[Detection of methicillin-resistant Staphylococcus aureus by the Showa disk]

The Showa disk susceptibility test using two penicillinase-resistant penicillins (PRPs: methicillin and oxacillin) and four cephalosporins (cefazolin, cefmetazole, ceftizoxime, and latamoxef) was evaluated to discriminate between the strains of Staphylococcus aureus resistant to methicillin (MRSA) and those susceptible (MSSA). Among 129 MRSA and 112 MSSA strains, significant numbers of MRSA found to be false-susceptible to PRPs, especially when incubated at 37 degrees C. Using agar plates supplemented with 5% NaCl, oxacillin disk correctly categorized all the MRSA and MSSA strains, but two MSSA strains were mischaracterized with methicillin disk. All the MSSA strains found to be susceptible against cephalosporins tested, but significant numbers of MRSA were also interpreted to be susceptible. Only the result of ceftizoxime disk susceptibility test highly correlated with methicillin resistance, when incubated at 35 degrees C. All the MRSA did not produce any significant bacterial growth inhibitory zone around the disk, whereas all the MSSA found to be susceptible (19 mm or more), e.g., 100% correlation. With these data, we recommend that ceftizoxime disk susceptibility test, incubated at 35 degrees C, provides a more reliable method to characterize the strains of MRSA and MSSA. The interpretive breakpoint would be used as; less than or equal to 16 mm (greater than or equal to 25 micrograms/ml) for MRSA.