In vitro activity of new quinolones against clinical strains of Staphylococcus aureus of the wild type and with mutations characterized by gyrA, gyrB and grlA

The aim of this study was to determine the in vitro activity of five quinolones against clinical strains of methicillin-susceptible and -resistant Staphylococcus aureus clinical isolates characterized at the molecular level with respect to the presence of mutations in genes coding for resistance to quinolones (grlA, gyrA and gyrB). The relationship between the mutations found and the activities of these quinolones was also analyzed. Trovafloxacin was the most active against methicillin-susceptible S. aureus and showed good activity against methicillin-resistant S. aureus, with a MIC90 of 2 mg/l. The grlA-gyrA double mutation was the most frequent (55% of the strains). Single mutation in grlA was detected only in 5% of strains; 39% of strains showed a wild-type genotype. The grlA-gyrA double mutants presented a high level of resistance against the fluoroquinolones tested except for trovafloxacin, with the MIC ranging between 0.5 and 4 mg/l. Wild-type strains were susceptible to all the fluoroquinolones tested and the single grlA mutants had a low level of quinolone resistance but were still below the breakpoint for resistance. Trovafloxacin and sparfloxacin were less affected by this mutation.