Bacterial invasion: the paradigms of enteroinvasive pathogens

Species specificity of the Listeria monocytogenes InlB protein

InlA and InlB mediate L. monocytogenes entry into eukaryotic cells. InlA is required for the crossing of the intestinal and placental barriers. InlA uses E-cadherin as receptor in a species-specific manner. The human E-cadherin but not the mouse E-cadherin is a receptor for InlA. In human cells, InlB uses Met and gC1qR as receptors. By studying the role of InlB in vivo, we found that activation of Met by InlB is species-specific. In mice, InlB is important for liver and spleen colonization, but not for the crossing of the intestinal epithelium. Strikingly, the virulence of a DeltainlB deletion mutant is not attenuated in guinea pigs and rabbits. Guinea pig and rabbit cell lines do not respond to InlB, although expressing Met and gC1qR, but support InlB-mediated responses upon human Met gene transfection, indicating that InlB does not recognize or stimulate guinea pig and rabbit Met. In guinea pig cells, the effect of human Met gene transfection on InlB-dependent entry is increased upon cotransfection with human gc1qr gene, showing the additive roles of gC1qR and Met. These results unravel a second L. monocytogenes species specificity critical for understanding human listeriosis and emphasize the need for developing new animal models for studying InlA and InlB functions in the same animal model.

Bacterial entry into cells: a role for the endocytic machinery

Increasing evidence indicates that pathogens have evolved highly efficient strategies to induce their internalization within host cells. Viruses and bacteria express and expose on their surface, molecules that mimic endogenous ligands to cell receptors, thereby inducing specific intracellular signalling cascades. More recently it has become clear that, as most viruses, bacteria can enter cells via the clathrin-mediated pathway, indicating a key role for endocytosis in pathogens entry into cells. Here we review the pathways followed by Listeria monocytogenes to enter into non-phagocytic cells, as a model for the subversion of cellular functions to induce pathogens internalization.

Uptake, transport, delivery, and intracellular activity of antimicrobial agents

Antibiotic interactions with cells, including polymorphonuclear neutrophils, may influence therapeutic outcomes. Selected microbes (e.g., Legionella pneumophila) may survive ingestion by polymorphonuclear neutrophils and are thus protected from the action of antimicrobial agents that remain extracellular. Antibiotics that penetrate the cell can kill these microbes. Certain antibiotics are concentrated inside phagocytes, and when the phagocyte migrates toward the site of infection, the antibiotic-loaded cell carries the active agent to the infecting microbes. Active antibiotic may be released when the short-lived phagocyte dies. Even microbes considered to be extracellular pathogens, such as pneumococci, may survive high concentrations of antibiotic by entering cells. Antibiotics that penetrate and are active in cells may aid in enhancing therapeutic outcomes and in eliminating the carrier state for some pathogens.

The enteropathogenic Escherichia coli (EPEC) Map effector is imported into the mitochondrial matrix by the TOM/Hsp70 system and alters organelle morphology

Enteropathogenic Escherichia coli (EPEC) is a human intestinal pathogen and a major cause of diarrhoea, particularly among infants in developing countries. EPEC target the Map and EspF multifunctional effector proteins to host mitochondria - organelles that play crucial roles in regulating cellular processes such as programmed cell death (apoptosis). While both molecules interfere with the organelles ability to maintain a membrane potential, EspF plays the predominant role and is responsible for triggering cell death. To learn more about the Map-mitochondria interaction, we studied Map localization to mitochondria with purified mitochondria (from mammalian and yeast cells) and within intact yeast. This revealed that (i) Map targeting is dependent on the predicted N-terminal mitochondrial targeting sequence, (ii) the N-terminal 44 residues are sufficient to target proteins to mitochondria and (iii) Map import involves the mitochondrial outer membrane translocase (Tom22 and Tom40), the mitochondrial membrane potential, and the matrix chaperone, mtHsp70. These results are consistent with Map import into the mitochondria matrix via the classical import mechanism. As all known, Map-associated phenotypes in mammalian cells are independent of mitochondrial targeting, this may indicate that import serves as a mechanism to remove Map from the cytoplasm thereby regulating cytoplasmic function. Intriguingly, Map, but not EspF, alters mitochondrial morphology with deletion analysis revealing important roles for residues 101-152. Changes in mitochondrial morphology have been linked to alterations in the ability of these organelles to regulate cellular processes providing a possible additional role for Map import into mitochondria.

Bacterial adhesion and entry into host cells

Successful establishment of infection by bacterial pathogens requires adhesion to host cells, colonization of tissues, and in certain cases, cellular invasion-followed by intracellular multiplication, dissemination to other tissues, or persistence. Bacteria use monomeric adhesins/invasins or highly sophisticated macromolecular machines such as type III secretion systems and retractile type IV pili to establish a complex host/pathogen molecular crosstalk that leads to subversion of cellular functions and establishment of disease.

Structural microbiology at the pathogen-host interface

Bacterial pathogens achieve the internalization of a multitude of virulence factors into eukaryotic cells. Some secrete extracellular toxins which bring about their own entry, usually by hijacking cell surface receptors and endocytic pathways. Others possess specialized secretion and translocation systems to directly inject bacterial proteins into the host cytosol. Recent advances in the structural biology of these virulence factors has begun to reveal at the molecular level how these bacterial proteins are delivered and modulate host activities ranging from cytoskeletal structure to cell cycle progression.

PtdIns5P activates the host cell PI3-kinase/Akt pathway during Shigella flexneri infection

The virulence factor IpgD, delivered into nonphagocytic cells by the type III secretion system of the pathogen Shigella flexneri, is a phosphoinositide 4-phosphatase generating phosphatidylinositol 5 monophosphate (PtdIns5P). We show that PtdIns5P is rapidly produced and concentrated at the entry foci of the bacteria, where it colocalises with phosphorylated Akt during the first steps of infection. Moreover, S. flexneri-induced phosphorylation of host cell Akt and its targets specifically requires IpgD. Ectopic expression of IpgD in various cell types, but not of its inactive mutant, or addition of short-chain penetrating PtdIns5P is sufficient to induce Akt phosphorylation. Conversely, sequestration of PtdIns5P or reduction of its level strongly decreases Akt phosphorylation in infected cells or in IpgD-expressing cells. Accordingly, IpgD and PtdIns5P production specifically activates a class IA PI 3-kinase via a mechanism involving tyrosine phosphorylations. Thus, S. flexneri parasitism is shedding light onto a new mechanism of PI 3-kinase/Akt activation via PtdIns5P production that plays an important role in host cell responses such as survival.

