High frequency of MRSA in surgical site infections and elevated vancomycin MIC

Background

Methicillin-resistant staphylococcus aureus (MRSA) causes surgical site infections (SSIs) and complicates the treatment of these infections. Vancomycin is the drug of choice for infections due to MRSA but creep MIC of vancomycin may be associated with failure in antibiotic therapy. The aim of the present study was to determinate the prevalence, antibiotic susceptibility patterns of MRSA in patients suffering from SSIs and determination of vancomycin MIC in these isolates.

Methods

Detection of S. aureus in 100 aspirated specimens from SSIs was performed by the standard microbiology tests. MRSA was detected by phenotypic and molecular methods. The disk diffusion, E-test and agar dilution methods were performed for determination of antibiotic susceptibility patterns, the vancomycin MIC and oxacillin MIC. The results were interpreted according to guidelines of the Clinical and Laboratory Standards Institute (CLSI).

Results

Of 24 S. aureus isolates, 20 (83.33%) were recognized as the MRSA by 30 μg cefoxitin disk from 100 specimens and all were confirmed by PCR. High frequency of resistance was observed to clindamycin, erythromycin, gentamicin, ciprofloxacin and chloramphenicol while all isolates were susceptible to vancomycin, linezolid, and mupirocin. The range of vancomycin and oxacillin MICs were 0.25 to 2 and 0.5 to 128 μg/ml, respectively. Vancomycin-creep MIC was detected in two isolates from the superficial wound and surgical respiratory tract infections, and one of the patients died.

Conclusion

The findings of this study show a high frequency of MRSA in SSIs. Due to the elevated MIC of vancomycin, we suggest consumption of other effective drugs for the treatment of SSIs.