Binding of intimin from enteropathogenic Escherichia coli to Tir and to host cells

Why put yourself on a pedestal? The pathogenic role of the A/E pedestal

Certain Escherichia coli (E. coli) strains are attaching and effacing (A/E) lesion pathogens that primarily infect intestinal epithelial cells. They cause actin restructuring and polymerization within the host cell to create an actin-rich protrusion below the site of adherence, termed the pedestal. Although there is clarity on the pathways initiating pedestal formation, the underlying purpose(s) of the pedestal remains ambiguous. The conservation of pedestal-forming activity across multiple pathogens and redundancy in formation pathways indicate a pathogenic advantage. However, few decisive conclusions have been drawn, given that the results vary between model systems. Some research argues that the pedestal increases the colonization capability of the bacterium. These studies utilize A/E pathogens specifically deficient in pedestal formation to evaluate adhesion and intestinal colonization following infection. There have been many proposed mechanisms for the colonization benefit conferred by the pedestal. One suggested benefit is that the pedestal allows for direct cytosolic anchoring through incorporation of the established host cortical actin, causing a stable link between the pathogen and cell structure. The pedestal may confer enhanced motility, as enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC) are better able to migrate on the surface of host cells and infect neighboring cells in the presence of the pedestal. Additionally, some research suggests that the pedestal improves effector delivery. This review will investigate the purpose of pedestal formation using evidence from recent literature and will critically evaluate the methodology and model systems. Most importantly, we will contextualize the proposed functions to reconcile potential synergistic effects.

In situ deposition of nanobodies by an engineered commensal microbe promotes survival in a mouse model of enterohemorrhagic E. coli

Engineered smart microbes that deliver therapeutic payloads are emerging as treatment modalities, particularly for diseases with links to the gastrointestinal tract. Enterohemorrhagic Escherichia coli (EHEC) is a causative agent of potentially lethal hemolytic uremic syndrome. Given concerns that antibiotic treatment increases EHEC production of Shiga toxin (Stx), which is responsible for systemic disease, novel remedies are needed. EHEC encodes a type III secretion system (T3SS) that injects Tir into enterocytes. Tir inserts into the host cell membrane, exposing an extracellular domain that subsequently binds intimin, one of its outer membrane proteins, triggering the formation of attaching and effacing (A/E) lesions that promote EHEC mucosal colonization. Citrobacter rodentium (Cr), a natural A/E mouse pathogen, similarly requires Tir and intimin for its pathogenesis. Mice infected with Cr(ΦStx2dact), a variant lysogenized with an EHEC-derived phage that produces Stx2dact, develop intestinal A/E lesions and toxin-dependent disease. Stx2a is more closely associated with human disease. By developing an efficient approach to seamlessly modify the C. rodentium genome, we generated Cr_Tir-MEHEC(ΦStx2a), a variant that expresses Stx2a and the EHEC extracellular Tir domain. We found that mouse precolonization with HS-PROT3EcT-TD4, a human commensal E. coli strain (E. coli HS) engineered to efficiently secrete an anti-EHEC Tir nanobody, delayed bacterial colonization and improved survival after challenge with Cr_Tir-MEHEC(ΦStx2a). This study suggests that commensal E. coli engineered to deliver payloads that block essential virulence determinants can be developed as a new means to prevent and potentially treat infections including those due to antibiotic resistant microbes.

In situ deposition of nanobodies by an engineered commensal microbe promotes survival in a mouse model of enterohemorrhagic E. coli

Engineered smart microbes that deliver therapeutic payloads are emerging as treatment modalities, particularly for diseases with links to the gastrointestinal tract. Enterohemorrhagic E coli (EHEC) is a causative agent of potentially lethal hemolytic uremic syndrome. Given concerns that antibiotic treatment increases EHEC production of Shiga toxin (Stx), which is responsible for systemic disease, novel remedies are needed. EHEC encodes a type III secretion system (T3SS) that injects Tir into enterocytes. Tir inserts into the host cell membrane, exposing an extracellular domain that subsequently binds intimin, one of its outer membrane proteins, triggering the formation of attaching and effacing (A/E) lesions that promote EHEC mucosal colonization. Citrobacter rodentium (Cr), a natural A/E mouse pathogen, similarly requires Tir and intimin for its pathogenesis. Mice infected with Cr(ΦStx2dact), a variant lysogenized with an EHEC-derived phage that produces Stx2dact, develop intestinal A/E lesions and toxin-dependent disease. Stx2a is more closely associated with human disease. By developing an efficient approach to seamlessly modify the C. rodentium genome, we generated Cr_Tir-MEHEC(ΦStx2a), a variant that expresses Stx2a and the EHEC extracellular Tir domain. We found that mouse pre-colonization with HS-PROT3EcT-TD4, a human commensal E. coli strain (E. coli HS) engineered to efficiently secrete- an anti-EHEC Tir nanobody, delayed bacterial colonization and improved survival after challenge with Cr_Tir-MEHEC(ΦStx2a). This study provides the first evidence to support the efficacy of engineered commensal E. coli to intestinally deliver therapeutic payloads that block essential enteric pathogen virulence determinants, a strategy that may serve as an antibiotic-independent antibacterial therapeutic modality.

Single domain antibodies from camelids in the treatment of microbial infections

Infectious diseases continue to pose significant global health challenges. In addition to the enduring burdens of ailments like malaria and HIV, the emergence of nosocomial outbreaks driven by antibiotic-resistant pathogens underscores the ongoing threats. Furthermore, recent infectious disease crises, exemplified by the Ebola and SARS-CoV-2 outbreaks, have intensified the pursuit of more effective and efficient diagnostic and therapeutic solutions. Among the promising options, antibodies have garnered significant attention due to their favorable structural characteristics and versatile applications. Notably, nanobodies (Nbs), the smallest functional single-domain antibodies of heavy-chain only antibodies produced by camelids, exhibit remarkable capabilities in stable antigen binding. They offer unique advantages such as ease of expression and modification and enhanced stability, as well as improved hydrophilicity compared to conventional antibody fragments (antigen-binding fragments (Fab) or single-chain variable fragments (scFv)) that can aggregate due to their low solubility. Nanobodies directly target antigen epitopes or can be engineered into multivalent Nbs and Nb-fusion proteins, expanding their therapeutic potential. This review is dedicated to charting the progress in Nb research, particularly those derived from camelids, and highlighting their diverse applications in treating infectious diseases, spanning both human and animal contexts.

Transmembrane substrates of type three secretion system injectisomes

The type three secretion system injectisome of Gram-negative bacterial pathogens injects virulence proteins, called effectors, into host cells. Effectors of mammalian pathogens carry out a range of functions enabling bacterial invasion, replication, immune suppression and transmission. The injectisome secretes two translocon proteins that insert into host cell membranes to form a translocon pore, through which effectors are delivered. A subset of effectors also integrate into infected cell membranes, enabling a unique range of biochemical functions. Both translocon proteins and transmembrane effectors avoid cytoplasmic aggregation and integration into the bacterial inner membrane. Translocated transmembrane effectors locate and integrate into the appropriate host membrane. In this review, we focus on transmembrane translocon proteins and effectors of bacterial pathogens of mammals. We discuss what is known about the mechanisms underlying their membrane integration, as well as the functions conferred by the position of injectisome effectors within membranes.

The EcpD Tip Adhesin of the Escherichia coli Common Pilus Mediates Binding of Enteropathogenic E. coli to Extracellular Matrix Proteins

The attachment of enteropathogenic Escherichia coli (EPEC) to intestinal epithelial cells is facilitated by several adhesins; however, the individual host-cell receptors for pili-mediated adherence have not been fully characterized. In this study, we evaluated the hypothesis that the E. coli common pilus (ECP) tip adhesin protein EcpD mediates attachment of EPEC to several extracellular matrix (ECM) glycoproteins (fibronectin, laminin, collagens I and IV, and mucin). We found that the ΔecpA mutant, which lacks production of the EcpA filament but retains EcpD on the surface, adhered to these glycoproteins below the wild-type levels, while the ΔecpD mutant, which does not display EcpA or EcpD, bound significantly less to these host glycoproteins. In agreement, a purified recombinant EcpD subunit bound significantly more than EcpA to laminin, fibronectin, collagens I and IV, and mucin in a dose-dependent manner. These are compelling data that strongly suggest that ECP-producing EPEC may bind to host ECM glycoproteins and mucins through the tip adhesin protein EcpD. This study highlights the versatility of EPEC to bind to different host proteins and suggests that the interaction of ECP with the host’s ECM glycoproteins may facilitate colonization of the intestinal mucosal epithelium.

