[Reliability of disc-diffusion susceptibility testing for arbekacin, vancomycin and teicoplanin against...

We investigated the differences in judgments among four disc-diffusion methods on susceptibility testing of arbekacin (ABK), vancomycin (VCM) and teicoplanin (TEIC) against 37 strains of methicillin-resistant Staphylococcus aureus (MRSA). These results were compared with minimum inhibitory concentrations (MICs) obtained from micro broth-dilution method. A marked difference was noted in the judgment of susceptibility to TEIC in Tri-disc method, that is 2 strains (5.4%) fell into sensitive (+3) 34 strains (91.9%) into moderately sensitive (+2) and 1 strain (2.7%) into moderately resistant (+), while in Sensi-disc method all strains fell into sensitive (S). According to the MICs, no strain of the MRSA tested revealed resistance to ABK, VCM and TEIC. Consequently, these three antimicrobial agents were thought to be effective on MRSA infections. From these results, we concluded that Tri-disc method for glycopeptide against MRSA, especially for TEIC, is not recommendable as a disc-diffusion method in susceptibility testing.