Introduction: Methicillin-resistant Staphylococcus aureus (MRSA) was seldom isolated in our hospital until the first outbreak in 1999. A recently documented increase in antibiotic multiresistance in MRSA strains in our setting prompted the design of this molecular epidemiology study to investigate the basis for this tendency.
Methods: All MRSA isolates from clinical samples of patients admitted from July 2002 to June 2003 were studied. Susceptibility testing was performed by disk diffusion. Clonal relatedness of MRSA isolates was determined by pulsed-field gel electrophoresis (PFGE). Results were compared with data from MRSA isolates from patients admitted to the hospital in 1999-2000.
Results: MRSA was isolated in 110 patients (30% of patients with S. aureus-positive cultures). PFGE detected three major clones (in 93% of patients), all of which had been present in 1999-2000, although in different proportions. Whereas the predominant clone in 1999 was clone A (clone A 63%, clone B 20%), clone B was now found to predominate (clone B 58%, clone A 19%). None of these major clones were related to the five pandemic clones, including the Iberian clone, but two of them seemed to be related to the two most prevalent clones in Spain at this time. The new predominant clone was more resistant than the others, and showed uniform resistance to ciprofloxacin, erythromycin, clindamycin, and gentamicin.
Conclusion: In recent years, a formerly predominant MRSA clone has been replaced by a multiresistant S. aureus clone that is unrelated to the Iberian clone.