Pathogenic Escherichia coli

Assembly of CS1 pili: the role of specific residues of the major pilin, CooA

CS1 pili are important virulence factors of enterotoxigenic Escherichia coli strains associated with human diarrheal disease. They are the prototype for a family of pili that share extensive sequence similarity among their structural and assembly proteins. Only four linked genes, cooB, cooA, cooC, and cooD, are required to produce CS1 pili in E. coli K-12. To identify amino acids important for the function of the major pilin CooA, we used alanine substitution mutagenesis targeting conserved residues in the N and C termini of the protein. To test function, we examined cooA mutants for the ability to agglutinate bovine erythrocytes. Each hemagglutination-negative (HA(-)) cooA mutant was examined to identify its assembly pathway defect. CooA has been shown to be degraded in the absence of CooB (K. Voegele, H. Sakellaris, and J. R. Scott, Proc. Natl. Acad. Sci. USA 94:13257-13261, 1997). We found several HA(-) cooA mutants that produced no detectable CooA, suggesting that recognition by CooB is mediated by residues in both the N and C termini of CooA. In addition, we found that alanine substitution for some of the conserved residues in the C-terminal motif "AGxYxG(x(6))T," which is found in all subunits of this pilus family, had no effect on pilus formation. However, alanine substitution for some of the alternating hydrophobic residues within this motif prevented CooA from interacting with CooD, which serves as both the tip adhesin and nucleation protein for pilus formation. Thus, it appears that some, but not all, of the residues in both the N and C termini of CooA play a critical role in the intermolecular interactions of the major pilin with the other structural and assembly proteins. We anticipate that the results obtained here for CS1 pili in enterotoxigenic E. coli will help develop an understanding of the pilus assembly pathway used by CS1 family members in several important human pathogens.

Spontaneous recombination between homologous prophage regions causes large-scale inversions within the Escherichia coli O157:H7 chromosome

It is known that XbaI-digested chromosomal DNAs of strains of Escherichia coli O157:H7 exhibit a wide variety in pulsed-field gel electrophoresis (PFGE) fragment patterns, which is used for epidemiological surveillance of this important pathogen. The variety in the restriction enzyme-digestion patterns suggests a wide genomic diversity, however, only a few studies have been conducted to investigate involvement of large-scale chromosomal rearrangements in development of the diversity. In this study, through rounds of subculturing E. coli O157:H7 strain EDL933, naturally occurring genome variation in the isolated derivatives was investigated. By comparing the PFGE patterns among clonally related derivatives, we found five types of large-scale inversions taking place within the chromosome. The five inversions found were across the replication axis and ranged from 250-kb to 1.4-Mb long, and all the corresponding recombination sites were associated with prophages or phage-like regions. Four inversions out of the five were resulted from recombination between pairs of lambda-like prophages disturbing the symmetry of the origin and terminus of the replication axis. These observations indicate that those prophage regions represent some of the hot spots for intrachromosomal recombination within the E. coli O157:H7 chromosome, where recombination between the prophage regions results not only in the large chromosomal inversions but might also in generation of chimeric phages.

Evolution of foodborne pathogens via temperate bacteriophage-mediated gene transfer

Temperate bacteriophages have always been central to the evolution of bacteria, although their importance has been consistently underestimated compared to transformation and conjugation. In the last 20 years, as more gene and genome sequences have become available and researchers have more accurately determined bacteriophage populations in the environment, we are gaining a clearer picture of their role in the past and potential role in the future. The transductive and lysogenic capacities of this class of bacteriophages have contributed to the evolution and shaping of emerging foodborne pathogenic bacteria through the dissemination of virulence and antibiotic resistance genes. For example, the genome sequences of Shigella dysenteriae, Escherichia coli O157:H7, and the Stxencoding bacteriophages demonstrate the critical role bacteriophage-mediated gene transfer events played in the evolution of these high-profile human pathogens. In this review, we describe the basic genetic exchange mechanisms mediated by temperate bacteriophages and how these mechanisms have been central to the dissemination of virulence genes, such as toxins and antibiotics from one species to another (the shiga-like toxins, and multiple antibiotic resistance dissemination in Salmonella are used as specific examples). Data demonstrating the role of bacteriophages in the spread of antimicrobial resistance in bacteria, including interspecies transduction, are also presented. That temperate bacteriophages play a role in the on-going evolution of emerging pathogenic bacteria is obvious, but it is also clearly an on-going process with a breadth that must be appreciated as well as studied further if we are to be able to foresee what new challenges will arise to imperil food safety.

Quorum sensing in Escherichia coli and Salmonella

Quorum sensing in Escherichia coli and Salmonella has been an elusive topic for a long time. However, in the past 8 years, several research groups have demonstrated that these bacteria use several quorum-sensing systems, such as: the luxS/AI-2, AI-3/epinephrine/norepinephrine, indole, and the LuxR homolog SdiA to achieve intercellular signaling. The majority of these signaling systems are involved in interspecies communication, and the AI-3/epinephrine/norepinephrine signaling system is also involved in interkingdom communication. Both E. coli and Salmonella reside in the human intestine, which is the largest and most complex environment in the mammalian host. The observation that these bacteria evolved quorum-sensing systems primarily involved in interspecies communication may constitute an adaptation to this environment. The gastrointestinal tract harbors a high density and diversity of bacterial cells, with the majority of the flora residing in the colon (10(11)-10(12) bacterial cells/ml). Given the enormous number and diversity of bacteria inhabiting the gastrointestinal environment, it should not be surprising that the members of this community communicate amongst themselves and with the host itself to coordinate a variety of adaptive processes.

The global regulator Ler is necessary for enteropathogenic Escherichia coli colonization of Caenorhabditis elegans

Enteropathogenic Escherichia coli (EPEC) is an important cause of infant diarrhea in developing countries and is useful for general investigations of the bacterial infection process. However, the study of the molecular pathogenesis of EPEC has been hampered by the lack of genetically tractable, convenient animal models. We have therefore developed the use of the nematode Caenorhabditis elegans as a small animal model of infection for this diarrheal pathogen. We found that nematodes died faster on nematode growth medium in the presence of EPEC pathogens than in the presence of the laboratory control strain MG1655. Increased numbers of pathogens in the gut, determined by standard plate count assays and fluorescence microscopy using green fluorescent protein-expressing bacteria, correlated with killing. Deletion of the gene encoding the global regulator Ler severely reduced the ability of EPEC to colonize the nematode gut and could be complemented by providing the ler gene on a multicopy plasmid in trans. Neither the type III secretion system nor the type IV bundle-forming pilus was required for colonization. Combined, the similarities and distinct differences between EPEC infection of nematodes and that of humans offer a unique opportunity to study several stages of the infection process, namely, attachment, colonization, and persistence, in a genetically tractable, inexpensive, and convenient in vivo system.

