Evaluation of oxacillin resistance by traditional and molecular methods in nosocomial strains of coagulase negative staphylococci

Evaluation of Oxacillin Resistance by Traditional and Molecular Methods in Nosocomial Strainsof Coagulase Negative StaphylococciPEKSEL Hasan, MD.Advisors: YÜCE Ayşe, MD, Professor of Infectious Diseases and Clinical Microbiology and YAPAR Nur, MD, Assoc.Professor of Infectious Diseases and Clinical MicrobiologyDokuz Eylul University Faculty of Medicine, Infectious Diseases and Clinical Microbiology DepartmentIzmir / TurkeyPurpose: To detect the methicillin resistance in nosocomial strains of coagulase negativestaphylococci (CoNS) by different methods according to the Clinical Laboratory Institute (CLSI)suggestions.Materials and methods: Nosocomial strains of 105 CoNS were isolated from blood culturesbetween April to September 2005. To detect the methicillin resistance; oxacillin and cefoxitin discdiffusion tests, oxacillin agar dilution test and mecA polymerase chain reaction (PCR) were conductedaccording to the CLSI suggestions.Results: By the oxacillin disc diffusion test; 82 isolates were resistant and 23 were susceptibleto oxacillin. By the cefoxitin disc diffusion test; 80 isolates were resistant and 25 were susceptible tocefoxitin. By the oxacillin agar dilution test; 84 isolates were resistant and 21 were susceptible tooxacillin. By the method mecA-PCR; 83/105 of isolates were mecA positive and 22/105 were mecAnegative.Statistical analysis: By the data gained from the four-filled table; the sensitivity of oxacillindisc diffusion test, cefoxitin disc diffusion test and oxacillin agar dilution tests when compared withthe gold standard test mecA-PCR were; 97.59%; 96.38% and 100% respectively. The specificity of thetests were; 95.45%; 100% and 95.45% respectively. By the results of the ROC analysis, cefoxitin discdiffusion test was the most concordant test reflecting the results of gold standard method mecA-PCRConclusion: Evaluating methicillin resistance in staphylococci by phenotypic methods willmostly cause inaccuracy. The results of our study showed that the cefoxitin disc diffusion test ispreferable to the oxacillin disc diffusion method. But it?s considered that the gold standard methodmecA-PCR must be used in the routine laboratory to show the methicillin resistance in both CoNS andS.aureus.Key words: Coagulase-negative Staphylococci, susceptibility testing, oxacillin resistance