Subversion of actin dynamics by EPEC and EHEC

During the course of infection, enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC, respectively) subvert the host cell signalling machinery and hijack the actin cytoskeleton to tighten their interaction with the gut epithelium, while avoiding phagocytosis by professional phagocytes. Much progress has been made recently in our understanding of how EPEC and EHEC regulate the pathways leading to local activation of two regulators of actin cytoskeleton dynamics, the Wiskott-Aldrich syndrome protein (N-WASP) and the Arp2/3 complex. A recent highlight is the unravelling of functions for effector proteins (particularly Tir, TccP, Map and EspG/EspG2) that are injected into the host cell by a type III secretion system.

Intracellular survival of Shigella

Bacterial invasion of eukaryotic cells and host recognition and killing of the invading bacteria are a key issue in determining the fate of bacterial infection. Once inside host cells, pathogenic bacteria often modify the phagosomal compartment or enter the host cytosol to escape from the lytic compartment and gain a replicative niche. Cytosolic invaders, however, are monitored by host innate immune systems, such as mediated by Nod/CARD family proteins, which induce inflammatory responses via activation of NF-kappaB. Furthermore, recent studies indicate that autophagy, a major cytoplasmic degradation system that eliminates cytosolic protein and organelles, also recognizes invading bacteria. Indeed, unless they are able to circumvent entrapping by autophagic membranes, bacteria targeted by autophagy ultimately undergo degradation by delivery into autolysosomes. In this article, we review recent advances in understanding of Shigella strategies to infect epithelial cells, and then focus on recent studies of an intriguing bacterial survival strategy against autophagic degradation.

Molecular motors hijacking by intracellular pathogens

Cargoes are transported intracellularly along cytoskeletal tracks composed of actin or tubulin. Their movement involves the action of molecular motor proteins that generate directed movement along microtubules or actin filaments. The three classes of molecular motors--kinesins, dyneins and myosins--are involved in a multiplicity of biological movements such as mitosis, positioning of organelles, intracellular transports and also vesicular sorting through membrane tubulation and fission and delivery to their target compartment. Intracellular pathogens use this molecular machinery to reach their site of replication, to leave their host or to control the dynamics of membrane exchanges with their replication compartment.

Meningitis-Associated Escherichia coli

Escherichia coliis the most common Gram-negative organism causing neonatal meningitis. Neonatal E. colimeningitis continues to be an important cause of mortality and morbidity throughout the world. Our incomplete knowledge of its pathogenesis and pathophysiology contributes to such mortality and morbidity. Recent reports of neonatal meningitis caused by E. coli strains producing CTX-M-type or TEM-type extended-spectrum β-lactamases create a challenge. E. colipenetration into the brain, the essential step in the development of E. coli meningitis, requires a high-degree of bacteremia and penetration of the blood-brain barrier as live bacteria, but the underlying mechanisms remain incompletely understood. Recent functional genomic approaches of meningitis-causing E. coli in both in vitro and in vivo models of the blood-brain barrier (e.g., human brain microvascular endothelial cells and animal models of experimental hematogenousE. colimeningitis, respectively) have identified several E. coli factors contributing to a high-degree of bacteremia, as well as specific microbial factors contributing to E. coli invasion of the blood-brain barrier. In addition, E. coli penetration of the blood-brain barrier involves specific host factors as well as microbe- and host-specific signaling molecules. Blockade of such microbial and host factors and host cell signaling molecules is efficient in preventing E. coli penetration into the brain. Continued investigation of the microbial and host factors contributing to E. colibacteremia andinvasion of the blood-brain barrier is likely to identify new targets for prevention and therapy of E. coli meningitis, thereby limiting the exposure to emerging antimicrobial-resistant E. coli.

Identification of a Bacterial Type III Effector Family with G Protein Mimicry Functions

Many bacterial pathogens use the type III secretion system to inject "effector" proteins into host cells. Here, we report the identification of a 24 member effector protein family found in pathogens including Salmonella, Shigella, and enteropathogenic E. coli. Members of this family subvert host cell function by mimicking the signaling properties of Ras-like GTPases. The effector IpgB2 stimulates cellular responses analogous to GTP-active RhoA, whereas IpgB1 and Map function as the active forms of Rac1 and Cdc42, respectively. These effectors do not bind guanine nucleotides or have sequences corresponding the conserved GTPase domain, suggesting that they are functional but not structural mimics. However, several of these effectors harbor intracellular targeting sequences that contribute to their signaling specificities. The activities of IpgB2, IpgB1, and Map are dependent on an invariant WxxxE motif found in numerous effectors leading to the speculation that they all function by a similar molecular mechanism.

Intercellular spreading of Porphyromonas gingivalis infection in primary gingival epithelial cells

Porphyromonas gingivalis, an important periodontal pathogen, is an effective colonizer of oral tissues. The organism successfully invades, multiplies in, and survives for extended periods in primary gingival epithelial cells (GECs). It is unknown whether P. gingivalis resides in the cytoplasm of infected cells throughout the infection or can spread to adjacent cells over time. We developed a technique based on flow cytofluorometry and fluorescence microscopy to study propagation of the organism at different stages of infection of GECs. Results showed that P. gingivalis spreads cell to cell and that the amount of spreading increases gradually over time. There was a very low level of propagation of bacteria to uninfected cells early in the infection (3 h postinfection), but there were 20-fold and 45-fold increases in the propagation rate after 24 h and 48 h, respectively, of infection. Immunofluorescence microscopy of infected cells suggested that intercellular translocation of P. gingivalis may be mediated through actin-based membrane protrusions, bypassing the need for release of bacteria into extracellular medium. Consistent with these observations, cytochalasin D treatment of infected cells resulted in significant inhibition of bacterial spreading. This study shows for the first time that P. gingivalis disseminates from cell to cell without passing through the extracellular space. This mechanism of spreading may allow P. gingivalis to colonize oral tissues without exposure to the humoral immune response.

