Neisseria meningitidis App, a new adhesin with autocatalytic serine protease activity

Identification of an autotransporter peptidase of Rickettsia rickettsii responsible for maturation of surface exposed autotransporters

Members of the spotted fever group rickettsia express four large, surface-exposed autotransporters, at least one of which is a known virulence determinant. Autotransporter translocation to the bacterial outer surface, also known as type V secretion, involves formation of a β-barrel autotransporter domain in the periplasm that inserts into the outer membrane to form a pore through which the N-terminal passenger domain is passed and exposed on the outer surface. Two major surface antigens of Rickettsia rickettsii, are known to be surface exposed and the passenger domain cleaved from the autotransporter domain. A highly passaged strain of R. rickettsii, Iowa, fails to cleave these autotransporters and is avirulent. We have identified a putative peptidase, truncated in the Iowa strain, that when reconstituted into Iowa restores appropriate processing of the autotransporters as well as restoring a modest degree of virulence.

Phylogenetic Classification and Functional Review of Autotransporters

Autotransporters are the core component of a molecular nano-machine that delivers cargo proteins across the outer membrane of Gram-negative bacteria. Part of the type V secretion system, this large family of proteins play a central role in controlling bacterial interactions with their environment by promoting adhesion to surfaces, biofilm formation, host colonization and invasion as well as cytotoxicity and immunomodulation. As such, autotransporters are key facilitators of fitness and pathogenesis and enable co-operation or competition with other bacteria. Recent years have witnessed a dramatic increase in the number of autotransporter sequences reported and a steady rise in functional studies, which further link these proteins to multiple virulence phenotypes. In this review we provide an overview of our current knowledge on classical autotransporter proteins, the archetype of this protein superfamily. We also carry out a phylogenetic analysis of their functional domains and present a new classification system for this exquisitely diverse group of bacterial proteins. The sixteen phylogenetic divisions identified establish sensible relationships between well characterized autotransporters and inform structural and functional predictions of uncharacterized proteins, which may guide future research aimed at addressing multiple unanswered aspects in this group of therapeutically important bacterial factors.

The Host-Pathogen Interactions and Epicellular Lifestyle of Neisseria meningitidis

Neisseria meningitidis is a gram-negative diplococcus and a transient commensal of the human nasopharynx. It shares and competes for this niche with a number of other Neisseria species including N. lactamica, N. cinerea and N. mucosa. Unlike these other members of the genus, N. meningitidis may become invasive, crossing the epithelium of the nasopharynx and entering the bloodstream, where it rapidly proliferates causing a syndrome known as Invasive Meningococcal Disease (IMD). IMD progresses rapidly to cause septic shock and meningitis and is often fatal despite aggressive antibiotic therapy. While many of the ways in which meningococci survive in the host environment have been well studied, recent insights into the interactions between N. meningitidis and the epithelial, serum, and endothelial environments have expanded our understanding of how IMD develops. This review seeks to incorporate recent work into the established model of pathogenesis. In particular, we focus on the competition that N. meningitidis faces in the nasopharynx from other Neisseria species, and how the genetic diversity of the meningococcus contributes to the wide range of inflammatory and pathogenic potentials observed among different lineages.

Conserved G-Quadruplex Motifs Regulate Gene Expression in Neisseria meningitidis

The noncanonical structures, G-quadruplexes (GQs), formed in the guanine-rich region of nucleic acids regulate various biological and molecular functions in prokaryotes and eukaryotes. Neisseria meningitidis is a commensal residing in a human's upper respiratory tract but occasionally becomes virulent, causing life-threatening septicemia and meningitis. The factors causing these changes in phenotypes are not fully understood. At the molecular level, regulatory components help in a clearer understanding of the pathogen's virulence and pathogenesis. Herein, genome analysis followed by biophysical assays and cell-based experiments revealed the presence of conserved GQ motifs in N. meningitidis. These GQs are linked to the essential genes involved in cell adhesion, pathogenesis, virulence, transport, DNA repair, and recombination. Primer extension stop assay, reporter assays, and quantitative real-time polymerase chain reaction (qRT-PCR) further affirmed the formation of stable GQs in vitro and in vivo. These results support the existence of evolutionarily conserved GQ motifs in N. meningitidis and uphold the usage of GQ-specific ligands as novel antimeningococcal therapeutics.