Microbiology and Epidemiology of Escherichia albertii—An Emerging Elusive Foodborne Pathogen

Escherichia albertii, a close relative of E. coli, is an emerging zoonotic foodborne pathogen associated with watery diarrhea mainly in children and immunocompromised individuals. E. albertii was initially classified as eae-positive Hafnia alvei, however, as more genetic and biochemical information became available it was reassigned to its current novel taxonomy. Its infections are common under conditions of poor hygiene with confirmed transmission via contaminated water and food, mainly poultry-based products. This pathogen has been isolated from various domestic and wild animals, with most isolates being derived from birds, implying that birds among other wild animals might act as its reservoir. Due to the absence of standardized isolation and identification protocols, E. albertii can be misidentified as other Enterobacteriaceae. Exploiting phenotypes such as its inability to ferment rhamnose and xylose and PCR assays targeting E. albertii-specific genes such as the cytolethal distending toxin and the DNA-binding transcriptional activator of cysteine biosynthesis encoding genes can be used to accurately identify this pathogen. Several gaps exist in our knowledge of E. albertii and need to be bridged. A deeper understanding of E. albertii epidemiology and physiology is required to allow the development of effective measures to control its transmission and infections. Overall, current data suggest that E. albertii might play a more significant role in global infectious diarrhea cases than previously assumed and is often overlooked or misidentified. Therefore, simple, and efficient diagnostic tools that cover E. albertii biodiversity are required for effective isolation and identification of this elusive agent of diarrhea.

Identification of Translocation Inhibitors Targeting the Type III Secretion System of Enteropathogenic Escherichia coli

Infections with enteropathogenic Escherichia coli (EPEC) cause severe diarrhea in children. The noninvasive bacteria adhere to enterocytes of the small intestine and use a type III secretion system (T3SS) to inject effector proteins into host cells to modify and exploit cellular processes in favor of bacterial survival and replication. Several studies have shown that the T3SSs of bacterial pathogens are essential for virulence. Furthermore, the loss of T3SS-mediated effector translocation results in increased immune recognition and clearance of the bacteria. The T3SS is, therefore, considered a promising target for antivirulence strategies and novel therapeutics development. Here, we report the results of a high-throughput screening assay based on the translocation of the EPEC effector protein Tir (translocated intimin receptor). Using this assay, we screened more than 13,000 small molecular compounds of six different compound libraries and identified three substances which showed a significant dose-dependent effect on translocation without adverse effects on bacterial or eukaryotic cell viability. In addition, these substances reduced bacterial binding to host cells, effector-dependent cell detachment, and abolished attaching and effacing lesion formation without affecting the expression of components of the T3SS or associated effector proteins. Moreover, no effects of the inhibitors on bacterial motility or Shiga-toxin expression were observed. In summary, we have identified three new compounds that strongly inhibit T3SS-mediated translocation of effectors into mammalian cells, which could be valuable as lead substances for treating EPEC and enterohemorrhagic E. coli infections.

NleB2 from enteropathogenic Escherichia coli is a novel arginine-glucose transferase effector

During infection, enteropathogenic Escherichia coli (EPEC) and enterohaemorrhagic E. coli (EHEC) directly manipulate various aspects of host cell function through the translocation of type III secretion system (T3SS) effector proteins directly into the host cell. Many T3SS effector proteins are enzymes that mediate post-translational modifications of host proteins, such as the glycosyltransferase NleB1, which transfers a single N-acetylglucosamine (GlcNAc) to arginine residues, creating an Arg-GlcNAc linkage. NleB1 glycosylates death-domain containing proteins including FADD, TRADD and RIPK1 to block host cell death. The NleB1 paralogue, NleB2, is found in many EPEC and EHEC strains but to date its enzymatic activity has not been described. Using in vitro glycosylation assays combined with mass spectrometry, we found that NleB2 can utilize multiple sugar donors including UDP-glucose, UDP-GlcNAc and UDP-galactose during glycosylation of the death domain protein, RIPK1. Sugar donor competition assays demonstrated that UDP-glucose was the preferred substrate of NleB2 and peptide sequencing identified the glycosylation site within RIPK1 as Arg603, indicating that NleB2 catalyses arginine glucosylation. We also confirmed that NleB2 catalysed arginine-hexose modification of Flag-RIPK1 during infection of HEK293T cells with EPEC E2348/69. Using site-directed mutagenesis and in vitro glycosylation assays, we identified that residue Ser252 in NleB2 contributes to the specificity of this distinct catalytic activity. Substitution of Ser252 in NleB2 to Gly, or substitution of the corresponding Gly255 in NleB1 to Ser switches sugar donor preference between UDP-GlcNAc and UDP-glucose. However, this switch did not affect the ability of the NleB variants to inhibit inflammatory or cell death signalling during HeLa cell transfection or EPEC infection. NleB2 is thus the first identified bacterial Arg-glucose transferase that, similar to the NleB1 Arg-GlcNAc transferase, inhibits host protein function by arginine glycosylation.

Bacterial gut pathogens including enteropathogenic E. coli (EPEC) and enterohaemorrhagic E. coli (EHEC), manipulate host cell function by using a type III secretion system to inject ‘effector’ proteins directly into the host cell cytoplasm. We and others have shown that many of these effectors are novel enzymes, including NleB1, which transfers a single N-acetylglucosamine (GlcNAc) sugar to arginine residues, mediating Arg-GlcNAc glycosylation. Here, we found that a close homologue of NleB1 that is also present in EPEC and EHEC termed NleB2, uses a different sugar during glycosylation. We demonstrated that in contrast to NleB1, the preferred nucleotide-sugar substrate of NleB2 is UDP-glucose and we identified the amino acid residue within NleB2 that dictates this unique catalytic activity. Substitution of this residue in NleB2 and NleB1 switches the sugar donor usage of these enzymes but does not affect their ability to inhibit host cell signalling. Thus, NleB2 is the first identified bacterial arginine-glucose transferase, an activity which has previously only been described in plants and algae.

Multipathogen Analysis of IgA and IgG Antigen Specificity for Selected Pathogens in Milk Produced by Women From Diverse Geographical Regions: The INSPIRE Study

Breastfeeding provides defense against infectious disease during early life. The mechanisms underlying this protection are complex but likely include the vast array of immune cells and components, such as immunoglobulins, in milk. Simply characterizing the concentrations of these bioactives, however, provides only limited information regarding their potential relationships with disease risk in the recipient infant. Rather, understanding pathogen and antigen specificity profiles of milk-borne immunoglobulins might lead to a more complete understanding of how maternal immunity impacts infant health and wellbeing. Milk produced by women living in 11 geographically dispersed populations was applied to a protein microarray containing antigens from 16 pathogens, including diarrheagenic E. coli, Shigella spp., Salmonella enterica serovar Typhi, Staphylococcus aureus, Streptococcus pneumoniae, Mycobacterium tuberculosis and other pathogens of global health concern, and specific IgA and IgG binding was measured. Our analysis identified novel disease-specific antigen responses and suggests that some IgA and IgG responses vary substantially within and among populations. Patterns of antibody reactivity analyzed by principal component analysis and differential reactivity analysis were associated with either lower-to-middle-income countries (LMICs) or high-income countries (HICs). Antibody levels were generally higher in LMICs than HICs, particularly for Shigella and diarrheagenic E. coli antigens, although sets of S. aureus, S. pneumoniae, and some M. tuberculosis antigens were more reactive in HICs. Differential responses were typically specific to canonical immunodominant antigens, but a set of nondifferential but highly reactive antibodies were specific to antigens possibly universally recognized by antibodies in human milk. This approach provides a promising means to understand how breastfeeding and human milk protect (or do not protect) infants from environmentally relevant pathogens. Furthermore, this approach might lead to interventions to boost population-specific immunity in at-risk breastfeeding mothers and their infants.

