Pathogenic neisseriae: surface modulation, pathogenesis and infection control

Neisseria meningitidis induces platelet inhibition and increases vascular endothelial permeability via nitric oxide regulated pathways

Despite antibiotic therapy, infections with Neisseria meningitidis still demonstrate a high rate of morbidity and mortality even in developed countries. The fulminant septicaemic course, named Waterhouse-Friderichsen syndrome, with massive haemorrhage into the adrenal glands and widespread petechial bleeding suggest pathophysiological inhibition of platelet function. Our data show that N. meningitidis produces the important physiological platelet inhibitor and cardiovascular signalling molecule nitric oxide (NO), also known as endothelium-derived relaxing factor (EDRF). N. meningitidis -derived NO inhibited ADP-induced platelet aggregation through the activation of soluble guanylyl cyclase (sGC) followed by an increase in platelet cyclic nucleotide levels and subsequent activation of platelet cGMP- and cAMP- dependent protein kinases (PKG and PKA). Furthermore, direct measurement of horseradish peroxidase (HRP) passage through a vascular endothelial cell monolayer revealed that N. meningitidis significantly increased endothelial monolayer permeability. Immunfluorescence analysis demonstrated NO dependent disturbances in the structure of endothelial adherens junctions after co-incubation with N. meningitidis . In contrast to platelet inhibition, the NO effects on HBMEC were not mediated by cyclic nucleotides. Our study provides evidence that NO plays an essential role in the pathophysiology of septicaemic meningococcal infection.

A functional microsatellite of the macrophage migration inhibitory factor gene associated with meningococcal disease

Macrophage migration inhibitory factor (MIF) is an abundantly expressed proinflammatory cytokine playing a critical role in innate immunity and sepsis and other inflammatory diseases. We examined whether functional MIF gene polymorphisms (-794 CATT(5-8) microsatellite and -173 G/C SNP) were associated with the occurrence and outcome of meningococcal disease in children. The CATT(5) allele was associated with the probability of death predicted by the Pediatric Index of Mortality 2 (P=0.001), which increased in correlation with the CATT(5) copy number (P=0.04). The CATT(5) allele, but not the -173 G/C alleles, was also associated with the actual mortality from meningoccal sepsis [OR 2.72 (1.2-6.4), P=0.02]. A family-based association test (i.e., transmission disequilibrium test) performed in 240 trios with 1 afflicted offspring indicated that CATT(5) was a protective allele (P=0.02) for the occurrence of meningococcal disease. At baseline and after stimulation with Neisseria meningitidis in THP-1 monocytic cells or in a whole-blood assay, CATT(5) was found to be a low-expression MIF allele (P=0.005 and P=0.04 for transcriptional activity; P=0.09 and P=0.09 for MIF production). Taken together, these data suggest that polymorphisms of the MIF gene affecting MIF expression are associated with the occurrence, severity, and outcome of meningococcal disease in children.

Structure of the C-terminal domain of Neisseria heparin binding antigen (NHBA), one of the main antigens of a novel vaccine against Neisseria meningitidis

Neisseria heparin binding antigen (NHBA), also known as GNA2132 (genome-derived Neisseria antigen 2132), is a surface-exposed lipoprotein from Neisseria meningitidis that was originally identified by reverse vaccinology. It is one the three main antigens of a multicomponent vaccine against serogroup B meningitis (4CMenB), which has just completed phase III clinical trials in infants. In contrast to the other two main vaccine components, little is known about the origin of the immunogenicity of this antigen, and about its ability to induce a strong cross-bactericidal response in animals and humans. To characterize NHBA in terms of its structural/immunogenic properties, we have analyzed its sequence and identified a C-terminal region that is highly conserved in all strains. We demonstrate experimentally that this region is independently folded, and solved its three-dimensional structure by nuclear magnetic resonance. Notably, we need detergents to observe a single species in solution. The NHBA domain fold consists of an 8-strand β-barrel that closely resembles the C-terminal domains of N. meningitidis factor H-binding protein and transferrin-binding protein B. This common fold together with more subtle structural similarities suggest a common ancestor for these important antigens and a role of the β-barrel fold in inducing immunogenicity against N. meningitidis. Our data represent the first step toward understanding the relationship between structural, functional, and immunological properties of this important vaccine component.

Pathogenic Neisseria hitchhike on the uropod of human neutrophils

Polymorphonuclear neutrophils (PMNs) are important components of the human innate immune system and are rapidly recruited at the site of bacterial infection. Despite the effective phagocytic activity of PMNs, Neisseria gonorrhoeae infections are characterized by high survival within PMNs. We reveal a novel type IV pilus-mediated adherence of pathogenic Neisseria to the uropod (the rear) of polarized PMNs. The direct pilus-uropod interaction was visualized by scanning electron microscopy and total internal reflection fluorescence (TIRF) microscopy. We showed that N. meningitidis adhesion to the PMN uropod depended on both pilus-associated proteins PilC1 and PilC2, while N. gonorrhoeae adhesion did not. Bacterial adhesion elicited accumulation of the complement regulator CD46, but not I-domain-containing integrins, beneath the adherent bacterial microcolony. Electrographs and live-cell imaging of PMNs suggested that bacterial adherence to the uropod is followed by internalization into PMNs via the uropod. We also present data showing that pathogenic Neisseria can hitchhike on PMNs to hide from their phagocytic activity as well as to facilitate the spread of the pathogen through the epithelial cell layer.

A naturally occurring single-residue mutation in the translocator domain of Neisseria meningitidis NhhA affects trimerization, surface localization, and adhesive capabilities

Neisseria meningitidis NhhA (Neisseria hia/hsf homologue A) is an oligomeric outer membrane protein belonging to the family of trimeric autotransporter adhesins. NhhA mediates the interaction of N. meningitidis with human epithelial cells and components of the extracellular matrix. The recombinant protein is able to induce bactericidal antibodies and hence has also been considered a potential vaccine candidate. In this study, we analyzed the production of NhhA in a large panel of N. meningitidis strains belonging to different serogroups and clonal complexes. We found that trimeric NhhA was produced at different levels by the various strains tested. In some strains belonging to the clonal complex ST41/44, the protein is detectable only as a monomer. Sequencing of the nhhA gene and generation of complementing strains in different genetic backgrounds have proved that a single mutation (Gly to Asp) in the translocator domain affected both trimerization and surface localization of NhhA. In vitro infection assays showed that this mutation impairs meningococcal NhhA-mediated adhesion, suggesting that strains carrying the mutation may rely on different strategies or molecules to mediate interaction with host cells. Finally, we demonstrated that N. meningitidis ST41/44 strains producing the mutated form did not induce killing mediated by NhhA-specific bactericidal antibodies. Our data help to elucidate the secretion mechanisms of trimeric autotransporters and to understand the contribution of NhhA in the evolutionary process of host-Neisseria interactions. Also, they might have important implications for the evaluation of NhhA as a vaccine candidate.

