Phenotypic and Genotypic Characterization of Antibiotic Resistance in Escherichia coli Strains Isolated from Broiler Chickens with Suspected Colibacillosis in Isfahan Province, Iran

The aim of this study was to evaluate genotypes and phenotypes of antibiotic resistance in Escherichia coli (E. coli) strains isolated from poultry farms in Isfahan province, Iran.

In this study, 50 E. coli strains isolated from pericarditis and perihepatitis lesions of broilers in Isfahan (central Iran) were selected. After microbiological and biochemical tests and confirmation of bacterial colonies, the colonies were purified. The pure colonies were cultured on Müeller-Hinton culture medium and then subjected to antibiotic susceptibility testing. In the next step, DNA was extracted from the purified bacteria, and the qnrA and sul1 genes were amplified with specific primers.

The results showed that 85% of E. coli isolates were resistant to at least two antibiotics, and 6% of E. coli strains were resistant to all 13 antibiotics used in this study. E. coli isolates showed the highest resistance to enrofloxacin (70%) and the lowest resistance to gentamicin (6%). Examination of resistance genes showed that about 54% of enrofloxacin-resistant E. coli strains contained the qnrA gene, and 48% of sulfonamide-resistant E. coli strains contained the sul1 gene.

In this study, some resistant strains lacked the resistance genes studied, indicating the importance of other resistance genes in inducing resistance against sulfonamides and fluoroquinolones. Also, the lack of resistance in some strains harboring qnrA and sul1 genes indicates the importance of gene expression in mediating resistance, and that the presence of resistance genes alone is not sufficient to induce antibiotic resistance in E. coli strains.