New recommendations for disk diffusion antimicrobial susceptibility tests for methicillin-resistant (heteroresistant) staphylococci

Abstract

The agar disc diffusion susceptibility test was reevaluated for its ability to discriminate between susceptible and resistant Staphylococcus aureus (128 strains) and coagulase-negative staphylococci (19 strains) when tested with methicillin, oxacillin and nafcillin. The current recommendations for disc potencies and interpretive zone diameters did not fit well with MIC [minimum inhibitory concentration] correlates that are now recommended. Based on data from this study, it is suggested that these parameters of the test be changed. For methicillin, a 10 .mu.g disc with breakpoints of .ltoreq. mm (.ltoreq. 16 .mu.g/ml) to indicate resistance and .gtoreq. 15 mm (.gtoreq. 4 .mu.g/ml) to indicate susceptibility is recommended. For oxacillin and nafcillin, 4 .mu.g discs with breakpoints of .gtoreq. 12 mm (.gtoreq. 8 .mu.g/ml) to indicate resistance and .gtoreq. 16 mm (.gtoreq. 2 .mu.g/ml) to indicate susceptibility are recommended. MIC breakpoints were from a broth microdilution system which used a medium containing salt. If 1 of these 3 penicillins were to be selected for routine tests, oxacillin is recommended based on data from this study; selection may also depend on the population of staphylococci within a particular hospital.