Role of intimin-tir interactions and the tir-cytoskeleton coupling protein in the colonization of calves and lambs by Escherichia coli O157:H7

Intimin facilitates intestinal colonization by enterohemorrhagic Escherichia coli O157:H7; however, the importance of intimin binding to its translocated receptor (Tir) as opposed to cellular coreceptors is unknown. The intimin-Tir interaction is needed for optimal actin assembly under adherent bacteria in vitro, a process which requires the Tir-cytoskeleton coupling protein (TccP/EspF(U)) in E. coli O157:H7. Here we report that E. coli O157:H7 tir mutants are at least as attenuated as isogenic eae mutants in calves and lambs, implying that the role of intimin in the colonization of reservoir hosts can be explained largely by its binding to Tir. Mutation of tccP uncoupled actin assembly from the intimin-Tir-mediated adherence of E. coli O157:H7 in vitro but did not impair intestinal colonization in calves and lambs, implying that pedestal formation may not be necessary for persistence. However, an E. coli O157:H7 tccP mutant induced typical attaching and effacing lesions in a bovine ligated ileal loop model of infection, suggesting that TccP-independent mechanisms of actin assembly may operate in vivo.

Enterocyte cytoskeleton changes are crucial for enhanced translocation of nonpathogenic Escherichia coli across metabolically stressed gut epithelia

Substantial data implicate the commensal flora as triggers for the initiation of enteric inflammation or inflammatory disease relapse. We have shown that enteric epithelia under metabolic stress respond to nonpathogenic bacteria by increases in epithelial paracellular permeability and bacterial translocation. Here we assessed the structural basis of these findings. Confluent filter-grown monolayers of the human colonic T84 epithelial cell line were treated with 0.1 mM dinitrophenol (which uncouples oxidative phosphorylation) and noninvasive, nonpathogenic Escherichia coli (strain HB101, 10(6) CFU) with or without pretreatment with various pharmacological agents. At 24 h later, apoptosis, tight-junction protein expression, transepithelial resistance (TER; a marker of paracellular permeability), and bacterial internalization and translocation were assessed. Treatment with stabilizers of microtubules (i.e., colchicine), microfilaments (i.e., jasplakinolide) and clathrin-coated pit endocytosis (i.e., phenylarsine oxide) all failed to block DNP+E. coli HB101-induced reductions in TER but effectively prevented bacterial internalization and translocation. Neither the TER defect nor the enhanced bacterial translocations were a consequence of increased apoptosis. These data show that epithelial paracellular and transcellular (i.e., bacterial internalization) permeation pathways are controlled by different mechanisms. Thus, epithelia under metabolic stress increase their endocytotic activity that can result in a microtubule-, microfilament-dependent internalization and transcytosis of bacteria. We speculate that similar events in vivo would allow excess unprocessed antigen and bacteria into the mucosa and could evoke an inflammatory response by, for example, the activation of resident or recruited immune cells.

Evaluation of data from the literature on the transport and survival of Escherichia coli and thermotolerant coliforms in aquifers under saturated conditions

Escherichia coli and thermotolerant coliforms are of major importance as indicators of fecal contamination of water. Due to its negative surface charge and relatively low die-off or inactivation rate coefficient, E. coli is able to travel long distances underground and is therefore also a useful indicator of fecal contamination of groundwater. In this review, the major processes known to determine the underground transport of E. coli (attachment, straining and inactivation) are evaluated. The single collector contact efficiency (SCCE), eta0, one of two parameters commonly used to assess the importance of attachment, can be quantified for E. coli using classical colloid filtration theory. The sticking efficiency, alpha, the second parameter frequently used in determining attachment, varies widely (from 0.003 to almost 1) and mainly depends on charge differences between the surface of the collector and E. coli. Straining can be quantified from geometrical considerations; it is proposed to employ a so-called straining correction parameter, alpha(str). Sticking efficiencies determined from field experiments were lower than those determined under laboratory conditions. We hypothesize that this is due to preferential flow mechanisms, E. coli population heterogeneity, and/or the presence of organic and inorganic compounds in wastewater possibly affecting bacterial attachment characteristics. Of equal importance is the inactivation or die-off of E. coli that is affected by factors like type of bacterial strain, temperature, predation, antagonism, light, soil type, pH, toxic substances, and dissolved oxygen. Modeling transport of E. coli can be separated into three steps: (1) attachment rate coefficients and straining rate coefficients can be calculated from Darcy flow velocity fields or pore water flow velocity fields, calculated SCCE fields, realistic sticking efficiency values and straining correction parameters, (2) together with the inactivation rate coefficient, total rate coefficient fields can be generated, and (3) used as input for modeling the transport of E. coli in existing contaminant transport codes. Areas of future research are manifold and include the effects of typical wastewater characteristics, including high concentrations of organic compounds, on the transport of E. coli and thermotolerant coliforms, and the upscaling of experiments to represent typical field conditions, possibly including preferential flow mechanisms and the aspect of population heterogeneity of E. coli.

Actin-dependent movement of bacterial pathogens

Listeria, Rickettsia, Burkholderia, Shigella and Mycobacterium species subvert cellular actin dynamics to facilitate their movement within the host cytosol and to infect neighbouring cells while evading host immune surveillance and promoting their intracellular survival. 'Attaching and effacing' Escherichia coli do not enter host cells but attach intimately to the cell surface, inducing motile actin-rich pedestals, the function of which is currently unclear. The molecular basis of actin-based motility of these bacterial pathogens reveals novel insights about bacterial pathogenesis and fundamental host-cell pathways.

Enteropathogenic Escherichia coli (EPEC) effector-mediated suppression of antimicrobial nitric oxide production in a small intestinal epithelial model system

In vivo studies with the mouse-specific member of the attaching and effacing (A/E) family of pathogens raised the possibility that these non-invasive enteric pathogens can specifically inhibit inducible nitric oxide synthase (iNOS) expression to prevent the production of antimicrobial nitric oxide (NO). In this study we use polarized Caco-2 cells, a model of human small intestinal epithelia, to (i) demonstrate conclusively that an A/E member, human specific enteropathogenic Escherichia coli (EPEC), can inhibit cytokine-induced iNOS expression, (ii) show that this activity is dependent on the delivery of effector molecules into host cells and (iii) investigate the mechanism of inhibition. Analysis of the level of iNOS-related mRNA, protein and NO production demonstrated that EPEC can inhibit iNOS expression at the transcriptional, by direct and indirect mechanisms, and post-transcriptional levels. This transcriptional block was linked to the failure of the iNOS-related transcriptional factor NF-kappaB, but not STAT1, to undergo phosphorylation-associated activation. A selective pressure to prevent iNOS production was evidenced by the finding that iNOS activity had a potent antimicrobial effect on adherent but not non-adherent bacteria. Moreover, given the central role NF-kappaB plays in transcribing genes associated with early host immune responses, this inhibitory mechanism presumably represents an important role in pathogenesis. Our study also provides insights into the nature of NO production in response to bacterial infection as well as the role of the locus of enterocyte effacement (LEE)-encoded effector molecules in inhibiting iNOS expression.

An avian pathogenic Escherichia coli isolate induces caspase 3/7 activation in J774 macrophages

Avian pathogenic Escherichia coli (APEC) strains, the etiological agent of colibacillosis in poultry, must resist the attack of incoming macrophages in order to cause disease. In this work, we show that an APEC strain (APEC17) remained viable inside J774 macrophages for at least 8 h and was cytotoxic to them 6-8 h after infection. APEC17 induced caspase 3/7 activation, the central caspases in apoptosis, in infected macrophages already at 2h after infection. Both cytotoxicity and caspase 3/7 activation were reduced when cells were infected with heat-killed APEC17, showing that bacteria must be viable to induce apoptosis. Our findings using APEC17 suggest that APEC may escape destruction by triggering macrophage apoptotic death.

