Genetic Relatedness of Multidrug-Resistant, Methicillin (Oxacillin)-ResistantStaphylococcus aureusBloodstream Isolates from SENTRY Antimicrobial Resistance Surveillance Centers Worldwide, 1998

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Abstract
We reviewed Staphylococcus aureus bloodstream infection isolates from SENTRY centers worldwide during 1998 to evaluate the molecular epidemiology of multiply drug-resistant methicillin ( oxacillin)-resistant S. aureus (MDR-MRSA). MDR-MRSA was defined as a S. aureus isolate with a MIC for oxacillin at >2 μg/ml and with four or more additional resistances. A total of 325 unique patient isolates of MDR-MRSA from five continents were analyzed using ribotyping and pulsed-field gel electrophoresis (PFGE). The frequency of MDR-MRSA among all S. aureus BSI isolates ranged from only 2.2% in Canada to 35.6% in the Asia-Pacific region. Forty-eight ribotypes (RT) were distinguished, but over 80% of the isolates were contained within the 10 most prevalent RTs. The most common RT, RT 184.5, which included 30% of all MDR-MRSA, was found on four of five continents. PFGE provided superior discrimination and identified numerous clusters of possible clonal dissemination of MDR-MRSA within individual medical centers and between institutions that are in geographic proximity. In four instances, strains with indistinguishable PFGE patterns were found on more than one continent. The predominant PFGE subtype in South America (RT 893.5/Ia) was isolated from patients at centers in Brazil, Argentina, and Portugal, and closely related subtypes were isolated in Chile and Italy. There is great geographic variation in rates of methicillin- and multidrug-resistance among S. aureus bloodstream isolates worldwide. Although many MDR-MRSA strains group geographically, a few closely related epidemic strains have wide regional and even global range.