Characterization of antimicrobial resistance of Escherichia coli on Zhongyangshan Island, Zhejiang Province, China under the concept of "One Health"

Objective: This study aimed to investigate antimicrobial resistance (AMR) in Escherichia ( E. ) coli , a recognized indicator bacterium for drug resistance, across animal, environmental and human compartments on Zhongyangshan Island under natural conditions. Methods: A total of 51 E. coli strains were isolated and identified from animal, environmental, and human samples on Zhongyangshan Island between 2022 and 2023. The drug resistance status of E. coli was analyzed using antimicrobial susceptibility testing, whole-genome sequencing analysis, multilocus sequence typing and pulsed-field gel electrophoresis. Results: Antimicrobial susceptibility testing revealed that the resistance rate of E. coli across the “animal-environment-human” interface on Zhongyangshan Island was ≤50%. Overall, AMR levels were low, with isolates from human sources exhibiting higher resistance to seven antibiotics and a higher prevalence of multidrug resistance compared to those from animal sources, while environmental isolates showed the lowest resistance levels. Of the 51 “animal-environment-human” E. coli strains, 31.37% (16/51) harbored five or more different antimicrobial resistance genes. Additionally, 78 transposons were detected, including 72 insertion sequences (IS), 3 compound transposons (Tn), and 3 other types of transposons, along with 18 plasmid replicon sequence types associated with AMR. Multilocus sequence typing and pulsed-field gel electrophoresis-based evolutionary analyses indicated the E. coli strains from the “animal-environment-human” continuum exhibited diverse sequence types and limited genetic relatedness. Further analysis of broiler-derived E. coli from Zhongyangshan Island showed significantly lower resistance rates to 12 out of 14 tested antibiotics and a lower overall multidrug resistance rate (46.67%), compared with isolates from intensive broiler farms in Zhejiang Province, except for amoxicillin/clavulanic acid and meropenem. whole-genome sequencing analysis confirmed that these strains carried fewer AMR genes. Conclusions: E. coli , serving as an indicator organism for AMR within the “animal-environment-human” system on Zhongyangshan Island, exhibits low AMR prevalence and limited clonal relatedness. These findings provide a valuable baseline for investigating the ecological mechanisms underlying antimicrobial resistance emergence and transmission under the “One Health” framework.