Multiple elements controlling adherence of enterohemorrhagic Escherichia coli O157:H7 to HeLa cells

Adherence of enterohemorrhagic Escherichia coli (EHEC) to the intestinal epithelium is essential for initiation of infection. Intimin is the only factor demonstrated to play a role in intestinal colonization by EHEC O157:H7. Other attempts to identify additional adhesion factors in vitro have been unsuccessful, suggesting that expression of these factors is under tight regulation. We sought to identify genes involved in the control of adherence of EHEC O157:H7 to cultured epithelial cells. A total of 5,000 independent transposon insertion mutants were screened for their ability to adhere to HeLa cells, and 7 mutants were isolated with a markedly enhanced adherence. The mutants adhered at levels 113 to 170% that of the wild-type strain, and analysis of the protein profiles of these mutants revealed several proteins differentially expressed under in vitro culture conditions. We determined the sequence of the differentially expressed proteins and further investigated the function of OmpA, whose expression was increased in a mutant with an insertionally inactivated tcdA gene. An isogenic ompA mutant showed reduced adherence compared to the parent strain. Disruption of the ompA gene in the tdcA mutant strain abolished the hyperadherent phenotype, and anti-OmpA serum inhibited adhesion of wild-type and tdcA mutant strains to HeLa cells. Enhanced adhesion mediated by OmpA was also observed with Caco-2 cells, and anti-OmpA serum blocked adherence to HeLa cells of other EHEC O157:H7 strains. Our results indicate that multiple elements control adherence and OmpA acts as an adhesin in EHEC O157:H7.     

Isolation and characterization of mini-Tn5Km2 insertion mutants of enterohemorrhagic Escherichia coli O157:H7 deficient in adherence to Caco-2 cells

Adherence of enterohemorrhagic Escherichia coli (EHEC) to intestinal epithelium is essential for initiation of the infection. To identify genes involved in adherence, an EHEC O157:H7 strain (O157Sakai) was mutagenized by mini-Tn5Km2, where Km refers to kanamycin resistance, and 4,677 insertion mutants were screened for their ability to form microcolonies (MC) on Caco-2 cells. The less adherent mutants were divided into three groups: those with no adherent ability (designated as class 1 mutants, n = 10), those less adherent than the wild type (class 2 mutants, n = 16), and those unable to form MC but which adhered in a diffuse manner (class 3 mutants, n = 1). The sites of insertion in class 1 mutants were all found within genes of the locus for enterocyte effacement (LEE) thought to be required for type III protein secretion. Indeed, the class 1 mutants failed to secrete type III secreted proteins such as EspA and Tir into the culture medium. The insertions in class 2 mutants were outside the LEE, and all the mutants except one were able to secrete type III proteins into the culture medium. The class 3 mutant had the insertion in the tir gene in the LEE and was deficient in Tir and intimin expression, suggesting that in the absence of intimin-Tir, O157Sakai can still adhere to Caco-2 cells but in a diffused manner. This was confirmed by construction of a nonpolar eae (encoding intimin) mutant. Examination of the eae mutant together with O157Sakai and one of the class 1 mutants for the ability to form MC revealed that EHEC initially adhered diffusely at 1.5 h after infection. Following washing out of the nonadherent bacteria, while wild-type EHEC bacteria developed MC for another 2 to 3 h on Caco-2 cells, the eae mutant diffusely adhered throughout the infection without forming MC. MC with O157Sakai but not the diffusely adherent eae mutant could evoke F-actin condensation beneath the bacterium. Our results suggest that EHEC encodes additional adherence-associated loci and that the type III secreted proteins are involved in the initial diffuse adherence, while the intimin-Tir interaction is required for the subsequent development of MC.     

