Phylogroups, virulence determinants and antimicrobial resistance in stx2 gene-carrying Escherichia coli isolated from aquatic environments

The aim of this study was to assess the prevalence of antibiotic-resistant stx2 gene-carrying Escherichia coli isolated from human and animal wastewater with regard to their animal/human origin, serotype, phylogenetic background and virulence factors. The isolates were characterized by PCR in relation to stx variant, phylogenetic group and other virulence genes (stx1, ehxA and saa). Antibiotic resistance was found in 92% of the stx2 gene-carrying E. coli strains, with 77% showing intermediate resistance or full resistance to more than one antibiotic. High levels of resistance were observed to chloramphenicol, tetracycline, sulfamethoxazole, streptomycin, trimethoprim, and trimethoprim + sulfamethoxazole, with resistance values of 79%, 69%, 63%, 58%, 47% and 42%, respectively, and a higher prevalence among those strains isolated from animal wastewater. There was no association between the E. coli serotype and/or phylogroup and the antimicrobial resistance profile displayed. However, those strains carrying the stx2 gene variant alone or in combination with other virulence factors (stx1, ehxA or saa gene) were susceptible to most of the tested antibiotics.