Interpretive criteria for disk diffusion susceptibility testing of mupirocin, a topical antibiotic

Five hundred gram-positive clinical bacterial isolates were tested for susceptibility to mupirocin by broth microdilution and disk diffusion methods (5- and 10-micrograms disks). All but 1 of 330 staphylococci (including 100 oxacillin-resistant Staphylococcus aureus) were susceptible to less than or equal to 1.0 micrograms of mupirocin per ml. With a susceptible MIC breakpoint of less than or equal to 2.0 micrograms/ml, the corresponding zone diameter breakpoints were greater than or equal to 14 mm for 5-micrograms disks and greater than or equal to 16 mm for 10-micrograms disks. With either disk potency, susceptible staphylococci were effectively separated from more resistant gram-positive species such as the enterococci and gram-positive bacilli.