The StcE protease contributes to intimate adherence of enterohemorrhagic Escherichia coli O157:H7 to host cells

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a diarrheal pathogen that causes attaching and effacing (A/E) lesions on intestinal epithelial cells. Strains of the O157 serogroup carry the large virulence plasmid pO157, which encodes the etp type II secretion system that secretes the genetically linked zinc metalloprotease StcE. The Ler regulator controls expression of many genes involved in A/E lesion formation, as well as StcE, suggesting StcE may be important at a similar time during colonization. Our laboratory has previously demonstrated that StcE cleaves C1-esterase inhibitor, a regulator of multiple inflammation pathways. Here we report two new substrates for StcE, mucin 7 and glycoprotein 340, and that purified StcE reduces the viscosity of human saliva. We tested the hypothesis that StcE contributes to intimate adherence of EHEC to host cells by cleavage of glycoproteins from the cell surface. The fluorescent actin stain (FAS) test was used to observe the intimate adherence represented by fluorescently stained bacteria colocalized with regions of bundled actin formed on HEp-2 cells. An E. coli O157:H7 strain with a stcE gene deletion was not affected in its ability to generally adhere to HEp-2 cells, but it did score threefold lower on the FAS test than wild-type or complemented strains. Addition of exogenous recombinant StcE increased intimate adherence of the mutant to wild-type levels. Thus, StcE may help block host clearance of E. coli O157:H7 by destruction of some classes of glycoproteins, and it contributes to intimate adherence of E. coli O157:H7 to the HEp-2 cell surface.     

Mouse model for colonization and disease caused by enterohemorrhagic Escherichia coli O157:H7.

Enterohemorrhagic Escherichia coli O157:H7 isolates produce Shiga-like toxins and carry a 60-megadalton plasmid which encodes an adhesin for Henle 407 intestinal cells. A streptomycin-treated mouse model was used to compare the intestinal colonizing capacity of E. coli O157:H7 strain 933 with that of its 60-megadalton plasmid-cured derivative, strain 933cu. When fed individually to mice, both 933 and 933cu maintained a stable number of organisms per gram of feces, and the greatest numbers of 933 or 933cu were isolated from cecal and proximal colonic epithelial cells. When 933 and 933cu were simultaneously fed to mice, 933cu was unable to maintain a stable level of colonization in about two-thirds of the mice tested. However, in one-third of the mice, the number of 933cu in feces began to increase rapidly until a stable level of co-colonization with 933 was attained. The isolate from these mice, 933cu-rev, was excreted in high numbers when fed alone to mice and was found on epithelial cells throughout the entire large bowel and distal small intestine. Moreover, 933cu-rev grew in mucus from all segments of the intestine and at higher levels than strain 933 or 933cu. Only mice fed strain 933cu-rev died. Histopathological studies confirmed that mice fed 933cu-rev died from bilateral renal cortical tubular necrosis consistent with toxic insult, perhaps due to Shiga-like toxins. The virulence of 933cu-rev may reflect its ability to grow well in mucus and colonize the small as well as large bowel.     

toxB gene on pO157 of enterohemorrhagic Escherichia coli O157:H7 is required for full epithelial cell adherence phenotype

Adherence of enterohemorrhagic Escherichia coli (EHEC) to the intestinal epithelium is critical for initiation of a bacterial infection. An in vitro infection study previously indicated that EHEC bacteria initially adhere diffusely and then proliferate to develop MC, a process that is mediated by various secreted proteins, such as EspA, EspB, EspD, Tir, and intimin, as well as other putative adherence factors. In the present study, we investigated the role of a large 93-kb plasmid (pO157) in the adherence of O157:H7 (O157Sakai) and found the toxB gene to be involved in the full adherence phenotype. A pO157-cured strain of O157Sakai (O157Cu) developed microcolonies on Caco-2 cells; however, the number of microcolonies was lower than that of O157Sakai, as were the production and secretion levels of EspA, EspB, and Tir. Introduction of a mini-pO157 plasmid (pIC37) composed of the toxB and ori regions restored full adherence capacity to O157Cu, including production and secretion of the proteins. In contrast, introduction of a pO157 mutant possessing toxB::Km into O157Cu could not restore the full adherence phenotype. Expression of truncated versions of His-tagged ToxB also promoted EspB production and/or secretion by O157Cu. These results suggest that ToxB contributes to the adherence of EHEC to epithelial cells through promotion of the production and/or secretion of type III secreted proteins.     

