Distribution Patterns of Methicillin Resistance Genes (mecA) in Staphylococcus aureus Isolated from Clinical Specimens

There is a growing concern about the application of molecular methods in epidemiological studies of infectious diseases. The objective of this study was to determine the genetic stability of methicillin resistance genes in Staphylococcus aureus for the evaluation of resistance strain distribution. One hundred and fifteen S. aureus isolates from patients with staphylococcal infection were collected. The isolates were screened for methicillin resistance by minimum inhibitory concentration (MIC) examination. The stability of methcillin resistance genes was examined by physical curing and PCR screening methods. The results showed that methicillin resistance Staphylococcus aureus (MRSA) had risen up to 43% in Nemazi Hospital (Shiraz, Iran). Indeed, the incidence of MRSA in our hospital was 10% during the last four years. The ability to lose (curability) of methicillin resistance genes (mecA) was examined by physical curing method in 49 isolates with MIC ³ 16 μg ml-1. No sign of curability of mecA gene was observed where 500 colonies from each strain have been studied and exhibited by the same MIC values before and after curing test. Positive PCR results for isolates with MIC ³ 16 μg ml-1 before and after curing experiment have been achieved. These data confirm the results of curing method, indicating that stable genetic determinants confer methicillin resistance. These results support the hypothesis that resistance isolates may be selected due to clonal selection under antibiotic pressure used in clinics rather than transmission of mobile genetic determinant. The high prevalence of MRSA immerged in our Hospital could be originated due to antibiotic pressure and poor control measures