ROLE OF MUTATIONS IN DNA GYRASE GENES IN CIPROFLOXACIN RESISTANCE OF PSEUDOMONAS-AERUGINOSA SUSCEPTIBLE OR RESISTANT TO IMIPENEM

In Pseudomonas aeruginosa, resistance to imipenem is mainly related to a lack of protein OprD and resistance to fluoroquinolones is mainly r elated to alterations in DNA gyrase. However, strains cross resistant to fluoroquinolones and imipenem have been selected in vitro and in vi vo with fluoroquinolones. We investigated the mechanisms of resistance to fluoroquinolones in 30 clinical strains of P. aeruginosa resistant to ciprofloxacin (mean MIC, >8 mu g/ml), 20 of which were also resist ant to imipenem (mean MIC, >16 mu g/ml). By immunoblotting, OprD level s were markedly decreased in all of the imipenem-resistant strains. Pl asmids carrying the wild-type gyrA gene (pPAW207) or gyrB gene (pPBW80 1) of Escherichia coli were introduced into each strain by transformat ion. MICs of imipenem did not change after transformation,,whereas tho se of ciprofloxacin and sparfloxacin dramatically decreased (25- to 70 -fold) for all of the strains. For 28 of them (8 susceptible and 20 re sistant to imipenem), complementation was obtained with pPAW207 but no t with pPBW801. After complementation, the geometric mean MICs of cipr ofloxacin and sparfloxacin (MICs of 0.3 mu g/ml and 0.5 mu g/ml, respe ctively) were as low as those for wild-type strains. Complementation w as obtained only with pPBW801 for one strain and with pPAW207 and pPBW 801 for one strain highly resistant to fluoroquinolones. These results demonstrate that in clinical practice, gyrA mutations are the major m echanism of resistance to fluoroquinolones even in the strains of P. a eruginosa resistant to imipenem and lacking OprD, concomitant resistan ce to these drugs being the result of the addition of at least two ind ependent mechanisms.