Meningococcal meningitis

OPINION STATEMENT

Meningococcal meningitis (MM) is the most common presentation of meningococcal disease and an important cause of morbidity and mortality worldwide. When MM is associated with shock, early recognition and treatment of shock is essential. No investigation should delay starting antibiotics once the diagnosis is suspected. Corticosteroids can be started at the same time as the antibiotics or just before, but this is not a specific recommendation for MM. Low-dose steroids should be used in meningococcal disease with refractory shock. Altered blood flow, cerebral edema, and raised intracranial pressure are problems that should be considered in all patients with MM and decreased consciousness level. When mechanical ventilation is required, the target carbon dioxide level is 4.0 to 4.5 kPa, with avoidance of hypocapnia. Seizures, although not frequent, can occur in MM and require prompt treatment. Other treatments, such as mannitol and activated protein C, should be avoided. Potential new treatments requiring further investigation include neuroprotection with hypothermia or glycerol.

Vaccines for gonorrhea: can we rise to the challenge?

Immune responses to the gonococcus after natural infection ordinarily result in little immunity to reinfection, due to antigenic variation of the gonococcus, and redirection or suppression of immune responses. Brinton and colleagues demonstrated that parenteral immunization of male human volunteers with a purified pilus vaccine gave partial protection against infection by the homologous strain. However, the vaccine failed in a clinical trial. Recent vaccine development efforts have focused on the female mouse model of genital gonococcal infection. Here we discuss the state of the field, including our unpublished data regarding efficacy in the mouse model of either viral replicon particle (VRP) vaccines, or outer membrane vesicle (OMV) vaccines. The OMV vaccines failed, despite excellent serum and mucosal antibody responses. Protection after a regimen consisting of a PorB-VRP prime plus recombinant PorB boost was correlated with apparent Th1, but not with antibody, responses. Protection probably was due to powerful adjuvant effects of the VRP vector. New tools including novel transgenic mice expressing human genes required for gonococcal infection should enable future research. Surrogates for immunity are needed. Increasing antimicrobial resistance trends among gonococci makes development of a vaccine more urgent.

Experimental Gonococcal Infection in Male Volunteers: Cumulative Experience with Neisseria gonorrhoeae Strains FA1090 and MS11mkC

Experimental infection of male volunteers with Neisseria gonorrhoeae is safe and reproduces the clinical features of naturally acquired gonococcal urethritis. Human inoculation studies have helped define the natural history of experimental infection with two well-characterized strains of N. gonorrhoeae, FA1090 and MS11mkC. The human model has proved useful for testing the importance of putative gonococcal virulence factors for urethral infection in men. Studies with isogenic mutants have improved our understanding of the requirements for gonococcal LOS structures, pili, opacity proteins, IgA1 protease, and the ability of infecting organisms to obtain iron from human transferrin and lactoferrin during uncomplicated urethritis. The model also presents opportunities to examine innate host immune responses that may be exploited or improved in development and testing of gonococcal vaccines. Here we review results to date with human experimental gonorrhea.

The Pathobiology of Neisseria gonorrhoeae Lower Female Genital Tract Infection

Infection and disease associated with Neisseria gonorrhoeae, the gonococcus, continue to be a global health problem. Asymptomatic and subclinical gonococcal infections occur at a high frequency in females; thus, the true incidence of N. gonorrhoeae infections are presumed to be severely underestimated. Inherent to this asymptomatic/subclinical diseased state is the continued prevalence of this organism within the general population, as well as the medical, economic, and social burden equated with the observed chronic, disease sequelae. As infections of the lower female genital tract (i.e., the uterine cervix) commonly result in subclinical disease, it follows that the pathobiology of cervical gonorrhea would differ from that observed for other sites of infection. In this regard, the potential responses to infection that are generated by the female reproductive tract mucosa are unique in that they are governed, in part, by cyclic fluctuations in steroid hormone levels. The lower female genital tract has the further distinction of being able to functionally discriminate between resident commensal microbiota and transient pathogens. The expression of functionally active complement receptor 3 by the lower, but not the upper, female genital tract mucosa; together with data indicating that gonococcal adherence to and invasion of primary cervical epithelial cells and tissue are predominately aided by this surface-expressed host molecule; provide one explanation for asymptomatic/subclinical gonococcal cervicitis. However, co-evolution of the gonococcus with its sole human host has endowed this organism with variable survival strategies that not only aid these bacteria in successfully evasion of immune detection and function but also enhance cervical colonization and cellular invasion. To this end, we herein summarize current knowledge pertaining to the pathobiology of gonococcal infection of the human cervix.

Transcriptome analysis of Neisseria meningitidis in human whole blood and mutagenesis studies identify virulence factors involved in blood survival

During infection Neisseria meningitidis (Nm) encounters multiple environments within the host, which makes rapid adaptation a crucial factor for meningococcal survival. Despite the importance of invasion into the bloodstream in the meningococcal disease process, little is known about how Nm adapts to permit survival and growth in blood. To address this, we performed a time-course transcriptome analysis using an ex vivo model of human whole blood infection. We observed that Nm alters the expression of ≈30% of ORFs of the genome and major dynamic changes were observed in the expression of transcriptional regulators, transport and binding proteins, energy metabolism, and surface-exposed virulence factors. In particular, we found that the gene encoding the regulator Fur, as well as all genes encoding iron uptake systems, were significantly up-regulated. Analysis of regulated genes encoding for surface-exposed proteins involved in Nm pathogenesis allowed us to better understand mechanisms used to circumvent host defenses. During blood infection, Nm activates genes encoding for the factor H binding proteins, fHbp and NspA, genes encoding for detoxifying enzymes such as SodC, Kat and AniA, as well as several less characterized surface-exposed proteins that might have a role in blood survival. Through mutagenesis studies of a subset of up-regulated genes we were able to identify new proteins important for survival in human blood and also to identify additional roles of previously known virulence factors in aiding survival in blood. Nm mutant strains lacking the genes encoding the hypothetical protein NMB1483 and the surface-exposed proteins NalP, Mip and NspA, the Fur regulator, the transferrin binding protein TbpB, and the L-lactate permease LctP were sensitive to killing by human blood. This increased knowledge of how Nm responds to adaptation in blood could also be helpful to develop diagnostic and therapeutic strategies to control the devastating disease cause by this microorganism.