Categorizing sequences of concern by function to better assess mechanisms of microbial pathogenesis

To identify sequences with a role in microbial pathogenesis, we assessed the adequacy of their annotation by existing controlled vocabularies and sequence databases. Our goal was to regularize descriptions of microbial pathogenesis for improved integration with bioinformatic applications. Here, we review the challenges of annotating sequences for pathogenic activity. We relate the categorization of more than 2,750 sequences of pathogenic microbes through a controlled vocabulary called Functions of Sequences of Concern (FunSoCs). These allow for an ease of description by both humans and machines. We provide a subset of 220 fully annotated sequences in the supplemental material as examples. The use of this compact (∼30 terms), controlled vocabulary has potential benefits for research in microbial genomics, public health, biosecurity, biosurveillance, and the characterization of new and emerging pathogens.

The outer-membrane protein MafA of Neisseria meningitidis constitutes a novel protein secretion pathway specific for the fratricide protein MafB

MafB proteins are toxins secreted by Neisseria spp. which are involved in interbacterial competition. Their secretion mechanism has so far not been elucidated. Each strain can produce several MafB variants. On the chromosome, the mafB genes are localized on genomic islands also containing mafA genes. MafA proteins have a role in virulence with reported activities in adhesion and transcytosis of pathogenic Neisseria, a priori unrelated to MafB activities. In this study, we investigated the possible involvement of MafA in the transport of MafB across the outer membrane of Neisseria meningitidis. In wild-type strains, proteolytic fragments of MafB proteins were detected in the extracellular medium. In the absence of MafA, secretion was abrogated, and, in the case of MafB_I, full-length and truncated polypeptides were detected inside the cells and inside outer-membrane vesicles. MafB_I secretion required its cognate MafA, whereas MafB_III could use any MafA. Heterologous expression in Escherichia coli showed that MafB_III is transported to a cell-surface-exposed, i.e. protease-accessible, location in a MafA-dependent way. MafA itself was found to be localized to the outer membrane, forming large oligomeric complexes. As homologs were found in diverse bacteria, the Maf system represents a new protein secretion system in Gram-negative bacteria.

Meningococcal disease: A paradigm of type-IV pilus dependent pathogenesis

Neisseria meningitidis (meningococcus) is a Gram-negative bacterium responsible for two devastating forms of invasive diseases: purpura fulminans and meningitis. Interaction with both peripheral and cerebral microvascular endothelial cells is at the heart of meningococcal pathogenesis. During the last two decades, an essential role for meningococcal type IV pili in vascular colonisation and disease progression has been unravelled. This review summarises 20 years of research on meningococcal type IV pilus-dependent virulence mechanisms, up to the identification of promising anti-virulence compounds that target type IV pili.

Single Domain Antibodies Targeting Receptor Binding Pockets of NadA Restrain Adhesion of Neisseria meningitidis to Human Brain Microvascular Endothelial Cells

Neisseria adhesin A (NadA), one of the surface adhesins of Neisseria meningitides (NM), interacts with several cell types including human brain microvascular endothelial cells (hBMECs) and play important role in the pathogenesis. Receptor binding pockets of NadA are localized on the globular head domain (A³³ to K⁶⁹) and the first coiled-coil domain (L¹²¹ to K¹⁵⁸). Here, the phage display was used to develop a variable heavy chain domain (VHH) that can block receptor binding sites of recombinant NadA (rec-NadA). A phage library displaying VHH was panned against synthetic peptides (NadA-gd^A33−K69 or NadA-cc^L121−K158), gene encoding VHH was amplified from bound phages and re-cloned in the expression vector, and the soluble VHHs containing disulfide bonds were overexpressed in the SHuffle E. coli. From the repertoire of 96 clones, two VHHs (VHH_F3–binding NadA-gd^A33−K69 and VHH_G9–binding NadA-cc^L121−K158) were finally selected as they abrogated the interaction between rec-NadA and the cell receptor. Preincubation of NM with VHH_F3 and VHH_G9 significantly reduced the adhesion of NM on hBMECs in situ and hindered the traversal of NM across the in-vitro BBB model. The work presents a phage display pipeline with a single-round of panning to select receptor blocking VHHs. It also demonstrates the production of soluble and functional VHHs, which blocked the interaction between NadA and its receptor, decreased adhesion of NM on hBMECs, and reduced translocation of NM across BBB in-vitro. The selected NadA blocking VHHs could be promising molecules for therapeutic translation.

