Molecular epidemiology of methicillin-resistant Staphylococcus aureus in a Veterans Administration Medical Center

OBJECTIVES: To determine whether patients who were colonized or infected with methicillin-resistant Staphylococcus aureus (MRSA) persistently carried the same strain and to identify the extent of strain variation within a population of patients. DESIGN: Molecular typing by pulsed-field gel electrophoresis (PFGE) of stored MRSA isolates. SETTING: A Veterans Administration Medical Center with 288 hospital, 45 intermediate-care, and 75 extended-care beds. ISOLATES: Between January 1991 and March 1993, 91 patients had MRSA identified in routine cultures. One hundred isolates from 57 patients (63%) were available for typing. RESULTS: Before 1988, only occasional MRSA isolates were identified. By 1993, 5096 of S. aureus isolates from unique patients were resistant to methicillin. PFGE identified 7 MRSA strains, 3 of which were identified in specimens from 1 patient each. The most common strains were SD4 (20 patients), SDI (12 patients), SD2 (12 patients), and SD5a (5 patients). Twenty patients had 2 or more isolates obtained at least 1 week apart (mean, 30.7 weeks: range, 1 to 102 weeks). Of these patients, 12 were colonized or infected with only one strain (mean time observed, 25.1 weeks; range, 1 to 82 weeks). Eight patients had at least 2 different strains (mean time observed, 39 weeks; range, 2 to 102 weeks). CONCLUSION: Numerous MRSA strains circulated in this endemic setting; 40% of patients observed over time were colonized or infected with more than one strain. Molecular typing was an essential tool for evaluating the epidemiology of MRSA in this setting.