Resistance of Neisseria gonorrhoeae to neutrophils

Infection with the human-specific bacterial pathogen Neisseria gonorrhoeae triggers a potent, local inflammatory response driven by polymorphonuclear leukocytes (neutrophils or PMNs). PMNs are terminally differentiated phagocytic cells that are a vital component of the host innate immune response and are the first responders to bacterial and fungal infections. PMNs possess a diverse arsenal of components to combat microorganisms, including the production of reactive oxygen species and release of degradative enzymes and antimicrobial peptides. Despite numerous PMNs at the site of gonococcal infection, N. gonorrhoeae can be cultured from the PMN-rich exudates of individuals with acute gonorrhea, indicating that some bacteria resist killing by neutrophils. The contribution of PMNs to gonorrheal pathogenesis has been modeled in vivo by human male urethral challenge and murine female genital inoculation and in vitro using isolated primary PMNs or PMN-derived cell lines. These systems reveal that some gonococci survive and replicate within PMNs and suggest that gonococci defend themselves against PMNs in two ways: they express virulence factors that defend against PMNs' oxidative and non-oxidative antimicrobial components, and they modulate the ability of PMNs to phagocytose gonococci and to release antimicrobial components. In this review, we will highlight the varied and complementary approaches used by N. gonorrhoeae to resist clearance by human PMNs, with an emphasis on gonococcal gene products that modulate bacterial-PMN interactions. Understanding how some gonococci survive exposure to PMNs will help guide future initiatives for combating gonorrheal disease.

Neisseria meningitidis adhesin NadA targets beta1 integrins: functional similarity to Yersinia invasin

Meningococci are facultative-pathogenic bacteria endowed with a set of adhesins allowing colonization of the human upper respiratory tract, leading to fulminant meningitis and septicemia. The Neisseria adhesin NadA was identified in about 50% of N. meningitidis isolates and is closely related to the Yersinia adhesin YadA, the prototype of the oligomeric coiled-coil adhesin (Oca) family. NadA is known to be involved in cell adhesion, invasion, and induction of proinflammatory cytokines. Because of the enormous diversity of neisserial cell adhesins the analysis of the specific contribution of NadA in meningococcal host interactions is limited. Therefore, we used a non-invasive Y. enterocolitica mutant as carrier to study the role of NadA in host cell interaction. NadA was shown to be efficiently produced and localized in its oligomeric form on the bacterial surface of Y. enterocolitica. Additionally, NadA mediated a β1 integrin-dependent adherence with subsequent internalization of yersiniae by a β1 integrin-positive cell line. Using recombinant NadA(24-210) protein and human and murine β1 integrin-expressing cell lines we could demonstrate the role of the β1 integrin subunit as putative receptor for NadA. Subsequent inhibition assays revealed specific interaction of NadA(24-210) with the human β1 integrin subunit. Cumulatively, these results indicate that Y. enterocolitica is a suitable toolbox system for analysis of the adhesive properties of NadA, revealing strong evidence that β1 integrins are important receptors for NadA. Thus, this study demonstrated for the first time a direct interaction between the Oca-family member NadA and human β1 integrins.

New concepts in immunity to Neisseria gonorrhoeae: innate responses and suppression of adaptive immunity favor the pathogen, not the host

It is well-known that gonorrhea can be acquired repeatedly with no apparent development of protective immunity arising from previous episodes of infection. Symptomatic infection is characterized by a purulent exudate, but the host response mechanisms are poorly understood. While the remarkable antigenic variability displayed by Neisseria gonorrhoeae and its capacity to inhibit complement activation allow it to evade destruction by the host's immune defenses, we propose that it also has the capacity to avoid inducing specific immune responses. In a mouse model of vaginal gonococcal infection, N. gonorrhoeae elicits Th17-driven inflammatory–immune responses, which recruit innate defense mechanisms including an influx of neutrophils. Concomitantly, N. gonorrhoeae suppresses Th1- and Th2-dependent adaptive immunity, including specific antibody responses, through a mechanism involving TGF-β and regulatory T cells. Blockade of TGF-β alleviates the suppression of specific anti-gonococcal responses and allows Th1 and Th2 responses to emerge with the generation of immune memory and protective immunity. Genital tract tissues are naturally rich in TGF-β, which fosters an immunosuppressive environment that is important in reproduction. In exploiting this niche, N. gonorrhoeae exemplifies a well-adapted pathogen that proactively elicits from its host innate responses that it can survive and concomitantly suppresses adaptive immunity. Comprehension of these mechanisms of gonococcal pathogenesis should allow the development of novel approaches to therapy and facilitate the development of an effective vaccine.

Structural characterization of outer membrane components of the type IV pili system in pathogenic Neisseria

Structures of the type IV pili secretin complexes from Neisseria gonorrhoeae and Neisseria meningitidis, embedded in outer membranes were investigated by transmission electron microscopy. Single particle averaging revealed additional domains not observed previously. Secretin complexes of N. gonorrhoeae showed a double ring structure with a 14–15-fold symmetry in the central ring, and a 14-fold symmetry of the peripheral ring with 7 spikes protruding. In secretin complexes of N. meningitidis, the spikes were absent and the peripheral ring was partly or completely lacking. When present, it had a 19-fold symmetry. The structures of the complexes in several pil mutants were determined. Structures obtained from the pilC1/C2 adhesin and the pilW minor pilin deletion strains were similar to wild-type, whereas deletion of the homologue of N. meningitidis PilW resulted in the absence of secretin structures. Remarkably, the pilE pilin subunit and pilP lipoprotein deletion mutants showed a change in the symmetry of the peripheral ring from 14 to 19 and loss of spikes. The pilF ATPase mutant also lost the spikes, but maintained 14-fold symmetry. These results show that secretin complexes contain previously unidentified large and flexible extra domains with a probable role in stabilization or assembly of type IV pili.

