Comparison of different methods for determining methicillin resistance in staphylococci

Methicillin-resistant Staphylococcus strains cause hospital acquired infections and community acquired infections. Therefore rapid and accurate identification of methicillin resistant strains is important. In the present study, we aimed to investigate the detection of methicillin resistance in clinically isolated Staphylococcus strains by different methods and to compare the specifity and sensitivity of these methods. In this study, 84 S. aureus and 82 coagulase negative Staphylococcus (CNS) strains were used. We used to determine the methicillin resistance with disc diffusion, microdilution, oxacillin salt agar screening, chromogenic media and PBP2a latex agglutination tests. Polymerase chain reaction (PCR) technique was used as reference. Test results were evaluated according to the guidelines which are Clinical and Laboratory Standards Institute (CLSI), European Committee on Antimicrobial Susceptibility Testing (EUCAST) and British Society for Antimicrobial Chemotherapy (BSAC). In the oxacillin disk diffusion test, 29 (%34,5) of the S. aureus and 54 (%65,9) of the CNS isolates; in the cefoxitine disk diffusion test, 32 (%38,1) of the S. aureus and 48 (% 58,5) of the CNS isolates; in the oxacillin microdilution test, 33 (%39,3) of the S. aureus and 59 (%72,0) of the CNS isolates; in the cefoxitin microdilution test, of the 32 (%38,1) S. aureus and 33 (%39,3) of the CNS isolates; in the oxacillin salt screen agar test, 33 (%39,3) of the S. aureus and 45 (%54,9) of the CNS isolates; in the latex agglutination test, 30 (%37,5) of the S. aureus and 32 (%39,0) of the CNS isolates; in the chromogenic media test, 31 (%36,9) of the S. aureus isolates were found to be resistant. According to PCR technique 31 (% 36,9) of the S. aureus and 47 (%57,3) of the CNS isolates were found to be resistant. In our study, mecALGA251 gene was not found in any of the Staphylococci isolates. When the results obtained according to CLSI compared reference methods, the microdilution and cefoxitin disk diffusion test results were the same was found to be more suitable than other methods for determining the methicilin resistance in S.aureus isolates (sensitivity %100, spesificity %98,1). Cefoxitin disk diffusion test was found more suitable than other methods for CNS isolates. When the resuts provided by the study were evaluated according to the criteria of CLSI, EUCAST, and BSAC, and were compared to the reference method, CLSI and EUCAST criteria were found to be accordant with each other, and BSAC criteria showed variability in terms of sensitivity and specificity. In order to determining methicillin resistance in Staphylococci, EUCAST could also be used instead of CLSI which criteria that are being used in our country was concluded., Key Words: Staphylococcus, methicillin, mecA, mecALGA251, PBP2a