A minD mutant of enterohemorrhagic E. coli O157:H7 has reduced adherence to human epithelial cells

Adherence to epithelial cells is a prerequisite for intestinal colonization by the bacterial pathogen, enterohemorrhagic Escherichia coli (EHEC). The deletion of minD, a cell division gene, in EHEC caused reduced adherence to human epithelioid cervical carcinoma (HeLa) and human colonic adenocarcinoma (Caco-2) cells as compared to wild-type. The minD mutant formed minicells and filaments owing to aberrant cytokinesis. Moreover, its ability to form microcolonies as typically seen in the co-cultures of wild-type with Caco-2 cells, was abolished. In conclusion, the present study highlights the importance of minD in regards to EHEC adherence to human epithelial cells.     

Characterization of an RTX toxin from enterohemorrhagic Escherichia coli O157:H7.

A hemolytic determinant of enterohemorrhagic Escherichia coli O157:H7 is encoded on a 90-kbp plasmid (pO157). This enterohemorrhagic E. coli toxin (Ehx) is a newly described RTX cytotoxin. The prototype RTX toxin is the E. coli hemolysin (Hly) associated with extraintestinal E. coli infections. We expressed Ehx from E. coli K-12 strains harboring either pSK3, a pO157 derivative marked with Tn801 unlinked to Ehx, or a recombinant plasmid containing an 11.9-kbp subclone (pEO40) of pSK3. The Ehx activities and antibody reactivities were compared with those of Hly. Little Ehx was secreted extracellularly from the strain harboring pSK3; however, when the Hly transport genes hlyBD were supplied in trans, both intracellular and extracellular levels of Ehx were enhanced more than 15-fold. The strain harboring pEO40 secreted at least 140-fold more Ehx than did the strain harboring pSK3, and neither intracellular nor extracellular levels were significantly enhanced by the addition of hlyBD in trans. Polyclonal anti-HlyA antiserum and several anti-HlyA monoclonal antibodies, including the monoclonal antibody A10, which is panreactive for nearly all RTX toxins, reacted with EhxA antigen by immunoblot analysis. In hemolysis and 51Cr release assays, Ehx demonstrated similar efficiencies in lysis of BL-3 cells (cells from a bovine lymphoma cell line) and sheep and human erythrocytes. Surprisingly, it demonstrated very little activity against two human lymphoma cell lines. In contrast, Hly lysed all five cell types tested, each to a greater extent than that demonstrated by comparable amounts of Ehx. As with other RTX toxins, Ehx activity was calcium dependent and heat labile.     

Cortactin recruitment by enterohemorrhagic Escherichia coli O157:H7 during infection in vitro and ex vivo

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is an important human pathogen that colonizes the gut mucosa via attaching and effacing (A/E) lesions; A/E lesion formation in vivo and ex vivo is dependent on the type III secretion system (T3SS) effector Tir. Infection of cultured cells by EHEC leads to induction of localized actin polymerization, which is dependent on Tir and a second T3SS effector protein, TccP, also known as EspF_U. Recently, cortactin was shown to bind both the N terminus of Tir and TccP via its SH3 domain and to play a role in EHEC-triggered actin polymerization in vitro. In this study, we investigated the recruitment of cortactin to the site of EHEC adhesion during infection of in vitro-cultured cells and mucosal surfaces ex vivo (using human terminal ileal in vitro organ cultures [IVOC]). We have shown that cortactin is recruited to the site of EHEC adhesion in vitro downstream of TccP and N-WASP. Deletion of the entire N terminus of Tir or replacing the N-terminal polyproline region with alanines did not abrogate actin polymerization or cortactin recruitment. In contrast, recruitment of cortactin to the site of EHEC adhesion in IVOC is TccP independent. These results imply that cortactin is recruited to the site of EHEC adhesion in vitro and ex vivo by different mechanisms and suggest that cortactin might have a role during EHEC infection of mucosal surfaces.     