Identification of Nanobodies Blocking Intimate Adherence of Shiga Toxin-Producing Escherichia coli to Epithelial Cells

Therapeutic antibodies (Abs) inhibiting bacterial adhesion to host epithelia are an attractive option to reduce the load of Shiga toxin-producing E. coli (STEC) in the intestine of the patient and also in the bovine reservoir, thereby minimizing the risk of STEC contamination in the food chain. Of particular interest are recombinant single-domain Ab fragments called nanobodies (Nbs) derived from the variable domain of camelid heavy chain-only antibodies (VHH). The outer membrane adhesin intimin and the translocated intimin receptor (Tir) are essential for the attachment of STEC to host epithelia. In addition, EspA filaments of the bacterial type III protein secretion system are needed for Tir translocation into the host cell. Given their importance for bacterial adhesion and colonization, we developed Nbs against intimin, Tir and EspA proteins of STEC serotype O157:H7. Here, we report the screening methods used to isolate inhibitory Nbs blocking intimin-Tir protein-protein interaction, actin-pedestal formation, and intimate adhesion of STEC to epithelial cells in vitro. First, we describe how VHH gene repertoires can be produced as Nbs secreted by E. coli using the α-hemolysin (HlyA) protein secretion system. Next, we report the methods for identification of inhibitors of intimin-Tir protein-protein interaction and of STEC intimate adhesion to HeLa cells in culture. These methods can be adapted for the screening of Nbs against different adhesin-receptor complexes to block the adhesion of other pathogens to host cells.

β-arrestin 2 quenches TLR signaling to facilitate the immune evasion of EPEC

The protein translocated intimin receptor (Tir) from enteropathogenic Escherichia coli shares sequence similarity with the host cellular immunoreceptor tyrosine-based inhibition motifs (ITIMs). The ITIMs of Tir are required for Tir-mediated immune inhibition and evasion of host immune responses. However, the underlying molecular mechanism by which Tir regulates immune inhibition remains unclear. Here we demonstrated that β-arrestin 2, which is involved in the G-protein-coupled receptor (GPCR) signal pathway, interacted with Tir in an ITIM-dependent manner. For the molecular mechanism, we found that β-arrestin 2 enhanced the recruitment of SHP-1 to Tir. The recruited SHP-1 inhibited K63-linked ubiquitination of TRAF6 by dephosphorylating TRAF6 at Tyr288, and inhibited K63-linked ubiquitination and phosphorylation of TAK1 by dephosphorylating TAK1 at Tyr206, which cut off the downstream signal transduction and subsequent cytokine production. Moreover, the inhibitory effect of Tir on immune responses was diminished in β-arrestin 2-deficient mice and macrophages. These findings suggest that β-arrestin 2 is a key regulator in Tir-mediated immune evasion, which could serve as a new therapeutic target for bacterial infectious diseases.

Staying out or Going in? The Interplay between Type 3 and Type 5 Secretion Systems in Adhesion and Invasion of Enterobacterial Pathogens

Enteric pathogens rely on a variety of toxins, adhesins and other virulence factors to cause infections. Some of the best studied pathogens belong to the Enterobacterales order; these include enteropathogenic and enterohemorrhagic Escherichia coli, Shigella spp., and the enteropathogenic Yersiniae. The pathogenesis of these organisms involves two different secretion systems, a type 3 secretion system (T3SS) and type 5 secretion systems (T5SSs). The T3SS forms a syringe-like structure spanning both bacterial membranes and the host cell plasma membrane that translocates toxic effector proteins into the cytoplasm of the host cell. T5SSs are also known as autotransporters, and they export part of their own polypeptide to the bacterial cell surface where it exerts its function, such as adhesion to host cell receptors. During infection with these enteropathogens, the T3SS and T5SS act in concert to bring about rearrangements of the host cell cytoskeleton, either to invade the cell, confer intracellular motility, evade phagocytosis or produce novel structures to shelter the bacteria. Thus, in these bacteria, not only the T3SS effectors but also T5SS proteins could be considered "cytoskeletoxins" that bring about profound alterations in host cell cytoskeletal dynamics and lead to pathogenic outcomes.

Escherichia coli ST302: Genomic Analysis of Virulence Potential and Antimicrobial Resistance Mediated by Mobile Genetic Elements

aEPEC are associated with persistent diarrhea, and diarrheal outbreaks in both humans and animals worldwide. They are differentiated from typical EPEC by the lack of bundle-forming pili, and from EHEC by the lack of phage-mediated stx toxins. However, phylogenetic analyses often associate aEPEC with EHEC, promoting the hypothesis that aEPEC are the progenitors of EHEC, which is supported by aEPEC conversion to EHEC by stx-carrying phages. While aEPEC can cause disease outright, the potential to acquire stx, one of the most potent bacterial toxins known, merits close monitoring. Escherichia coli ST302 (O108:H9, O182:H9, O45:H9) are aEPEC that have been isolated from diarrheic human, pig and rabbit hosts, as well as in healthy pigs, however, no study to date has focused on E. coli ST302 strains. Through WGS and hybrid assembly we present the first closed chromosome, and two circularized plasmids of an ST302 strain - F2_18C, isolated from a healthy pig in Australia. A phylogenetic analysis placed E. coli ST302 strains in proximity to EHEC ST32 (O145:H28) strains. Public databases were interrogated for WGSs of E. coli ST302 strains and short-read gene screens were used to compare their virulence-associated gene (VAG) and antimicrobial resistance gene (ARG) cargo. E. coli ST302 strains carry diverse VAGs, including those that typically associated with extraintestinal pathogenic E. coli (ExPEC). Plasmid comparisons showed that pF2_18C_FIB shared homology with EHEC virulence plasmids such as pO103 while pF2_18C_HI2 is a large multidrug resistance IncHI2:ST3 plasmid. A comparison of 33 HI2:ST3 plasmids demonstrated that those of Australian origin have not acquired resistances to extended-spectrum beta-lactams, colistin, fosfomycin or rifampicin, unlike those originating from Asia. F2_18C was shown to carry two additional pathogenicity islands – ETT2, and the STEC-associated PAI_CL3, plasmid-associated heavy metal resistance genes, as well as several unoccupied stx-phage attachment sites. This study sheds light on the virulence and AMR potential of E. coli ST302 strains and informs AMR genomic surveillance.

Identification and prevalence of in vivo-induced genes in enterohaemorrhagic Escherichia coli

Enterohaemorrhagic Escherichia coli (EHEC) are food-borne pathogens responsible for bloody diarrhoea and renal failure in humans. While Shiga toxin (Stx) is the cardinal virulence factor of EHEC, its production by E. coli is not sufficient to cause disease and many Shiga-toxin producing E. coli (STEC) strains have never been implicated in human infection. So far, the pathophysiology of EHEC infection is not fully understood and more knowledge is needed to characterize the "auxiliary" factors that enable a STEC strain to cause disease in humans. In this study, we applied a recombinase-based in vivo expression technology (RIVET) to the EHEC reference strain EDL933 in order to identify genes specifically induced during the infectious process, using mouse as an infection model. We identified 31 in vivo-induced (ivi) genes having functions related to metabolism, stress adaptive response and bacterial virulence or fitness. Eight of the 31 ivi genes were found to be heterogeneously distributed in EHEC strains circulating in France these last years. In addition, they are more prevalent in strains from the TOP seven priority serotypes and particularly strains carrying significant virulence determinants such as Stx2 and intimin adhesin. This work sheds further light on bacterial determinants over-expressed in vivo during infection that may contribute to the potential of STEC strains to cause disease in humans.

A nanobody targeting the translocated intimin receptor inhibits the attachment of enterohemorrhagic E. coli to human colonic mucosa

Enterohemorrhagic E. coli (EHEC) is a human intestinal pathogen that causes hemorrhagic colitis and hemolytic uremic syndrome. No vaccines or specific therapies are currently available to prevent or treat these infections. EHEC tightly attaches to the intestinal epithelium by injecting the intimin receptor Tir into the host cell via a type III secretion system (T3SS). In this project, we identified a camelid single domain antibody (nanobody), named TD4, that recognizes a conserved Tir epitope overlapping the binding site of its natural ligand intimin with high affinity and stability. We show that TD4 inhibits attachment of EHEC to cultured human HeLa cells by preventing Tir clustering by intimin, activation of downstream actin polymerization and pedestal formation. Furthermore, we demonstrate that TD4 significantly reduces EHEC adherence to human colonic mucosa in in vitro organ cultures. Altogether, these results suggest that nanobody-based therapies hold potential in the development of much needed treatment and prevention strategies against EHEC infection.

Currently, there is no effective treatment or vaccine against enterohemorrhagic E. coli (EHEC), a bacterial pathogen that infects human colon after the ingestion of contaminated food. It thrives in the colon thanks to its ability to attach intimately to the intestinal epithelium. Here, we have identified and characterised a small antibody fragment (nanobody) that recognises Tir, a receptor injected by the bacterium into the host cell to mediate intimate attachment. This nanobody shows higher affinity against Tir than its natural bacterial ligand (intimin) and, most importantly, blocks the intimate attachment of the pathogen to the human colonic tissue. Our results show the potential of this nanobody to prevent and treat EHEC infection.

