Citrobacter rodentium possesses a functional type II secretion system necessary for successful host infection

Infectious diarrheal diseases are the third leading cause of mortality in young children, many of which are driven by Gram-negative bacterial pathogens. To establish successful host infections these pathogens employ a plethora of virulence factors necessary to compete with the resident microbiota, and evade and subvert the host defenses. The type II secretion system (T2SS) is one such conserved molecular machine that allows for the delivery of effector proteins into the extracellular milieu. To explore the role of the T2SS during natural host infection, we used Citrobacter rodentium, a murine enteric pathogen, as a model of human intestinal disease caused by pathogenic Escherichia coli such as Enteropathogenic and Enterohemorrhagic E. coli (EPEC and EHEC). In this study, we determined that the C. rodentium genome encodes one T2SS and 22 potential T2SS-secreted protein effectors, as predicted via sequence homology. We demonstrated that this system was functional in vitro, identifying a role in intestinal mucin degradation allowing for its utilization as a carbon source, and promoting C. rodentium attachment to a mucus-producing colon cell line. During host infection, loss of the T2SS or associated effectors led to a significant colonization defect and lack of systemic spread. In mice susceptible to lethal infection, T2SS-deficient C. rodentium was strongly attenuated, resulting in reduced morbidity and mortality in infected hosts. Together these data highlight the important role of the T2SS and its effector repertoire during C. rodentium pathogenesis, aiding in successful host mucosal colonization.

A novel pathway of levodopa metabolism by commensal Bifidobacteria

The gold-standard treatment for Parkinson's disease is levodopa (L-DOPA), which is taken orally and absorbed intestinally. L-DOPA must reach the brain intact to exert its clinical effect; peripheral metabolism by host and microbial enzymes is a clinical management issue. The gut microbiota is altered in PD, with one consistent and unexplained observation being an increase in Bifidobacterium abundance among patients. Recently, certain Bifidobacterium species were shown to have the ability to metabolize L-tyrosine, an L-DOPA structural analog. Using both clinical cohort data and in vitro experimentation, we investigated the potential for commensal Bifidobacteria to metabolize this drug. In PD patients, Bifidobacterium abundance was positively correlated with L-DOPA dose and negatively with serum tyrosine concentration. In vitro experiments revealed that certain species, including B. bifidum, B. breve, and B. longum, were able to metabolize this drug via deamination followed by reduction to the compound 3,4-dihydroxyphenyl lactic acid (DHPLA) using existing tyrosine-metabolising genes. DHPLA appears to be a waste product generated during regeneration of NAD +. This metabolism occurs at low levels in rich medium, but is significantly upregulated in nutrient-limited minimal medium. Discovery of this novel metabolism of L-DOPA to DHPLA by a common commensal may help inform medication management in PD.

Cross-feeding between intestinal pathobionts promotes their overgrowth during undernutrition

Child undernutrition is a global health issue associated with a high burden of infectious disease. Undernourished children display an overabundance of intestinal pathogens and pathobionts, and these bacteria induce enteric dysfunction in undernourished mice; however, the cause of their overgrowth remains poorly defined. Here, we show that disease-inducing human isolates of Enterobacteriaceae and Bacteroidales spp. are capable of multi-species symbiotic cross-feeding, resulting in synergistic growth of a mixed community in vitro. Growth synergy occurs uniquely under malnourished conditions limited in protein and iron: in this context, Bacteroidales spp. liberate diet- and mucin-derived sugars and Enterobacteriaceae spp. enhance the bioavailability of iron. Analysis of human microbiota datasets reveals that Bacteroidaceae and Enterobacteriaceae are strongly correlated in undernourished children, but not in adequately nourished children, consistent with a diet-dependent growth synergy in the human gut. Together these data suggest that dietary cross-feeding fuels the overgrowth of pathobionts in undernutrition.

Dietary Intervention Reverses Fatty Liver and Altered Gut Microbiota during Early-Life Undernutrition

Nonalcoholic fatty liver disease (NAFLD), largely studied as a condition of overnutrition, also presents in undernourished populations. Like NAFLD, undernutrition disrupts systemic metabolism and has been linked to gut microbiota dysbiosis. Indeed, chronic exposures to fecal microbes contribute to undernutrition pathology in regions with poor sanitation. Despite a growing prevalence of fatty liver disease, the influence of undernutrition and the gut microbiota remain largely unexplored. Here, we utilize an established murine model (C57BL/6J mice placed on a malnourished diet that received iterative Escherichia coli/Bacteroidales gavage [MBG mice]) that combines a protein/fat-deficient diet and iterative exposure to specific, fecal microbes. Fecal-oral contamination exacerbates triglyceride accumulation in undernourished mice. MBG livers exhibit diffuse lipidosis accompanied by striking shifts in fatty acid, glycerophospholipid, and retinol metabolism. Multiomic analyses revealed metabolomic pathways linked to the undernourished gut microbiome and hepatic steatosis, including phenylacetate metabolism. Intriguingly, fatty liver features were observed only in the early-life, but not adult, MBG model despite similar liver metabolomic profiles. Importantly, we demonstrate that dietary intervention largely mitigates aberrant metabolomic and microbiome features in MBG mice. These findings indicate a crucial window in early-life development that, when disrupted by nutritional deficiency, may significantly influence liver function. Our work provides a multifaceted study of how diet and gut microbes inform fatty liver progression and reversal during undernutrition.IMPORTANCE Nonalcoholic fatty liver disease (NAFLD) remains a global epidemic, but it is often studied in the context of obesity and aging. Nutritional deficits, however, also trigger hepatic steatosis, influencing health trajectories in undernourished pediatric populations. Here, we report that exposure to specific gut microbes impacts fatty liver pathology in mice fed a protein/fat-deficient diet. We utilize a multiomics approach to (i) characterize NAFLD in the context of early undernutrition and (ii) examine the impact of diet and gut microbes in the pathology and reversal of hepatic steatosis. We provide compelling evidence that an early-life, critical development window facilitates undernutrition-induced fatty liver pathology. Moreover, we demonstrate that sustained dietary intervention largely reverses fatty liver features and microbiome shifts observed during early-life malnutrition.

