Glycopeptide minimum inhibitory concentration creep among meticillin-resistant Staphylococcus aureus from 2006–2011 in China

Vancomycin minimum inhibitory concentration (MIC) creep has recently been demonstrated by many countries but is rarely reported in China. In this study, a total of 1411 meticillin-resistant Staphylococcus aureus (MRSA) isolates were collected from six hospitals in China during the period 2006–2011 and the MICs of vancomycin, teicoplanin and linezolid were determined by broth microdilution according to Clinical and Laboratory Standards Institute (CLSI) guidelines. MIC50 and MIC90 values (MICs required to inhibit the growth of 50% and 90% of organisms, respectively) as well as geometric mean (GM) MICs were calculated for all isolates in each year, and MIC creep for the drugs was evaluated. All of the MRSA isolates were susceptible to vancomycin and linezolid. Overall, the vancomycin GM MIC of MRSA isolates was 0.906, 0.952, 0.956, 0.947, 1.013 and 1.040 mg/L, with a significantly increasing trend over the years (P < 0.001). Percentages of MRSA isolates with a vancomycin MIC above 1 μg/mL (2 μg/mL ≥ MIC > 1 μg/mL) were 26.0%, 23.5%, 21.6%, 27.8%, 30.6% and 42.8% from 2006–2011, respectively, and increased over time (P < 0.005). The teicoplanin GM MIC increased rapidly from 0.749 mg/L in 2008 to 0.973 mg/L in 2011, and ca. 5% of isolates were resistant to teicoplanin in the period according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) criteria. MIC shifts were not found for linezolid (P > 0.05). In conclusion, a tendency towards decreasing susceptibility to glycopeptides in MRSA has emerged in China.