SPREAD AND MAINTENANCE OF A DOMINANT METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS (MRSA) CLONE DURING AN OUTBREAK OF MRSA DISEASE IN A SPANISH HOSPITAL

It was not until November 1989 that the 1,000-bed University-affiliate d Hospital de Bellvitge ''Princeps d'Espanya'' in Barcelona first acqu ired methicillin-resistant Staphylococcus aureus (MRSA). Since that ti me, the outbreak of MRSA disease has continued. We have analyzed by ge nomic DNA fingerprinting 189 MRSA isolates collected between late 1989 and the end of 1993. The isolates include both invasive and colonizin g strains as well as isolates from health-care workers and environment al sources, In addition, 52 clinical isolates of methicillin-susceptib le S. aureus (MSSA) collected in the same hospital were also analyzed. Isolates were classified into clonal types on the basis of molecular typing techniques. A single MRSA clone (I::B::a) belonging to ClaI typ e I, pulsed-field gel electrophoretic pattern B, and Tn554 pattern a w as responsible for the great majority of infections (73% of blood cult ures and 79% of specimens from other clinical sources). This clone app eared at the very beginning of the outbreak, spread throughout the hos pital wards, and was also carried by inpatients and health-care worker s and on environmental surfaces. In contrast, no dominant lineage was apparent among MSSA isolates (33 distinct pulsed-field gel electrophor etic patterns among 52 isolates). Two MSSA isolates seem to have origi nated from the dominant clone by deletion of the mecA gene and some ad ditional DNA. In several isolates, different mecA polymorphs were pres ent in identical chromosomal backgrounds or cells with distinct chromo somal backgrounds carried the same mecA polymorph, suggesting horizont al transfer of the mecA gene.