BopC is a novel type III effector secreted by Bordetella bronchiseptica and has a critical role in type III-dependent necrotic cell death

In Bordetella bronchiseptica, the functional type III secretion system (TTSS) is required for the induction of necrotic cell death in infected mammalian cells. To identify the factor(s) involved in necrotic cell death, type III-secreted proteins from B. bronchiseptica were analyzed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and electrospray ionization tandem mass spectrometry. We identified a 69-kDa secreted protein designated BopC. The gene encoding BopC is located outside of the TTSS locus and is also highly conserved in both Bordetella parapertussis and Bordetella pertussis. The results of a lactate dehydrogenase release assay and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling assay demonstrated that BopC is required for necrotic cell death. It has been reported that tyrosine-phosphorylated proteins (PY) of host cells are dephosphorylated during B. bronchiseptica infection in a TTSS-dependent manner. We found that BopC is also involved in PY dephosphorylation in infected host cells. It appears that the necrotic cell death triggered by BopC occurs prior to the PY reduction in host cells, because Bordetella-induced cell death was not affected even in the presence of a dephosphorylation inhibitor. Furthermore, a translocation assay showed that the signal sequence for both secretion into culture supernatant and translocation into the host cell is located in 48 amino acid residues of the BopC N terminus. This report reveals for the first time that a novel type III effector, BopC, is required for the induction of necrotic cell death during Bordetella infection.

Complexity of the Genomic Diversity in Enterohemorrhagic Escherichia coli O157 Revealed by the Combinational Use of the O157 Sakai OligoDNA Microarray and the Whole Genome PCR scanning

Escherichia coli O157, an etiological agent of hemorrhagic colitis and hemolytic uremic syndrome, is one of the leading worldwide public health threats. Genome sequencing of two O157 strains have revealed that the chromosome is comprised of a 4.1 Mb backbone shared by K-12 and a total of 1.4 Mb O157-specific sequences. Most of the large O157-specific sequences are prophages and prophage-like elements, which have carried many virulence genes into the O157 genome. This suggests that bacteriophages have played the key roles in the emergence of O157. The Whole Genome PCR Scanning (WGPScanning) analysis of O157 strains, on the other hand, revealed a high level of genomic diversity in O157. Variation of prophages has also been suggested as a major factor generating such diversity. In this study, we analyzed the gene content of O157 strains, by an oligoDNA microarray, using the same set of strains as examined by the WGPScanning method. Although most of the strains were typical O157 : H7, they differed remarkably in gene composition, particularly in those on prophages, and we identified more than 400 'variably absent or present' genes which included virulence-related genes. This confirms the role of prophages in generating the genomic diversity, and raises a possibility that some level of variation in potential virulence is present among O157 strains. Fine comparison of the two datasets obtained by microarray and WGPScanning provided much further details on the O157 genome diversity than illustrated by each method alone, indicating the usefulness of this combinational approach in the genomic comparison of closely related strains.

Comparative genomics reveals what makes an enterobacterial plant pathogen

The bacterial family Enterobacteriaceae contains some of the most devastating human and animal pathogens, including Escherichia coli, Salmonella enterica and species of Yersinia and Shigella. These are among the best-studied of any organisms, yet there is much to be learned about the nature and evolution of interactions with their hosts and with the wider environment. Comparative and functional genomics have fundamentally improved our understanding of their modes of adaptation to different ecological niches and the genes that determine their pathogenicity. In addition to animal pathogens, Enterobacteriaceae include important plant pathogens, such as Erwinia carotovora subsp. atroseptica (Eca), the first plant-pathogenic enterobacterium to be sequenced. This review focuses on genomic comparisons between Eca and other enterobacteria, with particular emphasis on the differences that exemplify or explain the plant-associated lifestyle(s) of Eca. Horizontal gene transfer in Eca may directly have led to the acquisition of a number of determinants that mediate its interactions, pathogenic or otherwise, with plants, offering a glimpse into its evolutionary divergence from animal-pathogenic enterobacteria.

Emergence of a 'hyperinfectious' bacterial state after passage of Citrobacter rodentium through the host gastrointestinal tract

Citrobacter rodentium belongs to a family of human and animal enteric pathogens that includes the clinically significant enterohaemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC). These pathogens exploit attaching and effacing (A/E) lesions to colonize the host gastrointestinal tract. However, both EHEC and EPEC are poorly pathogenic in mice. In contrast, C. rodentium, which is genetically highly related to E. coli, relies on A/E lesion formation as an essential step in both colonization and infection of the murine mucosa, providing an excellent in vivo model. In this study we have used bioluminescence imaging (BLI) to investigate the organ specificity and dynamics of colonization of mice by LB-grown and mouse-passaged C. rodentium in situ and in real time. We have demonstrated the appearance of a 'hyperinfectious' state after passage of C. rodentium through the murine gastrointestinal tract. The 'hyperinfectious' state was found to dramatically reduce the dose required to infect secondary individuals, and also influenced the tissue distribution of colonizing bacteria, removing the requirement for primary colonization of the caecal patch. In addition, the 'hyperinfectious' phenotype was found to be transient with one overnight passage in rich medium sufficient to return C. rodentium to 'culture' infectivity.

Assessing the expression of enterotoxigenic Escherichia coli-specific surface antigens in recombinant strains by transmission electron microscopy and immunolabeling

Infections with enterotoxigenic Escherichia coli (ETEC) are a major cause of travelers' diarrhea worldwide. Colonization of the small intestine mucosa is dependent on specific colonization factor antigens (CFA) and coli surface (CS) antigens. CFA/1, CS3, and CS6 are the most prevalent fimbrial antigens found in clinical isolates. The goal of our study was to visualize the morphology of CS3 and CS6 fimbriae in wild-type and recombinant E. coli strains by means of transmission electron microscopy in conjunction with negative staining and immunolabeling. Corresponding ETEC genes were cloned into E. coli K12 strain DH10B. Expression of fimbriae was dependent on culture conditions and sample handling. Specific immunolabeling of fimbriae unequivocally demonstrated the presence of all types of surface antigens investigated. Negative staining was effective in revealing CS3 but not CS6. In addition, this technique clearly demonstrated differences in the morphology of genetically and immunologically identical CS3 surface antigens in wild-type and recombinant strains. This paper provides a basis for the assessment of recombinant vaccines.

Tyrosine-phosphorylated bacterial effector proteins: the enemies within

The tyrosine phosphorylation of proteins has a central role during signal transduction in eukaryotes. Recent progress shows that tyrosine phosphorylation is also a common feature of several effector proteins translocated by bacterial type III and type IV secretion systems. The involvement of these secretion systems in disease development is exemplified by a variety of pathogenic processes: pedestal formation (Tir of EPEC and Citrobacter), cell scattering (CagA of Helicobacter), invasion (Tarp of Chlamydia) and possibly proinflammatory responses and cell proliferation (BepD-F of Bartonella). The discovery that different bacterial pathogens use this common strategy to subvert host-cell function suggests that more examples will soon emerge.