Neisseria meningitidis has two independent modes of recognizing its human receptor CEACAM1

BACKGROUND

Several human-restricted gram-negative bacteria exploit carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) for host colonization. For example, Neisseria meningitidis engages these human receptors via outer membrane proteins of the colony opacity-associated (Opa) protein family triggering internalization into non-phagocytic cells.

PRINCIPAL FINDINGS

We report that a non-opaque strain of N. meningitidis selectively interacts with CEACAM1, but not other CEACAM family members. Using functional assays of bacterial adhesion and internalisation, microscopic analysis, and a panel of CEACAM1 deletion mutants we demonstrate that the engagement of CEACAM1 by non-opaque meningococci occurs in a manner distinct from Opa protein-mediated association. In particular, the amino-terminal domain of CEACAM1 is necessary, but not sufficient for Opa protein-independent binding, which requires multiple extracellular domains of the human receptor in a cellular context. Knock-down of CEACAM1 interferes with binding to lung epithelial cells, whereas chemical or pharmacological disruption of host protein glycosylation does not abrogate CEACAM1 recognition by non-opaque meningococci. The previously characterized meningococcal invasins NadA or Opc do not operate in a CEACAM1-dependent manner.

CONCLUSIONS

The results demonstrate a mechanistically distinct, Opa protein-independent interaction between N. meningitidis and human CEACAM1. Our functional investigations suggest the presence of a second CEACAM1-binding invasin on the meningococcal surface that associates with the protein backbone and not the carbohydrate structures of CEACAM1. The redundancy in meningococcal CEACAM1-binding factors further highlights the important role of CEACAM recognition in the biology of this human-adapted pathogen.

Protein folding in bacterial adhesion: secretion and folding of classical monomeric autotransporters

Bacterial adhesins mediate the attachment of bacteria to their niches, such as the tissue of an infected host. Adhesins have to be transported across the cell envelope to become active and during this secretion process they fold into their final conformation. This chapter focuses on the biogenesis of the classical monomeric autotransporter proteins, which are the most ubiquitous class of secreted proteins in Gram-negative bacteria. They may function as adhesins, but other functions are also known. Autotransporter proteins have a modular structure and consist of an N-terminal signal peptide and a C-terminal translocator domain with in between the secreted passenger domain that harbours the functions. The signal peptide directs the transport across the inner membrane to the periplasm via the Sec machinery. The translocator domain inserts into the outer membrane and facilitates the transport of the passenger to the cell surface. In this chapter, I will review our current knowledge of the secretion of classical monomeric autotransporters and the methods that have been used to assess their folding during the translocation, both in vitro and in vivo.

Infections of people with complement deficiencies and patients who have undergone splenectomy

The complement system comprises several fluid-phase and membrane-associated proteins. Under physiological conditions, activation of the fluid-phase components of complement is maintained under tight control and complement activation occurs primarily on surfaces recognized as "nonself" in an attempt to minimize damage to bystander host cells. Membrane complement components act to limit complement activation on host cells or to facilitate uptake of antigens or microbes "tagged" with complement fragments. While this review focuses on the role of complement in infectious diseases, work over the past couple of decades has defined several important functions of complement distinct from that of combating infections. Activation of complement in the fluid phase can occur through the classical, lectin, or alternative pathway. Deficiencies of components of the classical pathway lead to the development of autoimmune disorders and predispose individuals to recurrent respiratory infections and infections caused by encapsulated organisms, including Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae. While no individual with complete mannan-binding lectin (MBL) deficiency has been identified, low MBL levels have been linked to predisposition to, or severity of, several diseases. It appears that MBL may play an important role in children, who have a relatively immature adaptive immune response. C3 is the point at which all complement pathways converge, and complete deficiency of C3 invariably leads to severe infections, including those caused by meningococci and pneumococci. Deficiencies of the alternative and terminal complement pathways result in an almost exclusive predisposition to invasive meningococcal disease. The spleen plays an important role in antigen processing and the production of antibodies. Splenic macrophages are critical in clearing opsonized encapsulated bacteria (such as pneumococci, meningococci, and Escherichia coli) and intraerythrocytic parasites such as those causing malaria and babesiosis, which explains the fulminant nature of these infections in persons with anatomic or functional asplenia. Paramount to the management of patients with complement deficiencies and asplenia is educating patients about their predisposition to infection and the importance of preventive immunizations and seeking prompt medical attention.

The role of glyceraldehyde 3-phosphate dehydrogenase (GapA-1) in Neisseria meningitidis adherence to human cells

Background

Glyceraldehyde 3-phosphate dehydrogenases (GAPDHs) are cytoplasmic glycolytic enzymes, which although lacking identifiable secretion signals, have also been found localized to the surface of several bacteria (and some eukaryotic organisms); where in some cases they have been shown to contribute to the colonization and invasion of host tissues. Neisseria meningitidis is an obligate human nasopharyngeal commensal which can cause life-threatening infections including septicaemia and meningitis. N. meningitidis has two genes, gapA-1 and gapA-2, encoding GAPDH enzymes. GapA-1 has previously been shown to be up-regulated on bacterial contact with host epithelial cells and is accessible to antibodies on the surface of capsule-permeabilized meningococcal cells. The aims of this study were: 1) to determine whether GapA-1 was expressed across different strains of N. meningitidis; 2) to determine whether GapA-1 surface accessibility to antibodies was dependant on the presence of capsule; 3) to determine whether GapA-1 can influence the interaction of meningococci and host cells, particularly in the key stages of adhesion and invasion.

Results

In this study, expression of GapA-1 was shown to be well conserved across diverse isolates of Neisseria species. Flow cytometry confirmed that GapA-1 could be detected on the cell surface, but only in a siaD-knockout (capsule-deficient) background, suggesting that GapA-1 is inaccessible to antibody in in vitro-grown encapsulated meningococci. The role of GapA-1 in meningococcal pathogenesis was addressed by mutational analysis and functional complementation. Loss of GapA-1 did not affect the growth of the bacterium in vitro. However, a GapA-1 deficient mutant showed a significant reduction in adhesion to human epithelial and endothelial cells compared to the wild-type and complemented mutant. A similar reduction in adhesion levels was also apparent between a siaD-deficient meningococcal strain and an isogenic siaD gapA-1 double mutant.