Long polar fimbriae of enterohemorrhagic Escherichia coli O157:H7 bind to extracellular matrix proteins

Adherence to intestinal cells is a key process in infection caused by enterohemorrhagic Escherichia coli (EHEC). Several adhesion factors that mediate the binding of EHEC to intestinal cells have been described, but the receptors involved in their recognition are not fully characterized. Extracellular matrix (ECM) proteins might act as receptors involved in the recognition of enteric pathogens, including EHEC. In this study, we sought to characterize the binding of EHEC O157:H7 to ECM proteins commonly present in the intestine. We found that EHEC prototype strains as well as other clinical isolates adhered more abundantly to surfaces coated with fibronectin, laminin, and collagen IV. Further characterization of this phenotype, by using antiserum raised against the LpfA1 putative major fimbrial subunit and by addition of mannose, showed that a reduced binding of EHEC to ECM proteins was observed in a long polar fimbria (lpf) mutant. We also found that the two regulators, H-NS and Ler, had an effect in EHEC Lpf-mediated binding to ECM, supporting the roles of these tightly regulated fimbriae as adherence factors. Purified Lpf major subunit bound to all of the ECM proteins tested. Finally, increased bacterial adherence was observed when T84 cells, preincubated with ECM proteins, were infected with EHEC. Taken together, these findings suggest that the interaction of Lpf and ECM proteins contributes to the EHEC colonization of the gastrointestinal tract.     

Outer membranes are competitive inhibitors of Escherichia coli O157:H7 adherence to epithelial cells.

Escherichia coli of serotype O157:H7 are Vero cytotoxin-producing enteric pathogens that have been associated recently with sporadic cases and outbreaks of hemorrhagic colitis and with the hemolytic-uremic syndrome. Adherence of many enteropathogenic bacteria to mucosal surfaces is a critical step in the pathogenesis of diarrheal disease. We showed previously that adherence of E. coli O157:H7 strain CL-56 to epithelial cells in vitro is inhibited by outer membranes. In this study we examined whether outer membranes from a series of E. coli O157:H7 strains mediated competitive inhibition of bacterial binding to epithelial cells grown in tissue culture. We also determined which constituents of the outer membrane mediated inhibition of CL-56 adherence. Binding of six O157:H7 strains to HEp-2 cells was determined by quantitating the number of adherent bacteria in the presence and absence of outer membranes which were extracted from each strain with N-lauroyl sarcosinate (1.7%, wt/vol). After separation of outer membranes by gel electrophoresis, four bands (94, 40, 36, and 30 kDa) were collected by electroelution. Immune sera were raised in rabbits to each of the four eluted bands. Outer membrane extracts from each of the six O157:H7 strains inhibited binding of homologous organisms to the HEp-2 cells. At dilutions which did not cause bacterial agglutination, antiserum raised against the 94-kDa outer membrane protein showed maximal inhibition of bacterial adherence (17.0 +/- 7.3% adherence of control levels). Growth of bacteria in iron-depleted broth did not affect their binding to HEp-2 cells, suggesting that iron-regulated outer membranes were not involved. Fluid accumulation in ileal ligated loops of rabbits in response to E. coli O157:H7 challenge was diminished following both parenteral immunization with outer membranes extracted from the homologous strain and coincubation of organisms with immune serum which contained antibodies to outer membrane extracts. These data indicate that outer membranes are competitive inhibitors of E. coli O157:H7 adherence. Specific constituents of the outer membrane may function as bacterial attachment factors (i.e., adhesins) for E. coli O157:H7 adherence to epithelial cell surfaces.     

