Molecular analysis of virulence profiles and Shiga toxin genes in food-borne Shiga toxin-producing Escherichia coli

Abstract

In this study, 75 Shiga toxin (Stx)-producing Escherichia coli (STEC) strains originating from foods (n = 73) and drinking water (n = 2) were analyzed for their stx genotype, as well as for further chromosome-, phage-, and plasmid-encoded virulence factors. A broad spectrum of stx genes was detected. Fifty-three strains (70.7%) contained stx(2) or stx(2) variants, including stx(2d), mucus-activatable stx(2d), stx(2e), and stx(2g). Seven strains (9.3%) harbored stx(1) or stx(1c), and 15 strains (20.0%) carried both stx(2) and/or stx(2) variants and stx(1) or stx(1c). Beside stx, the most abundant accessory virulence markers in STEC food isolates were iha (57.3%), ehxA (40.0%), espP (28.0%), and subAB (25.3%). Only four strains were eae positive; three of these belonged to the serogroups O26, O103, and O157 and contained a typical enterohemorrhagic E. coli virulence spectrum. The results of this study show that a number of STEC strains that occur in foods appear to be pathogenic for humans, based on their virulence profiles. Analysis of stx subtypes and detection of additional virulence factors in eae-negative strains may help to better assess the risk of such strains for causing human infection.

FIG. 1.

Phylogenetic dendrogram of the deduced amino acid sequences of the A subunit (panel A) and the B subunit (panel B) of stx₂ genes, based on the unweighted pair-group method with arithmetic mean algorithm using Mega 3.1 software (42). The shaded boxes show the alignment of the stx₂ sequences to reference sequences (see Discussion).

FIG. 2.

Alignment of the deduced amino acid sequences of the C-terminal ends of the A subunits (A) and B subunits (B) of 28 Stx2 sequences analyzed in this study. The amino acid positions of signature amino acids which are indicative for activation by mucus are labeled with boxes. Asterisks show stx₂ sequences which are activatable according to the RFLP analysis. Amino acid sequences in bold font show the typical activatable signature as analyzed by sequencing.