Bacterial stimulation of the TLR-MyD88 pathway modulates the homeostatic expression of ileal Paneth cell α-defensins

Paneth cell α-defensins are antimicrobial peptides involved in the control of the intestinal microbiota and immunological homeostasis. In mice, they are encoded by multiple, highly homologous genes (Defa). The transcriptional activity of ileal Defa genes was studied in response to pharmacological and genetic perturbations of the intestinal environment of C57BL/6 mice. Defa gene transcription was sensitive to oral antibiotic administration suggesting that commensal microbes regulate Defa expression. Ileal microbiota analysis showed that decreased transcription of Defa genes correlated with depletion of Lactobacillus. Defa expression was partially restored in vivo by lactobacillus administration to antibiotic-treated mice. Defa transcripts were less abundant in ex vivo, microbiota-free intestinal explants but recovered after explant exposure to UV-killed bacteria, Toll-like receptor (TLR)-2 or TLR4 agonists. Genetic deficiency of several TLRs or MyD88 led to dramatic drops in Defa transcription in vivo. These results show that Paneth cell Defa genes are regulated by commensal bacteria through TLR-MyD88 signaling and provide a further understanding of the dysregulation of intestinal homeostasis that occurs as a result of imbalances in the populations of commensal bacteria.

The role of dynamin 3 in the testis

We report here that dynamin 3 in the testis is associated with structures termed tubulobulbar complexes that internalize intact intercellular junctions during sperm release and turnover of the blood-testis barrier. The protein lies adjacent to an actin-Arp2/3 network that cuffs the double plasma membrane tubular invagination at the core of each complex. To explore the possible relationship between dynamin 3 and nectin-based adhesion junctions, we transiently transfected DsRed-tagged dynamin 3 into MDCK cells stably transfected with eGFP-tagged nectin 2, one of the adhesion molecules known to be expressed in Sertoli cells at adhesion junctions. Cells transfected with the dynamin 3 construct had less uniformly distributed nectin 2 at intercellular contacts when compared to control cells expressing only nectin 2 or transfected with the DsRed plasmid alone. Significantly, tubular extensions positive for nectin 2 were visible projecting into the cells from regions of intercellular contact. Our findings are consistent with the conclusion that dynamin 3 is involved with tubulobulbar morphogenesis. Dynamin 3 also occurs in concentrated deposits around the capitulum and striated columns in the connecting piece of sperm tails suggesting that the protein in these cells may function to stabilize the base of the tail or serve as a reservoir for use during or after fertilization.

Salmonella, the host and disease: a brief review

Salmonella species cause substantial morbidity, mortality and burden of disease globally. Infections with Salmonella species cause multiple clinical syndromes. Central to the pathophysiology of all human salmonelloses is the induction of a strong host innate immune/inflammatory response. Whether this ultimately reflects an adaptive advantage to the host or pathogen is not clear. However, it is evident that both the host and pathogen have evolved mechanisms of triggering host responses that are detrimental to the other. In this review, we explore some of the host and pathogenic mechanisms mobilized in the two predominant clinical syndromes associated with infection with Salmonella enterica species: enterocolitis and typhoid.

Bacterial virulence strategies that utilize Rho GTPases

The ability to modify central host cellular functions is a major advantage to many bacterial pathogens that use such strategies as part of their virulence mechanisms. Small GTPases, including Rho GTPases, make particularly attractive targets for pathogens because of their central roles in modulating cellular functions such as cytoskeletal control. Such modifications of these GTPases can include direct chemical modification of the GTPase or interfacing with some of the regulatory elements associated with GTPase control. Pathogens use these alterations in GTPase functions for a variety of functions, including killing the host cell, mediating bacterial uptake into the host cell (invasion), reprogramming actin to form a lesion in host cells underlying adherent bacteria, to mediate intracellular survival by affecting intracellular trafficking, or to provide polymerized actin mechanisms to propel microbes around inside host cells and into adjacent cells. Collectively, these examples represent many key microbial virulence mechanisms that have led to a much deeper understanding of both microbial pathogens and GTPase functions.

Pathogens: Bacterial Needles Ruled to Length and Specificity

The mechanisms behind length regulation of prokaryotic surface structures has long eluded microbiologists. The recent identification of a protein that functions as a 'molecular ruler' to determine the physical length of a bacterial extracellular needle advances our understanding of surface structure biogenesis.

Microbial pathogenesis: new niches for salmonella

Salmonella occupies a vacuolar compartment inside cells of its host. Recent studies have shown that the fate of this vacuole is different in various cell types, and that the outcome of colonization is determined by both the infecting bacterium and defense mechanisms of the host cell.

Structural and biochemical characterization of the type III secretion chaperones CesT and SigE

Several Gram-negative bacterial pathogens have evolved a type III secretion system to deliver virulence effector proteins directly into eukaryotic cells, a process essential for disease. This specialized secretion process requires customized chaperones specific for particular effector proteins. The crystal structures of the enterohemorrhagic Escherichia coli O157:H7 Tir-specific chaperone CesT and the Salmonella enterica SigD-specific chaperone SigE reveal a common overall fold and formation of homodimers. Site-directed mutagenesis suggests that variable, delocalized hydrophobic surfaces observed on the chaperone homodimers are responsible for specific binding to a particular effector protein. Isothermal titration calorimetry studies of Tir-CesT and enzymatic activity profiles of SigD-SigE indicate that the effector proteins are not globally unfolded in the presence of their cognate chaperones.

