Enterohaemorrhagic Escherichia coli (EHEC) O157:H7 causes gastrointestinal disease with the potential for life-threatening sequelae. Although Shiga-like toxins are responsible for much of the serious pathology in humans, the bacterium also possesses a type III protein secretion system that is responsible for intimate attachment to host intestinal mucosa. This sophisticated interaction requires co-ordination that is governed by environmental and genetic factors. Ongoing research supports the following model for how EHEC enables and controls this process: (i) specific environmental cues that are present in the host result in the expression of a number of adhesins, including fimbriae, which allow the initial binding to the mucosal surface. The same conditions support the expression of the basal type III secretion apparatus; (ii) targeting and assembly of the translocon requires both an mRNA signal and chaperones, with coupled translation and secretion of translocon proteins, EspA, B and D; (iii) opening up of a conduit between the bacterium and host cell releases a cytoplasmic pool of effector proteins. A consequence of this is increased expression of particular effector proteins. Potentially, different proteins could be released into the cell at different times or have activities modulated with time; (iv) intimate contact between the translocated intimin receptor (Tir) and the bacterial surface factor intimin requires translocon expression to be down-regulated and translocon filaments to be lost. Fluorescent protein fusions allow contact-mediated regulation and protein targeting through the type III secretion system to be studied in detail.

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