Fitness and competitive growth advantage of new gentamicin-susceptible MRSA clones spreading in French hospitals

Since 1991, new epidemic methicillin-resistant Staphylococcus aureus (MRSA) strains characterized by the unexpected reappearance of heterogeneous phen otypic expression of resistance to methicillin and by susceptibility to gen tamicin and various other antibiotics (GS-MRSA) have been reported in Franc e. GS-MRSA strains have progressively replaced MRSA clones expressing homog eneous resistance to methicillin and resistance to gentamicin (GR-MRSA). In this study, we investigated the physiological characteristics of these new clones. In particular, we evaluated and compared the maximal growth rate a nd the deduced generation times (related to fitness of strains) of the majo r French epidemic MRSA clones. The population studied consisted of 79 isola tes including (i) GR-MRSA that comprised six different types on the basis o f PFGE; (ii) GS-MRSA the majority of which clustered into two PFGE types, A l (usually resistant to erythromycin) and B (usually susceptible to erythro mycin); (iii) methicillin-susceptible S. aureus (MSSA). GS-NRSA-A1 and MSSA strains were shown to have a significant fitness benefit (about 20%) with shorter generation times (theta = 23.7 +/- 0.1 and 22.9 +/- 0.05 min, respe ctively) than GR-MRSA and GS-MRSA-B strains (theta = 30.3 +/- 0.2 and 32.5 +/- 0.5 min, respectively). These data suggest that a link exists between g enetic patterns, resistance profiles and physiological properties. In vitro competitive experiments indicated that GS-MRSA-A1 strains were able to rap idly outgrow GR-MRSA strains. The growth advantage observed should be taken into account in understanding the spread of some new clones of MRSA.