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?

Abstract

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.

FIG. 1.

Prevalence of SCCmec types at the LUKS hospitals over a 2-year period (2007 and 2008).

FIG. 2.

Dendrogram showing the PFGE patterns of MRSA strains representative of each type and subtype detected. Percent similarity was calculated by use of the Dice coefficient (with a tolerance of 1% and an optimization of 0.5%) and is represented by UPGMA. Bands larger than 600 kb and smaller than 36 kb were not analyzed, due to poor resolution. (a) PFGE pattern; (b) SCCmec type; (c) number of isolates sharing the same PFGE pattern and SCCmec type. Pattern 6B represents that for reference strain USA300.

FIG. 3.

Dendrogram of the predominant cluster of MRSA isolates showing the pattern PFGE type 18 and SCCmec type IV, together with the antibiograms. Susceptibility is depicted in black, and resistance is depicted in red (CIP, ciprofloxacin; CLI, clindamycin; ERY, erythromycin; GEN, gentamicin; RIF, rifampin; SXT, trimethoprim-sulfamethoxazole; TOB, tobramycin; VAN, vancomycin). Percent similarity was calculated by use of the Dice coefficient (with a tolerance of 1% and an optimization of 0.5%) and is represented by UPGMA. Bands larger than 600 kb and smaller than 36 kb were not analyzed, due to poor resolution.

FIG. 4.

Dendrogram of PVL-positive MRSA strains. Specific PFGE patterns are shown, as are the SCCmec types and antibiograms. Susceptibility is depicted in black, and resistance is depicted in red (CIP, ciprofloxacin; CLI, clindamycin; ERY, erythromycin; GEN, gentamicin; RIF, rifampin; SXT, trimethoprim-sulfamethoxazole; TOB, tobramycin; VAN, vancomycin). Percent similarity was calculated by use of the Dice coefficient (tolerance of 1% and an optimization of 0.5%) and is represented by UPGMA. Bands larger than 600 kb and smaller than 36 kb were not analyzed, due to poor resolution.