Enteropathogenic Escherichia coli virulence factor bundle-forming pilus has a binding specificity for phosphatidylethanolamine

The metabolic impact of bacterial infection in the gut

Bacterial infections of the gut are one of the major causes of morbidity and mortality worldwide. The interplay between the pathogen and the host is finely balanced, with the bacteria evolving to proliferate and establish infection. In contrast, the host mounts a response to first restrict and then eliminate the infection. The intestine is a rapidly proliferating tissue, and metabolism is tuned to cater to the demands of proliferation and differentiation along the crypt-villus axis (CVA) in the gut. As bacterial pathogens encounter the intestinal epithelium, they elicit changes in the host cell, and core metabolic pathways such as the tricarboxylic acid (TCA) cycle, lipid metabolism and glycolysis are affected. This review highlights the mechanisms utilized by diverse gut bacterial pathogens to subvert host metabolism and describes host responses to the infection.

The maternal blood lipidome is indicative of the pathogenesis of severe preeclampsia

Preeclampsia is a pregnancy-specific syndrome characterized by hypertension and proteinuria after 20 weeks of gestation. However, it is not well understood what lipids are involved in the development of this condition, and even less is known how these lipids mediate its formation. To reveal the relationship between lipids and preeclampsia, we conducted lipidomic profiling of maternal sera of 44 severe preeclamptic and 20 healthy pregnant women from a multiethnic cohort in Hawaii. Correlation network analysis showed that oxidized phospholipids have increased intercorrelations and connections in preeclampsia, whereas other lipids, including triacylglycerols, have reduced network correlations and connections. A total of 10 lipid species demonstrate significant changes uniquely associated with preeclampsia but not any other clinical confounders. These species are from the lipid classes of lysophosphatidylcholines, phosphatidylcholines (PCs), cholesteryl esters, phosphatidylethanolamines, lysophosphatidylethanolamines, and ceramides. A random forest classifier built on these lipids shows highly accurate and specific prediction (F1 statistic = 0.94; balanced accuracy = 0.88) of severe preeclampsia, demonstrating their potential as biomarkers for this condition. These lipid species are enriched in dysregulated biological pathways, including insulin signaling, immune response, and phospholipid metabolism. Moreover, causality inference shows that various PCs and lysophosphatidylcholines mediate severe preeclampsia through PC 35:1e. Our results suggest that the lipidome may play a role in the pathogenesis and serve as biomarkers of severe preeclampsia.

Modulation of the enterohemorrhagic E. coli virulence program through the human gastrointestinal tract

Enteric pathogens must not only survive passage through the gastrointestinal tract but must also coordinate expression of virulence determinants in response to localized microenvironments with the host. Enterohemorrhagic Escherichia coli (EHEC), a serious food and waterborne human pathogen, is well equipped with an arsenal of molecular factors that allows it to survive passage through the gastrointestinal tract and successfully colonize the large intestine. This review will explore how EHEC responds to various environmental cues associated with particular microenvironments within the host and how it employs these cues to modulate virulence factor expression, with a view to developing a conceptual framework for understanding modulation of EHEC’s virulence program in response to the host. In vitro studies offer significant insights into the role of individual environmental cues but in vivo studies using animal models as well as data from natural infections will ultimately provide a more comprehensive picture of the highly regulated virulence program of this pathogen.

Sticky situation: localized adherence of enteropathogenic Escherichia coli to the small intestine epithelium

Enteropathogenic Escherichia coli (EPEC) primarily cause gastrointestinal illness in neonates. They accomplish this by a complex coordinated multistage strategy, whereby the organisms colonize the epithelial lining of the small intestine. This process can be divided into four stages: first, localized, nonintimate adherence; second, type III secretion-mediated injection of effector proteins, third effacement of microvilli and, finally, intimate adherence. In this article, we review the history and current state of knowledge, as well as present potential future directions for further investigating the fascinating processes by which EPEC and related organisms colonize the human intestine and cause disease.

