Bacteriostatic versus bactericidal activity of ciprofloxacin in Escherichia coli assessed by flow cytometry using a novel far-red dye

As common microbiological methods for the assessment of bacteriostatic or bactericidal activities are very time-consuming, in this work we describe that the use of a novel far-red fluorescent stain, Vybrant DyeCycle Ruby (DCR) for the flow cytometric analysis of fluoroquinolone (ciprofloxacin) bacteriostatic and bactericidal activities in Escherichia coli proved to be specific for bacterial DNA and, after ciprofloxacin exposure, DNA distribution analysis was achieved using a 7.5 μM DCR concentration to stain 5 × 10⁵ ethanol-fixed bacterial cells. The analysis of the bacterial DNA histograms obtained from the ciprofloxacin concentrations tested, enabled the distinction between ciprofloxacin bacteriostatic and bactericidal activities.