Effect of diffusely adherent Escherichia coli strains isolated from diarrhoeal patients and healthy carriers on IL-8 secretion and tight junction barrier integrity of Caco-2 cells.

The pathogenesis of diffusely adherent Escherichia coli (DAEC) remains to be elucidated. Previously, we found that afimbrial adhesin gene (afa)-positive motile DAEC strains isolated from patients with diarrhoea induce high levels of IL-8 secretion in Caco-2 cells via toll-like receptor 5 (TLR-5), while non-motile strains did not. The aim of this study was to compare virulence properties, including the phylogenetic groups, afa subtypes, IL-8 secretion levels, and the effects on tight junctions, of DAEC strains isolated from healthy persons with those isolated from patients with diarrhoea. Induction of IL-8 secretion in Caco-2 cells was examined for a total of 36 afa-positive strains: 19 from diarrhoeal patients and 17 from healthy carriers. Irrespective of the source, all strains were classified into the phylogenetic group B2 or D, with the exception of two strains. All 7 motile strains isolated from diarrhoeal patients induced high levels of IL-8 secretion, while only 6 of 15 motile strains from healthy carriers induced IL-8 secretion to the same levels as the diarrhoeal strains. We speculated that additional virulence factors other than afa and motility cause the loosening of tight junctions that allows flagellin to reach TLR-5 located on the basolateral side of the epithelium. However, no differences in the TER and dextran permeability were observed between cells infected with diarrhoeal strains and those from healthy persons. Thus, diarrhoeagenic DAEC seems to possess additional factors, in addition to adhesin and flagellin, which can induce high IL-8 secretion.