Enterohaemorrhagic Escherichia coli (EHEC) are an important cause of diarrhoeal and renal disease in man. Studies of a single prototypic O157 : H7 strain have shown tropism for follicle-associated epithelium (FAE) of distal ileal Peyer's patches without colonization of either small or large intestine. This study determined tropism in a range of Shiga toxin (Stx)-negative EHEC strains and looked for factors that might induce colonic colonization using human in vitro intestinal organ culture (IVOC). An FAE-restricted colonization was confirmed in two strains; four strains additionally colonized ileal villous surfaces, and adhesion to proximal small intestinal FAE was observed. All strains showed minimal adhesion to non-FAE regions of proximal small intestinal and to the transverse colon. Extensive large-bowel IVOC studies using three O157 : H7 strains, an O26 : H11 and an O103 : H2 strain, and tissue from caecum to rectum found colonization and attaching/effacing lesion formation in only 4 of 113 (3.5 %) IVOCs. Colonic adhesion was not enhanced by altering the IVOC technique or environment. Co-incubation of O157 : H7-infected ileal FAE with colonic samples enhanced colonic colonization, producing a novel, non-intimate adhesive phenotype. Thus, in the initial stages of colonization Stx-negative EHEC preferentially infect FAE and villi of the terminal ileal region ex vivo; colonic colonization is infrequently observed as an initial event but may represent a subsequent stage of infection.
INTRODUCTIONEnterohaemorrhagic Escherichia coli (EHEC) cause acute gastroenteritis, bloody diarrhoea and haemorrhagic colitis and are responsible for serious disease outbreaks associated with contaminated food; 10 % of cases may develop severe complications including haemolytic uraemic syndrome with a 5 % case fatality (Nataro & Kaper, 1998;Tarr, 1995). Systemic diseases associated with EHEC infections are linked to the expression of the bacteriophage-associated Shiga toxin (Stx) (Karmali et al., 1983(Karmali et al., , 1985; however, an O157 : H7 strain depleted of the bacteriophage-encoded Stx is as pathogenic, in terms of diarrhoeal disease, as the parent EHEC strain in animal models (Li et al., 1993).The most prominent EHEC serotype is O157 : H7 (Slutsker et al., 1997). Although attaching/effacing (A/E) lesion formation has been shown for EHEC in animal models (Dean-Nystrom et al., 1997;Tzipori et al., 1986) and on cell culture (Knutton et al., 1989), in vivo examples in man have not been demonstrated. We demonstrated in vitro A/E lesion formation on human intestine in organ culture, and found it was restricted to the follicle-associated epithelium (FAE) from the Peyer's patch (PP) region of the distal ileum (Phillips et al., 2000). This tropism has also been demonstrated for an O103 : H2 EHEC strain (Fitzhenry et al., 2003), for the initial stages of infection in rabbit enteropathogenic E. coli (REPEC) strains (Cantey & Inman, 1981;Heczko et al., 2000), and for the mouse pathogen Citrobacter rodentium (Wiles et al., 2004). Ent...