CadA negatively regulates Escherichia coli O157:H7 adherence and intestinal colonization

Adherence of pathogenic Escherichia coli strains to intestinal epithelia is essential for infection. For enterohemorrhagic E. coli (EHEC) serotype O157:H7, we have previously demonstrated that multiple factors govern this pathogen's adherence to HeLa cells (A. G. Torres and J. B. Kaper, Infect. Immun. 71:4985-4995, 2003). One of these factors is CadA, a lysine decarboxylase, and this protein has been proposed to negatively regulate virulence in several enteric pathogens. In the case of EHEC strains, CadA modulates expression of the intimin, an outer membrane adhesin involved in pathogenesis. Here, we inactivated cadA in O157:H7 strain 86-24 to investigate the role of this gene in EHEC adhesion to tissue-cultured monolayers, global gene expression patterns, and colonization of the infant rabbit intestine. The cadA mutant did not possess lysine decarboxylation activity and was hyperadherent to tissue-cultured cells. Adherence of the cadA mutant was nearly twofold greater than that of the wild type, and the adherence phenotype was independent of pH, lysine, or cadaverine in the media. Additionally, complementation of the cadA defect reduced adherence back to wild-type levels, and it was found that the mutation affected the expression of the intimin protein. Disruption of the eae gene (intimin-encoding gene) in the cadA mutant significantly reduced its adherence to tissue-cultured cells. However, adherence of the cadA eae double mutant was greater than that of an 86-24 eae mutant, suggesting that the enhanced adherence of the cadA mutant is not entirely attributable to enhanced expression of intimin in this background. Gene array analysis revealed that the cadA mutation significantly altered EHEC gene expression patterns; expression of 1,332 genes was downregulated and that of 132 genes was upregulated in the mutant compared to the wild-type strain. Interestingly, the gene expression variation shows an EHEC-biased gene alteration including intergenic regions. Two putative adhesins, flagella and F9 fimbria, were upregulated in the cadA mutant, suggestive of their association with adherence in the absence of the Cad regulatory mechanism. In the infant rabbit model, the cadA mutant outcompeted the wild-type strain in the ileum but not in the cecum or mid-colon, raising the possibility that CadA negatively regulates EHEC pathogenicity in a tissue-specific fashion.     

GrlA of enterohemorrhagic Escherichia coli O157:H7 activates LEE1 by binding to the promoter region.

BACKGROUND AND PURPOSE: The locus of enterocyte effacement (LEE) of enterohemorrhagic Escherichia coli (EHEC) O157:H7 encodes virulence factors that lead cooperatively to an attaching and effacing lesion on host large intestine cells. Global regulator of LEE activator (GrlA), encoded by the open reading frame 3 in the EHEC LEE, is known to serve as a positive regulator of LEE expression. However, how it functions to orchestrate gene expression remains unclear. METHODS: A grlA-deleted mutant strain was created, and the determinants needed for the LEE activation were addressed by complementation experiments. A DNA electrophoresis mobility-shifting assay was used to test a hypothesis that the activation occurs via a direct binding on the putative promoter region. RESULTS: Activation of the major LEE operons could be rescued by an over-expression of LEE-encoded regulator (Ler), except for the LEE1 operon, expression of which absolutely required GrlA. Consistent with the latter observation, GrlA bound specifically to the putative LEE1 promoter region. Furthermore, determinants critical for this activity have been mapped to the N-terminal region of GrlA. CONCLUSION: GrlA upregulates the expression of LEE through binding of the LEE1 promoter, which in turn increases the level of Ler allowing it to function as a downstream activator. The opposing effect of global regulator of LEE repressor (GrlR) is explainable by in vitro findings that GrlR interacts with GrlA, suppressing the specific binding of GrlA on the LEE1 promoter.     