Virological Synapse for Cell-Cell Spread of Viruses

Cell-to-cell spread of retroviruses via virological synapse (VS) contributes to overall progression of disease. VS are specialized pathogen-induced cellular structures that facilitate cell-to-cell transfer of HIV-1 and HTLV-1. VS provide a mechanistic explanation for cell-associated retroviral replication. While VS share some common features with neurological or immunological synapses, they also exhibit important differences. The role of VS might not be limited to human retroviruses and the emerging role of a plant synapse suggests that VS might well be conserved structures for cell-cell spreading of both animal and plant viruses. Dissection of the VS is just at its beginning, but already offers ample information and fascinating insights into mechanisms of viral replication and cell-to-cell communication.

Candida albicans biofilm-defective mutants

Biofilm formation plays a key role in the life cycles and subsistence of many microorganisms. For the human fungal pathogen Candida albicans, biofilm development is arguably a virulence trait, because medical implants that serve as biofilm substrates are significant risk factors for infection. The development of C. albicans biofilms in vitro proceeds through an early phase, in which yeast cells populate a substrate, an intermediate phase, in which pseudohyphal and hyphal cell types are produced, and a maturation phase, in which continued cell growth is accompanied by accumulation of an extracellular matrix. Here we report the results of a screen for C. albicans biofilm-defective mutants, in which homozygous insertions in NUP85, MDS3, KEM1, and SUV3 were found to block biofilm development. Confocal microscopic examination suggests that nup85, suv3, and mds3 mutations cause early-phase arrest, whereas the kem1 mutation causes intermediate-phase arrest. All of the mutants are defective in hypha production in several media. Analysis of mixed-biofilm development indicates that all of the mutants are defective in the production of hyphae in the context of a biofilm. Because all of the mutants are defective in the retention of cells in the biofilm, we infer that hyphae provide an adherent scaffold that stabilizes the biofilm structure.

Invasion processes of pathogenic Escherichia coli

Pathogenic Escherichia coli causes extraintestinal infections such as urinary tract infection and meningitis, which are prevalent and associated with considerable morbidity. Previous investigations have identified common strategies evolved by pathogenic E. coli to exploit host cell function and cause extraintestinal infections, which include the invasion into non-phagocytic eukaryotic cells such as epithelial and endothelial cells and associated host cell actin cytoskeletal rearrangements. However, the mechanisms involved in pathogenic E. coli invasion of eukaryotic cells are shown to differ depending upon types of host tissues and microbial determinants. In this mini-review, invasion processes of pathogenic E. coli are discussed using E. coli K1 invasion of human brain microvascular endothelial cells (HBMEC) as a paradigm. E. coli K1 is the most common Gram-negative organism causing neonatal meningitis, and E. coli invasion of HBMEC is shown to be a prerequisite for E. coli traversal of the blood-brain barrier in vivo. Previous studies have demonstrated that E. coli translocation of the blood-brain barrier is the result of specific E. coli host interactions including specific signal transduction pathways and modulation of endocytic pathways. Recent studies using functional genomics have identified additional microbial determinants contributing to E. coli K1 invasion of HBMEC. Complete understanding of microbial-host interactions that are involved in E. coli K1 invasion of HBMEC should help in the development of new strategies to prevent E. coli meningitis.

Apical spore phagocytosis is not a significant route of infection of differentiated enterocytes by Encephalitozoon intestinalis

Encephalitozoon intestinalis is a microsporidian species that infects the intestinal mucosal epithelium, primarily in immunodeficient individuals. The present study employed undifferentiated and differentiated human colonic carcinoma cell lines to determine if this parasite species infected polarized epithelial cells by spore phagocytosis or by impalement with the deployed spore polar tube. Apical surface spore attachment differed between cell lines such that SW480>HT-29>Caco-2>HCT-8, with attachment being greater to undifferentiated Caco-2 cells than differentiated cells and greater to partially differentiated HCT-8 cells than differentiated HCT-8 cells. Attachment was inhibited by chondroitin sulfate A, suggesting that it was mediated by host cell sulfated glycoaminoglycans. Infection rates 3 days postinfection paralleled spore attachment in the various cell lines. The undifferentiated cell line SW480 and undifferentiated Caco-2 and HCT-8 cells exhibited modest spore phagocytosis while the more differentiated cell line HT29 and differentiated Caco-2 and HCT-8 cells did not. All cell lines were impaled by the polar tubes of germinating spores. When normalized to the number of spores attached to the apical membrane, such impalement was greatest in the more differentiated Caco-2 and HCT-8 cells. The host cell apical surface influenced parasite spore germination, as in populations of large undifferentiated Caco-2 cells to which >3 spores had attached, the frequency distribution of the percentages of spores germinated per cell was bimodal, indicating that the surface of some cells favored germination, while others did not. This study suggests that phagocytosis is not a biologically significant mode of infection in differentiated intestinal epithelial cells.