Gene array technology to determine host responses to Salmonella

Gene expression array technology is a powerful new tool that has already been used to expand our understanding of host-pathogen interactions. There has been a rapid increase in published reports describing use of this approach to profile host responses to pathogenic bacteria and viruses. The large number of array studies currently in progress coupled with increasing accessibility of this new technology promises a plethora of gene expression data on host response to infection in the near future. Recent insights into macrophage and epithelial cell responses to Salmonella infection garnered from array studies are outlined and used as a basis to discuss various future research directions using gene arrays that will advance the field of cellular microbiology. There is an exciting potential for the gene expression data generated in such studies to provide insights into host physiology, the pathophysiology of disease and novel therapeutics.

Salmonella and apoptosis: to live or let die?

A successful pathogen manipulates its host for its own benefit. One means to establish a successful infection, especially for intracellular pathogens, is to exploit host cell death pathways and alter the viability of host cells. Here we describe the manipulation of apoptosis by Salmonella and discuss the advantages that such actions may confer to the bacteria, and its implications in resistance to disease.

Introduction: microbiology and immunology: lessons learned from Salmonella

Salmonella enterica, a Gram-negative bacterium, causes significant morbidity and mortality worldwide, and is an excellent model to study bacterial pathogenesis and cellular immune responses. With the development of powerful new technologies, there has been a fusion of research on immunology, molecular biology and cellular microbiology of S. enterica infections. This multidisciplinary research will enhance our understanding of the basic mechanisms of bacterial infections and immunity; it also provides new approaches towards therapeutic and control measures.

Locus of enterocyte effacement from Citrobacter rodentium: sequence analysis and evidence for horizontal transfer among attaching and effacing pathogens

The family of attaching and effacing (A/E) bacterial pathogens, which includes diarrheagenic enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC), remains a significant threat to human and animal health. These bacteria intimately attach to host intestinal cells, causing the effacement of brush border microvilli. The genes responsible for this phenotype are encoded in a pathogenicity island called the locus of enterocyte effacement (LEE). Citrobacter rodentium is the only known murine A/E pathogen and serves as a small animal model for EPEC and EHEC infections. Here we report the full DNA sequence of C. rodentium LEE and provide a comparative analysis with the published LEEs from EPEC, EHEC, and the rabbit diarrheagenic E. coli strain RDEC-1. Although C. rodentium LEE shows high similarities throughout the entire sequence and shares all 41 open reading frames with the LEE from EPEC, EHEC, and RDEC-1, it is unique in its location of the rorf1 and rorf2/espG genes and the presence of several insertion sequences (IS) and IS remnants. The LEE of EPEC and EHEC is inserted into the selC tRNA gene. In contrast, the Citrobacter LEE is flanked on one side by an operon encoding an ABC transport system, and an IS element and sequences homologous to Shigella plasmid R100 and EHEC pO157 flank the other. The presence of plasmid sequences next to C. rodentium LEE suggests that the prototype LEE resided on a horizontally transferable plasmid. Additional sequence analysis reveals that the 3-kb plasmid in C. rodentium is nearly identical to p9705 in EHEC O157:H7, suggesting that horizontal plasmid transfer among A/E pathogens has occurred. Our results indicate that the LEE has been acquired by C. rodentium and A/E E. coli strains independently during evolution.

Enteropathogenic Escherichia coli infection induces expression of the early growth response factor by activating mitogen-activated protein kinase cascades in epithelial cells

Enteropathogenic Escherichia coli (EPEC) is an extracellular bacterial pathogen that infects the human intestinal epithelium and is a major cause of infantile diarrhea in developing countries. EPEC belongs to the group of attaching and effacing (A/E) pathogens. It uses a type III secretion system to deliver proteins into the host cell that mediate signal transduction events in host cells. We used gene array technology to study epithelial cell responses to EPEC infection at the level of gene expression. We found that EPEC induces the expression of several genes in infected HeLa cells by a lipopolysaccharide (LPS)-independent mechanism, including cytokines and early growth response factor 1 (Egr-1). The transcription factor Egr-1 is an immediate-early-induced gene that is activated in most cell types in response to stress. EPEC-induced upregulation of egr-1 is mediated by the activation of the MEK/extracellular signal-regulated kinase signal transduction pathway and is dependent on the type III secretion system. egr-1 is also induced during infection of mice by the A/E pathogen Citrobacter rodentium, suggesting that both Egr-1 and the activation of this mitogen-activated protein kinase signal transduction pathway may play a role in disease.

Enteropathogenic E. coli Tir binds Nck to initiate actin pedestal formation in host cells

Enteropathogenic Escherichia coli (EPEC) is a bacterial pathogen that causes infantile diarrhea worldwide. EPEC injects a bacterial protein, translocated intimin receptor (Tir), into the host-cell plasma membrane where it acts as a receptor for the bacterial outer membrane protein, intimin. The interaction of Tir and intimin triggers a marked rearrangement of the host actin cytoskeleton into pedestals beneath adherent bacteria. On delivery into host cells, EPEC Tir is phosphorylated on tyrosine 474 of the intracellular carboxy-terminal domain, an event that is required for pedestal formation. Despite its essential role, the function of Tir tyrosine phosphorylation has not yet been elucidated. Here we show that tyrosine 474 of Tir directly binds the host-cell adaptor protein Nck, and that Nck is required for the recruitment of both neural Wiskott-Aldrich-syndrome protein (N-WASP) and the actin-related protein (Arp)2/3 complex to the EPEC pedestal, directly linking Tir to the cytoskeleton. Cells with null alleles of both mammalian Nck genes are resistant to the effects of EPEC on the actin cytoskeleton. These results implicate Nck adaptors as host-cell determinants of EPEC virulence.

