We studied changes over four years in the characteristics of methicillin-resistant Staphylococcus aureus (MRSA) strains isolated in a French university hospital by analysis of antibiotic susceptibility and restriction fragment length polymorphism using pulsed field gel electrophoresis. Changes were considered in relation to regional and national frameworks. The proportion of gentamicin-susceptible methicillin-resistant S. aureus (GS-MRSA) among MRSA increased from 22.6% in 1994 to 86.8% in 1998. We first isolated strains of GS-MRSA susceptible to tobramycin and amikacin (TKS-MRSA) in 1996 from patients in eight wards. The number of TKS-MRSA strains per 100 MRSA was 6.1 in 1997, 14.5 in 1998, and 18.9 in 1999. Pattern A, the major DNA pattern identified, accounted for 78.6% of isolates in 1998. This pattern was found in TKS-MRSA and TKR-MRSA strains, but not in Gentamicin-resistant MRSA strains (GR-MRSA). Fitness analysis showed that GR-MRSA strains had much lower replication rates than GS-MRSA strains, but there was no difference between TKS-MRSA and TKR-MRSA strains. Aminoglycoside consumption remained constant between 1994 and 1998. The spread of TKS-MRSA in our hospital since 1996 may relate to the "total use threshold", as the level of tobramycin/amikacin use is below that required for selection of TKR-MRSA.
Copyright 2000 The Hospital Infection Society.