Background Methicillin-resistant Staphylococcus aureus (MRSA) is a major nosocomial pathogen and recently there has been the emergence of community-acquired MRSA (CA-MRSA). The impact of CA-MRSA genotypes (e.g. USA300) as a cause of bacteremia and nosocomial infections is poorly understood.
Purpose of Study To determine the molecular epidemiology of MRSA bloodstream infections (BSI) in a large urban hospital.
Methods Between January 1, 2004 and April 7, 2004, MRSA isolates recovered from blood of patients were collected. Pulsed field gel electrophoresis (PFGE) was performed on extracted DNA after SmaI-digestion. DNA fingerprints were compared to established pulsed-field type (PFT) clusters. Epidemiologic data were recorded.
Results A total of 49 patients had a MRSA BSI during the study period, for a rate of 5.7 per 1,000 patient admissions. The demographic and clinical characteristics of patients included mean age of 51 years (range 6 months-92 years); 25 (51%) were male; 44 (90%) were African-American; 14 (29%) were HIV-seropositive; and 15 (31%) had a current or prior history of substance abuse. The mean length of stay was 25.6 days (range 2-131 days), and 13 (27%) died during their hospitalization. Age was independently associated with an increased risk of death (OR 1.07 per year, 95% CI 1.01-1.12). Community-onset disease, defined by positive cultures within 48 hours of admission, occurred in 25 (51%) of the patients; nosocomial infections, defined by a positive culture > 48 hours after admission, occurred in 24 (49%). However, 48 (98%) of 49 patients had health care-associated infections as all but 1 with “community onset” had previous contact with health care facilities (e.g., dialysis, antibiotic use, surgery, and residence in long-term care facility within 1 year prior to admission). Molecular typing revealed that 21 (43%) of 49 MRSA isolates were PFT USA100, 16 (33%) were USA500, and 12 (24%) were USA300, which has been associated with CA-MRSA infections.
Conclusions MRSA is a common cause of bacteremia at our institution and is primarily health care-associated, even if the onset occurred in the community. The MRSA USA300 genotype, usually associated with skin and soft tissue infection, has emerged as a significant cause of nosocomial and health care-associated MRSA BSI in our institution, accounting for nearly one-fourth of all infections. The emergence of MRSA USA300 as a nosocomial pathogen presents new challenges to infection control programs.