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This study was aimed at investigating the resistance to antimicrobial agents and to assess the predominant molecular mechanisms of fluoroquinolone resistance in faecal E. coli strains isolated from chickens farmed in central Poland. Bacterial strains were isolated from faecal samples of chickens reared on four conventional and one organic farm. The disk-diffusion method was applied to assess antimicrobial resistance and the prevalence of particular resistance mechanisms to fluoroquinolones was determined using specific polymerase chain reactions and sequencing of the gyrA and parC genes. Rep-PCR was used to determine the intra-specific variation of E. coli strains. The greatest resistance was observed for ß-lactams (e.g. from 25 to 100% of strains resistant to amoxicillin/clavulanate) and the smallest – for cephalotin (0 to 18.75% resistant strains). Three out of four conventional farms were characterised by very high resistance rates, particularly to enrofloxacin (from 87 to 93.3% of resistant isolates). The majority of multidrug-resistant strains were also isolated from these farms. The presence of plasmidmediated quinolone resistance genes (qnrB and qnrS) was detected very frequently, even in strains that exhibited phenotypic susceptibility to fluoroquinolones. With respect to point mutations in quinolone resistance determining regions, Ser-83 substitution was observed in numerous strains. Some of the fluoroquinolone-resistant strains appeared to possess both qnr genes coupled with point mutations, which indicates that a high level of resistance can be affected by multiple factors. Nevertheless, excessive use of antimicrobial agents in food-producing animals decreases the susceptibility of commensal strains, even those that never had contact with antibiotics.

Screening of antimicrobial resistance and molecular detection of fluoroquinolone resistance mechanisms in chicken faeces-derived Escherichia coli