Intimate host attachment: enteropathogenic and enterohaemorrhagic Escherichia coli

Abstract

Enteropathogenic and enterohaemorrhagic Escherichia coli use a novel infection strategy to colonize the gut epithelium, involving translocation of their own receptor, Tir, via a type III secretion system and subsequent formation of attaching and effecting (A/E) lesions. Following integration into the host cell plasma membrane of cultured cells, and clustering by the outer membrane adhesin intimin, Tir triggers multiple actin polymerization pathways involving host and bacterial adaptor proteins that converge on the host Arp2/3 actin nucleator. Although initially thought to be involved in A/E lesion formation, recent data have shown that the known Tir-induced actin polymerization pathways are dispensable for this activity, but can play other major roles in colonization efficiency, in vivo fitness and systemic disease. In this review we summarize the roadmap leading from the discovery of Tir, through the different actin polymerization pathways it triggers, to our current understanding of their physiological functions.

Figure 1

Actin pedestals as visualized by transmission electron microscopy appear as electron dense protrusions beneath intimately attached bacteria (A). By immunofluorescence microscopy, actin pedestals can visualized with phalloidin conjugated fluorophores. EHEC (blue; DAPI stained) induces actin pedestals on infected HeLa cells (B) and HCT8 intestinal cells (C) stained with Alexa-488 conjugated phalloidin (green). White arrowheads point to typical actin pedestals for these cells types. Tir stained with Alexa-568 (red) colocalizes with actin at the tips of actin pedestals, forming yellow foci beneath intimately adhered bacteria.

Figure 2

EPEC1 and EHEC use different mechanisms of Tir activated signaling cascades that converge on common host actin nucleation factors, N-WASP and Arp2/3 to form actin pedestals. The major EPEC1 actin pedestal pathway requires phosphorylation of the critical Tir residue Y₄₇₄, which recruits Nck (pathway 1). Nck can activate N-WASP directly or through WIP. The major EHEC actin pedestal pathway requires Tir residues NPY₄₅₈ and the EHEC effector EspF_U/TccP (pathway 2). EspF_u/TccP is linked to Tir via host adaptors IRTKS and IRSp53 and activates N-WASP. Minor EPEC actin assembly pathways utilize Tir residues Y₄₇₄ or Y₄₅₄ to initiate Nck independent activation of N-WASP (pathways a and b). A minor actin assembly pathway for EHEC utilizes EspF_u, but does not require N-WASP for Arp2/3 activation (pathway c).

Figure 3

EPEC2 and non-O157 EHEC can use the major pathways of both typical EPEC1 and EHEC O157 to generate actin pedestals. Tir in these strains contain both an Y₄₇₄ equivalent, which initiates actin assembly through Nck (pathway 1), and an NPY₄₅₈ equivalent, which initiates actin assembly through IRTKS/IRSp53 and an EspF_U/TccP homologue, TccP2/EspF_M (pathway 2).