DNase Test HiCynth™ Agar w/ Toluidine blue

Principle And Interpretation

DNase Test HiCynth™ Agar w/ toluidine blue is used for detecting deoxyribonuclease activity of bacteria and fungi and particularly for identification of pathogenic Staphylococci. With added toluidine blue, it is used in differentiation and identification of non-pigmented Serratia species isolated from clinical sources that might be improperly identified as Enterobacter and Klebsiella species. DNase activity was observed by Weckman and Catlin (1) in Micrococci and found the correlation with coagulase activity as coagulase positive species were DNase positive. Di Salvo (2) confirmed the results of Weckman and Catlin and observed accurate correlation of DNase and coagulase activity. In his experiment Di Salvo incorporated DNA and calcium chloride to activate DNase enzyme. Schreier modified DNase medium by adding toluidine blue (3). Modified medium achieved faster identification of Serratia marcescens and could differentiate Serratia from other members of the Enterobacteriaceae. HiCynth™ Agar is the modification by using chemically defined peptones instead of animal peptones to avoid BSE/TSE risk.

HiCynth™ Peptone No.3 and HiCynth™ Peptone No.5 provide nitrogen, carbon compounds, long chain amino acids, vitamins and essential nutrients for growth. DNase depolymerizes the DNA resulting in the formation of a clear zone around the microbial growth which is visualized by flooding the plate with hydrochloric acid (4).

When toluidine blue is added to the medium itself, DNase activity results in the production of a bright pink reaction due to the metachromatic property of toluidine blue. Some strains of Staphylococci may be inhibited on DNase Test HiCynth™ Agar due to toluidine blue. Further confirmatory tests for the identification should be carried out.