Upon intestinal colonization, enterohemorrhagic Escherichia coli (EHEC) induces epithelial cells to generate actin “pedestals” beneath bound bacteria, lesions that promote colonization. To induce pedestals, EHEC utilizes a type III secretion system to translocate into the mammalian cell bacterial effectors such as translocated intimin receptor (Tir), which localizes in the mammalian cell membrane and functions as a receptor for the bacterial outer membrane protein intimin. Whereas EHEC triggers efficient pedestal formation during mammalian infection, EHEC cultured in vitro induces pedestals on cell monolayers with relatively low efficiency. To determine whether growth within the mammalian host enhances EHEC pedestal formation, we compared in vitro-cultivated bacteria with EHEC directly isolated from infected piglets. Mammalian adaptation by EHEC was associated with a dramatic increase in the efficiency of cell attachment and pedestal formation. The amounts of intimin and Tir were significantly higher in host-adapted than in in vitro-cultivated bacteria, but increasing intimin or Tir expression, or artificially increasing the level of bacterial attachment to mammalian cells, did not enhance pedestal formation by in vitro-cultivated EHEC. Instead, a functional assay suggested that host-adapted EHEC translocate Tir much more efficiently than does in vitro-cultivated bacteria. These data suggest that adaptation of EHEC to the mammalian intestine enhances bacterial cell attachment, expression of intimin and Tir, and translocation of effectors that promote actin signaling.

Attachment to host cells by enterohaemorrhagic Escherichia coli (EHEC) is associated with the formation of a highly organized cytoskeletal structure containing filamentous actin, termed an attaching and effacing (AE) lesion. Intimin, an outer membrane protein of EHEC, is required for the formation of AE lesions, as is Tir, a bacterial protein that is translocated into the host cell to function as a receptor for intimin. We established a yeast two-hybrid assay for intimin-Tir interaction and, after random mutagenesis, isolated 24 point mutants in intimin, which disrupted Tir recognition in this system. Analysis of 11 point mutants revealed a correlation between recognition of recombinant Tir and the ability to trigger AE lesions. Many of the mutations fell within a 50-residue region near the C-terminus of intimin. Alanine-scanning mutagenesis of this region revealed four residues (Ser890, Thr909, Asn916 and Asn927) that are critical for Tir recognition. Mapping the sequences of EHEC intimin and Tir onto the crystal structure of the intimin-Tir complex of enteropathogenic E. coli predicts that each of these four intimin residues lies at the intimin-Tir interface and contributes to a pocket that interacts with Ile298 of EHEC Tir. Thus, this genetic approach to intimin function both identified residues critical for Tir binding and demonstrated a correlation between the ability to bind Tir and the ability to trigger actin focusing.