The Evasive Enemy: Insights into the Virulence and Epidemiology of the Emerging Attaching and Effacing Pathogen Escherichia albertii

The diarrheic attaching and effacing (A/E) pathogen Escherichia albertii was first isolated from infants in Bangladesh in 1991, although the bacterium was initially classified as Hafnia alvei Subsequent genetic and biochemical interrogation of these isolates raised concerns about their initial taxonomic placement. It was not until 2003 that these isolates were reassigned to the novel taxon Escherichia albertii because they were genetically more closely related to E. coli, although they had diverged sufficiently to warrant a novel species name. Unfortunately, new isolates continue to be mistyped as enteropathogenic E. coli (EPEC) or enterohemorrhagic E. coli (EHEC) owing to shared traits, most notably the ability to form A/E lesions. Consequently, E. albertii remains an underappreciated A/E pathogen, despite multiple reports demonstrating that many provisional EPEC and EHEC isolates incriminated in disease outbreaks are actually E. albertii Metagenomic studies on dozens of E. albertii isolates reveal a genetic architecture that boasts an arsenal of candidate virulence factors to rival that of its better-characterized cousins, EPEC and EHEC. Beyond these computational comparisons, studies addressing the regulation, structure, function, and mechanism of action of its repertoire of virulence factors are lacking. Thus, the paucity of knowledge about the epidemiology, virulence, and antibiotic resistance of E. albertii, coupled with its misclassification and its ability to develop multidrug resistance in a single step, highlights the challenges in combating this emerging pathogen. This review seeks to synthesize our current but incomplete understanding of the biology of E. albertii.

Escherichia coli isolates from patients with inflammatory bowel disease: ExPEC virulence- and colicin-determinants are more frequent compared to healthy controls

A set of 178 Escherichia coli isolates taken from patients with inflammatory bowel disease (IBD) was analyzed for bacteriocin production and tested for the prevalence of 30 bacteriocin and 22 virulence factor determinants. Additionally, E. coli phylogenetic groups were also determined. Pulsed-field gel electrophoresis (PFGE) was used for exclusion of clonal character of isolates. Results were compared to data from a previously published analysis of 1283 fecal commensal E. coli isolates. The frequency of bacteriocinogenic isolates (66.9%) was significantly higher in IBD E. coli compared to fecal commensal E. coli isolates (54.2%, p < 0.01). In the group of IBD E. coli isolates, a higher prevalence of determinants for group B colicins (i.e., colicins B, D, Ia, Ib, M, and 5/10) (p < 0.01), including a higher prevalence of the colicin B determinant (p < 0.01) was found. Virulence factor determinants encoding fimbriae (fimA, 91.0%; pap, 27.5%), cytotoxic necrotizing factor (cnf1, 11.2%), aerobactin synthesis (aer, 43.3%), and the locus associated with invasivity (ial, 9.0%) were more prevalent in IBD E. coli (p < 0.05 for all five determinants). E. coli isolates from IBD mucosal biopsies were more frequently bacteriocinogenic (84.6%, p < 0.01) compared to fecal IBD isolates and fecal commensal E. coli. PFGE analysis revealed clusters specific for IBD E. coli isolates (n = 11), for fecal isolates (n = 13), and clusters containing both IBD and fecal isolates (n = 10). ExPEC (Extraintestinal Pathogenic E. coli) virulence and colicin determinants appear to be important characteristics of IBD E. coli isolates, especially the E. coli isolates obtained directly from biopsy samples.

Modulation of Host Cell Processes by T3SS Effectors

Two of the enteric Escherichia coli pathotypes-enteropathogenic E. coli (EPEC) and enterohaemorrhagic E. coli (EHEC)-have a conserved type 3 secretion system which is essential for virulence. The T3SS is used to translocate between 25 and 50 bacterial proteins directly into the host cytosol where they manipulate a variety of host cell processes to establish a successful infection. In this chapter, we discuss effectors from EPEC/EHEC in the context of the host proteins and processes that they target-the actin cytoskeleton, small guanosine triphosphatases and innate immune signalling pathways that regulate inflammation and cell death. Many of these translocated proteins have been extensively characterised, which has helped obtain insights into the mechanisms of pathogenesis of these bacteria and also understand the host pathways they target in more detail. With increasing knowledge of the positive and negative regulation of host signalling pathways by different effectors, a future challenge is to investigate how the specific effector repertoire of each strain cooperates over the course of an infection.

Strategizing for the purification of a multiple Big domain-containing protein in native conformation is worth it!

The reliability and accuracy of conformational or functional studies of any novel multidomain protein rely on the quality of protein. The bottleneck in structural studies with the complete Big_2 domain containing proteins like LigA, LigB or MpIBP is usually their large molecular size owing to their multidomain (>10-12 domains) architectures. Interestingly, a soil bacterium Paenarthrobacter aurescens TC1, harbours a gene that encodes a protein comprising of four predicted Big_2 domains. We report here the expression and purification of this novel, multiple Big_2 domains containing protein, Arig of P. aurescens TC1. During overexpression, recombinant Arig formed inclusion bodies and hence was purified by on-column refolding. The refolded Arig revealed a β-sheet conformation and a well-resolved near-UV CD spectra but did not exhibit a well-dispersed 2D [¹H-¹⁵N]-HSQC NMR spectrum, as expected for a well-folded β-sheet native conformation. We, therefore, further optimized Arig overexpression in the soluble fraction by including osmolytes. CD spectroscopic and 2D [¹H-¹⁵N]-HSQC analyses consolidate that Arig purified alternatively has a well-folded native conformation. While we describe different strategies for purification of Arig, we also present the spectral properties of this novel all-β-sheet protein.

Molecular characterization of locus of enterocyte effacement pathogenicity island in shigatoxic Escherichia coli isolated from human & cattle in West Bengal, India

BACKGROUND & OBJECTIVES

Shigatoxic Escherichia coli (STEC) recovered from dairy animals of Kolkata, India, harboured the putative virulence genes; however, the animals did not exhibit clinical symptoms. Similarly, human isolates in this locality also showed variations in degree of symptoms. Hence, this study was designed to know the presence of recognized gene(s) in the locus of enterocyte effacement (LEE) pathogenicity island in these STEC isolates and functional status of the cardinal gene (eae) related to pathogenicity.

METHODS

Genes were characterized using polymerase chain reaction (PCR) assays, and functional status of cardinal gene (eae) was evaluated by fluorescent actin staining (FAS) assay. Variation in eae gene was determined by intimin PCR.

RESULTS

Cattle STEC isolates carried 22 genes in LEE pathogenicity island in different frequencies ranging from 5.63 to 47.88 per cent of the isolates. In human isolates, the genes namely ler, escRSTU, orf 2, esc C, esc V, orf 3 and tir that are associated with secretory function, were found to be absent and rest of the genes were present in lower frequency. Further, the cardinal gene (eae) responsible for initiation of pathogenesis was in a very low frequency in human (n=2; 10.5%) and cattle (n=11; 15.5%) isolates. None of theseeae+ STEC isolates from human and cattle revealed positivity in FAS assay.

INTERPRETATION & CONCLUSIONS

Majority of human STEC isolates lacked the cardinal virulence gene (eae), and genes for secretory function that are essential for facilitating pathogenesis. This may partially be attributed to low occurrence of STEC in human clinical diarrhoea in this area. Although a few isolates (11 of 71) from cattle had eae gene, they did not express phenotypically. This could be one of the reasons for not appearing of clinical symptoms in the hosts.

