JAC Advance Access originally published online on April 28, 2005
Journal of Antimicrobial Chemotherapy 2005 55(6):938-943; doi:10.1093/jac/dki136
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Mutant prevention concentrations of ciprofloxacin for urinary tract infection isolates of Escherichia coli
1 Department of Cell and Molecular Biology, Box 596, Biomedical Center, Uppsala University, S-751 24, Uppsala, Sweden; 2 Department of Medical Sciences, Clinical Bacteriology, Box 552, Uppsala University, S-751 22, Uppsala, Sweden
* Corresponding author. Email: diarmaid.hughes{at}icm.uu.se
Objectives: To measure the mutant prevention concentration (MPC) of ciprofloxacin for a set of urinary tract infection (UTI) Escherichia coli isolates with different levels of susceptibility and determine whether MPC can be predicted from MIC.
Methods: MPC was defined as the lowest ciprofloxacin concentration that prevented the growth of resistant colonies when 1010 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 hipA and hipB were amplified by PCR from persisters and sequenced.
Results: 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.
Conclusions: 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 hipAB mutants.
Keywords: MPC , UTI , E. coli , mutant selection window
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