Contribution of the twin arginine translocation system to the virulence of enterohemorrhagic Escherichia coli O157:H7

Shiga toxin-producing Escherichia coli O157:H7 is a major food-borne infectious pathogen. In order to analyze the contribution of the twin arginine translocation (TAT) system to the virulence of E. coli O157:H7, we deleted the tatABC genes of the O157:H7 EDL933 reference strain. The mutant displayed attenuated toxicity on Vero cells and completely lost motility on soft agar plates. Further analyses revealed that the ΔtatABC mutation impaired the secretion of the Shiga toxin 1 (Stx1) and abolished the synthesis of H7 flagellin, which are two major known virulence factors of enterohemorrhagic E. coli O157:H7. Expression of the EDL933 stxAB₁ genes in E. coli K-12 conferred verotoxicity on this nonpathogenic strain. Remarkably, cytotoxicity assay and immunoblot analysis showed, for the first time, an accumulation of the holotoxin complex in the periplasm of the wild-type strain and that a much smaller amount of StxA₁ and reduced verotoxicity were detected in the ΔtatC mutant cells. Together, these results establish that the TAT system of E. coli O157:H7 is an important virulence determinant of this enterohemorrhagic pathogen.     

Truncated enterohemorrhagic Escherichia coli (EHEC) O157:H7 intimin (EaeA) fusion proteins promote adherence of EHEC strains to HEp-2 cells.

Intimin, the product of the eaeA gene in enterohemorrhagic Escherichia coli O157:H7 (EHEC), is required for intimate adherence of these organisms to tissue culture cells and formation of the attaching and effacing lesion in the gnotobiotic pig. Because of the importance of intimin in the pathogenesis of EHEC O157:H7 infection in this animal model, we began a structure-function analysis of EaeA. For this purpose, we constructed amino-terminal fusions of the intimin protein with six histidine residues to form two independent fusions. The longer fusion, RIHisEae, contained 900 of the 935 predicted amino acids and included all but the extreme amino terminus. The second fusion, RVHdHisEae, consisted of the carboxyl two-thirds of the protein. Purified extracts of either construct enhanced binding of wild-type 86-24 to HEp-2 cells and conferred HEp-2 cell adherence on 86-24eaeDelta10, an eaeA deletion mutant, and B2F1, an EHEC O91:1-121 eaeA mutant strain. When 86-24eaeDelta10 was transformed with either of the plasmids encoding the intimin fusion proteins, the transformant behaved like the wild-type parent strain and displayed localized adherence to HEp-2 cells, with positive fluorescent-actin staining. In addition, polyclonal antisera raised against RIHisEae reacted with both fusion constructs and recognized an outer membrane protein of the same mass as intimin (97 kDa) in EHEC and enteropathogenic E. coli but not E. coli K-12. The intimin-specific antisera also blocked adherence of EHEC to HEp-2 cells. Thus, intimin (i) is a 97-kDa outer membrane protein in EHEC that serves as a requisite adhesin for attachment of the bacteria to epithelial cells, even when the protein is truncated by one-third at its amino terminus and (ii) can be added exogenously to specifically facilitate HEp-2 cell adherence of EHEC but not E. coli K-12.     

Cytolethal distending toxin gene cluster in enterohemorrhagic Escherichia coli O157:H- and O157:H7: characterization and evolutionary considerations

We identified a cytolethal distending toxin (cdt) gene cluster in 87, 6, and 0% of sorbitol-fermenting (SF) enterohemorrhagic Escherichia coli (EHEC) O157:H(-), EHEC O157:H7, and E. coli O55:H7/H(-) strains, respectively. The toxin was expressed by the wild-type EHEC O157 strains and by a cdt-containing cosmid from a library of SF EHEC O157:H(-) strain 493/89. The cdt flanks in strain 493/89 were homologous to bacteriophages P2 and lambda. Our data demonstrate that cdt, encoding a potential virulence factor, is present in the EHEC O157 complex and suggest that cdt may have been acquired by phage transduction.     

