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

Abstract

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.

FIG. 1.

Distributions of major epidemic MRSA isolates between 1992 and 2000.

FIG. 2.

Classification of PFGE restriction patterns of MRSA pattern I, II, III, VI, and VII isolates (55 isolates) by using the Dice coefficient and unweighted pair grouping matching analysis.

FIG. 3.

PFGE restriction patterns of MRSA pattern VII and MSSA pattern ΔVII isolates. Lane 1, MRSA pattern VII strain from patient 1; lane 2, MRSA pattern VII strain from patient 2; lane 3, MSSA pattern ΔVII strain from patient 1; lane 5, DNA ladder (n × 48.5-kb fragments); lane 6, MSSA pattern ΔVII strain from patient 2; lanes 4, 7, 8, 9, and 10, MSSA pattern ΔVII strains from other patients