Are community-associated methicillin-resistant Staphylococcus aureus (MRSA) strains replacing traditional nosocomial MRSA strains?

Background. Recent studies have suggested that community-associated methicillin-resistant Staphylococcus aureus (MRSA) infection is encroaching on health care settings. We describe the epidemiology of hospital-onset community-associated MRSA bloodstream infections using phenotypic and genotypic analysis. Methods. Using an update of an established rule derived from antibiotic susceptibilities, we inferred genotypes (i.e., community [CG] or hospital [HG]) for 208 MRSA isolates from hospital-onset (> 72 h after hospital admission) bloodstream infections during 2000-2006. We compared demographic characteristics, risk factors, and outcomes of patients infected with CG or HG strains. Results. Total hospital-onset MRSA bloodstream infection incidence density rates for the periods January 2000-June 2003 and July 2003-December 2006 (0.215 cases per 1000 patient-days and 0.207 cases per 1000 patient-days, respectively) were stable (risk ratio, 1.0; 95% confidence interval, 0.7-1.3; P = .79, period 2 vs. period 1). However, the risk that these bloodstream infections were due to CG strains doubled (risk ratio, 1.9; 95% confidence interval, 1.2-3.1; P = .01), whereas the risk due to HG strains decreased (risk ratio, 0.7; 95% confidence interval, 0.46-0.93; P = .02). After adjustment for comorbidities in multivariate analysis, no significant risk factors for or outcomes of infections due to CG versus HG strains were detected. Patients infected with HG strains showed a trend toward later day of acquisition of a positive blood culture, and those infected with CG strains showed trend toward greater risk of intensive care unit admission. Conclusion. Although total hospital-onset MRSA bloodstream infection rates were relatively stable during 2000-2006, CG strains were responsible for an increasing proportion of cases (from 24% to 49%), suggesting replacement of traditional hospital-associated strains. For most risk factors and outcomes, patients infected with CG and HG strains were similar, suggesting that, thus far, CG strains are behaving like their traditional hospital-associated counterparts.