Improved recognition of MRSA case clusters by the application of molecularsubtyping using pulsed-field gel electrophoresis

Methicillin-resistant Staphylococcus aureus (MRSA) is increasingly common i n hospital and community populations, making the recognition of true nosoco mial outbreaks more difficult. We have used pulsed-field gel electrophoresi s (PFGE) with Sma I digestion to analyse retrospectively two perceived outb reaks of epidemic methicillin-resistant Staphylococcus aureus 15 (EMRSA 15) colonization. The first cluster of cases in patients and staff on a genera l ward (ward D) revealed three different antibiograms based on differences in ciprofloxacin and rifampicin sensitivities. All isolates typed using PFG E, which was more discriminatory than phage-typing. One PFGE banding profil e labelled type 5 was predominant, but 12 isolates proved to be subtypes of type 5 and two were PFGE type 11. Four staff members carried a strain not found in patients, three carried strains found in patients and transient ca rriage was highlighted as a problem when screening staff. PFGE enhanced the epidemiological data and proved that the cases on this ward did not compri se one large outbreak but numerous sporadic cases and smaller clusters. In contrast, isolates from a second cluster of cases which occurred on ward F were indistinguishable using antibiograms, phage-typing and PFGE, confirmin g this was more likely to be a true outbreak of colonization. We conclude t hat PFGE usefully augments epidemiological information and allows more logi cal infection control decisions to be made, with better utilization of scar ce resources.