Nine-year surveillance of methicillin-resistant Staphylococcus aureus in a hospital suggests instability of mecA DNA region in an epidemic strain

The distributions of the antibiotic resistance patterns in a population of Staphylococcus aureus isolates from a teaching hospital were studied over a 9-year period. The results indicate the existence of successive major epidemic methicillin-resistant strains and the emergence of a methicillin-susceptible strain with an unusual resistance pattern. Our findings suggest that this methicillin-susceptible S. aureus strain could be derived from the dominant gentamicin-susceptible methicillin-resistant S. aureus strain with the loss of a 40-kb DNA fragment.