Phenotypic and genotypic characterization of fecal Escherichia coli isolates with decreased susceptibility to fluoroquinolones:: Results from a large hospital-based surveillance initiative

Background. The prevalence of fecal colonization with Escherichia coli that has reduced susceptibility to fluoro-quinolones is unknown. A detailed characterization of such isolates is limited. Methods. We conducted 3 annual fecal surveillance initiatives at 2 hospitals from 2002 to 2004. All E. coli isolates with reduced susceptibility to fluoroquinolones ( minimum inhibitory concentration [ MIC] to levofloxacin, >= 0.125 mu g/mL) were identified. We characterized gyrA and parC mutations and organic solvent tolerance (OST). Isolates were compared using pulsed-field gel electrophoresis. Results. Of 789 fecal samples, 149 (18.9%) revealed E. coli with reduced susceptibility to fluoroquinolones. Of 149 isolates, 102 (68.5%) had a MIC >= 8 mu g/mL, 138 (92.6%) had >= 1 gyrA mutation, 101 (67.8%) had >= 1 parC mutation, and 59 (39.6%) demonstrated OST. Isolates with a MIC >= 8 versus < 8 mu g/mL had more target mutations (median, 3 vs. 1; P <.001) and more often exhibited OST (51% vs. 15%; P <.001). Of 149 isolates, 144 (96.6%) demonstrated a MIC >= 16 mu g/mL to nalidixic acid. The prevalence of OST differed across study years (P =.01). There was no clonal spread of isolates. Conclusions. Colonization by E. coli with reduced fluoroquinolone susceptibility is common, and fluoroquinolone-resistance characteristics differ significantly over time. Resistance to nalidixic acid may be useful in the identification of E. coli with early resistance mutations.