We compared 58 Escherichia coli isolates obtained from various environmental sources in Slovenia (N = 26; seawater, river water, karst cave waters, and dishwasher rubber seals) and from clinical lower respiratory tract samples (N = 32). The isolates were assigned to phylogenetic groups using both the original and the revised Clermont methods, and their clonality was assessed using ERIC-PCR. Using molecular methods, we examined the presence of 20 virulence genes, genes encoding β-lactamases, genes associated with plasmid-mediated resistance to fluoroquinolones, and selected sequence types. Biofilm-forming ability was assessed under various conditions by altering nutrient availability, concentrations of trace elements, and the presence of subinhibitory concentrations of antimicrobial compounds.The results revealed pronounced differences between the two groups. Clinical isolates predominantly belonged to phylogenetic groups characteristic of more virulent and pathogenic strains (B2; 68.75%) and more frequently carried a greater number of virulence-associated genes. Biofilm formation varied significantly among the isolates and depended on the biofilm cultivation method. In the nutrient-rich LB medium, differences in biofilm formation between isolates were smaller, whereas in the minimal medium with 0.2 % glucose, more biofilm was produced regardless of the isolates’ origin. Clinical isolates were strong biofilm producers, especially in the minimal medium with 0.2 % final glucose concentration (96.9 %), while environmental isolates showed greater diversity.
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