Clonal Diversity, Low-Level and Heterogeneous Oxacillin Resistance of Oxacillin Sensitive MRSA

Purpose: This study investigates the phenotypic and genotypic resistance features of OS-MRSA clinical isolates and their distinctive sensitivities to oxacillin. Methods: 1200 clinical isolates of Staphylococcus aureus were enrolled in this study. Automated antibiotics susceptibility tests on VITEK-2 and BD Phoenix-100 (TM), cefoxitin disc diffusion method, oxacillin broth microdilution method, mecA, and mecC gene detection were performed to identify OS-MRSA. MLST, PFGE, SCCmec, and spa typing methods were employed to determine genotypes of OS-MRSA isolates. Heterogeneous resistance of OS-MRSA isolates was detected using the population analysis profiling method, and PBP2a latex agglutination assay was used to detect the expression of PBP2a protein for 14 OS-MRSA isolates and their highly resistant subpopulations. Results: A total of 14 OS-MRSA isolates (1.17%, 14/1200) were identified, and all of the isolates were confirmed to be positive with the mecA gene and negative with the mecC gene. All of the 14 OS-MRSA isolates were identified as MSSA by VITEK-2, BD Phonenix-100, and oxacillin broth microdilution methods, while 21.43% (3/14) isolates were determined to be MRSA by the cefoxitin disk diffusion method. Genotypes of the 14 OS-MRSA isolates were diverse, and no dominant clones were identified. The prevalence of pvl gene among 14 OS-MRSA isolates was high up to 64.29% (9/14). All of the isolates showed heterogeneous resistance to oxacillin, while frequencies of the oxacillin-resistant subpopulations ranged from 10(-9 )to 10(-5) and differed significantly among different isolates. Conclusion: The overall prevalence of OS-MRSA was relatively lower, but lower oxacillin MICs, low testing sensitivity of routine antibiotics susceptibility testing methods and weak PBP2a protein expression were observed in this study. 14 OS-MRSA showed diverse genotypes and universal heterogeneous resistance, and inaccurate laboratory identification and improper antimicrobial usage may promote the induction of highly resistant subpopulations and lead to treatment failure.

Automated summary

This study found a relatively low prevalence of OS-MRSA, but with universal heterogeneous resistance and weak PBP2a protein expression. The frequency of resistant subpopulations varied among different isolates, and no dominant clones were identified.