Vancomycin, teicoplanin, daptomycin, and linezolid MIC creep in methicillin-resistant Staphylococcus aureus is associated with clonality

The purpose of this study is to evaluate the susceptibility trend of vancomycin, teicoplanin, daptomycin, and linezolid against methicillin-resistant Staphylococcus aureus (MRSA) blood isolates of different clones over an 11-year period. From 2000 to 2010, all bloodstream MRSA isolates from Chang Gung Memorial Hospital in Taiwan were prospectively collected. Three periods, namely 2000 to 2001, 2004 to 2005, and 2010, were included and 124 MRSA isolates were selected from each period. Minimum inhibitory concentrations (MICs) were determined by E-test. All the isolates were molecularly characterized. MRSA molecular epidemiology evolved from 1 predominant pulsotype (type A) to 5 major pulsotypes of 3 clonal complexes (CC). Vancomycin, teicoplanin, and daptomycin MICs creep were observed, particularly for pulsotype A-CC 239-staphylococcal cassette chromosome mec (SCCmec) III though its prevalence dramatically decreased since 2004 to 2005. Throughout the study period, the overall vancomycin modal MIC was stable at 1.5mg/L, but teicoplanin and linezolid modal MIC increased to 2 and 2mg/L, respectively. Isolates with teicoplanin and linezolid ≥2ug/mL belonged to multiple clones. Pulsotype F-ST5-SCCmec II with a high rate of teicoplanin MIC ≥2ug/mL continued clonal spread. Teicoplanin MIC had a high correlation with linezolid MIC. Molecular epidemiology MRSA bloodstream isolates in northern Taiwan evolved from 2000 throughout 2010, which was subsequently associated with the changing distribution of antibiotic MICs. While vancomycin MIC level remained unchanged, teicoplanin, daptomycin, and linezolid MIC levels increased. The impact of these changes on clinical treatment response deserves further investigations.