Enterohaemorrhagic and enteropathogenic Escherichia coli use a different Tir-based mechanism for pedestal formation

Enterohaemorrhagic Escherichia coli (EHEC) adheres to the host intestinal epithelium, resulting in the formation of actin pedestals beneath adhering bacteria. EHEC and a related pathogen, enteropathogenic E. coli (EPEC), insert a bacterial receptor, Tir, into the host plasma membrane, which is required for pedestal formation. An important difference between EPEC and EHEC Tir is that EPEC but not EHEC Tir is tyrosine phosphorylated once delivered into the host. In this study, we assessed the role of Tir tyrosine phosphorylation in pedestal formation by EPEC and EHEC. In EPEC, pedestal formation is absolutely dependent on Tir tyrosine phosphorylation and is not complemented by EHEC Tir. The protein sequence surrounding EPEC Tir tyrosine 474 is critical for Tir tyrosine phosphorylation and pedestal formation by EPEC. In contrast, Tir tyrosine phosphorylation is not required for pedestal formation by EHEC. EHEC forms pedestals with both wild-type EPEC Tir and the non-tyrosine-phosphorylatable EPEC Tir Y474F. Pedestal formation by EHEC requires the type III delivery of additional EHEC factors into the host cell. These findings highlight differences in the mechanisms of pedestal formation by these closely related pathogens and indicate that EPEC and EHEC modulate different signalling pathways to affect the host actin cytoskeleton.

Characterization of Salmonella-induced filaments (Sifs) reveals a delayed interaction between Salmonella-containing vacuoles and late endocytic compartments

Salmonella typhimurium is a facultative intracellular pathogen that colonizes host cells throughout the course of infection. A unique feature of this pathogen is its ability to enter into (invade) epithelial cells and elongate the vacuole within which it resides into tubular structures called Salmonella-induced filaments (Sifs). In this study we sought to characterize the mechanism of Sif formation by immunofluorescence analysis using subcellular markers. The late endosomal lipid lysobisphosphatidic acid associated in a punctate pattern with the Salmonella-containing vacuole, starting 90 min after infection and increasing thereafter. Lysobisphosphatidic acid-rich vesicles were also found to interact with Sifs, at numerous sites along the tubules. Similarly, cholesterol-rich vesicles were also found in association with intracellular bacteria and Sifs. The lysosomal hydrolase cathepsin D was present in Sifs, both in a punctate pattern and, at later times, predominantly in an uninterrupted linear pattern. Rab7 associated with Sifs and expression of the N125I dominant negative mutant of this GTPase inhibited Sif formation. Transfection of HeLa cells with a vector encoding SifA fused to the green fluorescent protein caused swelling and aggregation of lysobisphosphatidic acid-containing compartments, suggesting that this virulence factor directs membrane fusion events involving late endosomes. Our findings demonstrate that Sif formation involves fusion of late endocytic compartments with the Salmonella-containing vacuole, and suggest that SifA modulates this event.

Pathogenic trickery: deception of host cell processes

Microbial pathogens cause a spectrum of diseases in humans. Although the disease mechanisms vary considerably, most pathogens have developed virulence factors that interact with host molecules, often usurping normal cellular processes, including cytoskeletal dynamics and vesicle targeting. These virulence factors often mimic host molecules, and mediate events as diverse as bacterial invasion, antiphagocytosis, and intracellular parastism.

Decreased apoptosis in the ileum and ileal Peyer's patches: a feature after infection with rabbit enteropathogenic Escherichia coli O103

Significant changes occur in intestinal epithelial cells after infection with enteropathogenic Escherichia coli (EPEC). However, it is unclear whether this pathogen alters rates of apoptosis. By using a naturally occurring weaned rabbit infection model, we determined physiological levels of apoptosis in rabbit ileum and ileal Peyer's patches (PP) and compared them to those found after infection with adherent rabbit EPEC (REPEC O103). Various REPEC O103 strains were first tested in vitro for characteristic virulence features. Rabbits were then inoculated with the REPEC O103 strains that infected cultured cells the most efficiently. After experimental infection, intestinal samples were examined by light and electron microscopy. Simultaneously, ileal apoptosis was assessed by using terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) and caspase 3 assays and by apoptotic cell counts based on morphology (hematoxylin-and-eosin staining). The highest physiological apoptotic indices were measured in PP germinal centers (median = 14.7%), followed by PP domed villi (8.1%), tips of absorptive villi (3.8%), and ileal crypt regions (0.5%). Severe infection with REPEC O103 resulted in a significant decrease in apoptosis in PP germinal centers (determined by TUNEL assay; P = 0.01), in the tips of ileal absorptive villi (determined by H&E staining; P = 0.04), and in whole ileal cell lysates (determined by caspase 3 assay; P = 0.001). We concluded that REPEC O103 does not promote apoptosis. Furthermore, we cannot rule out the possibility that REPEC O103, in fact, decreases apoptotic levels in the rabbit ileum.

Recruitment of cytoskeletal and signaling proteins to enteropathogenic and enterohemorrhagic Escherichia coli pedestals

Enteropathogenic Escherichia coli (EPEC) is a human pathogen that attaches to intestinal epithelial cells and causes chronic watery diarrhea. A close relative, enterohemorrhagic E. coli (EHEC), causes severe bloody diarrhea and hemolytic-uremic syndrome. Both pathogens insert a protein, Tir, into the host cell plasma membrane where it binds intimin, the outer membrane ligand of EPEC and EHEC. This interaction triggers a cascade of signaling events within the host cell and ultimately leads to the formation of an actin-rich pedestal upon which the pathogen resides. Pedestal formation is critical in mediating EPEC- and EHEC-induced diarrhea, yet very little is known about its composition and organization. In EPEC, pedestal formation requires Tir tyrosine 474 phosphorylation. In EHEC Tir is not tyrosine phosphorylated, yet the pedestals appear similar. The composition of the EPEC and EHEC pedestals was analyzed by examining numerous cytoskeletal, signaling, and adapter proteins. Of the 25 proteins examined, only two, calpactin and CD44, were recruited to the site of bacterial attachment independently of Tir. Several others, including ezrin, talin, gelsolin, and tropomyosin, were recruited to the site of EPEC attachment independently of Tir tyrosine 474 phosphorylation but required Tir in the host membrane. The remaining proteins were recruited to the pedestal in a manner dependent on Tir tyrosine phosphorylation or were not recruited at all. Differences were also found between the EPEC and EHEC pedestals: the adapter proteins Grb2 and CrkII were recruited to the EPEC pedestal but were absent in the EHEC pedestal. These results demonstrate that although EPEC and EHEC recruit similar cytoskeletal proteins, there are also significant differences in pedestal composition.