Investigation of enterohemorrhagic Escherichia coli O157:H7 adherence characteristics and invasion potential reveals a new attachment pattern shared by intestinal E. coli.

In this study, the interactions of enterohemorrhagic Escherichia coli (EHEC) O157 strains with human ileocecal (HCT-8) epithelial cells and HEp-2 cells were examined. EHEC adhered to, but did not invade, HCT-8 cells by the localized adherence mechanism and a heretofore unrecognized pattern which we called log jam. The log jam formation was (i) not observed on HEp-2 cells, (ii) independent of the EHEC eaeA gene required for localized adherence, and (iii) shared by pathogenic and nonpathogenic E. coli strains but not K-12 strains. The log jam phenotype may represent a basal means by which E. coli bacteria attach to the human intestine.     

Characterization of flagella purified from enterohemorrhagic, vero-cytotoxin-producing Escherichia coli serotype O157:H7.

Escherichia coli of the serotype O157:H7 has recently been isolated in human fecal specimens in association with sporadic cases and outbreaks of hemorrhagic colitis and with the hemolytic uremic syndrome. The aim of this study was to characterize the flagellin protein subunit constituents of flagellar filaments from E. coli O157:H7 strain CL-56. Flagellin isolated from a reference Salmonella enteritidis strain was used for comparison. Flagella were dissociated by incubation of bacteria under acidic conditions, centrifugation, and differential ammonium sulfate precipitation. Reconstituted flagellar filaments were demonstrated by three complementary methods: transmission electron microscopy, antigenic reactivity with H7 antiserum by a dot blot immunoassay, and immunogold localization of antiserum raised to the purified antigen to intact flagella on whole E. coli O157:H7. On sodium dodecyl sulfate-polyacrylamide gels flagellin proteins from E. coli O157:H7 demonstrated an apparent Mr of 66,000. The isoelectric point of E. coli O157:H7 flagellin was 5.42. By immunoblotting, H7 flagellin proteins were shown to be immunogenic. They induced a systemic immune response both in rabbits challenged with whole bacteria and in a human previously infected with E. coli O157:H7.     

TccP2 of O157:H7 and non-O157 enterohemorrhagic Escherichia coli (EHEC): challenging the dogma of EHEC-induced actin polymerization

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 and enteropathogenic E. coli (EPEC) trigger actin polymerization at the site of bacterial adhesion by inducing different signaling pathways. Actin assembly by EPEC requires tyrosine phosphorylation of Tir, which subsequently binds the host adaptor protein Nck. In contrast, Tir(EHEC O157) is not tyrosine phosphorylated and instead of Nck utilizes the bacterially encoded Tir-cytoskeleton coupling protein (TccP)/EspF(U), which mimics the function of Nck. tccP is carried on prophage CP-933U/Sp14 (TccP). Typical isolates of EHEC O157:H7 harbor a pseudo-tccP gene that is carried on prophage CP-933 M/Sp4 (tccP2). Here we report that atypical, beta-glucuronidase-positive and sorbitol-fermenting, strains of EHEC O157 harbor intact tccP and tccP2 genes, both of which are secreted by the LEE-encoded type III secretion system. Non-O157 EHEC strains, including O26, O103, O111, and O145, are typically tccP negative and translocate a Tir protein that encompasses an Nck binding site. Unexpectedly, we found that most clinical non-O157 EHEC isolates carry a functional tccP2 gene that encodes a secreted protein that can complement an EHEC O157:H7 DeltatccP mutant. Using discriminatory, allele-specific PCR, we have demonstrated that over 90% of tccP2-positive non-O157 EHEC strains contain a Tir protein that can be tyrosine phosphorylated. These results suggest that the TccP pathway can be used by both O157 and non-O157 EHEC and that non-O157 EHEC can also trigger actin polymerization via the Nck pathway.