From alpha to beta: identification of amino acids required for the N-acetyllactosamine-specific lectin-like activity of bundlin

Bundle-forming pili (BFP) promote the adherence of typical enteropathogenic Escherichia coli (EPEC) to human intestinal epithelial cells. BFP are polymers of bundlin and nine bundlin alleles have been identified in EPEC isolated from diverse sources. These alleles are divided into two main groups, alpha and beta, based on their amino acid sequences. Alpha bundlins are also N-acetyllactosamine- (LacNAc) specific lectins and bind to HEp-2 cells, whereas beta bundlins do not display these characteristics. The four surface-exposed regions of amino acid sequence heterogeneity between alpha and beta bundlin were therefore investigated as potential LacNAc-specific carbohydrate-binding domains in a bundlin. Mutation of one of these domains, 137-GENNI-141, in alpha(1) bundlin to that of beta bundlin (136-SPDST-140) resulted in BFP that no longer bound to LacNAc or HEp-2 cells. Conversely, mutating the beta3 bundlin gene to encode the alpha bundlin sequence at this domain resulted in the gain of HEp-2 cell adherence. The importance of this domain in carbohydrate binding is supported by the finding that introducing the mutation GENNI-->GENNT altered the alpha1 bundlin carbohydrate-binding specificity from LacNAc to the Lewis X glycan sequence.

The Escherichia coli common pilus and the bundle-forming pilus act in concert during the formation of localized adherence by enteropathogenic E. coli

Although the bundle-forming pilus (BFP) of enteropathogenic Escherichia coli (EPEC) mediates microcolony formation on epithelial cells, the adherence of BFP-deficient mutants is significantly abrogated, but the mutants are still adherent due to the presence of intimin and possibly other adhesins. In this study we investigated the contribution of the recently described E. coli common pilus (ECP) to the overall adherence properties of EPEC. We found that ECP and BFP structures can be simultaneously observed in the course (between zero time and 7 h during infection) of formation of localized adherence on cultured epithelial cells. These two pilus types colocalized at different levels of the microcolony topology, tethering the adhering bacteria. No evidence of BFP disappearance was found after prolonged infection. When expressed from a plasmid present in nonadherent E. coli HB101, ECP rendered this organism highly adherent at levels comparable to those of HB101 expressing the BFP. Purified ECP bound in a dose-dependent manner to epithelial cells, and the binding was blocked with anti-ECP antibodies, confirming that the pili possess adhesin properties. An ECP mutant showed only a modest reduction in adherence to cultured cells due to background expression levels of BFP and intimin. However, isogenic mutants not expressing EspA or BFP were significantly less adherent when the ecpA gene was also deleted. Furthermore, a DeltaespA DeltaecpA double mutant (unable to translocate Tir and to establish intimate adhesion) was at least 10-fold less adherent than the DeltaespA and DeltaecpA single mutants, even in the presence of BFP. A Delta bfp DeltaespA DeltaecpA triple mutant showed the least adherence compared to the wild type and all the isogenic mutant strains tested, suggesting that ECP plays a synergistic role in adherence. Our data indicate that ECP is an accessory factor that, in association with BFP and other adhesins, contributes to the multifactorial complex interaction of EPEC with host epithelial cells.

Altered Proteome in Burkholderia pseudomallei rpoE Operon Knockout Mutant: Insights into Mechanisms of rpoE Operon in Stress Tolerance, Survival, and Virulence

We have previously shown that the alternative sigma factor sigmaE (RpoE), encoded by rpoE, is involved in stress tolerance and survival of Burkholderia pseudomallei. However, its molecular and pathogenic mechanisms remain unclear. In the present study, we applied gel-based, differential proteomics to compare the cellular proteome of an rpoE operon knockout mutant (RpoE Mut) to that of wild-type (K96243 WT) B. pseudomallei. Quantitative intensity analysis (n = 5 gels from 5 individual culture flasks in each group) revealed significantly differential expression of 52 proteins, which were subsequently identified by Q-TOF MS/MS. These included oxidative, osmotic, and other stress response proteins; chaperones; transcriptional/translational regulators; metabolic enzymes; proteins involved in cell wall synthesis, fatty synthesis, glycogen synthesis, and storage; exported proteins; secreted proteins; adhesion molecule; protease/peptidase; protease inhibitor; signaling proteins; and other miscellaneous proteins. The down-regulation of several stress response proteins, chaperones, transcriptional/translational regulators, and proteins involved in cell wall synthesis in RpoE Mut provided some new insights into the mechanisms of the rpoE operon for the stress tolerance and survival of B. pseudomallei. In addition, the proteomic data and in vivo study indicated that the rpoE operon is also involved in the virulence of B. pseudomallei. Our findings underscore the usefulness of proteomics for unraveling pathogenic mechanisms of diseases at the molecular level.

