Antibacterial action of ciprofloxacin

The mechanisms by which quinolones rapidly kill are ill defined. We have investigated the action of ciprofloxacin on Escherichia coli KL16 with a combination of traditional and flow cytometric methods and have analyzed cells for changes in membrane potential, membrane integrity, oxidative metabolism, morphology, and viability. Log-phase cultures were exposed to various concentrations (0.1, 1, 10, and 100 times the MIC) of ciprofloxacin and analyzed at regular intervals over 120 min. We also measured protein synthesis in the related strain PQ37 cultured under the same conditions over 300 min, using a colorimetric assay for beta-galactosidase release. Despite a 3-log order decrease in CFU after 60-min exposure to 10 and 100 times the MIC of ciprofloxacin, there was no equivalent decrease in bacterial numbers as determined by both light microscopy and flow cytometry. Furthermore, while these bacteria showed concentration-dependent morphological changes, most were capable not only of excluding the fluorescent nucleic acid-binding dye propidium iodide, but also of reducing the tetrazolium dye cyanoditodyl tetrazolium chloride. Over 90% of the bacteria maintained a membrane potential [as determined by exclusion of bis-[1,3-dibutylbarbituric acid) trimethine oxonol] when exposed to ciprofloxacin for 120 min, except at 100 times the MIC, when this figure fell to < 10%. Finally, protein synthesis was either maintained or induced at all concentrations of ciprofloxacin up to 5 h postexposure. Taken together, these results demonstrate the continuing physical and metabolic survival of ciprofloxacin-exposed bacteria; we suggest parallels with the concept of the viable nonculturable state.