Cholesterol-enriched membrane microdomains are required for inducing host cell cytoskeleton rearrangements in response to attaching-effacing Escherichia coli

The diarrheal pathogens enterohemorrhagic Escherichia coli (EHEC) O157:H7 strain CL56 and enteropathogenic Escherichia coli (EPEC) O127:H6 strain E2348/69 adhere intimately to epithelial cells through attaching-effacing lesions, which are characterized by rearrangements of the host cytoskeleton, intimate adherence, and destruction of microvilli. These cytoskeletal responses require activation of host signal transduction pathways. Lipid rafts are signaling microdomains enriched in sphingolipid and cholesterol in the plasma membrane. The effect of perturbing plasma membrane cholesterol on bacterial intimate adherence was assessed. Infection of both HEp-2 cells and primary skin fibroblasts with strains CL56 and E2348/69 caused characteristic rearrangements of the cytoskeleton at sites of bacterial adhesion. CL56- and E2348/69-induced cytoskeletal rearrangements were inhibited following cholesterol depletion. Addition of exogenous cholesterol to depleted HEp-2 cells restored cholesterol levels and rescued bacterially induced alpha-actinin mobilization. Quantitative bacterial adherence assays showed that EPEC adherence to HEp-2 cells was dramatically reduced following cholesterol depletion, whereas the adherence of EHEC remained high. Cytoskeletal rearrangements on skin fibroblasts obtained from children with Niemann-Pick type C disease were markedly reduced. These findings indicate that host membrane cholesterol contained in lipid rafts is necessary for the cytoskeletal rearrangements following infection with attaching-effacing Escherichia coli. Differences in initial adherence indicate divergent roles for host membrane cholesterol in the pathogenesis of EHEC and EPEC infections.

Diarrhea caused by primarily non-gastrointestinal infections

Direct infections of the gastrointestinal tract cause most cases of diarrhea, but diarrhea can also be caused by systemic infections or infections that affect other organ systems. The authors of this Review discuss clinically relevant infectious diseases that do not primarily affect the gastrointestinal tract but commonly cause diarrhea, and note that they should be included in the differential diagnosis and diagnostic approach to diarrhea.

Infectious diseases that do not primarily affect the gastrointestinal tract can cause severe diarrhea. The pathogenesis of this kind of diarrhea includes cytokine action, intestinal inflammation, sequestration of red blood cells, apoptosis and increased permeability of endothelial cells in the gut microvasculature, and direct invasion of gut epithelial cells by various infectious agents. Of the travel-associated systemic infections presenting with fever, diarrhea occurs in patients with malaria, dengue fever and SARS. Diarrhea also occurs in patients with community-acquired pneumonia, when it is suggestive of legionellosis. Diarrhea can also occur in patients with systemic bacterial infections. In addition, although diarrhea is rare in patients with early Lyme borreliosis, the incidence is higher in those with other tick-borne infections, such as ehrlichiosis, tick-borne relapsing fever and Rocky Mountain spotted fever. Unfortunately, it is often not established whether diarrhea is an initial symptom or develops during the course of the disease. The real incidence of diarrhea in some infectious diseases must also be questioned because it could represent an adverse reaction to antibiotics.

Protection against hemorrhagic colitis in an animal model by oral immunization with isogeneic rabbit enteropathogenic Escherichia coli attenuated by truncating intimin

Strains of Shiga toxin (Stx)-producing Escherichia coli, also called enterohemorrhagic E. coli (EHEC), are important food-borne pathogens for humans. Most EHEC strains intimately adhere to the intestinal mucosa in a characteristic attaching and effacing (A/E) pattern, which is mediated by the bacterial adhesin intimin. Subsequent release of Stx1 and/or Stx2 leads to the frequent development of hemorrhagic colitis and, less commonly, to hemolytic-uremic syndrome. The aim of the present study was to develop an attenuated A/E E. coli strain for use as a vaccine against EHEC infection encoding a truncated intimin lacking adhesive capacity, but which would still express somatic antigens, other products of the locus of enterocyte effacement pathogenicity island, and an immunogenic remnant of the intimin molecule. A single-nucleotide deletion was generated in the eae gene in the prototype rabbit A/E E. coli strain RDEC-1 (O15:H-), which resulted in truncation of intimin by 81 C-terminal residues (860 to 939 amino acids) containing a disulfide loop. Inoculation of rabbits with large doses of the truncated intimin mutant (RDEC-1Deltaeae(860-939)) was well tolerated, as observed by the absence of clinical signs of disease or evidence of intestinal A/E lesions. The efficacy of RDEC-1Deltaeae(860-939) as a vaccine was evaluated by orogastric inoculation of rabbits with RDEC-1Deltaeae(860-939) followed by challenge with the virulent strain RDEC-H19A, an Stx1-producing derivative of wild-type RDEC-1 capable of inducing hemorrhagic colitis in rabbits. Following RDEC-H19A challenge, nonimmunized control rabbits exhibited characteristic weight loss with watery to bloody diarrhea and demonstrated intimate bacterial attachment, effacement of microvilli, submucosal edema, mucosal heterophile infiltrates, and Shiga toxin-induced vascular lesions. In contrast, the RDEC-1Deltaeae(860-939)-immunized rabbits showed no clinical signs of disease, maintained normal weight gain, had reduced fecal shedding of challenge organisms, and showed an absence of gross or microscopic lesions in the intestinal mucosa. Serum antibodies specific to intimin were detected among rabbits immunized with RDEC-1Deltaeae(860-939), indicating that truncation of the intimin functional domain not only attenuated bacterial virulence, but also retained at least some of the immunogenicity of native intimin. Although it is not possible to gauge the exact contribution of residual intimin immunity to protection, this attenuation strategy for A/E E. coli strains shows promise for the development of effective vaccines to prevent EHEC infection in humans and animals.

Imaging Techniques for the Study of Escherichia coli and Salmonella Infections

Infectious diseases are among the leading causes of mortality worldwide, and numerous bacterial species are included in the vast array of causative agents. This review describes microscopy-based techniques that can be used to study interactions between bacteria and infected host cells, bacterial gene expression in the infected animal, and bacteria-induced cell signaling in eukaryotic cells. As infectious model systems, urinary tract infections caused by uropathogenic Escherichia coli (UPEC) and a mouse model of typhoid fever caused by Salmonella enterica serovar Typhimurium are used. To study the interaction mechanism between bacteria and eukaryotic cells, one commonly uses cell lines, primary cells, and animal models. Within the host, bacteria can be located in various organs where they are exposed to different cell types, ranging from epithelial cells at the mucosal linings to phagocytic white blood cells. In each site, bacteria are exposed to specific sets of innate immune defense mechanisms, and to survive these threats, bacteria must rapidly adapt their gene expression profile to maximize their chance of survival in any situation. The rapid development of fluorescent reporter proteins and advances in microscopy-based techniques have provided new and promising approaches not only to locate bacteria in tissues, but also to analyze expression of virulence factors in individual bacteria and host cells during the progression of disease. These techniques enable, for the first time, studies of the complex microenvironments within infected organs and will reveal the alterations of bacterial physiology that occur during bacterial growth within a host.