Bacterial penetration of the mucosal barrier by targeting lipid rafts

Several traditionally extracellular pathogens not known to possess invasive capacity have been shown to invade various mucosal epithelial cells. The mucosal epithelium performs an important barrier function and is not typically amenable to bacterial invasion. Valuable clues to the underlying basis for bacterial invasion have emerged from recent studies examining the invasion of bladder epithelial cells by uropathogenic Escherichia coli and alveolar epithelial cells by Pseudomonas aeruginosa. In both cases, bacterial invasion is achieved through targeting of molecules specifically found within distinct glycosphingolipid- and cholesterol-enriched microdomains called lipid rafts. The importance of lipid rafts in promoting bacterial invasion was shown as disruptors of lipid rafts blocked cellular invasion by both E. coli and P. aeruginosa. In addition, molecular components of lipid rafts were found to be highly enriched in membranes encasing these intracellular bacteria. Furthermore, caveolin proteins, which serve to stabilize and organize lipid raft components, are necessary for bacterial entry. Taken together, targeting of lipid rafts appears to be an effective but poorly recognized mechanism used by pathogenic bacteria to circumvent the mucosal barriers of the host.

The rhoptry neck protein RON4 relocalizes at the moving junction during Toxoplasma gondii invasion

Host cell invasion in the Apicomplexa is unique in its dependency on a parasite actin-driven machinery and in the exclusion of most host cell membrane proteins during parasitophorous vacuole (PV) formation. This exclusion occurs at a junction between host cell and parasite plasma membranes that has been called the moving junction, a circumferential zone which forms at the apical tip of the parasite, moves backward and eventually pinches the PV from the host cell membrane. Despite having been described by electron microscopic studies 30 years ago, the molecular nature of this singular structure is still enigmatic. We have obtained a monoclonal antibody that recognizes the moving junction of invading tachyzoites of Toxoplasma gondii, in a pattern clearly distinct from those described so far for microneme and rhoptry proteins. The protein recognized by this antibody has been affinity purified. Mass spectrometry analysis showed that it is a rhoptry neck protein (RON4), a hypothetical protein with homologues restricted to Apicomplexa. Our findings reveals for the first time the participation of rhoptry neck proteins in moving junction formation and strongly suggest the conservation of this structure at the molecular level among Apicomplexa.

Ku70, a Component of DNA-Dependent Protein Kinase, Is a Mammalian Receptor for Rickettsia conorii

Rickettsia conorii, a strictly intracellular and category C priority bacterial pathogen (NIAID), invades different mammalian cells. Although some signaling events involved in bacterial entry have been documented, the bacterial and host proteins mediating entry were not known. We report the identification of the Ku70 subunit of DNA-dependent protein kinase (DNA-PK) as a receptor involved in R. conorii internalization. Ku70 is recruited to R. conorii entry sites, and inhibition of Ku70 expression impairs R. conorii internalization. Bacterial invasion is dependent on the presence of cholesterol-enriched microdomains containing Ku70. R. conorii infection stimulates the ubiquitination of Ku70. In addition, the ubiquitin ligase c-Cbl is recruited to R. conorii entry foci, and downregulation of endogenous c-Cbl blocks bacterial invasion and Ku70 ubiquitination. An affinity chromatography approach identified the rickettsial protein rOmpB as a ligand for Ku70. This is the first report of a receptor-ligand interaction involved in the internalization of any rickettsial species.

Abl kinases regulate actin comet tail elongation via an N-WASP-dependent pathway

Microbial pathogens have evolved diverse strategies to modulate the host cell cytoskeleton to achieve a productive infection and have proven instrumental for unraveling the molecular machinery that regulates actin polymerization. Here we uncover a mechanism for Shigella flexneri-induced actin comet tail elongation that links Abl family kinases to N-WASP-dependent actin polymerization. We show that the Abl kinases are required for Shigella actin comet tail formation, maximal intracellular motility, and cell-to-cell spread. Abl phosphorylates N-WASP, a host cell protein required for actin comet tail formation, and mutation of the Abl phosphorylation sites on N-WASP impairs comet tail elongation. Furthermore, we show that defective comet tail formation in cells lacking Abl kinases is rescued by activated forms of N-WASP. These data demonstrate for the first time that the Abl kinases play a role in the intracellular motility and intercellular dissemination of Shigella and uncover a new role for Abl kinases in the regulation of pathogen motility.

The many roads to cross-presentation

Cross-presentation of extracellular antigens by MHC class I molecules is required for priming cytotoxic T lymphocytes (CTLs) at locations remote from the site of infection. Various mechanisms have been proposed to explain cross-presentation. One such mechanism involves the fusion of the endoplasmic reticulum (ER) with the endosomal-phagosomal system, in which the machinery required for peptide loading of MHC class I molecules is introduced directly into the phagosome. Here, we discuss the evidence for and against the ER-phagosome concept as well as other possible mechanisms of cross-presentation.

Break on through to the Other Side-Biophysics and Cell Biology Shed Light on Cell-Penetrating Peptides

Cell-penetrating peptides (CPPs) have become widely used vectors for the cellular import of molecules in basic and applied biomedical research. Despite the broad acceptance of these molecules as molecular carriers, the details of the mode of cellular internalization and membrane permeation remain elusive. Within the last two years endocytosis has been demonstrated to be a route of uptake shared by several CPPs. These findings had a significant impact on CPP research. State-of-the-art cell biology is now required to advance the understanding of the intracellular fate of the CPP and cargo molecules. Owing to their presumed ability to cross lipid bilayers, CPPs also represent highly interesting objects of biophysical research. Numerous studies have investigated structure-activity relationships of CPPs with respect to their ability to bind to a lipid bilayer or to cross this barrier. Endocytosis route only relocates the membrane permeation from the cell surface to endocytic compartments. Therefore, biophysical experiments are key to a mechanistic molecular understanding of the cellular uptake of CPPs. However, biophysical investigations have to consider the molecular environment encountered by a peptide inside and outside a cell. In this contribution we will review biophysical and cell-biology data obtained for several prominent CPPs. Furthermore, we will summarize recent findings on the cell-penetrating characteristics of antimicrobial peptides and the antimicrobial properties of CPPs. Peptides of both groups have overlapping characteristics. Therefore, both fields may greatly benefit from each other. The review will conclude with a perspective of how biophysics and cell biology may synergize even more efficiently in the future.

