Characterization of Fluoroquinolone and Carbapenem Susceptibilities in Clinical Isolates of Levofloxacin-Resistant Pseudomonas aeruginosa

Background:Pseudomonas aeruginosa can rapidly acquire resistance to antibiotics, including fluoroquinolones and carbapenems. Methods: We characterized fluoroquinolone, carbapenem and other β-lactam susceptibilities and analyzed fluoroquinolone and carbapenem resistance in 16 clinical isolates of levofloxacin-resistant P. aeruginosa. Results: All levofloxacin-resistant isolates showed high MICs (≧32 µg/ml) for fluoroquinolones including norfloxacin, levofloxacin, sparfloxacin, gatifloxacin and pazufloxacin, whereas the MICs for sitafloxacin were between 2 and 16 µg/ml. These isolates had both a Thr83Ile mutation in GyrA and a Ser87Leu mutation in ParC. An additional mutation, Glu469Asp in GyrB, was detected in 3 isolates. Three of 16 isolates found during antibiotic therapy showed resistance to carbapenems (MIC, 16–32 µg/ml) because of a reduced production of OprD. Fluoroquinolones, β-lactams and sulfamethoxazole-trimethoprim were used for 3 months before the isolation of levofloxacin-resistant P.aeruginosa. Conclusions: Emergence of resistant isolates to both fluoroquinolones and carbapenems during antibiotic therapy is a serious clinical problem. Our results suggest that susceptibilities to fluoroquinolones as well as carbapenems should be monitored during a prolonged course of antibiotic therapy against P. aeruginosa infection.