Molecular Characterization of Plasmid-Mediated Fluoroquinolone Resistance Genes (qnr, oqxAB, aac(6′)-Ib-cr, qepA) in Avian Pathogenic Escherichia coli from Broiler Flocks with Colibacillosis in Northern Iran

Abstract

Colibacillosis caused by avian pathogenic Escherichia coli (APEC) is a major health and stewardship concern in poultry. We conducted a study in Mazandaran Province, northern Iran, which was chosen because it is a major hub for broiler production and frequently reports veterinary cases of colibacillosis. From 106 diagnostic submissions with compatible necropsy lesions, 81 isolates of Escherichia coli were recovered using EMB culture and confirmed by IMViC. Plasmid-mediated quinolone resistance (PMQR) genes were screened by conventional PCR for qnrA, qnrB, qnrS, oqxAB, qepA, and aac(6′)-Ib-cr, with one representative amplicon per target Sanger-confirmed. Fluoroquinolone susceptibility was assessed by Kirby–Bauer with ciprofloxacin, enrofloxacin, norfloxacin, and nalidixic acid. PMQR carriage was common: oqxAB in 53.1% of isolates and qnrS in 34.6%, with lower frequencies of aac(6′)-Ib-cr (13.6%) and qnrB (4.9%); qnrA and qepA were not detected. Non-susceptibility in disk diffusion was highest for enrofloxacin (76.5%) and also high for nalidixic acid (60.5%), while ciprofloxacin was lowest (23.5%), indicating substantial but heterogeneous fluoroquinolone pressure in this setting. Genotype–phenotype discordance occurred: five PMQR-positive isolates were fully susceptible to all tested fluoroquinolones, whereas nine non-susceptible isolates lacked the screened PMQR genes, consistent with alternative mechanisms such as gyrA/parC mutations or non-oqxAB efflux. These Mazandaran-specific data link PMQR genotypes to clinically relevant phenotypes in a high-priority poultry region, providing a baseline for surveillance and targeted stewardship to curb empirical fluoroquinolone use while expanding monitoring of both plasmid-borne and chromosomal resistance.