DEVELOPMENT OF MULTIPLE-ANTIBIOTIC-RESISTANT (MAR) MUTANTS OF PSEUDOMONAS-AERUGINOSA AFTER SERIAL EXPOSURE TO FLUOROQUINOLONES

Laboratory-derived fluoroquinolone-resistant mutants were created by s erially passaging wild-type Pseudomonas aeruginosa on fluoroquinolone- containing agar to obtain high-level fluoroquinolone resistance (e.g., ciprofloxacin MIC of 1,024 mu g/ml). With increases of 4- to 32-fold in MICs of fluoroquinolones, these organisms demonstrated (relative to wild-type) normal morphology, resistance to fluoroquinolones only, no change in fluoroquinolone uptake, and no change in lipopolysaccharide profiles or outer membrane protein profiles. Complementation with wil d-type Escherichia coli gyrA restored fluoroquinolone susceptibility, suggesting that these were gyrA mutants. After 4- to 32-fold increases in fluoroquinolone MICs (with continued passage on fluoroquinolone-co ntaining agar) isolates demonstrated altered morphology, a multiple-an tibiotic-resistant (Mar) phenotype (including cross resistance to beta -lactams, chloramphenicol, and tetracycline), reduced fluoroquinolone uptake and altered outer membrane proteins (reductions in the 25- and 38-kDa bands as well as several bands in the 43- to 66-kDa region). Co mplementation with wild-type E. coli gyrA partially reduced the level of fluoroquinolone resistance by approximately 8- to 32-fold, suggesti ng that these mutants displayed both gyrA and non-gyrA mutations.