Molecular epidemiology of quinolone resistance and comparative in vitro activities of new quinolones against European Staphylococcus aureus isolates

New fluoroquinolones (FQ) may possibly be used as alternative therapeutic options for Staphylococcus aureus infections. Our objectives were: (1) to define the in vitro activities of seven FQs in a collection of 434 methicillin-susceptible and 457 methicillin-resistant S. aureus from 23 European university hospitals; (2) to characterise the prevalence of mutations in the grlA and gyrA genes in all ciprofloxacin-resistant (n=433) isolates of S. aureus; (3) to determine the percentage of ciprofloxacin-resistant S. aureus strains with measurable quinolone efflux. Methods: (1) The in vitro activities of different FQs were determined by microdilution tests. (2) PCR-amplified DNA was sequenced. (3) Ciprofloxacin minimum inhibitory concentrations (MIC) were determined in the presence and absence of reserpine, which inhibits efflux pumps. Results: (1) Irrespective of the methicillin resistance of the isolates, sitafloxacin and clinafloxacin showed the best in vitro activities. (2) All ciprofloxacin-resistant isolates exhibited GrlA alterations, namely Ser-80-->Phe or Tyr or Glu-84-->Lys or Ala-116-->Glu or Pro or a combination of Ser-80-->Phe and Glu-84-->Val. These alterations in GrlA were combined with alterations in GyrA, namely Ser-84-->Leu or Lys or Glu-88-->Lys or Val. (3) Reserpine reduced ciprofloxacin MIC values in ca. 30% of the clinical isolates tested. Conclusions: (1) This current European overview of mutations involved in FQ resistance demonstrates that only a limited number of classical mutations in grlA and gyrA contributed to resistance in clinical isolates. (2) An efflux pump is involved in ca. 30% of ciprofloxacin-resistant S. aureus isolates. (3) Sitafloxacin and clinafloxacin are two very promising new FQs with good anti-staphylococcal activity. New FQs, perhaps in combination with efflux pump inhibitors, might play a role in the treatment of S. aureus infections.