A synaptojanin-homologous region of Salmonella typhimurium SigD is essential for inositol phosphatase activity and Akt activation

The Ser-Thr kinase Akt is activated in epithelial cells by Salmonella enterica serovar typhimurium. The bacterial effector SigD, which is translocated into host cells via the specialized type III secretion system, is essential for Akt activation. Here, we investigated the inositol phospholipid substrate preferences of SigD. Recombinant SigD preferentially dephosphorylated phosphatidylinositol 3,5-biphosphate and phosphatidylinositol 3,4,5-triphosphate over other phosphatidylinositol lipids. Phosphatidylinositol 3-phosphate was not a substrate, suggesting the 5' phosphate moiety is one of the preferred substrates. Database searches revealed that SigD bears a small region of homology to the mammalian type II inositol 5-phosphatase synaptojanin. Mutation of two conserved residues in this region, Lys527 and Lys530, decreased or abrogated phosphatase activity, respectively. The Shigella flexneri SigD homologue, IpgD, displayed a similar activity in vitro and also activated Akt when used to complement a DeltasigD Salmonella strain. A mutation in IpgD at Lys507, analogous to Lys530 of SigD, also failed to activate Akt. Thus, we have characterized a region near the carboxyl-terminus of SigD which is important for phosphatase activity. We discuss how dephosphorylation of inositol phospholipids by SigD in vivo might contribute to the activation of Akt.

Bacterial pathogenesis: the answer to virulence is in the pore

A wide variety of Gram-negative bacterial pathogens use a 'type III' protein secretion system to deliver bacterial virulence factors into host cells. Recent results suggest that Gram-positive pathogens may employ similar methods to deliver virulence factors into host cells.

Gut feelings: enteropathogenic E. coli (EPEC) interactions with the host

Enteropathogenic Escherichia coli (EPEC) is a gram-negative bacterial pathogen that adheres to human intestinal epithelial cells, resulting in watery, persistent diarrhea. It subverts the host cell cytoskeleton, causing a rearrangement of cytoskeletal components into a characteristic pedestal structure underneath adherent bacteria. In contrast to other intracellular pathogens that affect the actin cytoskeleton from inside the host cytoplasm, EPEC remains extracellular and transmits signals through the host cell plasma membrane via direct injection of virulence factors by a "molecular syringe," the bacterial type III secretion system. One injected factor is Tir, which functions as the plasma membrane receptor for EPEC adherence. Tir directly links extracellular EPEC through the epithelial membrane and firmly anchors it to the host cell actin cytoskeleton, thereby initiating pedestal formation. In addition to stimulating actin nucleation and polymerization in the host cell, EPEC activates several other signaling pathways that lead to tight junction disruption, inhibition of phagocytosis, altered ion secretion, and immune responses. This review summarizes recent developments in our understanding of EPEC pathogenesis and discusses similarities and differences between EPEC pedestals, focal contacts, and Listeria monocytogenes actin tails.

Requirement for N-ethylmaleimide-sensitive factor activity at different stages of bacterial invasion and phagocytosis

Bacterial invasion, like the process of phagocytosis, involves extensive and localized protrusion of the host cell plasma membrane. To examine the molecular mechanisms of the membrane remodeling that accompanies bacterial invasion, soluble NSF attachment protein receptor (SNARE)-mediated membrane traffic was studied in cultured cells during infection by Salmonella typhimurium. A green fluorescent protein-tagged chimera of VAMP3, a SNARE characteristic of recycling endosomes, was found to accumulate at sites of Salmonella invasion. To analyze the possible role of SNARE-mediated membrane traffic in bacterial infection, invasion was measured in cells expressing a dominant-negative form of N-ethylmaleimide-sensitive factor (NSF), an essential regulator of membrane fusion. Inhibition of NSF activity did not affect cellular invasion by S. typhimurium nor the associated membrane remodeling. By contrast, Fcgamma receptor-mediated phagocytosis was greatly reduced in the presence of the mutant NSF. Most important, dominant-negative NSF significantly impaired the fusion of Salmonella-containing vacuoles with endomembranes. These observations indicate that the membrane protrusions elicited by Salmonella invasion, unlike those involved in phagocytosis, occur via an NSF-independent mechanism, whereas maturation of Salmonella-containing vacuoles is NSF-dependent.

SifA permits survival and replication of Salmonella typhimurium in murine macrophages

SifA was originally identified as a virulence factor required for formation of Salmonella-induced filaments (Sifs), elongated tubules rich in lysosomal glycoproteins that extend from the Salmonella-containing vacuole in infected epithelial cells. Here, we demonstrate that deletion mutants of ssaR, a component of the SPI-2 type III secretion system, do not form Sifs in HeLa epithelial cells. This suggests that SifA is a translocated effector of this system, acting within host cells to form Sifs. In support of this hypothesis, transfection of HeLa cells with a vector encoding SifA fused to the green fluorescent protein caused extensive vacuolation of LAMP-1-positive compartments. Filamentous tubules that closely resembled Sifs were also observed in transfected cells, demonstrating that SifA is sufficient to initiate alteration of host cell endosomal structures. deltasifA mutants were impaired in their ability to survive/replicate in RAW 264.7 murine macrophages, a phenotype similar to ssaR mutants. Our findings suggest that SifA is an effector of the SPI-2 type III secretion system and allows colonization of murine macrophages, the host niche exploited during systemic phases of disease in these animals. A family of SifA-related proteins and their importance to Salmonella pathogenesis is also discussed.

