Isolation and molecular characterization of fluoroquinolone-resistant Escherichia coli from poultry litter

Nineteen fluoroquinolone-resistant Escherichia coli strains were isolated from poultry litter. Sixteen of the 19 strains were serotyped to groups 6, 8, 53, 56, 153, and 174. Three strains were not serotyped to any known group. All isolates were resistant to multiple antibiotics. Most strains were resistant to gentamicin, kanamycin, chloramphenicol, and streptomycin. Ribotyping of the multidrug-resistant isolates with restriction enzyme PvuII showed 5 different ribogroups, suggesting independent development of resistance instead of clonal spread. Quinolone resistance was associated with mutations of the quinolone resistance-determining region (QRDR) of the gyr A gene in all cases. To determine the incidence of gyr A mutations in fluoroquinolone-resistant E. coli isolates, a rapid PCR-based assay was used by amplifying a 164-bp region of the gyr A gene containing the mutation sites followed by digestion of the PCR product with restriction enzyme HinfI. A higher level of resistance to ciprofloxacin [minimum inhibitory concentration (MIC) >4 microg] was associated with double mutations, but the mutants with a low level of resistance (MIC <2 microg) had only a single mutation. Those strains that were ciprofloxacin-resistant (MIC <2 microg) had a single mutation of a C-to-T transition at position 248 (Ser 83-->Leu) or a G-to-A transition at position 259 (Asp 87-->Asn). The ciprofloxacin-resistant (MIC >4 microg) isolates had mutations at both positions. Fluoroquinolone resistance was present among different serotypes and ribotypes, and drug resistance profiles suggest that the incidence of resistance does not indicate a clonal population in avian E. coli.