Enteropathogenic Escherichia coli Tir translocation and pedestal formation requires membrane cholesterol in the absence of bundle-forming pili

Enteropathogenic Escherichia coli (EPEC) is a significant cause of paediatric diarrhoea worldwide. Virulence requires adherence to intestinal epithelial cells, mediated in part through type IV bundle-forming pili (BFP), and the EPEC protein Tir. Tir is inserted into the enterocyte plasma membrane (PM), resulting in the formation of actin-rich pedestals. Tir is translocated by the type III secretion system (TTSS), through a pore comprised of EPEC proteins inserted into the PM. Here, we demonstrate that in the absence of BFP, EPEC adherence, effector translocation and pedestal formation are dependent on lipid rafts. Lipid raft disruption using methyl-beta-cyclodextrin (MbetaCD) decreased adherence by an EPEC BFP-deficient strain from 85% to 1%. Translocation of the effectors Tir and EspF was blocked by MbetaCD treatment, although the TTSS pore still formed. MbetaCD treatment after Tir delivery decreased pedestal formation by EPEC from 40% to 5%, but not by the related pathogen E. coli O157:H7 which uses a different Tir-based mechanism. In contrast, EPEC expressing the BFP can circumvent the requirement for membrane cholesterol. This suggests that lipid rafts play a role in virulence of this medically important pathogen.

Externalization of host cell protein kinase C during enteropathogenic Escherichia coli infection

Enteropathogenic Escherichia coli (EPEC) is a common cause of diarrhea in children in developing countries. Protein kinase C (PKC), a serine- and threonine-directed protein kinase, is rapidly activated following EPEC infection and this is accompanied by its translocation to a membrane-bound location where it is tightly bound to phosphatidylserine (PS). EPEC infection causes host cell death, one of whose features is externalization of PS. We hypothesized that externalization of PS would be accompanied by externalization of PKC as well. We report that EPEC infection triggers the externalization of PKC to the outer surface of the host cell. Ecto-PKC remains firmly tethered to the cell but can be released by incubation with peptide or protein substrates for the enzyme. Ecto-PKC is intact and biologically active and able to phosphorylate protein substrates on the surface of the host cell. Phosphorylation of whole EPEC bacteria or EPEC-secreted proteins could not be detected. Externalization of PKC could be reproduced by the combination of an apoptotic stimulus (ultraviolet (UV) irradiation) and phorbol myristate acetate (PMA), a procedure which resulted in externalization of >25% of the total cellular content of PKC-alpha. In the presence of ATP, ecto-PKC inhibited UV-induced cell shrinkage, membrane blebbing, and propidium iodide uptake but not the activation of caspases 3 and 7. This is the first report that expression of an ecto-protein kinase is altered by a microbial pathogen and the first to note that externalization of PKC can accompany apoptosis.

Enteropathogenic Escherichia coli infection triggers host phospholipid metabolism perturbations

Enteropathogenic Escherichia coli (EPEC) specifically recognizes phosphatidylethanolamine (PE) on the outer leaflet of host epithelial cells. EPEC also induces apoptosis in epithelial cells, which results in increased levels of outer leaflet PE and increased bacterial binding. Consequently, it is of interest to investigate whether EPEC infection perturbs host cell phospholipid metabolism and whether the changes play a role in the apoptotic signaling. Our findings indicate that EPEC infection results in a significant increase in the epithelial cell PE level and a corresponding decrease in the phosphatidylcholine (PC) level. PE synthesis via both the de novo pathway and the serine decarboxylation pathway was enhanced, and de novo synthesis of phosphatidylcholine via CDP-choline was reduced. The changes were transitory, and the maximum change was noted after 4 to 5 h of infection. Addition of exogenous PC or CDP-choline to epithelial cells prior to infection abrogated EPEC-induced apoptosis, suggesting that EPEC infection inhibits the CTP-phosphocholine cytidylyltransferase step in PC synthesis, which is reportedly inhibited during nonmicrobially induced apoptosis. On the other hand, incorporation of exogenous PE by the host cells enhanced EPEC-induced apoptosis and necrosis without increasing bacterial adhesion. This is the first report that pathogen-induced apoptosis is associated with significant changes in PE and PC metabolism, and the results suggest that EPEC adhesion to a host membrane phospholipid plays a role in disruption of host phospholipid metabolism.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

