Escherichia coli are a common inhabitant of the gastrointestinal tract of mammals and birds; nevertheless, they may be associated with a variety of severe and invasive infections. Whereas fluoroquinolones (FQ) have been banned in the United States for use in poultry production, the use of these antimicrobials in poultry husbandry is still possible in the European Union, although with some restrictions. The aim of this study was to investigate the FQ resistance of 235 E. coli isolates recovered from chickens and turkeys. Minimum inhibitory concentrations were determined by a microdilution method, whereas mutations in the quinolone resistance-determining regions of the target genes, gyrA and parC, were detected by a PCR-based method. High resistance rates (>60%) were observed for nalidixic acid, flumequine, and difloxacin, whereas resistance to ciprofloxacin, danofloxacin, enrofloxacin, marbofloxacin, and sarafloxacin was less frequently reported (<40%). Sixty-four isolates (27.2%) showed full susceptibility toward the tested FQ, but 57 isolates (24.2%) were resistant to all tested FQ. The remaining 114 E. coli isolates (48.5%) were grouped in 5 different resistance patterns. Isolates resistant only to flumequine or nalidixic acid or both possessed 1 gyrA mutation, whereas isolates with further resistance to enrofloxacin, difloxacin, danofloxacin, and sarafloxacin had in addition 1 or 2 parC substitutions. Two gyrA mutations coupled with 1 substitution in parC were detected in isolates resistant to all tested FQ. The number of mutations and their correlation with the in vitro activity of FQ reflected the currently accepted model, according to which a single gyrA substitution is associated with resistance or decreased susceptibility to older quinolones, whereas further gyrA or parC substitutions are needed for a higher level of resistance.
Keywords: Escherichia coli; antimicrobial resistance; fluoroquinolone; poultry.