Adhesins of Enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) was first recognized as a cause of human disease in 1983 and is associated with diarrhea and hemorrhagic colitis, which may be complicated by life-threatening renal and neurological sequelae. EHEC are defined by their ability to produce one or more Shiga-like toxins (Stx), which mediate the systemic complications of EHEC infections, and to induce characteristic attaching and effacing lesions on intestinal epithelia, a phenotype that depends on the locus of enterocyte effacement. Acquisition of Stx-encoding bacteriophages by enteropathogenic E. coli is believed to have contributed to the evolution of EHEC, and consequently some virulence factors are conserved in both pathotypes. A key requirement for E. coli to colonize the intestines and produce disease is the ability to adhere to epithelial cells lining the gastrointestinal tract. Here, we review knowledge of the adhesins produced by EHEC and other Stx-producing E. coli, with emphasis on genetic, structural, and mechanistic aspects and their contribution to pathogenesis.

Binding of intimin with Tir on the bacterial surface is prerequisite for the barrier disruption induced by enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) infects intestinal epithelial cells and perturbs the intestinal barrier that limits the paracellular movement of molecules. The disruption of the barrier is mediated by the effectors translocated into the host cells through the bacterial type III secretion system (TTSS). A previous report has described the importance of a bacterial outer membrane protein, intimin, in EPEC-mediated disruption of the barrier, and proposed that intimin, in concert with a host intimin receptor, controls the activity of the translocated barrier-disrupting effectors [P. Dean, B. Kenny, Intestinal barrier dysfunction by enteropathogenic Escherichia coli is mediated by two effector molecules and a bacterial surface protein, Mol. Microbiol. 54 (2004) 665-675]. In this study, we found that the importance of intimin is in its ability to bind a bacterial intimin receptor, Tir. Additionally, the impaired ability of an intimin-negative mutant was not restored by co-infection with intimin-expressing TTSS mutants. Collectively, the results in this study favor an alternative scenario explaining the importance of intimin, that the binding of intimin with Tir on the bacterial surface triggers or promotes the translocation of factors required for the efficient disruption of the barrier. Thus, the interaction of intimin with Tir may serve as a molecular switch that controls the delivery of virulence factors into the host cells.

Adhesins of Diffusely Adherent and Enteroaggregative Escherichia coli

Epidemiological studies have implicated enteroaggregative Escherichia coli (EAEC) strains in acute and persistent diarrhea in children, in food-borne diarrhea outbreaks, and in traveler's diarrhea, and this group is recognized as an emerging pathotype of enteric disease. Diffusely adherent E. coli (DAEC) have been implicated as a cause of diarrhea, especially in children more than 2 years old, in both developing and developed countries. Although EAEC and DAEC strains appear to have different molecular equipment for attachment to host cell surfaces, identification and characterization of the gene clusters encoding adherence evidenced close relatedness between those determinants most frequently detected in isolates belonging to these two pathotypes of diarrheagenic E. coli. DAEC strains are a heterogeneous group of E. coli isolates, many of which express the related so-called Dr adhesins. The single designation is based on the identification of one similar cellular receptor for all these proteins. Although structurally different, they all recognize the Dr human blood group antigen on the decay-accelerating factor (DAF or CD55). These adhesins are encoded by a family of closely related operons, the first characterized and sequenced being the afa operon. Consequently, it has been suggested that this group of DAEC strains producing such adhesins be named the Afa/Dr DAEC family. Three distinct but closely related gene clusters coding for phenotypically and morphologically distinct aggregative adherence fimbriae (AAF) have been characterized. In each case, electron microscopy revealed that bacterial surfaces were surrounded by long, relatively flexible fimbrial structures.

The role of lipid rafts in the pathogenesis of bacterial infections

Numerous pathogens have evolved mechanisms of co-opting normal host endocytic machinery as a means of invading host cells. While numerous pathogens have been known to enter cells via traditional clathrin-coated pit endocytosis, a growing number of viral and bacterial pathogens have been recognized to invade host cells via clustered lipid rafts. This review focuses on several bacterial pathogens that have evolved several different mechanisms of co-opting clustered lipid rafts to invade host cells. Although these bacteria have diverse clinical presentations and many differences in their pathogenesis, they each depend on the integrity of clustered lipid rafts for their intracellular survival. Bacterial invasion via clustered lipid rafts has been recognized as an important virulence factor for a growing number of bacterial pathogens in their battle against host defenses.

Respiratory epithelial cells require Toll-like receptor 4 for induction of human beta-defensin 2 by lipopolysaccharide

Background

The respiratory epithelium is a major portal of entry for pathogens and employs innate defense mechanisms to prevent colonization and infection. Induced expression of human β-defensin 2 (HBD2) represents a direct response by the epithelium to potential infection. Here we provide evidence for the critical role of Toll-like receptor 4 (TLR4) in lipopolysaccharide (LPS)-induced HBD2 expression by human A549 epithelial cells.

Methods

Using RTPCR, fluorescence microscopy, ELISA and luciferase reporter gene assays we quantified interleukin-8, TLR4 and HBD2 expression in unstimulated or agonist-treated A549 and/or HEK293 cells. We also assessed the effect of over expressing wild type and/or mutant TLR4, MyD88 and/or Mal transgenes on LPS-induced HBD2 expression in these cells.

Results

We demonstrate that A549 cells express TLR4 on their surface and respond directly to Pseudomonas LPS with increased HBD2 gene and protein expression. These effects are blocked by a TLR4 neutralizing antibody or functionally inactive TLR4, MyD88 and/or Mal transgenes. We further implicate TLR4 in LPS-induced HBD2 production by demonstrating HBD2 expression in LPS non-responsive HEK293 cells transfected with a TLR4 expression plasmid.

Conclusion

This data defines an additional role for TLR4 in the host defense in the lung.

Nck-independent actin assembly is mediated by two phosphorylated tyrosines within enteropathogenic Escherichia coli Tir

Enteropathogenic Escherichia coli (EPEC) stimulates tyrosine-kinase signalling cascades to trigger localized actin assembly within mammalian cells. During actin 'pedestal' formation, the EPEC effector protein Tir is translocated into the plasma membrane, becomes phosphorylated on tyrosine-474 (Y474) and promotes recruitment of the mammalian adaptor protein Nck to efficiently activate N-WASP-Arp2/3-mediated actin polymerization. Tir also triggers localized actin assembly in the absence of Nck, but the Tir sequences involved in this signalling cascade have not been defined. To identify and characterize the phosphotyrosines that contribute to Nck-independent pedestal formation, we investigated the regulation of Tir tyrosine phosphorylation and found that phosphorylation is stimulated by Tir clustering. In addition to Y474, residue Y454 is also phosphorylated, although at lower efficiency. These tyrosines differentially contribute to actin polymerization in a fashion reminiscent of actin 'tail' formation mediated by the vaccinia virus envelope protein A36R, which utilizes two similarly spaced phosphotyrosines to recruit the adaptors Nck and Grb2, respectively, in order to stimulate N-WASP. Neither phosphorylated Y454 nor Y474 directly bind Grb2, but Tir derivatives harbouring these residues ultimately recruit N-WASP and Arp2/3 independently of Nck, suggesting that EPEC exploits additional phosphotyrosine-binding adaptors capable of initiating actin assembly.

