The accumulation of five quinolone antibacterial agents by Escherichia coli

The accumulation of five radiolabelled quinolone antibacterial agents by Escherichia coli KL16 was examined using a vacuum filtration method. Preliminary experiments were performed to determine the optimum quinolone concentration, inoculum of cells, filter washing regimen and filter type. All five quinolones showed a similar biphasic pattern of accumulation with high radioactive counts cell-associated during the first ten seconds of the assay, followed by steadily increasing accumulation over 30 min. Analysis of the mean accumulation after 30 min for each quinolone showed that there was no direct relationship between quinolone accumulation and antibacterial activity (as quantified by the MIC or bactericidal activity). Mechanistic investigations showed that accumulation was decreased by low reaction temperatures, acid pH and the presence of the metabolic inhibitors 2,4-dinitrophenol, potassium cyanide and sodium azide. These results suggest that quinolone accumulation by E. coli KL16 is partly dependent on cell metabolism and may proceed by an active transport mechanism. Treatment of cells with EDTA was found not to increase quinolone accumulation, suggesting that the outer membrane of E. coli KL16 does not act as a permeability barrier to these quinolones. The implication of these results in terms of possible mechanisms of bacterial resistance to quinolones is discussed.