Frequency of mecA Gene and Borderline Oxacillin Resistant Staphylococcus aureus in Nosocomial Acquired Methicillin Resistance Staphylococcus aureus Infections

Abstract

The aim of the study was to determine the frequency and type of MRSA strains and antibiotic susceptibility in Al-Zahra Hospital, Isfahan, Iran. In an analytic descriptive survey in 2005 and early 2006, patients admitted to the hospital who contracted S. aureus nosocomial infections were enrolled in the study. All isolates were identified by the conventional laboratory tests. Minimal Inhibitory Concentration (MIC) of oxacillin on isolated bacteria was determined by E-Test method. According to Clinical and Laboratory Standard Institute (CLSI) criteria all strains with MIC of > or = 4 microg for oxacillin were identified as MRSA. Intrinsic high level resistance (mecA positive) and borderline oxacillin resistant Staphylococcus aureus (BORSA) were detected by amoxicillin-clavulanate E-test strips. Strains with MIC of > or = 4 microg for oxacillin and > or = 8 microg for amoxicillin-clavulanate were identified as mecA positive MRSA. Other staphylococcus with MIC > or = 4 microg for oxacillin and < or = 4 for amoxicillin-clavulanate were identified as mecA negative MRSA (BORSA). MIC of vancomycin also was determined on isolated bacteria. Data were analyzed by SPSS version 13 and Who net version 5. Out of 134 Staphylococcus aureus samples which were isolated from nosocomial infections 90 (67.2%) were MRSA. Sixty seven out of 90 (74.5%) MRSA were mecA positive and 23 out of 90 (25.5%) were mecA negative (BORSA). Although most of the MRSA strains were isolated from surgical site infections, there were no statistically significant differences between types of Staphylococcus aureus growing from variant sites of infections. Only one (1.49) of the mecA positive MRSA had reduced susceptibility to vancomycin but all of the mecA-negative MRSA (BORSA) were sensitive to it. Because one fourth of our staphylococcus strains are mecA negative BORSA and there is no alternative for vancomycin against mecA positive MRSA and Enterococcus spp. in our hospital, vancomycin should be reserved only for life threatening infections due to these organisms. Thus MRSA typing should be done to choose appropriate antibiotic for optimal treatment of MRSA infections.