Changes in staphylococcal cassette chromosome type and antibiotic resistance profile in methicillin-resistant Staphylococcus aureus isolates from a French hospital over an 11 year period

Background: In this study, we investigated the relationship between changes in antibiotic resistance and distribution of staphylococcal cassette chromosome (SCC) types amongst methicillin-resistant Staphylococcus aureus (MRSA) isolates expressing the most frequently encountered profiles of antibiotic resistance over an 11 year period in the University Hospital of Rennes, France. Methods: Antibiotic susceptibilities were determined by agar diffusion. SCC typing was performed using PCR. PFGE demonstrated that isolates were phylogenetically related. Results: Fourteen profiles of antibiotic resistance were defined among MRSA isolates. For each resistance profile, only one SCC type was associated: four patterns corresponded to SCC type I or IA, nine to SCC type IV or IVA, and none to types II and III. One was not typed. PFGE indicated that isolates with SCC type I or IA belong to a single genetic lineage, which also includes most of the epidemic isolates, which carry SCC type IVA. In contrast to type I or IA, isolates with SCC type IV or IVA were found to be associated with several different PFGE clusters, although not all of these represent epidemic isolates. Conclusions: During the course of the study, the spectrum of antibiotic resistance in MRSA isolates decreased. This occurred due to the emergence of strains with SCC type IV or IVA, which are susceptible to more antibiotics than type I or IA strains. The greater prevalence of such isolates could not be linked conclusively to the presence of SCC type IV or IVA, or to one particular PFGE cluster.