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

Abstract

Roushan Liu,1,* Jian Zhang,2,* Xiaoli Du,3 Yingying Lv,1 Xiangyu Gao,1 Yanyan Wang,1 Junrui Wang1 1Department of Laboratory Medicine, Affiliated Hospital of Inner Mongolian Medical University, Hohhot, 010050, People’s Republic of China; 2Department of Laboratory Medicine, Bayannur People’s Hospital, Bayannur City, 015000, People’s Republic of China; 3National Institute for Communicable Disease Control and Prevention, Chinese Center for Disease Control and Prevention, Beijing, 102206, People’s Republic of China*These authors contributed equally to this workCorrespondence: Junrui WangDepartment of Laboratory Medicine, Affiliated Hospital of Inner Mongolian Medical University, Hohhot, 010050, People’s Republic of ChinaTel [phone]Email [email protected]: 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-100TM, 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.Keywords: oxacillin sensitive MRSA, molecular typing, heterogeneous resistance

Keywords

• oxacillin sensitive mrsa
• molecular typing
• heterogeneous resistance