494 Microbiologic Characteristics of Methicillin-resistant Staphylococcus aureus (MRSA) Bacteremia in a Large Urban Hospital

Saturday, March 20, 2010
Grand Hall (Hyatt Regency Atlanta)
Emily RM Sydnor, MD , Johns Hopkins University School of Medicine, Baltimore, MD
Tsigereda Tekle, MT , Johns Hopkins Hospital, Baltimore, MD
Paul Stamper , Johns Hopkins University School of Medicine, Baltimore, MD
Karen C. Carroll, MD , Johns Hopkins Hospital, Baltimore, MD
Pranita D. Tamma, MD , Johns Hopkins University, Baltimore, MD
Sara E. Cosgrove, MD, MS , The Johns Hopkins Hospital, Baltimore, MD
Background: Strains of MRSA associated with community-acquired infection (CAI) have been increasingly reported as causes of healthcare-associated infections (HCAI). However, the clinical significance of this epidemiologic transition is unclear.

Objective: We aimed to describe the microbiologic epidemiology of MRSA bacteremia at The Johns Hopkins Hospital and compare clinical and microbiologic characteristics to assess for significant differences among strain types.

Methods: We assessed clinical and microbiologic characteristics of all cases of MRSA bacteremia between 7/1/07 and 5/31/08. Clinical characteristics and outcomes for 54 of 138 cases were extracted retrospectively from medical records. Cases were classified as CAI, HCAI (not nosocomial) or nosocomial-acquired infection (NAI) based on standard definitions. Molecular characterization of MRSA isolates was performed by pulsed-field gel electrophoresis. The presence of panton-valentine leukocidin (PVL) toxin was determined by real-time PCR assay. Detection of hetero-resistant vancomycin intermediate Staphylococcus aureus (hVISA) was determined by glycopeptide resistance detection (GRD) E-test method on all isolates with a vancomycin E-test MIC of 2µg/ml or 3µg/ml and a subset of isolates with an MIC of 1.5µg/ml.

Results: We identified 138 cases of MRSA bacteremia. Community-associated strains of MRSA were more common in both CAIs and HCAIs as compared to NAIs {OR 8.96 (95% CI, 3.25-24.67) and OR 2.10 (95% CI, 0.87-5.09), respectively}. (Table 1) PVL toxin production also was associated with CAI. Forty of 138 (29.0%) isolates tested were characterized as non-USA300-900 strains. The majority of CAIs were due to intravenous drug use (IVDU) or skin and soft tissue infections, in contrast to HCAIs and NAIs, which were most commonly secondary to long-term catheter usage. Mortality among patients with CAI and HCAI (2.6% and 3.4%) was lower than mortality in patients with NAI (11.9%). Of isolates tested for hVISA, 3 of 12(25.0%) with vancomycin MICs of 2µg/ml and 2 of 11(18.2%) with an MIC of 1.5µg/ml were found to be hVISA. hVISA was detected in one healthcare-associated MRSA strain and four of the non-USA300-900 strains.

Conclusions: Patients with CAI and HCAI MRSA bacteremia were more likely to have community-associated strains than patients with NAI, although 24% of NAI MRSA bacteremia was due to community-associated strains at our institution. A third of our isolates were non-USA300-900 and the majority of hVISA detected was in these strains. Additional characteristics of and outcomes associated with these strains warrant future research.