Conclusions

Our data demonstrates that meningococcal GapA-1 is a constitutively-expressed, highly-conserved surface-exposed protein which is antibody-accessible only in the absence of capsule. Mutation of GapA-1 does not affect the in vitro growth rate of N. meningitidis, but significantly affects the ability of the organism to adhere to human epithelial and endothelial cells in a capsule-independent process suggesting a role in the pathogenesis of meningococcal infection.

Bacterial interactions with the host epithelium

The gastrointestinal epithelium deploys multiple innate defense mechanisms to fight microbial intruders, including epithelial integrity, rapid epithelial cell turnover, quick expulsion of infected cells, autophagy, and innate immune responses. Nevertheless, many bacterial pathogens are equipped with highly evolved infectious stratagems that circumvent these defense systems and use the epithelium as a replicative foothold. During replication on and within the gastrointestinal epithelium, gastrointestinal bacterial pathogens secrete various components, toxins, and effectors that can subvert, usurp, and exploit host cellular functions to benefit bacterial survival. In addition, bacterial pathogens use a variety of mechanisms that balance breaching the epithelial barrier with maintaining the epithelium in order to promote bacterial colonization. These complex strategies represent a new paradigm of bacterial pathogenesis.

Meningococcal internalization into human endothelial and epithelial cells is triggered by the influx of extracellular L-glutamate via GltT L-glutamate ABC transporter in Neisseria meningitidis

Meningococcal internalization into human cells is likely to be a consequence of meningococcal adhesion to human epithelial and endothelial cells. Here, we identified three transposon mutants of Neisseria meningitidis that were primarily defective in the internalization of human brain microvascular endothelial cells (HBMEC), with insertions occurring in the gltT (a sodium-independent L-glutamate transporter) gene or its neighboring gene, NMB1964 (unknown function). NMB1964 was tentatively named gltM in this study because of the presence of a mammalian cell entry (MCE)-related domain in the deduced amino acid sequences. The null ΔgltT-ΔgltM N. meningitidis mutant was also defective in the internalization into human umbilical vein endothelial cells and the human lung carcinoma epithelial cell line A549, and the defect was suppressed by transcomplementation of the mutants with gltT(+)-gltM(+) genes. The intracellular survival of the ΔgltT-ΔgltM mutant in HBMEC was not largely different from that of the wild-type strain under our experimental conditions. Introduction of a1-bp deletion and amber or ochre mutations in gltT-gltM genes resulted in the loss of efficient internalization into HBMEC. The defect in meningococcal internalization into HBMEC and L-glutamate uptake in the ΔgltT-ΔgltM mutant were suppressed only in strains expressing both GltT and GltM proteins. The efficiency of meningococcal invasion to HBMEC decreased under L-glutamate-depleted conditions. Furthermore, ezrin, a key membrane-cytoskeleton linker, accumulated beneath colonies of the gltT(+)-gltM(+) N. meningitidis strain but not of the ΔgltT-ΔgltM mutant. These findings suggest that l-glutamate influx via the GltT-GltM L-glutamate ABC transporter serves as a cue for N. meningitidis internalization into host cells.

Evidence for a non-replicative intracellular stage of nontypable Haemophilus influenzae in epithelial cells

Nontypable Haemophilus influenzae (NTHi) is a Gram-negative, non-capsulated human bacterial pathogen, a major cause of a repertoire of respiratory infections, and intimately associated with persistent lung bacterial colonization in patients suffering from chronic obstructive pulmonary disease (COPD). Despite its medical relevance, relatively little is known about its mechanisms of pathogenicity. In this study, we found that NTHi invades the airway epithelium by a distinct mechanism, requiring microtubule assembly, lipid rafts integrity, and activation of phosphatidylinositol 3-kinase (PI3K) signalling. We found that the majority of intracellular bacteria are located inside an acidic subcellular compartment, in a metabolically active and non-proliferative state. This NTHi-containing vacuole (NTHi-CV) is endowed with late endosome features, co-localizing with LysoTracker, lamp-1, lamp-2, CD63 and Rab7. The NTHi-CV does not acquire Golgi- or autophagy-related markers. These observations were extended to immortalized and primary human airway epithelial cells. By using NTHi clinical isolates expressing different amounts of phosphocholine (PCho), a major modification of NTHi lipooligosaccharide, on their surfaces, and an isogenic lic1BC mutant strain lacking PCho, we showed that PCho is not responsible for NTHi intracellular location. In sum, this study indicates that NTHi can survive inside airway epithelial cells.

Human-restricted bacterial pathogens block shedding of epithelial cells by stimulating integrin activation

Colonization of mucosal surfaces is the key initial step in most bacterial infections. One mechanism protecting the mucosa is the rapid shedding of epithelial cells, also termed exfoliation, but it is unclear how pathogens counteract this process. We found that carcinoembryonic antigen (CEA)-binding bacteria colonized the urogenital tract of CEA transgenic mice, but not of wild-type mice, by suppressing exfoliation of mucosal cells. CEA binding triggered de novo expression of the transforming growth factor receptor CD105, changing focal adhesion composition and activating beta1 integrins. This manipulation of integrin inside-out signaling promotes efficient mucosal colonization and represents a potential target to prevent or cure bacterial infections.

Analogies and homologies in lipopolysaccharide and glycoprotein biosynthesis in bacteria

Bacteria generate and attach countless glycan structures to diverse macromolecules. Despite this diversity, the mechanisms of glycoconjugate biosynthesis are often surprisingly similar. The focus of this review is on the commonalities between lipopolysaccharide (LPS) and glycoprotein assembly pathways and their evolutionary relationship. Three steps that are essential for both pathways are completed by membrane proteins. These include the initiation of glycan assembly through the attachment of a first sugar residue onto the lipid carrier undecaprenyl pyrophosphate, the translocation across the plasma membrane and the final transfer onto proteins or lipid A-core. Two families of initiating enzymes have been described: the polyprenyl-P N-acetylhexosamine-1-P transferases and the polyprenyl-P hexosamine-1-P transferases, represented by Escherichia coli WecA and Salmonella enterica WbaP, respectively. Translocases are either Wzx-like flippases or adenosine triphosphate (ATP)-binding cassette transporters (ABC transporters). The latter can consist either of two polypeptides, Wzt and Wzm, or of a single polypeptide homolog to the Campylobacter jejuni PglK. Finally, there are two families of conjugating enzymes, the N-oligosaccharyltransferases (N-OTase), best represented by C. jejuni PglB, and the O-OTases, including Neisseria meningitidis PglL and the O antigen ligases involved in LPS biosynthesis. With the exception of the N-OTases, probably restricted to glycoprotein synthesis, members of all these transmembrane protein families can be involved in the synthesis of both glycoproteins and LPS. Because many translocation and conjugation enzymes display relaxed substrate specificity, these bacterial enzymes could be exploited in engineered living bacteria for customized glycoconjugate production, generating potential vaccines and therapeutics.

