Antimicrobial resistance and virulence-associated genes in Escherichia coli from young healthy chickens

Escherichia coli is a commensal intestinal bacterium of humans and animals and is frequently associated with antimicrobial resistance (AMR). This study investigated the phenotypic and genotypic AMR and virulence factors in E. coli isolated from 120 young healthy chickens never treated with antibiotics and coccidiostats. The disk diffusion method employed on all 120 isolates revealed the highest resistance (11.67%) to chloramphenicol and trimethoprim-sulfamethoxazole. All isolates were susceptible to aztreonam and gentamicin, whereas over 95% showed susceptibility to amikacin, cefoxitin, ciprofloxacin, and ertapenem. Minimum inhibition concentration test found 7 (5.83%) isolates resistant to colistin. Eight strains (6.67%) were found to be multidrug-resistant (MDR). Molecular analyses conducted on resistant and intermediate isolates showed that 11/13 penicillin-resistant strains carried the blaTEM gene, 12/12 tetracycline-resistant strains had tetA, 4/14 chloramphenicol-resistant and 1/2 chloramphenicol-intermediate strains had cmlA. The astA gene, potentially indicative of enteroaggregative E. coli (EAEC), was found in 15% of isolates. Virulence genes characterizing other pathotypes investigated were not detected. Chickens reared in optimal conditions and never treated with antibiotics may harbor antimicrobial-resistant and pathogenic bacterial strains that could become a public health hazard. Good farm management and hygiene are fundamental to reducing the circulation of pathogenic strains and the need for antimicrobial treatments.