Comparison of oxascillin-cefoxitin disc diffusion, oxacillin agar screening, latex aglutination, mecA gene detection and an automatized system for detection of methicillin resistance in Staphylococcus aureus

Purpose: In this study, we aimed to compare the phenotypic methods that are employed at the routine microbiology laboratory of Turgut Ozal Medical Center in determining methicillin-resistant Staphylococcus aureus (MRSA), MRSA latex agglutination test, and an automatized system.Materials and Methods: The of 214 isolates that were included in the study, were verified as Staphylococcus aureus using colony morphology, Gram staining, catalase, mannitol fermentation, and coagulase tests. Determination of mecA gen by PCR was referred to define MRSA. Later on, oxacillin and cefoxitin disc diffusion, oxacillin agar screening, MRSA latex agglutination test, and the automatized system were applied to all species. Sensitivity and specificity of the methods were evaluated.Results: Of the species included in the study, 40% were determined as MRSA since they were mecA positive. The sensitivity of oxacillin disc diffusion, cefoxitin disc diffusion, oxacillin agar screening, MRSA latex agglutination test, and the automatized system in determining MRSA were 95.1%, 96.3%, 95.1%, 96.3% and 98.7%, respectively; the specifity of oxacillin disc diffusion, cefoxitin disc diffusion, oxacillin agar screening, MRSA latex agglutination test, and the automatized system in determining MRSA were 95.3%, 98.4%, 98.4%, 98.4% ve 97.6%, respectively. Three species were mecA positive and their oxacillin MIC were greater than 2 µg/ml; and they were determined as sensitive to methicillin by disc diffusion, agar screening and latex agglutination. Another 4 species were determined as sensitive to methicillin by all tests, though they were mecA positive. The MIC values to oxacillin of these 4 species were lower than 2 µg/ml and these species were determined as oxacillin- sensitive MRSAs (OS-MRSA).Conclusion: The automatized system had the highest sensitivity values in determining the MRSA species. Also, cefoxitin disc diffusion and latex agglutination tests showed the same sensitivity values. The rapid results achieved by automatized system and latex agglutination tests suggests us that they are superior to the other phenotypic methods. Nevertheless, cefoxitin disc diffusion test seems to be more appropriate to perform at routine laboratories in terms of cost and efficiency. Furthermore, presence of mecA and oxacillin MIC values should be performed simultaneously to determine OS-MRSA species.