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Description:
Weckman & Catlin1 suggested that DNase activity could be used to identify pathogenic staphylococci after they had established a close correlation with coagulase production. Jeffries et al.2 incorporated DNA in the agar medium to provide a simple method of detecting DNase activity. Organisms are streaked on to the surface of the agar medium and incubated. The growth on the surface of the agar is then flooded with 1N hydrochloric acid. Polymerised DNA precipitates in the presence of 1N HCl and makes the medium opaque. If the organisms produce D Nase enzymes, in sufficient quantity to hydrolyse the DNA, then clear zones are seen around the colonies.
Good correlation was shown between DNase production and coagulase activity when testing Staphylococcus aureus strains from clinical samples2,3,4. Both Staphylococcus aureus and Staphylococcus epidermidis produce extracellular DNase5,6,7 but Staphylococcus aureus produces greater quantities1,7.
A modification of the medium is to add mannitol (1% w/v) and phenol red or bromothymol blue (0.0025% w/v) as an indicator of mannitol fermentation9. The pH reaction around the colonies must be read before the plate is flooded with acid.
The DNase reaction helps in the differentiation and identification of non -pigmented Serratia marcescens8(positive DNase reaction) from Klebsiella-Enterobacter (negative DNase reaction).
Normal HCl is bactericidal and the organisms cannot be recovered from the surface of the agar after flooding. The incorporation of dyes into the medium which can distinguish hydrolysis of DNA is a fur ther modification which avoids the use of acid. Toluidine blue8 and methyl green10 form coloured complexes with polymerised DNA; these colours change as the DNA is hydrolysed.
It should be noted that toluidine blue inhibits Gram positive organisms and it i s used to detect DNase production by the Enterobacteriaceae. It has been used with ampicillin (30 mg/litre) to demonstrate DNase production by Aeromonas hydrophila from faeces11.
Specification:
50g