Phage regulatory circuits and virulence gene expression

In many pathogenic bacteria, genes that encode virulence factors are located in the genomes of prophages. Clearly bacteriophages are important vectors for disseminating virulence genes, but, in addition, do phage regulatory circuits contribute to expression of these genes? Phages of the lambda family that have genes encoding Shiga toxin are found in certain pathogenic Escherichia coli (known as Shiga toxin producing E. coli) and the filamentous phage CTXphi, that carries genes encoding cholera toxin (CTX), is found in Vibrio cholerae. Both the lambda and CTXphi phages have repressor systems that maintain their respective prophages in a quiescent state, and in both types of prophages this repressed state is abolished when the host cell SOS response is activated. In the lambda type of prophages, only binding of the phage-encoded repressor is involved in repression and this repressor ultimately controls Shiga toxin production and/or release. In the CTXphi prophage, binding of LexA, the bacterial regulator of SOS, in addition to binding of the repressor is involved in repression; the repressor has only limited control over CTX production and has no influence on its release.

Blueprinting the regulatory response of Escherichia coli to the urinary tract

Recent work has shown how comparative genomic and microarray analyses can provide insights into the transcriptional state of uropathogenic Escherichia coli (UPEC) during infection. This study will serve as an important platform from which to identify virulence determinants and the principle mechanisms of adaptation to the urinary tract.

Enterohaemorrhagic E. coli in veterinary medicine

The emergence of enterohaemorrhagic Escherichia coli (EHEC) as major human pathogens began with the identification of serotype O157:H7 in the early 1980s as the cause of various food-borne outbreaks of severe intestinal disease. The key virulence factors include verocytotoxins (Vt) and effectors and adhesins associated with type III secretion systems. Tracing the origins of human outbreaks reveals that the primary source of this organism is the ruminant gastro-intestinal tract and a variety of transmission routes to humans have been identified. The epidemiology of E. coli O157:H7 within cattle and other ruminants has been studied extensively and the prevalence of non-O157:H7 serotypes contrasts with the observed dominance of E. coli O157:H7 amongst human EHEC isolates. Although there is some evidence that EHEC cause disease in young animals, the high prevalence of Vt within healthy ruminants suggests that this is not a virulence factor within these species. An understanding of the mechanisms underpinning EHEC persistence within their natural reservoir hosts and the development of a molecular understanding of EHEC biology and evolution could eventually allow a reduction in the incidence of human disease and may reduce future threats. The use of animal models to replicate and study human EHEC pathogenesis is described.

Escherichia coli mediated urinary tract infections: are there distinct uropathogenic E. coli (UPEC) pathotypes?

A variety of virulence genes are associated with Escherichia coli mediated urinary tract infections. Particular sets of virulence factors shared by bacterial strains directing them through a particular pathogenesis process are called a "pathotype." Comparison of co-occurrence of potential urinary tract infection (UTI) virulence genes among different E. coli isolates from fecal and UTI collections provides evidence for multiple pathotypes of uropathogenic E. coli, but current understanding of critical genetic differences defining the pathotypes is limited. Discovery of additional E. coli genes involved in uropathogenesis and determination of their distribution and co-occurrences will further define UPEC pathotypes and allow for a more detailed analysis of how these pathotypes might differ in how they cause disease.

Cytotoxic necrotizing factor type 1 production by uropathogenic Escherichia coli modulates polymorphonuclear leukocyte function

Many strains of uropathogenic Escherichia coli (UPEC) produce cytotoxic necrotizing factor type 1 (CNF1), a toxin that constitutively activates the Rho GTPases RhoA, Rac1, and Cdc42. We previously showed that CNF1 contributes to the virulence of UPEC in a mouse model of ascending urinary tract infection and a rat model of acute prostatitis and that a striking feature of the histopathology of the mouse bladders and rat prostates infected with CNF1-positive strains is an elevation in levels of polymorphonuclear leukocytes (PMNs). We also found that CNF1 synthesis leads to prolonged survival of UPEC in association with human neutrophils. Here, we tested the hypothesis that CNF1 production by UPEC diminishes the antimicrobial capacity of mouse PMNs by affecting phagocyte function through targeting Rho family GTPases that are critical to phagocytosis and the generation of reactive oxygen species. We found that, as with human neutrophils, CNF1 synthesis provided a survival advantage to UPEC incubated with mouse PMNs. We also observed that CNF1-positive UPEC down-regulated phagocytosis, altered the distribution of the complement receptor CR3 (CD11b/CD18), enhanced the intracellular respiratory burst, and increased levels of Rac2 activation in PMNs. From these results, we conclude that modulation of PMN function by CNF1 facilitates UPEC survival during the acute inflammatory response.

Bacteria-host-cell interactions at the plasma membrane: stories on actin cytoskeleton subversion

Exploitation of the host-cell actin cytoskeleton is pivotal for many microbial pathogens to enter cells, to disseminate within and between infected tissues, to prevent their uptake by phagocytic cells, or to promote intimate attachment to the cell surface. To accomplish this, these pathogens have evolved common as well as unique strategies to modulate actin dynamics at the plasma membrane, which will be discussed here, exemplified by a number of well-studied bacterial pathogens.

Pathoadaptive mutation that mediates adherence of shiga toxin-producing Escherichia coli O111

The adherence of pathogenic Escherichia coli strains to intestinal epithelium is essential for initiation of infection. The cad operon encodes the lysine decarboxylase (LDC) system responsible for metabolizing lysine, and this operon has been proposed as an antivirulence mechanism in enteroinvasive E. coli and Shigella flexneri and as a factor mediating E. coli O157:H7 adherence. We sought to determine whether the LDC activity was present in a phylogenetically characterized collection of diarrheagenic E. coli (DEC) strains and to establish whether its expression was associated with their adherence to tissue culture cells. LDC activity was found in most of the pathogenic E. coli strains tested and was absent from Shiga toxin-producing E. coli (STEC) O111 strains (DEC pathotype 8). Analysis of the cad region in these O111 strains indicates that the operon has been rearranged and some of the genes are either missing or disrupted. A similar rearrangement was found in an E. coli O111:H8 strain recently isolated from an outbreak in Texas. Complementation of the LDC-negative strains with the cad operon in trans restored the LDC activity and resulted in a reduction in adherence to tissue culture cells. Initial analysis of the protein profiles on the surface of the O111 strains indicates that the LDC activity has an effect on the expression of the adhesin intimin. Cadaverine had a slight effect on LDC-negative strain adhesion but none on intimin expression. Our data suggest that this pathoadaptive mutation is an important mechanism to control functions potentially implicated in the pathogenesis of these organisms.

