Molecular characterization of methicillin-resistant Staphylococcus aureus (MRSA) in a university hospital in Italy

The molecular epidemiology of methicillin-resistant Staphylococcus aureus ( MRSA) in a university hospital in Italy was studied in a five-month period in 1996, during which all S. aureus isolated were collected. All MRSA isola tes (95) and a sample of methicillin-susceptible S. aureus (20) were typed with a variety of phenotypic and genotypic methods. Clonal identities were determined by pulsed-field gel electrophoresis (PFGE) of chromosomal SmaI d igests and, for MRSA isolates, by probing ClaI digests with a mecA probe an d a Tn554 probe. Overall, MRSA represented 32.3 % of all isolates, with ver y high percentages from the intensive care units (adult and neonatal). PFGE after restriction with SmaI resolved ge nomic DNA of 95 MRSA strains into 26 major PFGE patterns. The use of southern blot hybridization of ClaI geno mic digests with mecA and Tn554 allowed us a significant increase in discri mination, differentiating at least 32 different clones. Two major clones, h owever, each sharing common ClaI-mecA and Tn554 type and PFGE pattern as we ll as a common resistance phenotype, represented more than 50% of all MRSA isolates. The recovery of these two clones in the majority of the isolates of adult and neonatal intensive care units, respectively, is indicative of typical nosocomial outbreaks and clonal spread. It is concluded that intens ive care units are major areas requiring preventative interventions.