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-affiliated Hospital de Bellvitge "Princeps d'Espanya" in Barcelona first acquired methicillin-resistant Staphylococcus aureus (MRSA). Since that time, the outbreak of MRSA disease has continued. We have analyzed by genomic DNA fingerprinting 189 MRSA isolates collected between late 1989 and the end of 1993. The isolates include both invasive and colonizing strains as well as isolates from health-care workers and environmental sources. In addition, 52 clinical isolates of methicillin-susceptible 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 type I, pulsed-field gel electrophoretic pattern B, and Tn554 pattern a was responsible for the great majority of infections (73% of blood cultures and 79% of specimens from other clinical sources). This clone appeared at the very beginning of the outbreak, spread throughout the hospital wards, and was also carried by inpatients and health-care workers and on environmental surfaces. In contrast, no dominant lineage was apparent among MSSA isolates (33 distinct pulsed-field gel electrophoretic patterns among 52 isolates). Two MSSA isolates seem to have originated from the dominant clone by deletion of the mecA gene and some additional DNA. In several isolates, different mecA polymorphs were present in identical chromosomal backgrounds or cells with distinct chromosomal backgrounds carried the same mecA polymorph, suggesting horizontal transfer of the mecA gene.