Frequency and diversity of molecular epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) isolates from patients of a South West German teaching hospital.

We have determined the frequency and the genetic diversity of methicillin-resistant Staphylococcus aureus (MRSA) strains isolated from inpatients of a South West German teaching hospital. The frequency of primary MRSA isolates remained constant over the two-year surveillance period: 108 of the total number of 1450 S. aureus isolates (7.5%) were MRSA in 2001 and the corresponding figures were 110 MRSA out of 1412 S. aureus isolates (7.8%) in 2002. Six clusters of nosocomial MRSA infection were observed during 2001 and 2002. Cluster infections involved 65 patients on four different wards. Molecular typing of the 65 cluster isolates demonstrated that 51 (79%) belonged to a single predominant clone. The remaining 14 isolates represented seven genetically unrelated MRSA clones. Molecular typing of 50 additional non-cluster, sporadic isolates demonstrated 37 different pulsed-field gel electrophoresis (PFGE) banding patterns. Our results demonstrate that primary MRSA strains from hospitalized patients show a high degree of genetic heterogeneity. In contrast, however, MRSA isolates from nosocomial clusters were mostly derived from a single predominant clone, thus indicating a significant intra-ward and intra-hospital distribution. Molecular typing methods may lead to an improved understanding of MRSA epidemiology and thus allow the establishment of effective preventive measures.