Tyrosine-phosphorylated caveolin-1 blocks bacterial uptake by inducing Vav2-RhoA-mediated cytoskeletal rearrangements

During the early stages of infection, Neisseria gonorrhoeae triggers a phosphotyrosine-dependent Cav1-Vav2-RhoA signaling cascade that promotes the pathogen's extracellular state.

Certain bacterial adhesins appear to promote a pathogen's extracellular lifestyle rather than its entry into host cells. However, little is known about the stimuli elicited upon such pathogen host-cell interactions. Here, we report that type IV pili (Tfp)-producing Neisseria gonorrhoeae (P⁺GC) induces an immediate recruitment of caveolin-1 (Cav1) in the host cell, which subsequently prevents bacterial internalization by triggering cytoskeletal rearrangements via downstream phosphotyrosine signaling. A broad and unbiased analysis of potential interaction partners for tyrosine-phosphorylated Cav1 revealed a direct interaction with the Rho-family guanine nucleotide exchange factor Vav2. Both Vav2 and its substrate, the small GTPase RhoA, were found to play a direct role in the Cav1-mediated prevention of bacterial uptake. Our findings, which have been extended to enteropathogenic Escherichia coli, highlight how Tfp-producing bacteria avoid host cell uptake. Further, our data establish a mechanistic link between Cav1 phosphorylation and pathogen-induced cytoskeleton reorganization and advance our understanding of caveolin function.

Like many bacterial pathogens, successful attachment of Neisseria gonorrhoeae—the causative agent of the sexually transmitted disease gonorrhoea—to its host cells depends on specialized structures on the bacterial surface called type IV pili (Tfp). Pathogen attachment induces changes within host cells that may facilitate and promote infection. In this study, we identify some of the earliest cellular signals elicited by N. gonorrhoeae during infection, which, in this case, prevent the organism from entering the cell precociously. After attachment to host cells the bacteria form microcolonies on the cell surface. Underneath these microcolonies, so-called cortical plaques form within the host cell—these contain the cytoskeleton protein actin and a range of signaling proteins. We show that N. gonorrhoeae recruits a host cell protein called caveolin-1 to the cell membrane where the bacteria are attached; here, caveloin-1 effectively impedes uptake of the bacteria by activating a signaling cascade that involves its phosphorylation on a tyrosine residue and subsequent interactions with proteins that regulate the cytoskeleton. Thus, these proteins play a pivotal role in maintaining N. gonorrhoeae in the extracellular milieu. By extrapolating our findings to another Tfp-producing bacterium, the enteropathogenic Escherichia coli, we argue that the establishment and maintenance of this extracellular state benefits certain pathogens by giving them time to express proteins required for subsequent steps of infection.

Comparative genome biology of a serogroup B carriage and disease strain supports a polygenic nature of meningococcal virulence

Neisseria meningitidis serogroup B strains are responsible for most meningococcal cases in the industrialized countries, and strains belonging to the clonal complex ST-41/44 are among the most prevalent serogroup B strains in carriage and disease. Here, we report the first genome and transcriptome comparison of a serogroup B carriage strain from the clonal complex ST-41/44 to the serogroup B disease strain MC58 from the clonal complex ST-32. Both genomes are highly colinear, with only three major genome rearrangements that are associated with the integration of mobile genetic elements. They further differ in about 10% of their gene content, with the highest variability in gene presence as well as gene sequence found for proteins involved in host cell interactions, including Opc, NadA, TonB-dependent receptors, RTX toxin, and two-partner secretion system proteins. Whereas housekeeping genes coding for metabolic functions were highly conserved, there were considerable differences in their expression pattern upon adhesion to human nasopharyngeal cells between both strains, including differences in energy metabolism and stress response. In line with these genomic and transcriptomic differences, both strains also showed marked differences in their in vitro infectivity and in serum resistance. Taken together, these data support the concept of a polygenic nature of meningococcal virulence comprising differences in the repertoire of adhesins as well as in the regulation of metabolic genes and suggest a prominent role for immune selection and genetic drift in shaping the meningococcal genome.

Systematic functional analysis reveals that a set of seven genes is involved in fine-tuning of the multiple functions mediated by type IV pili in Neisseria meningitidis

Type IV pili (Tfp), which mediate multiple phenotypes ranging from adhesion to motility, are one of the most widespread virulence factors in bacteria. However, the molecular mechanisms of Tfp biogenesis and associated functions remain poorly understood. One of the underlying reasons is that the roles played by the numerous genes involved in Tfp biology are unclear because corresponding mutants have been studied on a case-by-case basis, in different species, and using different assays, often generating heterogeneous results. Therefore, we have recently started a systematic functional analysis of the genes involved in Tfp biology in a well-characterized clinical isolate of the human pathogen Neisseria meningitidis. After previously studying 16 genes involved in Tfp biogenesis, here we report the characterization of 7 genes that are dispensable for piliation and potentially involved in Tfp biology. Using a battery of assays, we assessed piliation and each of the Tfp-linked functions in single mutants, double mutants in which filament retraction is abolished by a concurrent mutation in pilT, and strains overexpressing the corresponding proteins. This showed that each of the seven genes actually fine-tunes a Tfp-linked function(s), which brings us one step closer to a global view of Tfp biology in the meningococcus.

Virulence determinants involved in differential host niche adaptation of Neisseria meningitidis and Neisseria gonorrhoeae

Neisseria meningitidis and Neisseria gonorrhoeae are the only pathogenic species of the genus Neisseria. Although these two species are closely related, they specialized on survival in completely different environments within the human host-the nasopharynx in the case of N. meningitidis versus the urogenital tract in the case of N. gonorrhoeae. The genetic background of these differences has not yet been determined. Here, we present a comparison of all characterized transcriptional regulators in these species, delineating analogous functions and disclosing differential functional developments of these DNA-binding proteins with a special focus on the recently characterized regulator FarR and its contribution to divergent host niche adaptation in the two Neisseria spp. Furthermore, we summarize the present knowledge on two-partner secretion systems in meningococci, highlighting their overall expression among meningococcal strains in contrast to the complete absence in gonococci. Concluding, the decisive role of these two entirely different factors in host niche adaptation of the two human pathogenic Neisseria species is depicted, illuminating another piece of the puzzle to locate the molecular basis of their differences in preferred colonization sites and pathogenicity.

