A deletion mutation in nfxB of in vitro-induced moxifloxacin-resistant Pseudomonas aeruginosa confers multidrug resistance.

The modulation of efflux pump functions under fluoroquinolone (FQ) exposure is of great concern as it could result in occurrence of multidrug-resistant (MDR) bacterial strains. In this study, MDR mechanism in Pseudomonas aeruginosa induced via moxifloxacin (MOX) pressure was investigated. After serial MOX [concentration of 0.5 × the minimum inhibitory concentration (MIC)] exposure, the fully susceptible P. aeruginosa ATCC 9027 strain has increased its MIC not only toward MOX (1→128 mg/L) but also to other antibiotics. Furthermore, this MOX-exposed strain did not revert to antibiotic-sensitive phenotype when being cultured in antibiotic-free medium for 12 days. No mutation was observed for FQ-target (gyrA and parC) or most investigated efflux regulatory genes (mexT, mexR, and nalC) except nfxB in which a 100-bp deletion was found. This associated with the elevated expression of multidrug efflux pump operon (mexCD-oprJ) which could directly result in MDR phenotype.