The bacterial arginine glycosyltransferase effector NleB preferentially modifies Fas-associated death domain protein (FADD)

The inhibition of host innate immunity pathways is essential for the persistence of attaching and effacing pathogens such as enteropathogenic Escherichia coli (EPEC) and Citrobacter rodentium during mammalian infections. To subvert these pathways and suppress the antimicrobial response, attaching and effacing pathogens use type III secretion systems to introduce effectors targeting key signaling pathways in host cells. One such effector is the arginine glycosyltransferase NleB1 (NleB^CR in C. rodentium) that modifies conserved arginine residues in death domain-containing host proteins with N-acetylglucosamine (GlcNAc), thereby blocking extrinsic apoptosis signaling. Ectopically expressed NleB1 modifies the host proteins Fas-associated via death domain (FADD), TNFRSF1A-associated via death domain (TRADD), and receptor-interacting serine/threonine protein kinase 1 (RIPK1). However, the full repertoire of arginine GlcNAcylation induced by pathogen-delivered NleB1 is unknown. Using an affinity proteomic approach for measuring arginine-GlcNAcylated glycopeptides, we assessed the global profile of arginine GlcNAcylation during ectopic expression of NleB1, EPEC infection in vitro, or C. rodentium infection in vivo NleB overexpression resulted in arginine GlcNAcylation of multiple host proteins. However, NleB delivery during EPEC and C. rodentium infection caused rapid and preferential modification of Arg¹¹⁷ in FADD. This FADD modification was extremely stable and insensitive to physiological temperatures, glycosidases, or host cell degradation. Despite its stability and effect on the inhibition of apoptosis, arginine GlcNAcylation did not elicit any proteomic changes, even in response to prolonged NleB1 expression. We conclude that, at normal levels of expression during bacterial infection, NleB1/NleB^CR antagonizes death receptor-induced apoptosis of infected cells by modifying FADD in an irreversible manner.

The L-Arginine Transporter Solute Carrier Family 7 Member 2 Mediates the Immunopathogenesis of Attaching and Effacing Bacteria

Solute carrier family 7 member 2 (SLC7A2) is an inducible transporter of the semi-essential amino acid L-arginine (L-Arg), which has been implicated in immune responses to pathogens. We assessed the role of SLC7A2 in murine infection with Citrobacter rodentium, an attaching and effacing enteric pathogen that causes colitis. Induction of SLC7A2 was upregulated in colitis tissues, and localized predominantly to colonic epithelial cells. Compared to wild-type mice, Slc7a2^–/–mice infected with C. rodentium had improved survival and decreased weight loss, colon weight, and histologic injury; this was associated with decreased colonic macrophages, dendritic cells, granulocytes, and Th1 and Th17 cells. In infected Slc7a2^–/–mice, there were decreased levels of the proinflammatory cytokines G-CSF, TNF-α, IL-1α, IL-1β, and the chemokines CXCL1, CCL2, CCL3, CCL4, CXCL2, and CCL5. In bone marrow chimeras, the recipient genotype drove the colitis phenotype, indicative of the importance of epithelial, rather than myeloid SLC7A2. Mice lacking Slc7a2 exhibited reduced adherence of C. rodentium to the colonic epithelium and decreased expression of Talin-1, a focal adhesion protein involved in the attachment of the bacterium. The importance of SLC7A2 and Talin-1 in the intimate attachment of C. rodentium and induction of inflammatory response was confirmed in vitro, using conditionally-immortalized young adult mouse colon (YAMC) cells with shRNA knockdown of Slc7a2 or Tln1. Inhibition of L-Arg uptake with the competitive inhibitor, L-lysine (L-Lys), also prevented attachment of C. rodentium and chemokine expression. L-Lys and siRNA knockdown confirmed the role of L-Arg and SLC7A2 in human Caco-2 cells co-cultured with enteropathogenic Escherichia coli. Overexpression of SLC7A2 in human embryonic kidney cells increased bacterial adherence and chemokine expression. Taken together, our data indicate that C. rodentium enhances its own pathogenicity by inducing the expression of SLC7A2 to favor its attachment to the epithelium and thus create its ecological niche.

Intestinal infections by attaching and effacing (A/E) bacteria widely impact human health, with major social and economic repercussions. Mucosal immunity plays a critical role in determining the outcome of these infections. The amino acid L-arginine regulates inflammatory responses to bacterial pathogens. We studied the role of the L-arginine transporter solute carrier family 7 member 2 (SLC7A2) during infection with the A/E pathogen Citrobacter rodentium. SLC7A2 is induced in colonic epithelial cells during the infection and facilitates the intimate attachment of the bacteria, thus initiating the inflammatory response of the infected mucosa. These data were confirmed in vitro using C. rodentium-infected mouse cells and human colonic epithelial cells infected with enteropathogenic Escherichia coli. Our work describes a mechanism by which A/E bacteria manipulate host response to favor their colonization, thereby positioning SLC7A2 as an unrecognized therapeutic target to limit infection with enterobacteria.

The Locus of Enterocyte Effacement and Associated Virulence Factors of Enterohemorrhagic Escherichia coli

A subset of Shiga toxin-producing Escherichia coli strains, termed enterohemorrhagic E. coli (EHEC), is defined in part by the ability to produce attaching and effacing (A/E) lesions on intestinal epithelia. Such lesions are characterized by intimate bacterial attachment to the apical surface of enterocytes, cytoskeletal rearrangements beneath adherent bacteria, and destruction of proximal microvilli. A/E lesion formation requires the locus of enterocyte effacement (LEE), which encodes a Type III secretion system that injects bacterial proteins into host cells. The translocated proteins, termed effectors, subvert a plethora of cellular pathways to the benefit of the pathogen, for example, by recruiting cytoskeletal proteins, disrupting epithelial barrier integrity, and interfering with the induction of inflammation, phagocytosis, and apoptosis. The LEE and selected effectors play pivotal roles in intestinal persistence and virulence of EHEC, and it is becoming clear that effectors may act in redundant, synergistic, and antagonistic ways during infection. Vaccines that target the function of the Type III secretion system limit colonization of reservoir hosts by EHEC and may thus aid control of zoonotic infections. Here we review the features and functions of the LEE-encoded Type III secretion system and associated effectors of E. coli O157:H7 and other Shiga toxin-producing E. coli strains.

Expression, purification, crystallization and crystallographic analysis of the N-terminal domain of translocated intimin receptor

Translocated intimin receptor (Tir) is an Escherichia coli-encoded protein that is transported into the host cell through a sophisticated bacterial type III secretion system (T3SS). Tir anchors the infected cell membrane twice using both its N- and C-termini from inside the host cytoplasm for signalling. It plays a key role in enterohemorrhagic Escherichia coli (EHEC) infection, attaching and effacing (A/E) lesions and intracellular signal transduction. Here, the overexpression, purification and crystallization of its N-terminal intracellular domain are reported. The crystal belonged to the orthorhombic space group I4122, with unit-cell parameters a = b = 59.79, c = 183.11 Å. The asymmetric unit contained one molecule, with a solvent content of 51% and a VM of 2.55 Å(3) Da(-1).

Adhesins of Enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) strains induce morphological changes in infected epithelial cells. The resulting attaching and effacing (A/E) lesion is characterized by intimate bacterial adherence to epithelial cells, with microvillus destruction, cytoskeletal rearrangement, and aggregation of host cytoskeletal proteins. This review presents an overview of the adhesion mechanisms used for the colonization of the human gastrointestinal tract by EPEC. The mechanisms underlying EPEC adhesion, prior to and during the formation of the A/E lesion, and the host cytosolic responses to bacterial infection leading to diarrheal disease are discussed.

Tir Triggers Expression of CXCL1 in Enterocytes and Neutrophil Recruitment during Citrobacter rodentium Infection

The hallmarks of enteropathogenic Escherichia coli (EPEC) infection are formation of attaching and effacing (A/E) lesions on mucosal surfaces and actin-rich pedestals on cultured cells, both of which are dependent on the type III secretion system effector Tir. Following translocation into cultured cells and clustering by intimin, Tir Y474 is phosphorylated, leading to recruitment of Nck, activation of N-WASP, and actin polymerization via the Arp2/3 complex. A secondary, weak, actin polymerization pathway is triggered via an NPY motif (Y454). Importantly, Y454 and Y474 play no role in A/E lesion formation on mucosal surfaces following infection with the EPEC-like mouse pathogen Citrobacter rodentium. In this study, we investigated the roles of Tir segments located upstream of Y451 and downstream of Y471 in C. rodentium colonization and A/E lesion formation. We also tested the role that Tir residues Y451 and Y471 play in host immune responses to C. rodentium infection. We found that deletion of amino acids 382 to 462 or 478 to 547 had no impact on the ability of Tir to mediate A/E lesion formation, although deletion of amino acids 478 to 547 affected Tir translocation. Examination of enterocytes isolated from infected mice revealed that a C. rodentium strain expressing Tir_Y451A/Y471A recruited significantly fewer neutrophils to the colon and triggered less colonic hyperplasia on day 14 postinfection than the wild-type strain. Consistently, enterocytes isolated from mice infected with C. rodentium expressing Tir_Y451A/Y471A expressed significantly less CXCL1. These result show that Tir-induced actin remodeling plays a direct role in modulation of immune responses to C. rodentium infection.