Structural basis for solute transport, nucleotide regulation, and immunological recognition of Neisseria meningitidis PorB

PorB is the second most prevalent outer membrane protein in Neisseria meningitidis. PorB is required for neisserial pathogenesis and can elicit a Toll-like receptor mediated host immune response. Here, the x-ray crystal structure of PorB has been determined to 2.3 A resolution. Structural analysis and cocrystallization studies identify three putative solute translocation pathways through the channel pore: One pathway transports anions nonselectively, one transports cations nonselectively, and one facilitates the specific uptake of sugars. During infection, PorB likely binds host mitochondrial ATP, and cocrystallization with the ATP analog AMP-PNP suggests that binding of nucleotides regulates these translocation pathways both by partial occlusion of the pore and by restricting the motion of a putative voltage gating loop. PorB is located on the surface of N. meningitidis and can be recognized by receptors of the host innate immune system. Features of PorB suggest that Toll-like receptor mediated recognition outer membrane proteins may be initiated with a nonspecific electrostatic attraction.

Biosynthesis and production of polysialic acids in bacteria

Polysialic acids (PA) are protective capsular sialohomopolymers present in some bacteria which can invade the mammalian host and cause lethal bacteremia and meningitis. Biosynthesis and translocation of PA to the cell surface are equivalent in different species and bacterial strains which are produced. The diversity in PA structure is derived from the PA linkages and is a consequence of the specific sialyltransferase activities. The monomer acetylation and the polymer length could be important factors in the potential virulence. In vivo PA production is affected by different physical and chemical factors. The temperature of cellular growth strictly regulates PA genesis through a molecular complex and multifactorial mechanism that operate to transcription level.

Entry of Neisseria meningitidis into mammalian cells requires the Src family protein tyrosine kinases

Neisseria meningitidis, the causative agent of meningitis and septicemia, is able to attach to and invade a variety of cell types. In a previous study we showed that entry of N. meningitidis into human brain microvascular endothelial cells (HBMEC) is mediated by fibronectin bound to the outer membrane protein Opc, which forms a molecular bridge to alpha 5 beta 1-integrins. This interaction results in cytoskeletal remodeling and uptake of the bacteria. In this study we identified and characterized the intracellular signals involved in integrin-initiated uptake of N. meningitidis. We determined that the Src protein tyrosine kinases (PTKs) are activated in response to contact with N. meningitidis. Inhibition of Src PTK activity by the general tyrosine kinase inhibitor genistein and the specific Src inhibitor PP2 reduced Opc-mediated invasion of HBMEC and human embryonic kidney (HEK) 293T cells up to 90%. Moreover, overexpression of the cellular Src antagonist C-terminal Src kinase (CSK) also significantly reduced N. meningitidis invasion. Src PTK-deficient fibroblasts were impaired in the ability to internalize N. meningitidis and showed reduced phosphorylation of the cytoskeleton and decreased development of stress fibers. These data indicate that the Src family PTKs, particularly the Src protein, along with other proteins, are important signal proteins that are responsible for the transfer of signals from activated integrins to the cytoskeleton and thus mediate the endocytosis of N. meningitidis into brain endothelial cells.

Genomic content of Neisseria species

The physical properties of most bacterial genomes are largely unexplored. We have previously demonstrated that the strict human pathogen Neisseria gonorrhoeae is polyploid, carrying an average of three chromosome copies per cell and only maintaining one pair of replication forks per chromosome (D. M. Tobiason and H. S. Seifert, PLos Biol. 4:1069-1078, 2006). We are following up this initial report to test several predictions of the polyploidy model of gonococcal chromosome organization. We demonstrate that the N. gonorrhoeae chromosomes exist solely as monomers and not covalently linked dimers, and in agreement with the monomer status, we show that distinct nucleoid regions can be detected by electron microscopy. Two different approaches to isolate heterozygous N. gonorrhoeae resulted in the formation of merodiploids, showing that even with more than one chromosome copy, these bacteria are genetically haploid. We show that the closely related bacterium Neisseria meningitidis is also polyploid, while the commensal organism Neisseria lactamica maintains chromosomes in single copy. We conclude that the pathogenic Neisseria strains are homozygous diploids.

Cellular and molecular biology of Neisseria meningitidis colonization and invasive disease

The human species is the only natural host of Neisseria meningitidis, an important cause of bacterial meningitis globally, and, despite its association with devastating diseases, N. meningitidis is a commensal organism found frequently in the respiratory tract of healthy individuals. To date, antibiotic resistance is relatively uncommon in N. meningitidis isolates but, due to the rapid onset of disease in susceptible hosts, the mortality rate remains approx. 10%. Additionally, patients who survive meningococcal disease often endure numerous debilitating sequelae. N. meningitidis strains are classified primarily into serogroups based on the type of polysaccharide capsule expressed. In total, 13 serogroups have been described; however, the majority of disease is caused by strains belonging to one of only five serogroups. Although vaccines have been developed against some of these, a universal meningococcal vaccine remains a challenge due to successful immune evasion strategies of the organism, including mimicry of host structures as well as frequent antigenic variation. N. meningitidis express a range of virulence factors including capsular polysaccharide, lipopolysaccharide and a number of surface-expressed adhesive proteins. Variation of these surface structures is necessary for meningococci to evade killing by host defence mechanisms. Nonetheless, adhesion to host cells and tissues needs to be maintained to enable colonization and ensure bacterial survival in the niche. The aims of the present review are to provide a brief outline of meningococcal carriage, disease and burden to society. With this background, we discuss several bacterial strategies that may enable its survival in the human respiratory tract during colonization and in the blood during infection. We also examine several known meningococcal adhesion mechanisms and conclude with a section on the potential processes that may operate in vivo as meningococci progress from the respiratory niche through the blood to reach the central nervous system.

