Methicillin resistance in staphylococci: an evaluation of conditions for detection

Ninety-four strains of coagulase positive staphylococci and 73 strains of coagulase negative staphylococci were tested for their susceptibility to methicillin, using agar dilution and disc diffusion techniques with Diagnostic Sensitivity Test agar (DST) and DST supplemented with salt, incubated at 30 degrees C and 37 degrees C. Disc diffusion on salt-incorporated DST at 30 degrees C was most reliable for detecting methicillin resistance in both types of staphylococci. The agar dilution methods were particularly poor for detecting resistance with coagulase negative staphylococci. The simultaneous testing of coagulase positive and negative staphylococci by this method indicated the need for both DST and salt-incorporated DST agar plates, incubated at 30 degrees C. However, sensitive strains should still be tested by disc diffusion for confirmation.