EHEC Adhesins

Adhesins are a group of proteins in enterohemorrhagic Escherichia coli (EHEC) that are involved in the attachment or colonization of this pathogen to abiotic (plastic or steel) and biological surfaces, such as those found in bovine and human intestines. This review provides the most up-to-date information on these essential adhesion factors, summarizing important historical discoveries and analyzing the current and future state of this research. In doing so, the proteins intimin and Tir are discussed in depth, especially regarding their role in the development of attaching and effacing lesions and in EHEC virulence. Further, a series of fimbrial proteins (Lpf1, Lpf2, curli, ECP, F9, ELF, Sfp, HCP, and type 1 fimbriae) are also described, emphasizing their various contributions to adherence and colonization of different surfaces and their potential use as genetic markers in detection and classification of different EHEC serotypes. This review also discusses the role of several autotransporter proteins (EhaA-D, EspP, Saa and Sab, and Cah), as well as other proteins associated with adherence, such as flagella, EibG, Iha, and OmpA. While these proteins have all been studied to varying degrees, all of the adhesins summarized in this chapter have been linked to different stages of the EHEC life cycle, making them good targets for the development of more effective diagnostics and therapeutics.

Citrobacter rodentium: infection, inflammation and the microbiota

Citrobacter rodentium is a mucosal pathogen of mice that shares several pathogenic mechanisms with enteropathogenic Escherichia coli (EPEC) and enterohaemorrhagic E. coli (EHEC), which are two clinically important human gastrointestinal pathogens. Thus, C. rodentium has long been used as a model to understand the molecular basis of EPEC and EHEC infection in vivo. In this Review, we discuss recent studies in which C. rodentium has been used to study mucosal immunology, including the deregulation of intestinal inflammatory responses during bacteria-induced colitis and the role of the intestinal microbiota in mediating resistance to colonization by enteric pathogens. These insights should help to elucidate the roles of mucosal inflammatory responses and the microbiota in the virulence of enteric pathogens.

The ability of an attaching and effacing pathogen to trigger localized actin assembly contributes to virulence by promoting mucosal attachment

Enterohaemorrhagic Escherichia coli (EHEC) colonizes the intestine and causes bloody diarrhoea and kidney failure by producing Shiga toxin. Upon binding intestinal cells, EHEC triggers a change in host cell shape, generating actin 'pedestals' beneath bound bacteria. To investigate the importance of pedestal formation to disease, we infected genetically engineered mice incapable of supporting pedestal formation by an EHEC-like mouse pathogen, or wild type mice with a mutant of that pathogen incapable of generating pedestals. We found that pedestal formation promotes attachment of bacteria to the intestinal mucosa and vastly increases the severity of Shiga toxin-mediated disease.

Estimating the prevalence of potential enteropathogenic Escherichia coli and intimin gene diversity in a human community by monitoring sanitary sewage

Presently, the understanding of bacterial enteric diseases in the community and their virulence factors relies almost exclusively on clinical disease reporting and examination of clinical pathogen isolates. This study aimed to investigate the feasibility of an alternative approach that monitors potential enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC) prevalence and intimin gene (eae) diversity in a community by directly quantifying and characterizing target virulence genes in the sanitary sewage. The quantitative PCR (qPCR) quantification of the eae, stx1, and stx2 genes in sanitary sewage samples collected over a 13-month period detected eae in all 13 monthly sewage samples at significantly higher abundance (93 to 7,240 calibrator cell equivalents [CCE]/100 ml) than stx1 and stx2, which were detected sporadically. The prevalence level of potential EPEC in the sanitary sewage was estimated by calculating the ratio of eae to uidA, which averaged 1.0% (σ = 0.4%) over the 13-month period. Cloning and sequencing of the eae gene directly from the sewage samples covered the majority of the eae diversity in the sewage and detected 17 unique eae alleles belonging to 14 subtypes. Among them, eae-β2 was identified to be the most prevalent subtype in the sewage, with the highest detection frequency in the clone libraries (41.2%) and within the different sampling months (85.7%). Additionally, sewage and environmental E. coli isolates were also obtained and used to determine the detection frequencies of the virulence genes as well as eae genetic diversity for comparison.

Comparative genomics reveals that a fish pathogenic bacterium Edwardsiella tarda has acquired the locus of enterocyte effacement (LEE) through horizontal gene transfer

Background

Edwardsiella tarda is an enterobacterium which causes edwardsiellosis, a fatal disease of cultured fishes such as red sea bream, eel, and flounder. Preventing the occurrence of E. tarda infection has thus been an important issue in aquaculture. E. tarda has been isolated from other animals and from many environments; however, the relationship between the genotype and evolutionary process of this pathogen is not fully understood. To clarify this relationship, we sequenced and compared the genomes of pathogenic and non-pathogenic E. tarda strains isolated from fish, human, and eel pond using next-generation sequencing technology.

Results

Eight strains of E. tarda were sequenced with high accuracy (>99.9%) with coverages from 50- to 400-fold. The obtained reads were mapped to a public reference genome. By comparing single nucleotide and insertion/deletion polymorphisms, we found that an attenuated strain of E. tarda had a loss-of-function mutation in a gene related to the type III secretion system (T3SS), suggesting that this gene is involved in the virulence of E. tarda. A comprehensive gene comparison indicated that fish pathogenic strains possessed a type VI secretion system (T6SS) and pilus assembly genes in addition to the T3SS. Moreover, we found that an E. tarda strain isolated from red sea bream harbored two pathogenicity islands of T3SS and T6SS, which were absent in other strains. In particular, this T3SS was homologous to the locus of enterocyte effacement (LEE) in enteropathogenic and enterohemorrhagic Escherichia coli. Evolutionary analysis suggested that this locus, here named Et-LEE (E. tarda LEE), was introgressed into the E. tarda genome through horizontal transfer.

Conclusions

We found significant differences in the presence/absence of virulence-related genes among E. tarda strains, reflecting their evolutionary relationship. In particular, a single genotype previously proposed for fish-pathogenic strains may be further divided into two subgroups. Furthermore, the current study demonstrated, for the first time, that a fish pathogenic bacterium carried a LEE-like pathogenicity island which was previously reported only in zoonotic pathogenic enterobacteria. These findings will contribute to the exploration of strain-specific drug targets against E. tarda in aquafarms, while also shedding light on the evolution of pathogenesis in enterobacteria.

Exploitation of eukaryotic subcellular targeting mechanisms by bacterial effectors

Several bacterial species have evolved specialized secretion systems to deliver bacterial effector proteins into eukaryotic cells. These effectors have the capacity to modulate host cell pathways in order to promote bacterial survival and replication. The spatial and temporal context in which the effectors exert their biochemical activities is crucial for their function. To fully understand effector function in the context of infection, we need to understand the mechanisms that lead to the precise subcellular localization of effectors following their delivery into host cells. Recent studies have shown that bacterial effectors exploit host cell machinery to accurately target their biochemical activities within the host cell.

Autotransporter secretion: varying on a theme

Autotransporters are widely distributed among Gram-negative bacteria. They can have a large variety of functions and many of them have a role in virulence. They are synthesized as large precursors with an N-terminal signal sequence that mediates transport across the inner membrane via the Sec machinery and a translocator domain that mediates the transport of the connected passenger domain across the outer membrane to the bacterial cell surface. Like integral outer membrane proteins, the translocator domain folds in a β-barrel structure and requires the Bam machinery for its insertion into the outer membrane. After transport across the outer membrane, the passenger may stay connected via the translocator domain to the bacterial cell surface or it is proteolytically released into the extracellular milieu. Based on the size of the translocator domain and its position relative to the passenger in the precursor, autotransporters are divided into four sub-categories. We review here the current knowledge of the biogenesis, structure and function of various autotransporters.

