pathogenic staphylococci, based on the ability of coagulase-positive species to split DNA. DNases produced by the organisms hydrolyse the DNA molecule to a mixture of smaller mono and poly nucleotides. Perfect correlation between coagulase activity and DN’ase production using S. aureus strains from clinical specimens can be observed with this medium.  There are many publications, which have also reported a close correlation.

 

Method

 

Weigh 42 grams of powder, disperse in 1 litre of deionised water. Allow to soak for 10 minutes, swirl to mix then sterilise by autoclaving at 121°C for 15 minutes. Allow to cool to 47°C then pour into petri dishes.

 

 

Q.C. organisms: S. aureus NCIMB 50080, S. epidermidis NCIMB 50082

 

Storage of Prepared Medium: Plates  up to 7 days: at 2-8°C in

the dark. Capped container up to 1 month at 4°C in the dark.

 

Inoculation: Use a heavy inoculum on a small area. Four or more organisms can be tested on one 90mm petri dish.

 

Incubation: 37°C aerobically for 18-24 hours.

 

Interpretation:

Having obtained good growth flood the plate with 1N hydrochloric acid. This will precipitate the DNA in the medium. DN’ase producing organisms will be surrounded by a clear area where the DNA has been broken down into fractions which are not precipitated by the Hydrochloric acid. Gram positive, catalase positive cocci that produce DN’ase can be provisionally classified as S. aureus, and confirmed by tube coagluase or thermostable DN’ase tests. D.N.’ase is also produced by some Gram negative bacilli such as Serratia marcescens, Pseudomonas aeruginosa. Some corynebacteria and streptococci may also produce DN’ase.

 

References

Messinova, O. V., Yusupova, D. V. and Shamsutdinov, N. S. 1963. Deoxyribonuclease activity of Corynebacterium and its relation to virulence. Fed. Proc. 22, T1033.

Streitfeld, M. M., Hoffmann, E. M. and Janklow, H. M. 1962.