Genetic incorporation of non-canonical amino acid photocrosslinkers in Neisseria meningitidis: New method provides insights into the physiological function of the function-unknown NMB1345 protein

Although whole-genome sequencing has provided novel insights into Neisseria meningitidis, many open reading frames have only been annotated as hypothetical proteins with unknown biological functions. Our previous genetic analyses revealed that the hypothetical protein, NMB1345, plays a crucial role in meningococcal infection in human brain microvascular endothelial cells; however, NMB1345 has no homology to any identified protein in databases and its physiological function could not be elucidated using pre-existing methods. Among the many biological technologies to examine transient protein-protein interaction in vivo, one of the developed methods is genetic code expansion with non-canonical amino acids (ncAAs) utilizing a pyrrolysyl-tRNA synthetase/tRNAPyl pair from Methanosarcina species: However, this method has never been applied to assign function-unknown proteins in pathogenic bacteria. In the present study, we developed a new method to genetically incorporate ncAAs-encoded photocrosslinking probes into N. meningitidis by utilizing a pyrrolysyl-tRNA synthetase/tRNAPyl pair and elucidated the biological function(s) of the NMB1345 protein. The results revealed that the NMB1345 protein directly interacts with PilE, a major component of meningococcal pili, and further physicochemical and genetic analyses showed that the interaction between the NMB1345 protein and PilE was important for both functional pilus formation and meningococcal infectious ability in N. meningitidis. The present study using this new methodology for N. meningitidis provides novel insights into meningococcal pathogenesis by assigning the function of a hypothetical protein.

Neisseria gonorrhoeae NGO2105 Is an Autotransporter Protein Involved in Adhesion to Human Cervical Epithelial Cells and in vivo Colonization

Autotransporters are important virulence factors in the outer membrane of gram-negative bacteria. Although several autotransporters have been identified in Neisseria meningitidis, only IgA1 protease has been identified in Neisseria gonorrhoeae. A sequence analysis showed a marked difference in the distribution of autotransporters between the two strains. It has been speculated that only two autotransporters, the IgA1 protease and the NGO2105 protein, might be encoded by N. gonorrhoeae. Here, we describe the identification of NGO2105, a new autotransporter in N. gonorrhoeae. A sequence alignment showed that NGO2105 is highly similar to the adhesion and penetration protein (App) in N. meningitidis. We found that NGO2105 is exported to the outer membrane, cleaved and released into the culture supernatant by endogenous serine protease activity in N. gonorrhoeae and E. coli. The site-directed mutagenesis of S267A in the predicted enzyme catalytic triad abolished autoproteolytic cleavage to allow secretion. The NGO2105 β-barrel shows the ability to translocate the heterologous Hbp passenger domain. NGO2105 is involved in gonococcal adherence to and invasion into human cervical epithelial cells. Furthermore, antibodies raised against NGO2105 are able to block gonococcal adherence to human cervical epithelial cells. The Δngo2105 mutant and anti-NGO2105 antiserum significantly attenuated the colonization of N. gonorrhoeae in mice. Collectively, our results suggest that the newly identified serine protease autotransporter NGO2105 represents a novel virulence factor of gonococcus and a potential vaccine target.

Atypical, Yet Not Infrequent, Infections with Neisseria Species

Neisseria species are extremely well-adapted to their mammalian hosts and they display unique phenotypes that account for their ability to thrive within niche-specific conditions. The closely related species N. gonorrhoeae and N. meningitidis are the only two species of the genus recognized as strict human pathogens, causing the sexually transmitted disease gonorrhea and meningitis and sepsis, respectively. Gonococci colonize the mucosal epithelium of the male urethra and female endo/ectocervix, whereas meningococci colonize the mucosal epithelium of the human nasopharynx. The pathophysiological host responses to gonococcal and meningococcal infection are distinct. However, medical evidence dating back to the early 1900s demonstrates that these two species can cross-colonize anatomical niches, with patients often presenting with clinically-indistinguishable infections. The remaining Neisseria species are not commonly associated with disease and are considered as commensals within the normal microbiota of the human and animal nasopharynx. Nonetheless, clinical case reports suggest that they can behave as opportunistic pathogens. In this review, we describe the diversity of the genus Neisseria in the clinical context and raise the attention of microbiologists and clinicians for more cautious approaches in the diagnosis and treatment of the many pathologies these species may cause.

