Identification of methicillin-resistant Staphylococcus aureus by disk diffusion method, determination of MIC and PCR for mecA gene

Staphylococcus aureus is an important cause of serious infection in both hospital and the community. Methicillin-resistant S. aureus (MRSA) is associated with high morbidity and mortality rates with rapid development of resistance. There is a need for early and reliable detection of MRSA infection to direct antibiotic therapy, and more effectively control cross-infection. In this study, resistance to methicillin was detected by a disk diffusion method, the determination of MIC, and the PCR for mecA gene. A total of 174 S.aureus strains were isolated from different clinical specimens from three teaching Hospitals. Antibiotic susceptibility was determined by disk diffusion method, MIC for oxacillin was made by the agar dilution, and mecA gene was identified by specific primers. The prevalence of MRSA by three methods ranged from 47% to 50%, and mecA positive isolates were more resistant to all of the antibiotic tested than mecA negative isolates. All S. aureus isolates were resistant to penicillin, and susceptible to vancomycin. The results of agar dilution test indicated a low-level resistance to methicillin (MIC>64mg/l). The distribution of MRSA isolates were uniform between three hospitals, and there were not significant differences in the presence of MRSA between isolates from different clinical specimens. The PCR method was the best test for routine detection of MRSA in the present study. An additional benefit of the mecA PCR is the potential to generate a susceptibility report, 24h earlier than the time of generation of results of conventional susceptibility testing methods.