Transcriptional and functional analysis of the Neisseria gonorrhoeae Fur regulon

To ensure survival in the host, bacteria have evolved strategies to acquire the essential element iron. In Neisseria gonorrhoeae, the ferric uptake regulator Fur regulates metabolism through transcriptional control of iron-responsive genes by binding conserved Fur box (FB) sequences in promoters during iron-replete growth. Our previous studies showed that Fur also controls the transcription of secondary regulators that may, in turn, control pathways important to pathogenesis, indicating an indirect role for Fur in controlling these downstream genes. To better define the iron-regulated cascade of transcriptional control, we combined three global strategies--temporal transcriptome analysis, genomewide in silico FB prediction, and Fur titration assays (FURTA)--to detect genomic regions able to bind Fur in vivo. The majority of the 300 iron-repressed genes were predicted to be of unknown function, followed by genes involved in iron metabolism, cell communication, and intermediary metabolism. The 107 iron-induced genes encoded hypothetical proteins or energy metabolism functions. We found 28 predicted FBs in FURTA-positive clones in the promoters and within the open reading frames of iron-repressed genes. We found lower levels of conservation at critical thymidine residues involved in Fur binding in the FB sequence logos of FURTA-positive clones with intragenic FBs than in the sequence logos generated from FURTA-positive promoter regions. In electrophoretic mobility shift assay studies, intragenic FBs bound Fur with a lower affinity than intergenic FBs. Our findings further indicate that transcription under iron stress is indirectly controlled by Fur through 12 potential secondary regulators.

Targeting of whole killed bacteria to gastrointestinal M-cells induces humoral immunity in the female reproductive tract

Recently, we demonstrated that oral delivery of whole killed bacteria, when agglutinated by an M-cell targeting lectin, resulted in an enhanced systemic and mucosal antibody response, as well as a protective immunity, against the gut pathogens Helicobacter pylori and Campylobacter jejuni. Importantly, this protection was achieved without the addition of exogenous adjuvant. Here, in this addendum, we extend this initial study by reporting on the vaginal antibody response induced by these vaccinations. These data show that the targeting of M-cells within the gastrointestinal tract also induces the secretion of antigen-specific antibodies (IgG and IgA) at a distal mucosal site, namely the vaginal mucosa. This observation raises the possibility that oral delivery of a whole, killed bacteria vaccine that target intestinal M-cells could potentially provide a strategy for inducing protective immunity against pathogenic bacteria that infect mucosal sites outside the gastrointestinal tract.

Breaking the wall: targeting of the endothelium by pathogenic bacteria

The endothelium lining blood and lymphatic vessels is a key barrier separating body fluids from host tissues and is a major target of pathogenic bacteria. Endothelial cells are actively involved in host responses to infectious agents, producing inflammatory cytokines, controlling coagulation cascades and regulating leukocyte trafficking. In this Review, a range of bacteria and bacterial toxins are used to illustrate how pathogens establish intimate interactions with endothelial cells, triggering inflammatory responses and coagulation processes and modifying endothelial cell plasma membranes and junctions to adhere to their surfaces and then invade, cross and even disrupt the endothelial barrier.

Neisseria meningitidis differentially controls host cell motility through PilC1 and PilC2 components of type IV Pili

Neisseria meningitidis is a strictly human pathogen that has two facets since asymptomatic carriage can unpredictably turn into fulminant forms of infection. Meningococcal pathogenesis relies on the ability of the bacteria to break host epithelial or endothelial cellular barriers. Highly restrictive, yet poorly understood, mechanisms allow meningococcal adhesion to cells of only human origin. Adhesion of encapsulated and virulent meningococci to human cells relies on the expression of bacterial type four pili (T4P) that trigger intense host cell signalling. Among the components of the meningococcal T4P, the concomitantly expressed PilC1 and PilC2 proteins regulate pili exposure at the bacterial surface, and until now, PilC1 was believed to be specifically responsible for T4P-mediated meningococcal adhesion to human cells. Contrary to previous reports, we show that, like PilC1, the meningococcal PilC2 component is capable of mediating adhesion to human ME180 epithelial cells, with cortical plaque formation and F-actin condensation. However, PilC1 and PilC2 promote different effects on infected cells. Cellular tracking analysis revealed that PilC1-expressing meningococci caused a severe reduction in the motility of infected cells, which was not the case when cells were infected with PilC2-expressing strains. The amount of both total and phosphorylated forms of EGFR was dramatically reduced in cells upon PilC1-mediated infection. In contrast, PilC2-mediated infection did not notably affect the EGFR pathway, and these specificities were shared among unrelated meningococcal strains. These results suggest that meningococci have evolved a highly discriminative tool for differential adhesion in specific microenvironments where different cell types are present. Moreover, the fine-tuning of cellular control through the combined action of two concomitantly expressed, but distinctly regulated, T4P-associated variants of the same molecule (i.e. PilC1 and PilC2) brings a new model to light for the analysis of the interplay between pathogenic bacteria and human host cells.

Structural and kinetic characterizations of the polysialic acid O-acetyltransferase OatWY from Neisseria meningitidis

The neuroinvasive pathogen Neisseria meningitidis has 13 capsular serogroups, but the majority of disease is caused by only 5 of these. Groups B, C, Y, and W-135 all display a polymeric sialic acid-containing capsule that provides a means for the bacteria to evade the immune response during infection by mimicking host sialic acid-containing cell surface structures. These capsules in serogroups C, Y, and W-135 can be further acetylated by a sialic acid-specific O-acetyltransferase, a modification that correlates with decreased immunoreactivity and increased virulence. In N. meningitidis serogroup Y, the O-acetylation reaction is catalyzed by the enzyme OatWY, which we show has clear specificity toward the serogroup Y capsule ([Glc-(alpha1-->4)-Sia](n)). To understand the underlying molecular basis of this process, we have performed crystallographic analysis of OatWY with bound substrate as well as determined kinetic parameters of the wild type enzyme and active site mutants. The structure of OatWY reveals an intimate homotrimer of left-handed beta-helix motifs that frame a deep active site cleft selective for the polysialic acid-bearing substrate. Within the active site, our structural, kinetic, and mutagenesis data support the role of two conserved residues in the catalytic mechanism (His-121 and Trp-145) and further highlight a significant movement of Tyr-171 that blocks the active site of the enzyme in its native form. Collectively, our results reveal the first structural features of a bacterial sialic acid O-acetyltransferase and provide significant new insight into its catalytic mechanism and specificity for the capsular polysaccharide of serogroup Y meningococci.