Diffusion of meticillin-resistant Staphylococcus aureus USA300 strains in central Italy

Meticillin-resistant Staphylococcus aureus (MRSA) is an outstanding, clonally evolving pathogen that in recent years, under the selective pressure of antibiotics, has acquired the crucial ability to infect people outside of hospitals. MRSA USA300 has progressively become synonymous with severe community-associated staphylococcal disease worldwide. Whilst spreading worldwide, these clones have progressively acquired resistance to several antibiotics and have gained the ability to cause infections in hospital settings. Recently, USA300-related strains showing resistance to several antibiotics have been isolated from community-acquired infections in Italy. This paper reports the high frequency of isolation of USA300-related strains both from community- and hospital-acquired infections in central Italy as well as their genotypic characteristics and antibiotic susceptibility. Analysis of these characteristics by partial least squares discriminant analysis enabled it to be demonstrated that whilst moving from the community to the hospital setting these isolates underwent an adaptive process that generated clones showing distinctive characteristics. These observations further support the hypothesis that the threatening generation of strains combining both resistance and virulence is becoming a reality, and stress the necessity of constant molecular epidemiological surveillance of MRSA.