Characterization of enterohemorrhagic Escherichia coli strains based on acid resistance phenotypes

Acid resistance is perceived to be an important property of enterohemorrhagic Escherichia coli strains, enabling the organisms to survive passage through the acidic environment of the stomach so that they may colonize the mammalian gastrointestinal tract and cause disease. Accordingly, the organism has developed at least three genetically and physiologically distinct acid resistance systems which provide different levels of protection. The glutamate-dependent acid resistance (GDAR) system utilizes extracellular glutamate to protect cells during extreme acid challenges and is believed to provide the highest protection from stomach acidity. In this study, the GDAR system of 82 pathogenic E. coli isolates from 34 countries and 23 states within the United States was examined. Twenty-nine isolates were found to be defective in inducing GDAR under aerobic growth conditions, while five other isolates were defective in GDAR under aerobic, as well as fermentative, growth conditions. We introduced rpoS on a low-copy-number plasmid into 26 isolates and were able to restore GDAR in 20 acid-sensitive isolates under aerobic growth conditions. Four isolates were found to be defective in the newly discovered LuxR-like regulator GadE (formerly YhiE). Defects in other isolates could be due to a mutation(s) in a gene(s) with an as yet undefined role in acid resistance since GadE and/or RpoS could not restore acid resistance. These results show that in addition to mutant alleles of rpoS, mutations in gadE exist in natural populations of pathogenic E. coli. Such mutations most likely alter the infectivity of individual isolates and may play a significant role in determining the infective dose of enterohemorrhagic E. coli.

Probiotics reduce enterohemorrhagic Escherichia coli O157:H7- and enteropathogenic E. coli O127:H6-induced changes in polarized T84 epithelial cell monolayers by reducing bacterial adhesion and cytoskeletal rearrangements

The aim of this study was to determine if probiotics reduce epithelial injury following exposure to Escherichia coli O157:H7 and E. coli O127:H6. The pretreatment of intestinal (T84) cells with lactic acid-producing bacteria reduced the pathogen-induced drop in transepithelial electrical resistance. These findings demonstrate that probiotics prevent epithelial injury induced by attaching-effacing bacteria.

Identification and characterization of the locus for diffuse adherence, which encodes a novel afimbrial adhesin found in atypical enteropathogenic Escherichia coli

The O26 serogroup of enteropathogenic Escherichia coli (EPEC) is one of the serogroups most frequently implicated in infant diarrhea and is also common among enterohemorrhagic E. coli (EHEC) strains. The most common O26 strains belong to EPEC/EHEC serotype O26:H11 and are generally Shiga toxin (Stx) positive. Stx-negative E. coli strains that are negative for the EPEC EAF plasmid and bundle-forming pilus (Bfp) are classified as atypical EPEC. Here, we report a novel adhesin present in an stx-negative bfpA-negative atypical EPEC O26:H11 strain isolated from an infant with diarrhea. A cloned 15-kb genomic region from this strain, designated the locus for diffuse adherence (lda), confers diffuse adherence on HEp-2 cells when expressed in E. coli K-12. Sequence analysis of lda revealed a G+C content of 46.8% and 15 open reading frames sharing homology with the E. coli K88 fae and CS31A clp fimbrial operons. The lda region is part of a putative 26-kb genomic island inserted into the proP gene of the E. coli chromosome. Hybridization studies have demonstrated the prevalence of the minor structural subunit gene, ldaH, across E. coli serogroups O5, O26, O111, and O145. A second plasmid-encoded factor that contributed to the Hep-2 adherence of this strain was also identified but was not characterized. Null mutations that abolish adherence to HEp-2 cells can be restored by plasmid complementation. Antiserum raised against the major structural subunit, LdaG, recognizes a 25-kDa protein from crude heat-extracted protein preparations and inhibits the adherence of the E. coli DH5alpha lda(+) clone to HEp-2 cells. Electron microscopy revealed a nonfimbrial structure surrounding the bacterial cell.

CesT is a multi-effector chaperone and recruitment factor required for the efficient type III secretion of both LEE- and non-LEE-encoded effectors of enteropathogenicEscherichia coli

Enteropathogenic Escherichia coli (EPEC) is an intestinal attaching and effacing pathogen that utilizes a type III secretion system (T3SS) for the delivery of effectors into host cells. The chaperone CesT has been shown to bind and stabilize the type III translocated effectors Tir and Map in the bacterial cytoplasm prior to their delivery into host cells. In this study we demonstrate a role for CesT in effector recruitment to the membrane embedded T3SS. CesT-mediated effector recruitment was dependent on the presence of the T3SS membrane-associated ATPase EscN. EPEC DeltacesT carrying a C-terminal CesT variant, CesT(E142G), exhibited normal cytoplasmic Tir stability function, but was less efficient in secreting Tir, further implicating CesT in type III secretion. In vivo co-immunoprecipitation studies using CesT-FLAG containing EPEC lysates demonstrated that CesT interacts with Tir and EscN, consistent with the notion of CesT recruiting Tir to the T3SS. CesT was also shown to be required for the efficient secretion of several type III effectors encoded within and outside the locus of enterocyte effacement (LEE) in addition to Tir and Map. Furthermore, a CesT affinity column was shown to specifically retain multiple effector proteins from EPEC culture supernatants. These findings indicate that CesT is centrally involved in recruiting multiple type III effectors to the T3SS via EscN for efficient secretion, and functionally redefine the role of CesT in multiple type III effector interactions.

Direct and indirect transcriptional activation of virulence genes by an AraC-like protein, PerA from enteropathogenic Escherichia coli

The plasmid-encoded Per regulatory locus of enteropathogenic Escherichia coli (EPEC) is generally considered to consist of three genes, perA, perB and perC. PerA, a member of the AraC-like family of transcriptional regulators, is known to be an activator of its own promoter (autoactivation) as well as of the plasmid-located bfp operon encoding bundle-forming pili, but its role in activation of the chromosomal locus of enterocyte effacement (LEE) pathogenicity island, which confers the property of intimate adherence on EPEC, requires clarification. Here, we show that PerA is also required for activation of the master regulatory LEE operon, LEE1, but that this activation is indirect, being achieved via autoactivation of the per promoter which ensures sufficient production of the PerC protein to activate LEE1. In contrast, PerA-dependent activation of the per and bfp promoters is direct and does not require the other Per proteins, but is modulated by the nucleoid-associated protein H-NS. The closely related VirF regulator from Shigella flexneri cannot substitute for PerA to activate these promoters, despite being able to bind their upstream regions in vitro. PerA can bind the per and bfp promoter fragments to form multiple complexes, while VirF forms only a single complex. Site-directed mutagenesis of the PerA protein suggests that, like VirF, it may use both of its carboxy-terminal helix-turn-helix motifs for DNA interaction, and may also make direct contacts with RNA polymerase. In addition, we have isolated mutations in the poorly characterized amino-terminal domain of PerA which affect its ability to activate gene expression.

Genomic peculiarity of coding sequences and metabolic potential of probiotic Escherichia coli strain Nissle 1917 inferred from raw genome data

Probiotic Escherichia coli strain Nissle 1917 (O6:K5:H1) is a commensal E. coli isolate that has a long tradition in medicine for the treatment of various intestinal disorders in humans. To elucidate the molecular basis of its probiotic nature, we started sequencing the genome of this organism with a whole-genome shotgun approach. A 7.8-fold coverage of the genomic sequence has been generated and is now in the finishing stage. To exploit the genome data as early as possible and to generate hypotheses for functional studies, the unfinished sequencing data were analyzed in this work using a new method [Sun, J., Zeng, A.P., 2004. IdentiCS--identification of coding sequence and in silico reconstruction of the metabolic network directly from unannotated low-coverage bacterial genome sequence. BMC Bioinformatics 5, 112] which is particularly suitable for the prediction of coding sequences (CDSs) from unannotated genome sequence. The CDSs predicted for E. coli Nissle 1917 were compared with those of all five other sequenced E. coli strains (E. coli K-12 MG1655, E. coli K-12 W3110, E. coli CFT073, EHEC O157:H7 EDL933 and EHEC O157:H7 Sakai) published to date. Five thousand one hundred and ninety-two CDSs were predicted for E. coli Nissle 1917, of which 1065 were assigned with enzyme EC numbers. The comparison of all predicted CDSs of E. coli Nissle 1917 to the other E. coli strains revealed 108 CDSs specific for this isolate. They are organized as four big genome islands and many other smaller gene clusters. Based on CDSs with EC numbers for enzymes, the potential metabolic network of Nissle 1917 was reconstructed and compared to those of the other five E. coli strains. Overall, the comparative genomic analysis sheds light on the genomic peculiarity of the probiotic E. coli strain Nissle 1917 and is helpful for designing further functional studies long before the sequencing project is completely finished.

