Phenotypic and molecular typing of nosocomial methicillin-resistant Staphylococcus aureus strains susceptible to gentamicin isolated in France from 1995 to 1997

Methicillin-resistant strains susceptible to gentamicin (Gm(s) MRSA) have e merged since 1993 in several French hospitals. To study whether particular clones have spread in various French cities and whether some clones are rel ated to gentamicin-resistant (Gm(r)) MRSA strains, various methods (antibio typing, phage typing, determination of SmaI macrorestriction patterns befor e and after hybridization with IS256 transposase and aacA-aphD probes) were used to compare 62 Gm(s) MRSA strains isolated from 1995 to 1997 in nine c ities and 15 Gm(r) MRSA strains. Eighteen major SmaI genotypes were identif ied, of which 11 included only Gm(s) MRSA strains and 5 included only Gm(r) MRSA strains. Each of the Gm(r) MRSA strains contained 6 to 13 SmaI fragme nts hybridizing with the insertion sequence IS256, of which a single band a lso hybridized with the aacA-aphD gene. No such hybridizing sequences were detected in 60 of the 62 Gm(s) MRSA strains. Thus, the divergence between G m(r) and Gm(s) MRSA strains is revealed, not only by their distributions in distinct SmaI genotypes but also by the differences in hybridization patte rns. Two of the 62 Gm(s) MRSA strains had the uncommon feature of carrying several SmaI bands hybridizing,vith IS256, suggesting that they are possibl y related to the Gm(r) MRSA strains grouped in the same SmaI genotype, Five of the 11 SmaI genotypes including only Gm(r) MRSA strains contained strai ns from diverse cities, isolated during different Sears and with different antibiograms, suggesting that some clones have spread beyond their cities o f origin and persisted.