Population-Based Community Prevalence of Methicillin-Resistant Staphylococcus aureus in the Urban Poor of San Francisco

The study objective was to determine the prevalence and risk factors for nasal colonization with Staphylococcus aureus and methicillin resistance among the urban poor and to compare antibiotic resistance and genetic similarity to concurrently collected clinical isolates of methicillin-resistant S. aureus (MRSA). A population-based community sample of 833 homeless and marginally housed adults were cultured and compared with 363 clinical isolates of MRSA; 22.8% of the urban poor were colonized with S. aureus. Of S. aureus isolates, 12.0% were methicillin resistant. Overall prevalence of MRSA was 2.8%. Significant multivariate risk factors for MRSA were injection drug use (odds ratio [OR], 9.7), prior endocarditis (OR, 4.1), and prior hospitalization within 1 year (OR, 2.4). Resistance to antimicrobials other than beta-lactams was uncommon. Only 2 individuals (0.24%) with MRSA had no known risk factors. A total of 22 of 23 community MRSA genotypically matched clinical MRSA isolates, with 15 of 23 isolates identical to MRSA clones endemic among hospitalized patients.