Associations between virulence factors of Shiga toxin-producing Escherichia coli and disease in humans

Abstract

Associations between known or putative virulence factors of Shiga toxin-producing Escherichia coli and disease in humans were investigated. Univariate analysis and multivariate logistic regression analysis of a set of 237 isolates from 118 serotypes showed significant associations between the presence of genes for intimin (eae) and Shiga toxin 2 (stx2) and isolates from serotypes reported in humans. Similar associations were found with isolates from serotypes reported in hemorrhagic colitis and hemolytic-uremic syndrome. The enterohemorrhagic E. coli (EHEC) hemolysin gene was significantly associated with isolates from serotypes found in severe diseases in univariate analysis but not in multivariate logistic regression models. A strong association between the intimin and EHEC-hemolysin genes may explain the lack of statistical significance of EHEC hemolysin in these multivariate models, but a true lack of biological significance of the hemolysin in humans or in disease cannot be excluded. This result warrants further investigations of this topic. Multivariate analysis revealed an interaction between the eae and stx2 genes, thus supporting the hypothesis of the synergism between the adhesin intimin and Shiga toxin 2. A strong statistical association was observed between the stx2 gene and severity of disease for a set of 112 human isolates from eight major serotypes. A comparison of 77 isolates of bovine origin and 91 human isolates belonging to six major serotypes showed significant associations of the genes for Shiga toxin 1 and EspP protease with bovine isolates and an increased adherence on HEp-2 cell cultures for human isolates, particularly from diarrheic patients and healthy persons.

FIG. 1

Graphical representation of sampling strategy used to obtain sets 1 to 3 of STEC isolates.

FIG. 2

Conceptual models of associations between ehxA, eae, and disease resulting in statistical association between ehxA and disease in univariate analysis but not in multivariate analysis (for reasons of simplicity, stx₂ was not included in these graphical representations). (A) Confounding effect of eae, i.e., strong association of eae with disease, no direct association between ehxA and disease, and strong association of ehxA with eae. (B) Collinearity of eae and ehxA, i.e., strong association of eae with disease, strong association of ehxA with disease, and strong association of ehxA with eae.