No change in MRSA vancomycin MICs among isolates from paediatric infections (20002007)

Abstract number: O39

Zheng X., Qi C., O'Leary A., Arrieta M., Shulman S.

Objectives: Vancomycin remains one of the major options for treating methicillin-resistant S. aureus (MRSA) related infections. Some but not all studies have shown an increase in prevalence of MRSA isolates with elevated vancomycin MIC values among recent clinical isolates, so called "MIC creep". Although still within the susceptible range, higher MICs may be associated with increased chance of treatment failure. Because of the conflicting reports and lack of published data from paediatric patients, we sought to assess possible MIC change over time and to compare results generated by using different methodologies including Etest, agar dilution, and broth microdilution (MicroScan) methods.

Methods: We studied 318 MRSA isolates predominantly community acquired including all blood and normally sterile site isolates collected in our large Children's hospital in 2000/2001, 2003, 2005, and 2007/2008. In addition, MRSA from non-sterile sites included all collected in 2000/2001, and the first 70 from each of 2003, 2005, and 2007. Antimicrobial susceptibility testing was conducted with MicroScan Positive Combo Panel 29 plates. Vancomycin MIC was also measured by both Etest and standard agar dilution methods. Inducible clindamycin resistance was assessed by D-test. MRSA typing was performed by pulse field gel electrophoresis (PFGE).

Results: We found no vancomycin MIC increase among paediatric MRSA isolates collected from 2000 to 2007. The highest vancomycin MIC value observed was 2 ug/ml. We found significant inter-assay variations, with proportions of isolates with vancomycin MIC=2 ug/ml from 85–92% for Etest 43–48% for MicroScan and 2–4% for agar dilution. We found a progressive decrease in prevalence of inducible clindamycin resistance by D-test, from 65% in 2000 to only 5% in 2007. PFGE analysis showed that US300 accounted for 26% isolates in 2000/2001, and increased to 95% in 2007.

Conclusions: MRSA vancomycin MICs did not increase in our paediatric isolates from 2000 to 2007 despite the fact that US300 strains increased dramatically. In addition, inducible clindamycin resistance declined markedly from 2000 to 2007. Considerably different proportions of isolates with vancomycin MIC=2 ug/ml are found when different laboratory methodologies used, suggesting caution in their interpretation.

Session Details

Date: 16/05/2009
Time: 00:00-00:00
Session name: 19th European Congress of Clinical Microbiology and Infectious Diseases
Location: Helsinki, Finland, 16 - 19 May 2009
Presentation type:
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