A novel murine infection model for Shiga toxin-producing Escherichia coli

Enterohemorrhagic E. coli (EHEC) is an important subset of Shiga toxin-producing (Stx-producing) E. coli (STEC), pathogens that have been implicated in outbreaks of food-borne illness and can cause intestinal and systemic disease, including severe renal damage. Upon attachment to intestinal epithelium, EHEC generates "attaching and effacing" (AE) lesions characterized by intimate attachment and actin rearrangement upon host cell binding. Stx produced in the gut transverses the intestinal epithelium, causing vascular damage that leads to systemic disease. Models of EHEC infection in conventional mice do not manifest key features of disease, such as AE lesions, intestinal damage, and systemic illness. In order to develop an infection model that better reflects the pathogenesis of this subset of STEC, we constructed an Stx-producing strain of Citrobacter rodentium, a murine AE pathogen that otherwise lacks Stx. Mice infected with Stx-producing C. rodentium developed AE lesions on the intestinal epithelium and Stx-dependent intestinal inflammatory damage. Further, the mice experienced lethal infection characterized by histopathological and functional kidney damage. The development of a murine model that encompasses AE lesion formation and Stx-mediated tissue damage will provide a new platform upon which to identify EHEC alterations of host epithelium that contribute to systemic disease.

Sigma factor N, liaison to an ntrC and rpoS dependent regulatory pathway controlling acid resistance and the LEE in enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) is dependent on acid resistance for gastric passage and low oral infectious dose, and the locus of enterocyte effacement (LEE) for intestinal colonization. Mutation of rpoN, encoding sigma factor N (σ(N)), dramatically alters the growth-phase dependent regulation of both acid resistance and the LEE. This study reports on the determinants of σ(N)-directed acid resistance and LEE expression, and the underlying mechanism attributable to this phenotype. Glutamate-dependent acid resistance (GDAR) in TW14359ΔrpoN correlated with increased expression of the gadX-gadW regulatory circuit during exponential growth, whereas upregulation of arginine-dependent acid resistance (ADAR) genes adiA and adiC in TW14359ΔrpoN did not confer acid resistance by the ADAR mechanism. LEE regulatory (ler), structural (espA and cesT) and effector (tir) genes were downregulated in TW14359ΔrpoN, and mutation of rpoS encoding sigma factor 38 (σ(S)) in TW14359ΔrpoN restored acid resistance and LEE genes to WT levels. Stability, but not the absolute level, of σ(S) was increased in TW14359ΔrpoN; however, increased stability was not solely attributable to the GDAR and LEE expression phenotype. Complementation of TW14359ΔrpoN with a σ(N) allele that binds RNA polymerase (RNAP) but not DNA, did not restore WT levels of σ(S) stability, gadE, ler or GDAR, indicating a dependence on transcription from a σ(N) promoter(s) and not RNAP competition for the phenotype. Among a library of σ(N) enhancer binding protein mutants, only TW14359ΔntrC, inactivated for nitrogen regulatory protein NtrC, phenocopied TW14359ΔrpoN for σ(S) stability, GDAR and ler expression. The results of this study suggest that during exponential growth, NtrC-σ(N) regulate GDAR and LEE expression through downregulation of σ(S) at the post-translational level; likely by altering σ(S) stability or activity. The regulatory interplay between NtrC, other EBPs, and σ(N)-σ(S), represents a mechanism by which EHEC can coordinate GDAR, LEE expression and other cellular functions, with nitrogen availability and physiologic stimuli.

Escherichia coli virulence factors

Escherichia coli was described in 1885 by a German pediatrician, Theodor Escherich, in the faeces of a child suffering diarrhoea. In 1893, a Danish veterinarian postulated that the E. coli species comprises different strains, some being pathogens, others not. Today the E. coli species is subdivided into several pathogenic strains causing different intestinal, urinary tract or internal infections and pathologies, in animal species and in humans. Since this congress topic is the interaction between E. coli and the mucosal immune system, the purpose of this manuscript is to present different classes of adhesins (fimbrial adhesins, afimbrial adhesins and outer membrane proteins), the type 3 secretion system, and some toxins (oligopeptide, AB, and RTX pore-forming toxins) produced by E. coli, that can directly interact with the epithelial cells of the intestinal, respiratory and urinary tracts.

Recruitment and membrane interactions of host cell proteins during attachment of enteropathogenic and enterohaemorrhagic Escherichia coli

EPEC (enteropathogenic Escherichia coli) and EHEC (enterohaemorrhagic Escherichia coli) are attaching and effacing pathogens frequently associated with infectious diarrhoea. EPEC and EHEC use a T3SS (type III secretion system) to translocate effectors that subvert different cellular processes to sustain colonization and multiplication. The eukaryotic proteins NHERF2 (Na(+)/H(+) exchanger regulatory factor 2) and AnxA2 (annexin A2), which are involved in regulation of intestinal ion channels, are recruited to the bacterial attachment sites. Using a stable HeLa-NHERF2 cell line, we found partial co-localization of AnxA2 and NHERF2; in EPEC-infected cells, AnxA2 and NHERF2 were extensively recruited to the site of bacterial attachment. We confirmed that NHERF2 dimerizes and found that NHERF2 interacts with AnxA2. Moreover, we found that AnxA2 also binds both the N- and C-terminal domains of the bacterial effector Tir through its C-terminal domain. Immunofluorescence of HeLa cells infected with EPEC showed that AnxA2 is recruited to the site of bacterial attachment in a Tir-dependent manner, but independently of Tir-induced actin polymerization. Our results suggest that AnxA2 and NHERF2 form a scaffold complex that links adjacent Tir molecules at the plasma membrane forming a lattice that could be involved in retention and dissemination of other effectors at the bacterial attachment site.

The enteropathogenic E. coli effector EspH promotes actin pedestal formation and elongation via WASP-interacting protein (WIP)

Enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC) are diarrheagenic pathogens that colonize the gut mucosa via attaching-and-effacing lesion formation. EPEC and EHEC utilize a type III secretion system (T3SS) to translocate effector proteins that subvert host cell signalling to sustain colonization and multiplication. EspH, a T3SS effector that modulates actin dynamics, was implicated in the elongation of the EHEC actin pedestals. In this study we found that EspH is necessary for both efficient pedestal formation and pedestal elongation during EPEC infection. We report that EspH induces actin polymerization at the bacterial attachment sites independently of the Tir tyrosine residues Y474 and Y454, which are implicated in binding Nck and IRSp53/ITRKS respectively. Moreover, EspH promotes recruitment of neural Wiskott-Aldrich syndrome protein (N-WASP) and the Arp2/3 complex to the bacterial attachment site, in a mechanism involving the C-terminus of Tir and the WH1 domain of N-WASP. Dominant negative of WASP-interacting protein (WIP), which binds the N-WASP WH1 domain, diminished EspH-mediated actin polymerization. This study implicates WIP in EPEC-mediated actin polymerization and pedestal elongation and represents the first instance whereby N-WASP is efficiently recruited to the EPEC attachment sites independently of the Tir:Nck and Tir:IRTKS/IRSp53 pathways. Our study reveals the intricacies of Tir and EspH-mediated actin signalling pathways that comprise of distinct, convergent and synergistic signalling cascades.

Highly selective trapping of enteropathogenic E. coli on Fabry-Pérot sensor mirrors

Untreated recycled water, such as sewage and graywater, will almost always contain a wide range of agents that are likely to present risks to human health, including chemicals and pathogenic microorganisms. The microbial hazards, such as large numbers of enteric pathogens that can cause gastroenteric illness if ingested, are the main cause of concern for human health. The presence of the enteropathogenic Escherichia coli (EPEC) serotype is of particular concern, as this group of bacteria is responsible for causing severe infant and travelers' diarrhea, gastroenteritis and hemolytic uremic syndrome. A biosensing system based on an optical Fabry-Pérot (FP) cavity, capable of directly detecting the presence of EPEC within 5 min, has been developed using a simple micro-thin double-sided adhesive tape and two semi-transparent FP mirror plates. The system utilizes a poly(methyl methacrylate) (PMMA) or glass substrates sputtered by 40-nm-thick gold thin films serving as FP mirrors. Mirrors have been activated using 0.1M mercaptopropionic acid, influencing an immobilization density of the translocated intimin receptor (TIR) of 100 ng/cm(2). The specificity of recognition was confirmed by exposing TIR functionalized surfaces to four taxonomically related and/or distantly related bacterial strains. It was found that the TIR-functionalized surfaces did not show any bacterial capture for these other bacterial strains within a 15 min incubation period.

