Emerging Challenges in Methicillin Resistance of Coagulase-Negative Staphylococci

Objective: In the present study, we used phenotypic and molecular methods to determine susceptibility to oxacillin in coagulase-negative staphylococci (CoNS) and estimate the prevalence of strains with low-level resistance to oxacillin, mecA-positive oxacillin-susceptible methicillin-resistant (OS-MRCoNS), and borderline oxacillin-resistant (BORCoNS). Methods: One hundred one CoNS strains were screened for oxacillin and cefoxitin susceptibility using phenotypic (disk diffusion, agar dilution, latex agglutination, and chromagar) and molecular (detection of mecA, mecB, and mecC) methods. Staphylococcal cassette chromosome mec (SCCmec) typing was performed. Results: Sixteen (15.8%) CoNS strains were mecA-positive, and 85 (84.2%) were mec-negative. Seven (6.9%) were classified as OS-MRCoNS, accounting for 43.8% of all mecA-positive strains. Twelve (11.9%) mec-negative strains were classified as borderline oxacillin resistant (BORCoNS). Compared with MRCoNS and BORCoNS, OS-MRCoNS strains demonstrated lower resistance to non-beta-lactams. SCCmec type I cassette was predominant. The disc-diffusion method with oxacillin accurately predicted OS-MRCoNS strains but did not provide reliable results for BORCoNS strains. Meanwhile, the latex agglutination test and CHROMagar culture accurately identified BORCoNS but not OS-MRCoNS. Conclusions: Finally, our findings imply that the recognition of methicillin resistance in CoNS requires a meticulous approach and that further research is needed to develop unified laboratory diagnostic algorithms to prevent the misreporting of borderline CoNS.