Ciprofloxacin resistance rapidly declines in nfxB defective clinical strains of Pseudomonas aeruginosa

Antibiotic-resistant bacteria could be faced by identifying trade-offs of evolution, as high fitness costs, that may be harnessed to force reversion to susceptibility. A decline in antimicrobial resistance can occur through compensatory mutations or by genetic reversion to the wild-type allele, which reduce fitness costs associated with resistance. We analysed the impact of antibiotic-free environments on declining ciprofloxacin resistance in eight nfxB defective clinical strains of Pseudomonas aeruginosa spanning varied clone types and ciprofloxacin resistance levels. Ciprofloxacin resistance declined in just 100 generations, which was mainly caused by newly acquired mutations in the genes encoding the overproduced efflux pump MexCD-OprJ and not by the reversion of nfxB mutations of the parental strains. Importantly, pre-existing collateral sensitivity to tobramycin was preserved. The rapid reversion of ciprofloxacin resistance in P. aeruginosa suggests the potential for reusing this antibiotic and underlines the need to implement evolution-based approaches against nfxB defective resistant mutant strains.