Autophagy induction favours the generation and maturation of the Coxiella-replicative vacuoles

Pathogens evolved mechanisms to invade host cells and to multiply in the cytosol or in compositionally and functionally customized membrane-bound compartments. Coxiella burnetii, the agent of Q fever in man is a Gram-negative gamma-proteobacterium which multiplies in large, acidified, hydrolase-rich and fusogenic vacuoles with phagolysosomal-like characteristics. We reported previously that C. burnetii phase II replicative compartments are labelled by LC3, a protein specifically localized to autophagic vesicles. We show here that autophagy in Chinese hamster ovary cells, induced by amino acid deprivation prior to infection with Coxiella increased the number of infected cells, the size of the vacuoles, and their bacterial load. Furthermore, overexpression of GFP-LC3 or of GFP-Rab24 - a protein also localized to autophagic vacuoles - likewise accelerated the development of Coxiella-vacuoles at early times after infection. However, overexpression of mutants of those proteins that cannot be targeted to autophagosomes dramatically decreased the number and size of the vacuoles in the first hours of infection, although by 48 h the infection was similar to that of non-transfected controls. Overall, the results suggest that transit through the autophagic pathway increases the infection with Coxiella by providing a niche more favourable to their initial survival and multiplication.

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.

Characterization of ORF2 and its encoded protein of the Helicoverpa armigera nucleopolyhedrovirus

The open reading frame 2 (ha2) of the Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus (HaSNPV), a conserved gene in most baculoviruses from lepidopteran insects such as p78/83 of the Autographa californica MNPV, was characterized. It is 1,242 bp long and potentially encodes a 45.9 kDa. Ha2 is conserved among baculoviruses from lepidopteran insects. Ha2 transcripts were detected from 16 to 96 h post infection (hpi) of HzAM1 cells. Rabbit polyclonal antiserum against a GST-HA2 fusion protein reacted with three protein of 50, 46 and 35 kDa at 24-72 hpi of HzAM1 cells. Anti OpMNPV ORF2 (homologue of HA2) antibody reacted only with the 46 and 35 kDa proteins in HaSNPV-infected cells. These results demonstrate that Ha2 is modified at the mRNA or protein levels. Western blot analysis showed that only the 50 kDa product of HA2 is a structural component of proteins of both the budded virus (BV) and occlusion-derived virus (ODV) phenotypes. HA2-EGFP fusion protein showed that HA2 is localized primarily in the nucleus of HzAM1 infected cells. The HA2 was found to co-localize with actin by labelling of actin with Rhordamine-Phalloidin. In summary, the data indicated that HA2 is a structural protein and interacts with host cell actin.

Bacterial toxins activating Rho GTPases

The CNF1 toxin is produced by some uropathogenic (UPECs) andmeningitis-causing Escherichia coli strains. It belongs to a large family of bacterial virulence factors and toxins modifying cellular regulators of the actin cytoskeleton, namely the Rho GTPases. CNF1 autonomously enters the host cell cytosol, where it catalyzes the constitutive activation of Rho GTPases by deamidation. This activation is, however, attenuated because of activated Rho protein ubiquitin-mediated proteasomal degradation. Both Rho protein activation and deactivation confer phagocytic properties on epithelial and endothelial cells, as well as epithelial cell motility and cell-cell junction dynamics. Transcriptome analysis using DNA microarray revealed that endothelial cells respond to high doses of CNF1 by launching a genetic program of host alarm. This host cell reaction to CNF1 intoxication also indicates that degradation of activated Rho proteins by the proteasome may lead to a lowering of the threshold of the intoxicated cell inflammatory response. These results are consistent with growing evidence that Rho proteins control the cell inflammatory responses. It is tempting to assume that Rho deregulation may participate in various immunological disorders also involved in cancer.

The Mycobacterium tuberculosis ESAT-6 homologue in Listeria monocytogenes is dispensable for growth in vitro and in vivo

ESAT-6 is a virulence determinant in Mycobacterium tuberculosis and a member of a conserved group of proteins in a variety of other bacteria. A targeted deletion of the homologous gene in Listeria was generated, and in contrast to that observed for mycobacteria, this locus was not required for Listeria virulence.

Shigella and Salmonella: death as a means of survival

Shigella and Salmonella kill host cells and trigger inflammatory responses by mechanisms that are not fully understood. The goal of this review is to reevaluate key observations reported over the past 15 years and, whenever possible, to provide a chronological perspective as to how our understanding of the pathways by which Shigella and Salmonella kill host cells has evolved.

Microbial strategies to target, cross or disrupt epithelia

Epithelia are highly organized structures adapted to protect the underlying tissues from external aggressions, including microbial infections. Consequently, pathogens have evolved various strategies to target directly or indirectly intercellular junctions and/or components that maintain the structure of epithelia. Interestingly, some extracellular pathogens secrete enzymes that modify the extracellular part of junction components. Others produce toxins that are endocytosed and act from the inside of the cell to disrupt epithelial junctions. Other pathogens may directly inject into cells factors that are targeted to and destabilize the junctions, or that interact with signaling cascades that affect junction stability. Finally invasive bacteria or viruses may, by entering into cells, destabilize the junctions by targeting junction components directly or by inducing a series of events that lead to chemokine secretion, polymorphonuclear recruitment and inflammation.

Bacterial internalization in periodontitis

BACKGROUND

Bacterial invasion of host epithelial cells plays an important role in the pathogenesis of periodontitis; however, the exact mechanism of the invasion has not been investigated.

METHODS

Pocket epithelium biopsies in periodontitis were analysed via scanning and transmission electron microscopy using ultra-histochemical staining with ruthenium red for glycocalyx visualization.

RESULTS

We demonstrated that oral bacteria adhered via fimbriae-mediated adhesion only. The bacterial internalization in periodontitis was marked by the hallmark of the fimbriae-induced zipper mechanism--the phagocytic cup formation--but we found no sign of the trigger mechanism of internalization. In addition, we frequently observed apoptosis in the phagocytizing epithelial cells.

CONCLUSION

Fimbriae-mediated adhesion is a prerequisite for bacterial invasion in periodontitis. This occurs by the fimbriae-induced zipper mechanism of internalization. As internalization induces apoptosis, the subsequent exfoliation might play a significant role in the clearance of periodontal pathogens.

