Emergence and spread in French hospitals of methicillin-resistant Staphylococcus aureus with increasing susceptibility to gentamicin and other antibiotics

Oxacillin (methicillin) resistance in methicillin-resistant Staphylococcus aureus (MRSA) is associated with an increased incidence of resistance to ot her antibiotics, which has increased since it was first reported in 1969. I n 1992 a new phenotype of MRSA arose in France; this nas characterized by a heterogeneous expression of resistance to oxacillin and susceptibility to various antibiotics, including gentamicin but also tetracycline, minocyclin e, lincomycin, pristinamycin, co-trimoxazole, rifampin, and fusidic acid. I n French hospitals a longitudinal nationwide surveillance of antibiotic res istance in S. aureus has allowed for the detection of changes in antibiotic Susceptibility profiles, Seven French clinical laboratories (six from the mainland and one from the West Indies) reported the results of susceptibili ty testing of 57,347 S. aureus strains isolated in their institutes between 1992 and 1998. Over a 7-year period the incidence of isolation of gentamic in-susceptible MRSA (GS-MRSA) strains has steadily increased to represent, in 1998, 46.8 to 94.4% of the MRSA strains, irrespective of the overall inc idence of MRSA. Two predominant types recognized by pulsed-field gel electr ophoresis (PFGE) accounted for the majority of the GS-MRSA in different mai nland hospitals, both differing from the predominant type observed in the F rench West Indies. Some GS-MRSA and gentamicin-resistant MRSA (GR-MRSA) str ains had closely related PFGE profiles, and hybridization studies confirmed the lack in GS-MRSA of the aac6'-apha2 " gene, which confers resistance to all aminoglycosides, with conservation of the ant4' gene, which confers re sistance to kanamycin, tobramycin, and amikacin. Thus, it is likely that ce rtain GS-MRSA strains could have emerged from GR-MRSA strains by excision o r deletion of the aac6'-apha2 " gene.