Mutant prevention concentrations of ciprofloxacin for urinary tract infection isolates of Escherichia coli

<it>Objectives</it>: To measure the mutant prevention concentration (MPC) of ciprofloxacin for a set of urinary tract infection (UTI) <it>Escherichia coli</it> isolates with different levels of susceptibility and determine whether MPC can be predicted from MIC. <it>Methods</it>: MPC was defined as the lowest ciprofloxacin concentration that prevented the growth of resistant colonies when 10<cross-ref type="bib" refid="r10">10</cross-ref> bacteria were spread on solid medium and incubated for 96 h at 37°C. MIC was measured by Etest. Bacteria surviving (persisting) at MPC were isolated and quantified from agar plugs taken after 96 h. The genes <it>hipA</it> and <it>hipB</it> were amplified by PCR from persisters and sequenced. <it>Results</it>: Isolates with MICs above the NCCLS breakpoint for ciprofloxacin resistance (4 mg/L) typically have MPCs greater than 32 mg/L. Isolates with MICs below the breakpoint for ciprofloxacin susceptibility (1 mg/L) have MPCs up to 5 mg/L. MPC/MIC is ∼16 for most susceptible isolates but there are several notable exceptions (MPC/MIC > 100). Resistant colonies arising one dilution step below MPC often had MIC > MPC. In every case tested, a proportion of cells survived (persisted), but did not grow into colonies, at MPC, without any increase in MIC. <it>Conclusions</it>: MPCs were determined for all ciprofloxacin-susceptible isolates. MPC is not accurately predicted from MIC. Colonies selected below MPC frequently have MIC > MPC, suggesting multiple mutations. A small fraction of cells from all strains tested survived for 96 h at MPC, without any associated increase in MIC. These survivors/persisters are not <it>hipAB</it> mutants.