The Shigella flexneri effector OspG interferes with innate immune responses by targeting ubiquitin-conjugating enzymes

Bacteria of Shigella spp. are responsible for shigellosis in humans. They use a type III secretion system to inject effector proteins into host cells and induce their entry into epithelial cells or trigger apoptosis in macrophages. We present evidence that the effector OspG is a protein kinase that binds various ubiquitinylated ubiquitin-conjugating enzymes, including UbcH5, which belongs to the stem cell factor SCF(beta-TrCP) complex promoting ubiquitination of phosphorylated inhibitor of NF-kappaB type alpha (phospho-IkappaBalpha). Transfection experiments indicated that OspG can prevent phospho-IkappaBalpha degradation and NF-kappaB activation induced by TNF-alpha stimulation. Infection of epithelial cells by the S. flexneri wild-type strain, but not an ospG mutant, led to accumulation of phospho-IkappaBalpha, consistent with OspG inhibiting SCF(beta-TrCP) activity. Upon infection of ileal loops in rabbits, the ospG mutant induced a stronger inflammatory response than the wild-type strain. This finding indicates that OspG negatively controls the host innate response induced by S. flexneri upon invasion of the epithelium.

Host adaptor proteins Gab1 and CrkII promote InlB-dependent entry of Listeria monocytogenes

The bacterial surface protein InlB mediates internalization of Listeria monocytogenes into mammalian cells through interaction with the host receptor tyrosine kinase, Met. InlB/Met interaction results in activation of the host phosphoinositide (PI) 3-kinase p85-p110, an event required for bacterial entry. p85-p110 activation coincides with tyrosine phosphorylation of the host adaptor Gab1, and formation of complexes between Gab1 and the p85 regulatory subunit of PI 3-kinase. When phosphorylated in response to agonists, Gab1 is known to recruit several Src-homology 2 (SH2) domain-containing proteins including p85, the tyrosine phosphatase Shp2 and the adaptor CrkII. Here, we demonstrate that Gab1.p85 and Gab1.CrkII complexes promote entry of Listeria. Overexpression of wild-type Gab1 stimulated entry, whereas Gab1 alleles unable to recruit all SH2 proteins known to bind wild-type Gab1 inhibited internalization. Further analysis with Gab1 alleles defective in binding individual effectors suggested that recruitment of p85 and CrkII are critical for entry. Consistent with this data, overexpression of wild-type CrkII stimulated bacterial uptake. Experiments with mutant CrkII alleles indicated that both the first and second SH3 domains of this adaptor participate in entry, with the second domain playing the most critical role. Taken together, these findings demonstrate novel roles for Gab1 and CrkII in Listeria internalization.

Prediction of glycan structures from gene expression data based on glycosyltransferase reactions

MOTIVATION

Glycan chains are synthesized by a combination of several kinds of glycosyltransferases (GTs). Thus, once we know the repertoire of GTs in the genome, in the transcriptome or in the proteome, it should in principle be possible to predict the repertoire of possible glycan structures in an organism or at a specific stage of the cell. Here, we show that a repertoire of glycan structures can be predicted from the set of GTs in the transcriptome. That is, using knowledge about glycan structure characteristics, we can predict glycan structures from incomplete or noisy data such as DNA microarray data.

RESULTS

First, we constructed a reaction pattern library consisting of bond-formation patterns of GT reactions and investigated the co-occurrence frequencies of all reaction patterns in the glycan database. This was followed by the prediction of glycan structures using this library and a co-occurrence score. A penalty score was also implemented in the prediction method. Then we examined the performance of prediction by the leave-one-out cross validation method using individual reaction pattern profiles in the KEGG GLYCAN database as virtual expression profiles. The accuracy of prediction was 81%. Finally, we applied the prediction method to real expression data. Using expression profiles from the human carcinoma cell, glycan structures with sialic acid and sialyl Lewis X epitope were predicted, which corresponded well with experimental results.

Shigella effector IpaH9.8 binds to a splicing factor U2AF(35) to modulate host immune responses

Shigella effectors injected into the host cell via the type III secretion system are involved in various aspects of infection. Here, we show that one of the effectors, IpaH9.8, plays a role in modulating inflammatory responses to Shigella infection. In murine lung infection model, DeltaipaH9.8 mutant caused more severe inflammatory responses with increased pro-inflammatory cytokine production levels than did wild-type Shigella, which resulted in a 30-fold decrease in bacterial colonization. Binding assays revealed that IpaH9.8 has a specific affinity to U2AF(35), a mammalian splicing factor, which interferes with U2AF(35)-dependent splicing as assayed for IgM pre-mRNA. Reducing the U2AF(35) level in HeLa cells and infecting HeLa cells with wild-type caused a decrease in the expression of the il-8, RANTES, GM-CSF, and il-1beta genes as examined by RT-PCR. The results indicate that IpaH9.8 plays a role in Shigella infection to optimize the host inflammatory responses, thus facilitating bacterial colonization within the host epithelial cells.

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.

Going against the grain: chemotaxis and infection in Vibrio cholerae

Chemotaxis is the process by which motile cells move in a biased manner both towards favourable and away from unfavourable environments. The requirement of this process for infection has been examined in several bacterial pathogens, including Vibrio cholerae. The single polar flagellum of Vibrio species is powered by a sodium-motive force across the inner membrane, and can rotate to produce speeds of up to 60 cell-body lengths (approximately 60microm) per second. Investigating the role of the chemotactic control of rapid flagellar motility during V. cholerae infection has revealed some unexpected and intriguing results.

