Trend in vancomycin susceptibility and correlation with molecular characteristics of methicillin-resistant Staphylococcus aureus causing invasive infections in Taiwan: results from the Tigecycline in vitro Surveillance in Taiwan (TIST) study, 2006-2010.

Abstract This study was intended to investigate the trend in vancomycin susceptibility and correlation with molecular characteristics of methicillin-resistant Staphylococcus aureus (MRSA) causing invasive infections. A total of 670 MRSA isolates were collected from patients with invasive infections as part of bacterial collection in the Tigecycline in vitro Surveillance in Taiwan (TIST) from 2006 to 2010. MICs of the isolates to vancomycin were determined using the agar dilution method. Characteristics of staphylococcal cassette chromosome mec (SCC mec ), mec -associated hypervariable region ( dru ), and accessory gene regulator ( agr ) of the isolates were identified by polymerase chain reaction methods. MRSA isolates with SCC mec types I, II, and III were molecularly defined as hospital-associated MRSA (HA-MRSA), and those with SCC mec types IV, V, and V T were assigned as community-associated MRSA (CA-MRSA). All but 1 MRSA isolates exhibited vancomycin MICs ≤1 mg/L. A declining trend in vancomycin MICs among MRSA isolates was noted, which was associated with the decline in proportion of HA-MRSA. The percentage of CA-MRSA increased from 25.6% in 2006 to 46.0% in 2010. An increase in the geometric mean of vancomycin MICs was found in MRSA with particular molecular types such as SCC mec types II and III, agr groups I and II, and dru 10–14. A significant correlation among particular molecular types was found, including SCC mec II– agr group II– dru 4, SCC mec III– agr group I– dru 11–14, SCC mec IV– agr group II– dru 9, and SCC mec V T – agr group I– dru 9 and dru 11. There was no vancomycin creep among MRSA isolates, and the declining trend of vancomycin MIC against MRSA was attributed to the increasing prevalence of CA-MRSA over time.