Emergence of SCCmec type IV and SCCmec type V methicillin-resistant Staphylococcus aureus containing the Panton-Valentine leukocidin genes in a large academic teaching hospital in central Switzerland: external invaders or persisting circulators?

The hospital epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) has changed in the past few years due to the encroachment of community-associated MRSA (CA-MRSA) strains into health care settings. MRSA strains that were isolated during a 2-year period from patients of the Luzerner Kantonsspital were analyzed to elucidate their epidemiology. Moreover, extended surveillance of individuals who were contacts of those patients was carried out for 6 months to identify the routes of spread and to assess the quality of the infection control measures used in our setting. Patient data were collected to distinguish CA-MRSA strains from health care-associated MRSA (HA-MRSA) strains by epidemiological criteria, as defined by the Centers for Disease Control and Prevention (CDC). On the basis of the CDC definition, the majority of the strains were considered to be HA-MRSA. However, 87% of them belonged to staphylococcal cassette chromosome mec (SCCmec) types IV and V, which are traditionally associated with CA-MRSA. Surprisingly, classical nosocomial SCCmec types I and II represented a minority, whereas SCCmec type III was completely absent. By PFGE analysis, four predominant clonal lineages and 21 highly variable sporadic genotypes were detected. Twenty-eight percent of the MRSA strains studied carried the genes encoding the Panton-Valentine leukocidin (PVL), of which 21% and 83% were associated with SCCmec types IV and V, respectively. Among 289 contact individuals screened for MRSA carriage throughout the extended surveillance, a single secondary patient was discovered. The possibility of nosocomial transmission could be excluded. The high proportions of SCCmec type IV and V strains as well as PVL-positive strains suggest strong infiltration of CA-MRSA into our institution. Moreover, the low endemic prevalence of MRSA demonstrates that current infection control measures are sufficient to limit its spreading and the emergence of large epidemic outbreaks.