Towards a vaccine for attaching/effacing Escherichia coli: a LEE encoded regulator (ler) mutant of rabbit enteropathogenic Escherichia coli is attenuated, immunogenic, and protects rabbits from lethal challenge with the wild-type virulent strain

The ler (LEE encoded regulator) gene product is a central regulator for the genes encoded on the locus of enterocyte effacement (LEE) pathogenicity island of attaching/effacing (A/E) pathogens, including human enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC) as well as animal isolates. Although an in vivo role for Ler in bacterial virulence has not been documented, we hypothesized that a Ler deletion mutant should be attenuated for virulence but might retain immunogenicity. The goals of this study were to genetically characterize ler of a rabbit EPEC (rEPEC) strain (O103:H2), to examine the effect of ler on in vivo virulence, and to determine if intragastric inoculation of an attenuated rEPEC ler mutant was immunogenic and could protect rabbits against subsequent challenge with the wild-type virulent parent strain. The predicted ler gene product of rEPEC strain O103:H2 shares high homology (over 95% amino acid identity) with the Lers of another rEPEC strain RDEC-1 (O15:H-) and human EPEC and EHEC. A defined internal ler deletion mutant of rEPEC O103:H2 showed reduced production of secreted proteins. Although orogastric inoculation of rabbits with the virulent parent O103:H2 strain induced severe diarrhea, significant weight loss and early mortality with adherent mucosal bacteria found at sacrifice, the isogeneic ler mutant strain was well tolerated. Animals gained weight and showed no clinical signs of disease. Examination of histological sections of intestinal segments revealed the absence of mucosal bacterial adherence. This result demonstrates an essential role for Ler in in vivo pathogenicity of A/E E. coli. Single dose orogastric immunization with the rEPEC ler mutant induced serum IgG antibody to whole bacteria (but not to intimin). Immunized animals were protected against enteric infection with the WT virulent parent strain exhibiting normal weight gain, absence of diarrhea and absence of mucosally adherent bacteria at sacrifice. Such attenuated ler mutant strains may have potential for use as oral vaccines, or as vaccine vectors for delivery of foreign antigens. It remains to be determined whether such regulatory mutants can protect against infection with A/E bacteria of differing serotypes affecting different hosts.

Enteropathogenic Escherichia coli type III effectors EspG and EspG2 disrupt the microtubule network of intestinal epithelial cells

Enteropathogenic Escherichia coli infection of intestinal epithelial cells leads to localized depletion of the microtubule cytoskeleton, an effect that is dependent on delivery of type III translocated effector proteins EspG and Orf3 (designated EspG2) to the site of depletion. Microtubule depletion involved disruption rather than displacement of microtubules.

Inactivation of the pst system reduces the virulence of an avian pathogenic Escherichia coli O78 strain

Escherichia coli O78 strains are frequently associated with extraintestinal diseases, such as airsacculitis and septicemia, in poultry, livestock, and humans. To understand the influence of the pst operon in the virulence of E. coli, we introduced mutations into the pst genes of the avian pathogenic E. coli (APEC) O78:K80 strain chi7122 by allelic exchange. The mutation of pst genes led to the constitutive expression of the Pho regulon. Furthermore, the virulence of APEC strain chi7122 in a chicken infection model was attenuated by inactivation of the Pst system. The pst mutant caused significantly fewer extraintestinal lesions in infected chickens, and bacterial numbers isolated from different tissues after infection were significantly lower for the mutant than for the wild-type strain. Moreover, resistance to the bactericidal effects of rabbit serum and acid shock was impaired in the pst mutant, in contrast to the wild-type strain. In addition, the MIC of polymyxin was twofold lower for the mutant than for the wild-type strain. Although the pst mutant demonstrated an increased susceptibility to rabbit serum, this strain was not killed by chicken serum, suggesting the presence of differences in host innate immune defenses and complement-mediated killing. In APEC O78 strain chi7122, a functional Pst system is required for full virulence and resistance to acid shock and polymyxin. Our results suggest that the mutation of pst genes induces a deregulation of phosphate sensing and changes in the cell surface composition that lead to decreased virulence, indicating the importance of the Pst system for the virulence of pathogenic E. coli strains from different hosts.

Use of in vivo-induced antigen technology for identification of Escherichia coli O157:H7 proteins expressed during human infection

Using in vivo-induced antigen technology (IVIAT), a modified immunoscreening technique that circumvents the need for animal models, we directly identified immunogenic Escherichia coli O157:H7 (O157) proteins expressed either specifically during human infection but not during growth under standard laboratory conditions or at significantly higher levels in vivo than in vitro. IVIAT identified 223 O157 proteins expressed during human infection, several of which were unique to this study. These in vivo-induced (ivi) proteins, encoded by ivi genes, mapped to the backbone, O islands (OIs), and pO157. Lack of in vitro expression of O157-specific ivi proteins was confirmed by proteomic analysis of a mid-exponential-phase culture of E. coli O157 grown in LB broth. Because ivi proteins are expressed in response to specific cues during infection and might help pathogens adapt to and counter hostile in vivo environments, those identified in this study are potential targets for drug and vaccine development. Also, such proteins may be exploited as markers of O157 infection in stool specimens.

Events at the host-microbial interface of the gastrointestinal tract III. Cell-to-cell signaling among microbial flora, host, and pathogens: there is a whole lot of talking going on

Humans have an important association with their intestinal microbial flora. The microbial flora helps to shape the mammalian innate immune system, absorbs nutrients, and plays an intricate role on intestinal development. Microbes and mammals communicate with each other through an array of hormone and hormonelike chemical compounds. These "signals," however, are hijacked by bacterial pathogens, such as enterohemorrhagic Eschrichia coli (EHEC), to activate its virulence genes, colonize the host, and start the disease process. This review explores the cell-to-cell signaling events in the gastrointestinal tract that lead EHEC to regulate its virulence genes in a coordinate manner.

Important bacterial gastrointestinal pathogens in children: a pathogenesis perspective

This article focuses on the five most common bacterial enteropathogens of the developed world--Helicobacter pylori, Escherichia coli, Shigella, Salmonella, and Campylobacter--from the perspective of how they cause disease and how they relate to each other. Basic and recurring themes of bacterial pathogenesis, including mechanisms of entry, methods of adherence, sites of cellular injury, role of toxins, and how pathogens acquire particular virulence traits (and antimicrobial resistance), are discussed.