Sunday, 27 May 2018

Deoxyribonuclease test

The test is used primarily as a supplementary presumptive test to distinguish pathogenic staphylococci which produce large quantities of extracellular DNase. The DNase reacts with media containing DNA with the resulting hydrolysis of the DNA. The oligonucleotides liberated by the hydrolysis are soluble in acid and in a positive reaction, the addition of hydrochloric acid results in a clear zone around the inoculum. Due to the precipitation of DNA by hydrochloric acid, in a negative reaction, the solution becomes cloudy. In contrast to hydrochloric acid, toluidine blue produces much more clearly delineated zones of DNase activity.

Most strains of Staphylococcus aureus hydrolyse DNA and give positive reactions in this test, but some MRSA strains do not and some strains of the coagulase negative staphylococci may give weak reactions such as Staphylococcus capitis. Some strains of Staphylococcus intermedius are DNase positive. Subspecies of Staphylococcus schleiferi are DNase positive and produce heat stable nucleases.

This test also aids in the differentiation of closely related genera within the Klebsiella/Enterobacter-Serratia division of Enterobacteriaceae and several other pathogens, including Pseudomonas aeruginosa. Serratia and Moraxella species also produce deoxyribonuclease.

Public Health England has opened a consultation on the standard test method. For details see: UK Government Gateway:

Posted by Dr. Tim Sandle

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