Vancomycin MIC creep in MRSA isolates from 1997 to 2008 in a healthcare region in Hong Kong

Abstract

Objectives: To assess whether vancomycin MIC creeps among blood methicillin-resistant Staphylococcus aureus (MRSA) isolates recovered from 5 hospitals in Hong Kong from 1997 to 2008. Methods: Blood cultures MRSA isolates from 1997 to 1999 (period 1), 2004 (period 2) and 2006-2008 (period 3) were retrieved. Etest method was used to determine their vancomycin MIC. The genotypic features were determined by PCR and sequencing. Results: 247 blood MRSA isolates were studied. The vancomycin MIC were 0.375, 0.5, 0.75 and 1 mg/L for 15 (6.1%), 68 (27.5%), 89 (36%) and 75 (30.4%) isolates, respectively. There was an increase in the percentage of isolates with an MIC=1mg/L from 10.4% (5/48) during period 1 to 21.6% (8/37) during period 2 and 38.3% (62/162) during period 3 (period 1 vs. period 3, P<. 0.001). Molecular typing showed that this was due to increased percentages of clonal cluster (CC) 8/SCC. mec III/IIIA (agr group I), CC45/SCC. mec IV/V (agr group IV) and other minor clones with elevated MIC over time. Conclusion: This study found vancomycin MIC creep among blood MRSA isolates over time. As elevated MIC within the susceptible range may reduce vancomycin efficacy, clinical laboratories should adopt methods with the required precision to accurately determine MICs. © 2009 The British Infection Society.