Tir tyrosine phosphorylation and pedestal formation are delayed in enteropathogenic Escherichia coli sepZ::TnphoA mutant 30-5-1(3)

Enteropathogenic Escherichia coli (EPEC) strain 30-5-1(3) has been reported to form attaching and effacing (A/E) lesions without Tir tyrosine phosphorylation. In this study, we show that 30-5-1(3), which has a transposon insertion within the sepZ gene, forms wild-type A/E lesions including Tir tyrosine phosphorylation, but at a slower rate. A/E lesion formation by 30-5-1(3) occurs without detectable secretion of Tir or other EPEC Esp secreted proteins.

Activation of Akt/protein kinase B in epithelial cells by the Salmonella typhimurium effector sigD

The serine-threonine kinase Akt is a protooncogene involved in the regulation of cell proliferation and survival. Activation of Akt is initiated by binding to the phospholipid products of phosphoinositide 3-kinase at the inner leaflet of the plasma membranes followed by phosphorylation at Ser(473) and Thr(308). We have found that Akt is activated by Salmonella enterica serovar Typhimurium in epithelial cells. A bacterial effector protein, SigD, which is translocated into host cells via the specialized type III secretion system, is essential for Akt activation. In HeLa cells, wild type S. typhimurium induced translocation of Akt to membrane ruffles and phosphorylation at residues Thr(308) and Ser(473) and increased kinase activity. In contrast, infection with a SigD deletion mutant did not induce phosphorylation or activity although Akt was translocated to membrane ruffles. Complementation of the SigD deletion strain with a mutant containing a single Cys to Ser mutation (C462S), did not restore the Akt activation phenotype. This residue has previously been shown to be essential for inositol phosphatase activity of the SigD homologue, SopB. Our data indicate a novel mechanism of Akt activation in which the endogenous cellular pathway does not convert membrane-associated Akt into its active form. SigD is also the first bacterial effector to be identified as an activator of Akt.

Microbial pathogenesis: lipid rafts as pathogen portals

The route of initial entry influences how host cells respond to intracellular pathogens. Recent studies have demonstrated that a wide variety of pathogens target lipid microdomains in host cell membranes, known as lipid rafts, to enter host cells as an infectious strategy.

Characteristics of Helicobacter pylori attachment to human primary antral epithelial cells

Conventional cell lines are commonly used to study infection characteristics of the human gastric pathogen Helicobacter pylori. We sought to investigate bacterial attachment to human antral primary epithelial cells, a cell model that more closely resembles the human stomach than transformed cell lines. Primary cells were infected for 24 and 48 h with H. pylori. Morphological appearance of both the pathogen and the cells as well as features of colonization, attachment and internalization were evaluated by electron microscopy and compared to features observed with cultured AGS cells. H. pylori exhibited various shapes during colonization including the spiral, U-shaped, donut, and coccoid forms. The prevalence of each form seemed to be dependent on the infected donor tissue but, in general, changed with time to the coccoid form. Bacterial cell membranes progressively enlarged and appeared at times to be connected with microvilli. Bacterial attachment occurred to cells that were either unchanged, or had formed cup-like structures. Simultaneously, outer membrane vesicles were increasingly secreted from the bacteria, coinciding with increased cellular damage. We conclude that bacterial shape conversion, adherence and secretion of outer membrane vesicles are features of H. pylori infection. Primary gastric cell cultures closely imitate the antral environment and present an appropriate and useful model to study H. pylori pathogenesis.

An alpha-helical cationic antimicrobial peptide selectively modulates macrophage responses to lipopolysaccharide and directly alters macrophage gene expression

Certain cationic antimicrobial peptides block the binding of LPS to LPS-binding protein and reduce the ability of LPS to induce the production of inflammatory mediators by macrophages. To gain a more complete understanding of how LPS activates macrophages and how cationic peptides influence this process, we have used gene array technology to profile gene expression patterns in macrophages treated with LPS in the presence or the absence of the insect-derived cationic antimicrobial peptide CEMA (cecropin-melittin hybrid). We found that CEMA selectively blocked LPS-induced gene expression in the RAW 264.7 macrophage cell line. The ability of LPS to induce the expression of >40 genes was strongly inhibited by CEMA, while LPS-induced expression of another 16 genes was relatively unaffected. In addition, CEMA itself induced the expression of a distinct set of 35 genes, including genes involved in cell adhesion and apoptosis. Thus, CEMA, a synthetic alpha-helical peptide, selectively modulates the transcriptional response of macrophages to LPS and can alter gene expression in macrophages.

Vacuole acidification is not required for survival of Salmonella enterica serovar typhimurium within cultured macrophages and epithelial cells

Phagosome acidification is an important component of the microbicidal response by infected eukaryotic cells. Thus, intracellular pathogens that reside within phagosomes must either block phagosome acidification or be able to survive at low pH. In this work, we studied the effect of phagosomal acidification on the survival of intracellular Salmonella enterica serovar Typhimurium in different cell types. Bafilomycin A1, a specific inhibitor of the vacuolar proton-ATPases, was used to block acidification of salmonella-containing vacuoles. We found that in several epithelial cell lines, treatment with bafilomycin A1 had no effect on intracellular survival or replication. Furthermore, although acidification was essential for Salmonella intracellular survival in J774 cultured macrophages, as reported previously (13), it is not essential in other macrophage cell lines. These data suggest that vacuolar acidification may play a role in intracellular survival of salmonellae only under certain conditions and in specific cell types.

