Detection of Methicillin Resistance in Nosocomial Isolates of Staphylococcus aureus and Coagulase Negative Staphylococcus Strains by Different Methods

Objective: Methicillin resistance of nosocoimal Staphylococcus aureus and coagulase negative staphylococci was planned to be detected by oxacillin and cefoxitine disk diffusion tests, oxacillin E-test, salt agar screen and mecA gene which is responsible for the resistance by polymerase chain reaction.Material and Methods: In this study, 90 Staphylococcus aureus and 90 coagulase negative staphylococci strains isolated from patients who were hospitalized in Çukurova University Hospital between 2006-2007. The isolates were identified by the Vitek 2 automated system. mecA gene which is accepted as gold standard was tested by polymerase chain reaction.Results: In the oxacillin disk diffusion test, 45 (50 %) of the Staphylococcus aureus and 47 (52.2 %) of the coagulase negative staphylococci isolates, in the cefoxitine disk diffusion test 45 (50 %) of the Staphylococcus aureus and 46 (51.1 %) of the coagulase negative staphylococci isolates, in the oxacillin E-test 45 (50 %) of the Staphylococcus aureus and 47 (52.2 %) of the coagulase negative staphylococci isolates, in the salt agar screen 44 (48.9 % ) of the Staphylococcus aureus and, 41 (45.5 %) of the coagulase negative staphylococci isolates, and in the Vitek 2 automated system, 44 (48.9 %) of the Staphylococcus aureus isolates and 47 (52.2 %) coagulase negative staphylococcus isolates were found to be resistant. The mecA gene was positive in 44 (48.9 %) of the Staphylococcus aureus and, 45 (50 %) of the coagulase negative staphylococci isolates polymerase chain reaction studies.Conclusion: Detection of methicillin resistance in staphylococci by phenotypical methods often leads to false results. This problem is more evident in the coagulase negative staphylococci than the Staphylococcus aureus isolates. In our study, the results of the phenotypical methods, oxaciilin and cefoxitine disk diffusion tests, oxacillin E-test and salt agar screen test were found to be consistent (sensitivity 97.72 %, spesificity 95.65 %). Cefoxitine disk diffusion test was found to be the most accurate test reaching almost to the gold standard compared to the other methods in coagulase negative staphylococcus strains (sensitivity 97.77 %, spesificity 97.77 %). However, the most appropriate method for detecting the methicillin resistance for both Staphylococcus aureus and coagulase negative staphylococci isolates is detection of mecA gene by polymerase chain reaction.