During 2004-2009, 20004 isolates of Staphylococcus aureus were collected from the Tigecycline Evaluation and Surveillance Trial (T.E.S.T.). Of these isolates, 8249 (41.2%) were meticillin-resistant S. aureus (MRSA) and 11755 (58.8%) were meticillin-susceptible S. aureus (MSSA). A total of 4.0%, 5.3% and 3.0% of all S. aureus, MRSA and MSSA isolates, respectively, exhibited vancomycin minimum inhibitory concentrations (MICs) ≥ 2 μg/mL. Whilst no vancomycin-resistant S. aureus were encountered in this study and the majority of these isolates remained susceptible to vancomycin at the Clinical and Laboratory Standards (CLSI) breakpoint of 2 μg/mL, the total number of isolates with MICs creeping up to 2 μg/mL and above increased in all S. aureus from 4.0% in 2004 to 7.7% in 2009 (P < 0.001). Moreover, in MRSA this phenotype increased from 5.6% in 2004 to 11.1% in 2009 (P < 0.001). The increase was also notable for MSSA, which rose from 2.6% in 2004 to 5.6% in 2009 (P < 0.001). Of the 12 antibiotics tested, linezolid, minocycline, tigecycline and vancomycin were the most active agents by susceptibility against all S. aureus, all MRSA and all MSSA isolates. Against MRSA isolates with vancomycin MICs ≥ 2 μg/mL, susceptibility to vancomycin decreased from 100% in 2004 to 95.77% in 2009 (P > 0.05). Similarly, in MSSA isolates susceptibility to vancomycin decreased from 100% in 2004 to 91.07% in 2009 (P > 0.05). These data suggest that although the number of isolates of S. aureus with reduced susceptibility to vancomycin has increased significantly from 2004 to 2009, this upward creep of MICs has not yet impacted significantly on the overall susceptibility of vancomycin against either MRSA or MSSA.
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