O157:H7 and O104:H4 Vero/Shiga toxin-producing Escherichia coli outbreaks: respective role of cattle and humans

An enteroaggregative Verotoxin (Vtx)-producing Escherichia coli strain of serotype O104:H4 has recently been associated with an outbreak of haemolytic-uremic syndrome and bloody diarrhoea in humans mainly in Germany, but also in 14 other European countries, USA and Canada. This O104:H4 E. coli strain has often been described as an enterohaemorrhagic E. coli (EHEC), i.e. a Vtx-producing E. coli with attaching and effacing properties. Although both EHEC and the German O104:H4 E. coli strains indeed produce Vtx, they nevertheless differ in several other virulence traits, as well as in epidemiological characteristics. For instance, the primary sources and vehicles of typical EHEC infections in humans are ruminants, whereas no animal reservoir has been identified for enteroaggregative E. coli (EAggEC). The present article is introduced by a brief overview of the main characteristics of Vtx-producing E. coli and EAggEC. Thereafter, the O104:H4 E. coli outbreak is compared to typical EHEC outbreaks and the virulence factors and host specificity of EHEC and EAggEC are discussed. Finally, a renewed nomenclature of Vtx-producing E. coli is proposed to avoid more confusion in communication during future outbreaks and to replace the acronym EHEC that only refers to a clinical condition.

Escherichia coli-induced epithelial hyporesponsiveness to secretagogues is associated with altered CFTR localization

Both pathogenic and commensal strains of Escherichia coli colonize the human intestinal tract. Pathogenic strains differ only in the expression of virulence factors, many of which comprise a type III secretion system (TTSS). Little is known regarding the effect of E. coli on the intestinal epithelial response to the secretagogues that drive ion secretion, despite its importance in causing clinically significant diarrhoea. Using Ussing chambers to measure electrogenic ion transport of T84 intestinal epithelial cell monolayers, we found that all strains of E. coli tested (pathogenic, commensal, probiotic and lab strain) significantly reduced cAMP-dependent ion secretion after 4-8 h exposure. Enteropathogenic E. coli mutants lacking a functional TTSS caused similar hyposecretion while not causing significant apoptosis (as shown by caspase-3 cleavage) or necrosis (lactate dehydrogenase release), as did the commensal strain F18, indicating that epithelial cell death was not the cause of hyposecretion. Enteropathogenic E. coli and the TTSS mutant significantly reduced cell surface expression of the apical anion channel, cystic fibrosis transmembrane conductance regulator, which is likely the mechanism behind the pathogen-induced hyposecretion. However, F18 did not cause cystic fibrosis transmembrane conductance regulator mislocalization and the commensal-induced mechanism remains unclear.

THE ROLE OF SPECIFIC IgG AND COMPLEMENT IN COMBATING A PRIMARY MUCOSAL INFECTION OF THE GUT EPITHELIUM

The role of complement and complement-fixing IgG isotypes at mucosal surfaces is ill defined. Previous data have demonstrated that survival of an infection with the attaching and effacing pathogen Citrobacter rodentium requires production of systemic and CD4+ T cell-dependent IgG. We have found that both complement and complement-fixing IgG isotypes are needed to survive a C. rodentium infection. Our results indicate that both IgG and complement C3b enter the gut lumen and bind epithelially adherent, and fecally shed C. rodentium. Furthermore, C3-deficient mice demonstrate a profound survival defect, though means to replenish C3 in systemic or mucosal sites restores the protective capacity of complement in the host. Our data provide evidence that both IgG and complement interact constructively on both sides of the epithelium to fight colonizing mucosal infections.

Immunogenicity of a plant-derived edible chimeric EspA, Intimin and Tir of Escherichia coli O157:H7 in mice

Transgenic plants offer the possibility to produce and deliver an oral immunogen on a large-scale with low production costs and minimal purification or enrichment. Cattles are important reservoirs of Escherichia coli O157:H7 and developing a specific immunity in animals would be invaluable. Intimin, Tir, and EspA proteins are the virulence factors expressed by LEE locus of enterohemorrhagic E. coli. We hypothesized that the chimeric recombinant forms of these effectors delivered as an edible-base vaccine would reduce colonization of bacteria in mice. A synthetic gene (eit) composed of espA (e), eae (i) and tir (t) attached by linkers was constructed. The gene was codon optimized and cloned into plant expression vectors adjacent to CaMV35S and FAE promoters for expression in tobacco and canola plants. Of total soluble protein 0.2% and 0.3% (in average) were detected in transgenic tobacco leaves and canola seeds respectively. Mice immunized either subcutaneously or orally with recombinant EIT and challenged with E. coli O157:H7 significantly exhibited reduced bacterial shedding. Application of transgenic plants containing trivalent immunogen is an effective tool for protection against E. coli O157:H7.

A type III effector protease NleC from enteropathogenic Escherichia coli targets NF-κB for degradation

Many bacterial pathogens utilize a type III secretion system (T3SS) to inject virulence effector proteins into host cells during infection. Previously, we found that enteropathogenic Escherichia coli (EPEC) uses the type III effector, NleE, to block the inflammatory response by inhibiting IκB degradation and nuclear translocation of the p65 subunit of NF-κB. Here we screened further effectors with unknown function for their capacity to prevent p65 nuclear translocation. We observed that ectopic expression of GFP–NleC in HeLa cells led to the degradation of p65. Delivery of NleC by the T3SS of EPEC also induced degradation of p65 in infected cells as well as other NF-κB components, c-Rel and p50. Recombinant His⁶-NleC induced p65 and p50 cleavage in HeLa cell lysates and mutation of a consensus zinc metalloprotease motif, HEIIH, abrogated NleC proteolytic activity. NleC inhibited IL-8 production during prolonged EPEC infection of HeLa cells in a protease activity-dependent manner. A double nleE/nleC mutant was further impaired for its ability to inhibit IL-8 secretion than either a single nleE or a single nleC mutant. We conclude that NleC is a type III effector protease that degrades NF-κB thereby contributing the arsenal of bacterial effectors that inhibit innate immune activation.

Inhibition of NF-κB signaling in human dendritic cells by the enteropathogenic Escherichia coli effector protein NleE

Intestinal dendritic cells (DCs) send processes between epithelial cells into the gut lumen to sample pathogens. Noninvasive enteropathogenic Escherichia coli (EPEC) colonize the gut using a type three secretion system (T3SS) to inject effector proteins into epithelial cells. We hypothesized that EPEC might also inject proteins into DC processes to dampen immune recognition. Using a T3SS-linked fluorescence resonance energy transfer-based system we show that EPEC injects effectors into in vitro grown human myeloid DCs. Injected cells emit a blue signal due to cleavage of the green fluorescence resonance energy transfer-based substrate CCF2/AM by β-lactamase. When cultured with a mutant EPEC unable to translocate effector proteins, myeloid DCs show rapid activation of NF-κB, secrete large amounts of proinflammatory cytokines and increase expression of CD80, CD83, and CD86, whereas wild-type EPEC barely elicits cytokine production and shuts off nuclear translocation of NF-κB p65. By deleting effector protein genes, we identified NleE as being critical for this effect. Expression of NleE in HeLa cells completely prevented nuclear p65 accumulation in response to IL1-β, and luciferase production in an NF-κB reporter cell line. DCs cocultured with wild-type EPEC or NleE-complemented strains were less potent at inducing MLR. EPEC was also able to inject effectors into DCs sending processes through model gut epithelium in a transwell system and into Peyer's patch myeloid DCs. Thus, EPEC translocate effectors into human DCs to dampen the inflammatory response elicited by its own pathogen-associated molecular patterns.

In silico analysis of chimeric espA, eae and tir fragments of Escherichia coli O157:H7 for oral immunogenic applications

Background

In silico techniques are highly suited for both the discovery of new and development of existing vaccines. Enterohemorrhagic Escherichia coli O157:H7 (EHEC) exhibits a pattern of localized adherence to host cells, with the formation of microcolonies, and induces a specific histopathological lesion (attaching/effacing). The genes encoding the products responsible for this phenotype are clustered on a 35-kb pathogenicity island. Among these proteins, Intimin, Tir, and EspA, which are expressed by attaching-effacing genes, are responsible for the attachment to epithelial cell that leads to lesions.

Results

We designed synthetic genes encoding the carboxy-terminal fragment of Intimin, the middle region of Tir and the carboxy-terminal part of EspA. These multi genes were synthesized with codon optimization for a plant host and were fused together by the application of four repeats of five hydrophobic amino acids as linkers. The structure of the synthetic construct gene, its mRNA and deduced protein and their stabilities were analyzed by bioinformatic software. Furthermore, the immunogenicity of this multimeric recombinant protein consisting of three different domains was predicted.

Conclusion

a structural model for a chimeric gene from LEE antigenic determinants of EHEC is presented. It may define accessibility, solubility and immunogenecity.