The prevalence of fluoroquinolone resistance mechanisms in colonizing Escherichia coli isolates recovered from hospitalized patients

(See the article by Johnson et al, on pages 286–294.) Background. Fluoroquinolones are the most commonly prescribed antimicrobials. The epidemiology of fecal colonization with Escherichia coli demonstrating reduced susceptibility to fluoroquinolones remains unclear. Methods. During a 3-year period (15 September 2004 through 19 October 2007), all patients hospitalized for 13 days were approached for fecal sampling. All E. coli isolates with reduced susceptibility to fluoroquino-lones (minimum inhibitory concentration [MIC] of levofloxacin, 0.125 mg/mL) were identified. We charac-terized gyrA and parC mutations and organic solvent tolerance. Isolates were compared using pulsed-field gel electrophoresis. Results. Of 353 patients colonized with E. coli demonstrating reduced fluoroquinolone susceptibility, 300 (85.0%) had 1 gyrA mutation, 161 (45.6%) had 1 parC mutation, and 171 (48.6%) demonstrated organic solvent tolerance. The mean numbers of total mutations (ie, gyrA and parC) for E. coli isolates with a levofloxacin MIC of 8 mg/mL versus!8.0 mg/mL were 2.70 and 0.82 (P!.001). Of the 136 E. coli isolates with a levofloxacin MIC of 8 mg/mL, 90 (66.2%) demonstrated a nalidixic acid MIC of 16 mg/mL. Significant differences were found over time in the proportion of E. coli isolates demonstrating gyrA mutation, parC mutation, and organic solvent tolerance. There was little evidence of clonal spread of isolates