Binding of Actinobacillus pleuropneumoniae to phosphatidylethanolamine

The gram-negative bacterium Actinobacillus pleuropneumoniae is the causative agent of porcine fibrinohemorrhagic necrotizing pleuropneumonia, a disease that causes important economic losses to the swine industry worldwide. In general, the initial step of bacterial colonization is attachment to host cells. The purpose of the present study was to evaluate the binding of A. pleuropneumoniae serotype 1 to phospholipids, which are the major constituents of biological membranes. Phospholipids serve as receptors for several bacteria, including respiratory pathogens. To study this effect, we used thin-layer chromatography overlay binding assays to test commercial phospholipids such as phosphatidic acid, phosphatidylcholine, phosphatidylserine, phosphatidylinositol, phosphatidylglycerol, and phosphatidylethanolamine (PE). Our results indicate that A. pleuropneumoniae serotype 1 binds to PE but not to the other phospholipids tested. Serotypes 5b and 7, which, along with serotype 1, are the most prevalent serotypes of A. pleuropneumoniae in North America, share the ability to bind PE. Inhibition of binding with a monoclonal antibody against A. pleuropneumoniae serotype 1 O antigen and the use of isogenic lipopolysaccharide (LPS) mutants of A. pleuropneumoniae serotype 1 showed that the O antigen seems to be implicated in the binding to PE, at least for A. pleuropneumoniae serotype 1. A. pleuropneumoniae was also shown to bind to a phospholipid extracted from swine lungs by using the method of Folch. Chemical staining with molybdenum blue and ninhydrin, migration with neutral, acidic, and basic solvent systems, and mass spectrometry analysis all indicated that this lipid is PE. This study is, to the best of our knowledge, the first description of A. pleuropneumoniae binding to phospholipids. Our data also suggest that LPS O antigens could be involved in binding to PE.

Virulence of enteropathogenic Escherichia coli, a global pathogen

Enteropathogenic Escherichia coli (EPEC) remains an important cause of diarrheal disease worldwide. Research into EPEC is intense and provides a good virulence model of other E. coli infections as well as other pathogenic bacteria. Although the virulence mechanisms are now better understood, they are extremely complex and much remains to be learnt. The pathogenesis of EPEC depends on the formation of an ultrastructural lesion in which the bacteria make intimate contact with the host apical enterocyte membrane. The formation of this lesion is a consequence of the ability of EPEC to adhere in a localized manner to the host cell, aided by bundle-forming pili. Tyrosine phosphorylation and signal transduction events occur within the host cell at the lesion site, leading to a disruption of the host cell mechanisms and, consequently, to diarrhea. These result from the action of highly regulated EPEC secreted proteins which are released via a type III secretion system, many genes of which are located within a pathogenicity island known as the locus of enterocyte effacement. Over the last few years, dramatic increases in our knowledge of EPEC virulence have taken place. This review therefore aims to provide a broad overview of and update to the virulence aspects of EPEC.

Prevalent transfer of human colostral IgA antibody activity for the enteropathogenic Escherichia coli bundle-forming pilus structural repeating subunit A in neonates

The frequency of IgA antibody activity to the structural protein subunit BfpA of the enteropathogenic Escherichia coli bundle-forming pilus was determined in 40 mother-infant pairs by immunoblot analysis using affinity purified recombinant BfpA to monitor for IgA in maternal colostrum and in feces of the neonates. Fecal samples were collected from exclusively breastfed term infants < 24-h after the first breastmilk feeding and colostral samples from their mothers. Infants were monitored prospectively with monthly visits to ascertain dietary practices and diarrheal illnesses. The percentage of colostral anti-BfpA IgA positive patients that were also coproantibody positive was 67.5%. The median duration of lactation was 108 days and the incidence of infantile diarrheal disease was 7.5%. Thus, colostral anti-BfpA IgA antibody activity survives passage through the gut of breastfed neonates, persisting in their feces. It is suggested that oral passive immunotherapy may be used to prevent and/or treat typical EPEC infection during infancy.

