This in vitro cell-based study identified the contributing role of pyocyanin in the resistanceof Pseudomonas aeruginosa to ciprofloxacin. Problem statement: P. aeruginosa is the majorpathogen in the Cystic Fibrosis (CF) lung with pyocyanin being a critical component of its virulence.Prevalence is high and, once acquired, chronic infection is difficult to eliminate. Ciprofloxacin remainsa crucial oral agent effective against P. aeruginosa, but resistance is increasingly reported. Approach:Here we examined the extent to which exogenously added pyocyanin affected P. aeruginosasusceptibility to ciprofloxacin and the contribution of altered efflux activity and biofilm productionwith the aim of ultimately increasing sensitivity. Results: Metabolic conversion of resazurin toresorufin was used as an index of bacterial cell growth while fluorescent measurement of acriflavineefflux and crystal violet staining was used as markers of efflux activity and biofilm production,respectively. Pyocyanin (100
