Virulence traits associated with verocytotoxigenic Escherichia coli O157 recovered from freshwater biofilms

To investigate whether epilithic biofilms in freshwater streams in a mixed UK agricultural river catchment harbour Escherichia coli O157, and if so, whether they demonstrate an association with those excreted by grazing farm animals. Flint shingle, native to the study site, was used as a surface for biofilm development within cages of metal lath set into a stream bed at four locations on a chalkland farm. Shingle was collected from all sites once a month, as were pooled faecal samples from five farm animal populations. Subpopulations of E. coli, including E. coli O157 that demonstrated significant phenotypic and genotypic similarity with animal faecal isolates (t-test, P = 0·05) were isolated. Of 1002 E. coli isolates from biofilms and animal faeces, 48 were confirmed as the O157 strain by latex agglutination. The presence of five virulence traits associated with incidence of human disease was tested using PCR. Stx₂ was the most frequently isolated single gene (30 isolates), while stx₁ was the least frequently recovered (four isolates). Escherichia coli O157, expressing up to four virulence factors associated with human disease, reside within freshwater biofilms in this agricultural environment. Aquatic biofilms may potentially act as a reservoir for these pathogens, and the implications of the findings for the protection of drinking water resources should be further investigated.