Bacterial surface association of heat-labile enterotoxin through lipopolysaccharide after secretion via the general secretory pathway

Heat-labile enterotoxin (LT) is an important virulence factor expressed by enterotoxigenic Escherichia coli. The route of LT secretion through the outer membrane and the cellular and extracellular localization of secreted LT were examined. Using a fluorescently labeled receptor, LT was found to be specifically secreted onto the surface of wild type enterotoxigenic Escherichia coli. The main terminal branch of the general secretory pathway (GSP) was necessary and sufficient to localize LT to the bacterial surface in a K-12 strain. LT is a heteromeric toxin, and we determined that its cell surface localization was mediated by the its B subunit independent of an intact G(M1) ganglioside binding site and that LT binds lipopolysaccharide and G(M1) concurrently. The majority of LT secreted into the culture supernatant by the GSP in E. coli associated with vesicles. Only a mutation in hns, not overexpression of the GSP or LT, caused an increase in vesicle yield, supporting a specific vesicle formation machinery regulated by the nucleoid-associated protein HNS. We propose a model in which LT is secreted by the GSP across the outer membrane, secreted LT binds lipopolysaccharide via a G(M1)-independent binding region on its B subunit, and LT on the surface of released outer membrane vesicles interacts with host cell receptors, leading to intoxication. These data explain a novel mechanism of vesicle-mediated receptor-dependent delivery of a bacterial toxin into a host cell.

Expression and purification of the recombinant enteropathogenic Escherichia coli vaccine candidates BfpA and EspB

BfpA, the structural repeating protein subunit A of the bundle-forming pilus and EspB, a type-III-secreted pore-forming protein of enteropathogenic Escherichia coli (EPEC), both virulence factors central for EPEC pathogenesis, were overexpressed in E. coli DH5alpha and M15 laboratory strains, respectively, using the pQE-30 cloning expression system, as chimeric fusions to a NH(2)-terminal histidine hexapeptide (His(6)-tag) sequence. After isopropyl beta-d-thiogalactoside induction, the expression levels achieved were 11 and 40% of total soluble protein for BfpA and EspB, respectively. The His(6)-tagged recombinant proteins were purified (up to 98% homogeneity) by Ni-agarose affinity chromatography and produced yields varying from 0.65 to 3.1 mg of recombinant protein per gram of wet weight cells. The immunogenicity and antigenicity of the final products were tested in rabbits and using fecal specimens obtained from children suffering from acute watery diarrhea, respectively. The recombinant products correspond to antigenically authentic protein standards, useful in future epidemiological and neonatal vaccinology studies.

Induction of epithelial cell death including apoptosis by enteropathogenic Escherichia coli expressing bundle-forming pili

Infection with enteropathogenic Escherichia coli (EPEC) is a major cause of severe infantile diarrhea, particularly in parts of the developing world. The bundle-forming pilus (BFP) of EPEC is an established virulence factor encoded on the EPEC adherence factor plasmid (EAF) and has been implicated in both localized adherence to host cells and bacterial autoaggregation. We investigated the role of BFP in the ability of EPEC binding to kill host epithelial cells. BFP-expressing strains killed all three cell lines tested, comprising HEp-2 (laryngeal), HeLa (cervical), and Caco-2 (colonic) cells. Analysis of phosphatidylserine expression, internucleosomal cleavage of host cell DNA, and morphological changes detected by electron microscopy indicated evidence of apoptosis. The extent of cell death was significantly greater for BFP-expressing strains, including E2348/69, a wild-type clinical isolate, as well as for a laboratory strain, HB101, transformed with a bfp-carrying plasmid. Strains which did not express BFP induced significantly less cell death, including a bfpA disruptional mutant of E2348/69, EAF plasmid-cured E2348/69, HB101, and HB101 complemented with the locus of enterocyte effacement pathogenicity island. These results indicate a direct correlation between BFP expression and induction of cell death, including apoptosis, an event which may involve the targeting of host cell membrane phosphatidylethanolamine.