Limited clonality but widespread plasmid sharing of ESBL-producing E. coli between humans and the environment of northeastern Slovenia

Extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli is a critical antimicrobial-resistant pathogen. While well-studied in humans and animals, its presence in the environment remains underexplored. This study analyses the genotypes and plasmid composition of ESBL-producing E. coli from human and environmental sources in Northeastern Slovenia. Freshwater and sediment samples were collected bimonthly during one year at ten sites, including rivers, lakes, and wastewater treatment plant (WWTP) influents. Human isolates were collected during the same period and geographic area. Whole-genome sequencing and bioinformatic analyses were conducted to evaluate genetic relatedness, antimicrobial resistance genes, and plasmid composition. Among 414 sequenced isolates (230 human, 184 environmental), 108 multilocus sequence types (MLST-ST) were identified, with 20 STs shared between sources. Core-genome MLST (cgMLST) revealed 59 clonal clusters, six of which included both human and environmental isolates. The dominant ESBL gene was bla_CTX−M-15 (59 % of all ESBL isolates), and 54 % of all ESBL isolates carried ESBL genes on plasmids. Plasmid cluster AA474, found across different sources, STs, and locations, was linked to multiple ESBL genes, suggesting a key role in resistance dissemination. Despite shared STs, ESBL genes, and plasmids between humans and the environment, cgMLST analysis indicated limited clonal spread. This suggest that transmission between humans and the environment remains restricted and is more often linked to spread of plasmids than of strains.