Human response to Escherichia coli O157:H7 infection: antibodies to secreted virulence factors

Vaccination has been proposed for the prevention of disease due to enterohemorrhagic Escherichia coli (EHEC), but the immune response following human infection, including the choice of potential antigens, has not been well characterized. To study this, sera were obtained from five pediatric patients with acute diarrhea caused by E. coli O157:H7 0, 8, and 60 days after hospitalization. These sera were used to examine the immune response to four different EHEC virulence factors: Tir (translocated intimin receptor, which is inserted into the host cell membrane), intimin (bacterial outer membrane protein which binds to Tir), EspA (secreted protein which forms filamentous structures on EHEC surface), and EspB (inserted into the host membrane and cytoplasm). The response to O157:H7 lipopolysaccharide was also examined. Sera were assayed against purified recombinant proteins using immunoblot analysis and by enzyme-linked immunosorbent assay to determine the sera's titers to each of the antigens in all patients. We found that there was little reaction to EspA, EspB, and intimin in the acute-phase sera, although there was some reactivity to Tir. By day 8, titers of antibody to all four virulence factors were present in all patients, with a very strong response against Tir (up to a titer of 1:256,000), especially in hemolytic-uremic syndrome patients, and lesser strong responses to the other three antigens. The titer to the antigens 60 days after hospitalization was decreased but was still highest for Tir. These results suggest that there is a strong immune response to Tir, and to a lesser extent to the other three virulence factors, following EHEC disease, indicating that these bacterial molecules are potential vaccine candidates for preventing EHEC disease. They also suggest that bacterial virulence factors that are inserted into host cells during infection by type III secretion systems (Tir or EspB) are still recognized by the host immune response.

Exploitation of host cells by enteropathogenic Escherichia coli

Microbial pathogens have evolved many ingenious ways to infect their hosts and cause disease, including the subversion and exploitation of target host cells. One such subversive microbe is enteropathogenic Escherichia coli (EPEC). A major cause of infantile diarrhea in developing countries, EPEC poses a significant health threat to children worldwide. Central to EPEC-mediated disease is its colonization of the intestinal epithelium. After initial adherence, EPEC causes the localized effacement of microvilli and intimately attaches to the host cell surface, forming characteristic attaching and effacing (A/E) lesions. Considered the prototype for a family of A/E lesion-causing bacteria, recent in vitro studies of EPEC have revolutionized our understanding of how these pathogens infect their hosts and cause disease. Intimate attachment requires the type III-mediated secretion of bacterial proteins, several of which are translocated directly into the infected cell, including the bacteria's own receptor (Tir). Binding to this membrane-bound, pathogen-derived protein permits EPEC to intimately attach to mammalian cells. The translocated EPEC proteins also activate signaling pathways within the underlying cell, causing the reorganization of the host actin cytoskeleton and the formation of pedestal-like structures beneath the adherent bacteria. This review explores what is known about EPEC's subversion of mammalian cell functions and how this knowledge has provided novel insights into bacterial pathogenesis and microbe-host interactions. Future studies of A/E pathogens in animal models should provide further insights into how EPEC exploits not only epithelial cells but other host cells, including those of the immune system, to cause diarrheal disease.

Mechanical fractionation reveals structural requirements for enteropathogenic Escherichia coli Tir insertion into host membranes

Enteropathogenic Escherichia coli (EPEC) inserts its receptor for intimate adherence (Tir) into host cell membranes by using a type III secretion system. Detergents are frequently used to fractionate infected host cells to investigate bacterial protein delivery into mammalian cells. In this study, we found that the Triton X-100-soluble membrane fraction from EPEC-infected HeLa cells was contaminated with bacterial proteins. We therefore applied a mechanical method of cell lysis and ultracentrifugation to fractionate infected HeLa cells to investigate the biology and biochemistry of Tir delivery and translocation. This method demonstrates that the translocation of Tir into the host cell membrane requires its transmembrane domains, but not tyrosine phosphorylation or binding to Tir's ligand, intimin.

Crystal structure of enteropathogenic Escherichia coli intimin-receptor complex

Intimin and its translocated intimin receptor (Tir) are bacterial proteins that mediate adhesion between mammalian cells and attaching and effacing (A/E) pathogens. Enteropathogenic Escherichia coli (EPEC) causes significant paediatric morbidity and mortality world-wide. A related A/E pathogen, enterohaemorrhagic E. coli (EHEC; O157:H7) is one of the most important food-borne pathogens in North America, Europe and Japan. A unique and essential feature of A/E bacterial pathogens is the formation of actin-rich pedestals beneath the intimately adherent bacteria and localized destruction of the intestinal brush border. The bacterial outer membrane adhesin, intimin, is necessary for the production of the A/E lesion and diarrhoea. The A/E bacteria translocate their own receptor for intimin, Tir, into the membrane of mammalian cells using the type III secretion system. The translocated Tir triggers additional host signalling events and actin nucleation, which are essential for lesion formation. Here we describe the the crystal structures of an EPEC intimin carboxy-terminal fragment alone and in complex with the EPEC Tir intimin-binding domain, giving insight into the molecular mechanisms of adhesion of A/E pathogens.

Enteropathogenic E. coli translocated intimin receptor, Tir, interacts directly with alpha-actinin

Enteropathogenic Escherichia coli (EPEC) triggers a dramatic rearrangement of the host epithelial cell actin cytoskeleton to form an attaching and effacing lesion, or pedestal. The pathogen remains attached extracellularly to the host cell through the pedestal for the duration of the infection. At the tip of the pedestal is a bacterial protein, Tir, which is secreted from the bacterium into the host cell plasma membrane, where it functions as the receptor for an EPEC outer membrane protein, intimin [1]. Delivery of Tir to the host cell results in its tyrosine phosphorylation, followed by Tir-intimin binding. Tir is believed to anchor EPEC firmly to the host cell, although its direct linkage to the cytoskeleton is unknown. Here, we show that Tir directly binds the cytoskeletal protein alpha-actinin. alpha-Actinin is recruited to the pedestal in a Tir-dependent manner and colocalizes with Tir in infected host cells. Binding is mediated through the amino terminus of Tir. Recruitment of alpha-actinin occurs independently of Tir tyrosine phosphorylation. Recruitment of actin, VASP, and N-WASP, however, is abolished in the absence of this tyrosine phosphorylation. These results suggest that Tir plays at least three roles in the host cell during infection: binding intimin on EPEC; mediating a stable anchor with alpha-actinin through its amino terminus in a phosphotyrosine-independent manner; and recruiting additional cytoskeletal proteins at the carboxyl terminus in a phosphotyrosine-dependent manner. These findings demonstrate the first known direct linkage between extracellular EPEC, through the transmembrane protein Tir, to the host cell actin cytoskeleton via alpha-actinin.