Cholesterol binding by the bacterial type III translocon is essential for virulence effector delivery into mammalian cells

A ubiquitous early step in infection of man and animals by enteric bacterial pathogens like Salmonella, Shigella and enteropathogenic Escherichia coli (EPEC) is the translocation of virulence effector proteins into mammalian cells via specialized type III secretion systems (TTSSs). Translocated effectors subvert the host cytoskeleton and stimulate signalling to promote bacterial internalization or survival. Target cell plasma membrane cholesterol is central to pathogen-host cross-talk, but the precise nature of its critical contribution remains unknown. Using in vitro cholesterol-binding assays, we demonstrate that Salmonella (SipB) and Shigella (IpaB) TTSS translocon components bind cholesterol with high affinity. Direct visualization of cell-associated fluorescently labelled SipB and parallel immunogold transmission electron microscopy revealed that cholesterol levels limit both the amount and distribution of plasma membrane-integrated translocon. Correspondingly, cholesterol depletion blocked effector translocation into cultured mammalian cells by not only the related Salmonella and Shigella TTSSs, but also the more divergent EPEC system. The data reveal that cholesterol-dependent association of the bacterial TTSS translocon with the target cell plasma membrane is essential for translocon activation and effector delivery into mammalian cells.

Listeria hijacks the clathrin-dependent endocytic machinery to invade mammalian cells

The bacterial pathogen Listeria monocytogenes uses the surface protein InlB to invade a variety of cell types. The interaction of InlB with the hepatocyte growth-factor receptor, Met, is crucial for infection to occur. Remarkably, the ubiquitin ligase Cbl is rapidly recruited to InlB-activated Met. Recent studies have shown that ligand-dependent endocytosis of Met and other receptor tyrosine kinases is triggered by monoubiquitination of the receptor, a process that is mediated by Cbl. Here, we show that purified InlB induces the Cbl-dependent monoubiquitination and endocytosis of Met. We then demonstrate that the bacterium exploits the ubiquitin-dependent endocytosis machinery to invade mammalian cells. First, we show that L. monocytogenes colocalizes with Met, EEA1, Cbl, clathrin and dynamin during entry. Then, we assess the role of different proteins of the endocytic machinery during L. monocytogenes infection. Over-expression or down-regulation of Cbl, respectively, increases or decreases bacterial invasion. Furthermore, RNA interference-mediated knock-down of major components of the endocytic machinery (for example, clathrin, dynamin, eps15, Grb2, CIN85, CD2AP, cortactin and Hrs), inhibit bacterial entry, establishing that the endocytic machinery is key to the bacterial internalization process.

Gp96 is a receptor for a novel Listeria monocytogenes virulence factor, Vip, a surface protein

By comparative genomics, we have identified a gene of the intracellular pathogen Listeria monocytogenes that encodes an LPXTG surface protein absent from nonpathogenic Listeria species. This gene, vip, is positively regulated by PrfA, the transcriptional activator of the major Listeria virulence factors. Vip is anchored to the Listeria cell wall by sortase A and is required for entry into some mammalian cells. Using a ligand overlay approach, we identified a cellular receptor for Vip, the endoplasmic reticulum (ER) resident chaperone Gp96 recently shown to interact with TLRs. The Vip-Gp96 interaction is critical for bacterial entry into some cells. Comparative infection studies using oral and intravenous inoculation of nontransgenic and transgenic mice expressing human E-cadherin demonstrated a role for Vip in Listeria virulence, not only at the intestine level but also in late stages of the infectious process. Vip thus appears as a new virulence factor exploiting Gp96 as a receptor for cell invasion and/or signalling events that may interfere with the host immune response in the course of the infection.

A new release on life: emerging concepts in proteolysis and parasite invasion

Cell invasion by apicomplexan pathogens such as the malaria parasite and Toxoplasma is accompanied by extensive proteolysis of zoite surface proteins (ZSPs) required for attachment and penetration. Although there is still little known about the proteases involved, a conceptual framework is emerging for the roles of proteolysis in cell invasion. Primary processing of ZSPs, which includes the trimming of terminal peptides or segmentation into multiple fragments, is proposed to activate these adhesive ligands for tight binding to host receptors. Secondary processing, which occurs during penetration, results in the shedding of ZSPs by one of two mechanistically distinct ways, shaving or capping. Resident surface proteins are typically shaved from the surface whereas adhesive ligands mobilized from intracellular secretory vesicles are capped to the posterior end of the parasite before being shed during the final steps of penetration. Intriguingly, recent studies have revealed that ZSPs can be released either by being cleaved adjacent to the membrane anchor or actually within the membrane itself. Mounting evidence suggests that intramembrane cleavage is catalysed by one or more integral membrane serine proteases of the Rhomboid family and we propose that several malaria adhesive ligands may be potential substrates for these enzymes. We also discuss the evidence that the key reason for ZSP shedding during invasion is to break the connection between parasite surface ligands and host receptors. The sequential proteolytic events associated with invasion by pathogenic protozoa may represent vulnerable pathways for the future development of synergistic anti-protozoal therapies.

Cortactin: an Achilles' heel of the actin cytoskeleton targeted by pathogens

Cortactin is an actin-binding protein and a central regulator of the actin cytoskeleton. Importantly, cortactin is also a common target exploited by microbes during infection. Its involvement in disease development is exemplified by a variety of pathogenic processes, such as pedestal formation [enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC)], invasion (Shigella, Neisseria, Rickettsia, Chlamydia, Staphylococcus and Cryptosporidium), actin-based motility (Listeria, Shigella and vaccinia virus) and cell scattering (Helicobacter). Recent progress turns our attention to how cortactin function can be regulated by serine and tyrosine phosphorylation. This has an important impact on how pathogens abuse cortactin to modulate the architecture of the host actin cytoskeleton.

Manipulation of innate immunity by bacterial pathogens

The past decade has witnessed tremendous growth in two related fields: innate immunity and microbial pathogenesis. Many pathogens have evolved mechanisms to infect their hosts in the face of a fully functional innate immune system, and there are numerous examples by which pathogens avoid recognition and/or suppress inflammation. In this review, I suggest that pathogens not only survive the innate immune response, but use it to promote their pathogenesis.

Comment on "Nanorobotics Control Design: A Collective Behavior Approach for Medicine

The limitations on nanorobot design and activity imposed by Brownian motion events, communication problems, and the nature of the intercellular space are discussed. It is shown that severe problems exist for a nanorobot designed to enter tissues for therapeutic purposes when it is smaller than about 1 microm in any one of its dimensions.

Pathogenesis of Afa/Dr diffusely adhering Escherichia coli

Over the last few years, dramatic increases in our knowledge about diffusely adhering Escherichia coli (DAEC) pathogenesis have taken place. The typical class of DAEC includes E. coli strains harboring AfaE-I, AfaE-II, AfaE-III, AfaE-V, Dr, Dr-II, F1845, and NFA-I adhesins (Afa/Dr DAEC); these strains (i) have an identical genetic organization and (ii) allow binding to human decay-accelerating factor (DAF) (Afa/Dr(DAF) subclass) or carcinoembryonic antigen (CEA) (Afa/Dr(CEA) subclass). The atypical class of DAEC includes two subclasses of strains; the atypical subclass 1 includes E. coli strains that express AfaE-VII, AfaE-VIII, AAF-I, AAF-II, and AAF-III adhesins, which (i) have an identical genetic organization and (ii) do not bind to human DAF, and the atypical subclass 2 includes E. coli strains that harbor Afa/Dr adhesins or others adhesins promoting diffuse adhesion, together with pathogenicity islands such as the LEE pathogenicity island (DA-EPEC). In this review, the focus is on Afa/Dr DAEC strains that have been found to be associated with urinary tract infections and with enteric infection. The review aims to provide a broad overview and update of the virulence aspects of these intriguing pathogens. Epidemiological studies, diagnostic techniques, characteristic molecular features of Afa/Dr operons, and the respective role of Afa/Dr adhesins and invasins in pathogenesis are described. Following the recognition of membrane-bound receptors, including type IV collagen, DAF, CEACAM1, CEA, and CEACAM6, by Afa/Dr adhesins, activation of signal transduction pathways leads to structural and functional injuries at brush border and junctional domains and to proinflammatory responses in polarized intestinal cells. In addition, uropathogenic Afa/Dr DAEC strains, following recognition of beta(1) integrin as a receptor, enter epithelial cells by a zipper-like, raft- and microtubule-dependent mechanism. Finally, the presence of other, unknown virulence factors and the way that an Afa/Dr DAEC strain emerges from the human intestinal microbiota as a "silent pathogen" are discussed.

IpgB1 is a novel Shigella effector protein involved in bacterial invasion of host cells. Its activity to promote membrane ruffling via Rac1 and Cdc42 activation

Shigella, the causative agent of bacillary dysentery, is capable of inducing the large scale membrane ruffling required for the bacterial invasion of host cells. Shigella secrete a subset of effectors via the type III secretion system (TTSS) into the host cells to induce membrane ruffling. Here, we show that IpgB1 is secreted via the TTSS into epithelial cells and plays a major role in producing membrane ruffles via stimulation of Rac1 and Cdc42 activities, thus promoting bacterial invasion of epithelial cells. The invasiveness of the ipgB1 mutant was decreased to less than 50% of the wild-type level (100%) in a gentamicin protection or plaque forming assay. HeLa cells infected with the wild-type or a IpgB1-hyperproducing strain developed membrane ruffles, with the invasiveness and the scale of membrane ruffles being comparable with the level of IpgB1 production in bacteria. Upon expression of EGFP-IpgB1 in HeLa cells, large membrane ruffles are extended, where the EGFP-IpgB1 was predominantly associated with the cytoplasmic membrane. The IpgB1-mediated formation of ruffles was significantly diminished by expressing Rac1 small interfering RNA and Cdc42 small interfering RNA or by treatment with GGTI-298, an inhibitor of the geranylgeranylation of Rho GTPases. When IpgB1 was expressed in host cells or wild-type Shigella-infected host cells, Rac1 and Cdc42 were activated. The results thus indicate that IpgB1 is a novel Shigella effector involved in bacterial invasion of epithelial cells via the activation of Rho GTPases.

The new bacterial cell biology: moving parts and subcellular architecture

Recent advances have demonstrated that bacterial cells have an exquisitely organized and dynamic subcellular architecture. Like their eukaryotic counterparts, bacteria employ a full complement of cytoskeletal proteins, localize proteins and DNA to specific subcellular addresses at specific times, and use intercellular signaling to coordinate multicellular events. The striking conceptual and molecular similarities between prokaryotic and eukaryotic cell biology thus make bacteria powerful model systems for studying fundamental cellular questions.

Actin-based motility of intracellular pathogens

The actin cytoskeleton is harnessed by several pathogenic bacteria that are capable of entering into non-phagocytic cells, the so-called 'invasive bacteria'. Among them, a few also exploit the host actin cytoskeleton to move intra- and inter-cellularly. Our knowledge of the basic mechanisms underlying actin-based motility has dramatically increased and the list of bacteria that are able to move in this way is also increasing including not only Listeria, Shigella and Rickettsia species but also Mycobacterium marinum and Burkholderia pseudomallei. In all cases the central player is the Arp2/3 complex. Vaccinia virus moves intracellularly on microtubules and just after budding, triggers actin polymerization and the formation of protrusions similar to that of adherent enteropathogenic Escherichia coli, that involve the Arp2/3 complex and facilitate its inter-cellular spread.

Tyrosine kinase signaling and type III effectors orchestrating Shigella invasion

Upon epithelial cell contact, Shigella type III effectors activate complex signaling pathways that induce localized membrane ruffling, resulting in Shigella invasion. Bacterial induced membrane ruffles require a timely coordination of cytoskeletal processes, including actin polymerization, filament reorganization and depolymerization, orchestrated by Rho GTPases and tyrosine kinases. An emerging concept is that multiple Shigella effectors act in synergy to promote actin polymerization in membrane extensions at the site of bacterial entry. Recent advances point to the role of Abl/Arg and Src tyrosine kinases as key regulators of bacterial induced cytoskeletal dynamics.