Salmonella typhimurium infection and lipopolysaccharide stimulation induce similar changes in macrophage gene expression

Changes in macrophage phenotype induced during infection result from the recognition of bacterial products as well as the action of bacterial virulence factors. We used the unprecedented opportunity provided by gene arrays to simultaneously study the expression of hundreds of genes during Salmonella typhimurium infection of macrophages and to assess the contribution of the bacterial virulence factor, LPS, in initiating the host responses to Salmonella. We found that S. typhimurium infection caused significant changes in the expression of numerous genes encoding chemokines, cell surface receptors, signaling molecules, and transcriptional activators at 4 h postinfection of the RAW 264.7 murine macrophage cell line. Our results revealed changes in the expression of several genes that had not been previously implicated in the host responses to S. typhimurium infection, as well as changes in the expression of several genes previously shown to be regulated by S. typhimurium infection. An overlapping spectrum of genes was expressed in response to virulent S. typhimurium and purified S. typhimurium LPS, reinforcing the major role of this surface molecule in stimulating the early response of macrophages to bacterial infection. The macrophage gene expression profile was further altered by activation with IFN-gamma, indicating that host cell responses depend on the activation state of the cell.

Role of EspB in experimental human enteropathogenic Escherichia coli infection

Enteropathogenic Escherichia coli (EPEC), a leading cause of diarrhea among infants in developing countries, induces dramatic alterations in host cell architecture that depend on a type III secretion system. EspB, one of the proteins secreted and translocated to the host cytoplasm via this system, is required for numerous alterations in host cell structure and function. To determine the role of EspB in virulence, we conducted a randomized, double-blind trial comparing the ability of wild-type EPEC and an isogenic DeltaespB mutant strain to cause diarrhea in adult volunteers. Diarrhea developed in 9 of 10 volunteers who ingested the wild-type strain but in only 1 of 10 volunteers who ingested the DeltaespB mutant strain. Marked destruction of the microvillous brush border adjacent to adherent organisms was observed in a jejunal biopsy from a volunteer who ingested the wild-type strain but not from two volunteers who ingested the DeltaespB mutant strain. Humoral and cell-mediated immune responses to EPEC antigens were stronger among recipients of the wild-type strain. In addition, four of the volunteers who ingested the wild-type strain had lymphoproliferative responses to EspB. These results demonstrate that EspB is a critical virulence determinant of EPEC infections and suggest that EspB contributes to an immune response.

The NRAMP proteins of Salmonella typhimurium and Escherichia coli are selective manganese transporters involved in the response to reactive oxygen

NRAMPs (natural resistance-associated macrophage proteins) have been characterized in mammals as divalent transition metal transporters involved in iron metabolism and host resistance to certain pathogens. The mechanism of pathogen resistance is proposed to involve sequestration of Fe2+ and Mn2+, cofactors of both prokaryotic and eukaryotic catalases and superoxide dismutases, not only to protect the macrophage against its own generation of reactive oxygen species, but to deny the cations to the pathogen for synthesis of its protective enzymes. NRAMP homologues are also present in bacteria. We report the cloning and characterization of the single NRAMP genes in Escherichia coli and Salmonella enterica ssp. typhimurium, and the cloning of two distinct NRAMP genes from Pseudomonas aeruginosa and an internal fragment of an NRAMP gene in Burkholderia cepacia. The genes are designated mntH because the two enterobacterial NRAMPs encode H+-stimulated, highly selective manganese(II) transport systems, accounting for all Mn2+ uptake in each species under the conditions tested. For S. typhimurium MntH, the Km for 54Mn2+ ( approximately 0.1 microM) was pH independent, but maximal uptake increased as pH decreased. Monovalent cations, osmotic strength, Mg2+ and Ca2+ did not inhibit 54Mn2+ uptake. Ni2+, Cu2+ and Zn2+ inhibited uptake with Kis greater than 100 microM, Co2+ with a Ki of 20 microM and Fe2+ with a Ki that decreased from 100 microM at pH 7. 6 to 10 microM at pH 5.5. Fe3+ and Pb2+ inhibited weakly, exhibiting Kis of 50 microM, while Cd2+ was a potent inhibitor with a Ki of about 1 microM. E. coli MntH had a similar inhibition profile, except that Kis were three- to 10-fold higher. Both S. typhimurium and E. coli MntH also transport 55Fe2+ however, the Kms are equivalent to the Kis for Fe2+ inhibition of Mn2+ uptake, and are thus too high to be physiologically relevant. In both S. typhimurium and E. coli, mntH:lacZ constructs were strongly induced by hydrogen peroxide, weakly induced by EDTA and unresponsive to paraquat, consistent with the presence of Fur and OxyR binding sites in the promoters. Strains overexpressing mntH were more susceptible to growth inhibition by Mn2+ and Cd2+ than wild type, and strains lacking a functional mntH gene were more susceptible to killing by hydrogen peroxide. In S. typhimurium strain SL1344, mntH mutants showed no defect in invasion of or survival in cultured HeLa or RAW264.7 macrophage cells; however, expression of mntH:lacZ was induced severalfold by 3 h after invasion of the macrophages. S. typhimurium mntH mutants showed only a slight attenuation of virulence in BALB/c mice. Thus, the NRAMP Mn2+ transporter MntH and Mn2+ play a role in bacterial response to reactive oxygen species and possibly have a role in pathogenesis.