An Overview of Neisseria meningitidis

Neisseria meningitidis (the meningococcus) is a member of the normal nasopharyngeal microbiome in healthy individuals, but can cause septicemia and meningitis in susceptible individuals. In this chapter we provide an overview of the disease caused by N. meningitidis and the schemes used to type the meningococcus. We also review the adhesions, virulence factors, and phase variable genes that enable it to successfully colonize the human host. Finally, we outline the history and current status of meningococcal vaccines and highlight the importance of continued molecular investigation of the epidemiology and the structural analysis of the antigens of this pathogen to aid future vaccine development.

The Meningococcal Cysteine Transport System Plays a Crucial Role in Neisseria meningitidis Survival in Human Brain Microvascular Endothelial Cells

Neisseria meningitidis colonizes at a nasopharynx of human as a unique host and has many strains that are auxotrophs for amino acids for their growth. To cause invasive meningococcal diseases (IMD) such as sepsis and meningitis, N. meningitidis passes through epithelial and endothelial barriers and infiltrates into blood and cerebrospinal fluid as well as epithelial and endothelial cells. However, meningococcal nutrients, including cysteine, become less abundant when it more deeply infiltrates the human body even during inflammation, such that N. meningitidis has to acquire nutrients in order to survive/persist, disseminate, and proliferate in humans. This was the first study to examine the relationship between meningococcal cysteine acquisition and the pathogenesis of meningococcal infections. The results of the present study provide insights into the mechanisms by which pathogens with auxotrophs acquire nutrients in hosts and may also contribute to the development of treatments and prevention strategies for IMD.

While Neisseria meningitidis typically exists in an asymptomatic nasopharyngeal carriage state, it may cause potentially lethal diseases in humans, such as septicemia or meningitis, by invading deeper sites in the body. Since the nutrient compositions of human cells are not always conducive to meningococci, N. meningitidis needs to exploit nutrients from host environments. In the present study, the utilization of cysteine by the meningococcal cysteine transport system (CTS) was analyzed for the pathogenesis of meningococcal infections. A N. meningitidis strain deficient in one of the three cts genes annotated as encoding cysteine-binding protein (cbp) exhibited approximately 100-fold less internalization into human brain microvascular endothelial cells (HBMEC) than the wild-type strain. This deficiency was restored by complementation with the three cts genes together, and the infectious phenotype of HBMEC internalization correlated with cysteine uptake activity. However, efficient accumulation of ezrin was observed beneath the cbp mutant. The intracellular survival of the cbp mutant in HBMEC was markedly reduced, whereas equivalent reductions of glutathione concentrations and of resistance to reactive oxygens species in the cbp mutant were not found. The cbp mutant grew well in complete medium but not in synthetic medium supplemented with less than 300 μM cysteine. Taking cysteine concentrations in human cells and other body fluids, including blood and cerebrospinal fluid, into consideration, the present results collectively suggest that the meningococcal CTS is crucial for the acquisition of cysteine from human cells and participates in meningococcal nutrient virulence.

Uptake of Neisserial autotransporter lipoprotein (NalP) promotes an increase in human brain microvascular endothelial cell metabolic activity

Neisseria meningitidis is normally a human nasopharyngeal commensal but is also capable of causing life-threatening sepsis and meningitis. N. meningitidis secretes several virulence-associated proteins including Neisserial autotransporter lipoprotein (NalP), an immunogenic, type Va autotransporter harboring an S8-family serine endopeptidase domain. NalP has been previously characterized as a cell-surface maturation protease which processes other virulence-associated meningococcal surface proteins, and as a factor contributing to the survival of meningococci in human serum due to its ability to cleave complement factor C3. Here, recombinant NalP (rNalP) fragments were purified and used to investigate the interaction of NalP with host cells. Flow cytometry and confocal microscopy demonstrated binding and uptake of rNalP into different human cell types. High-resolution microscopy confirmed that internalized rNalP predominantly localized to the perinuclear region of cells. Abolition of rNalP protease activity using site-directed mutagenesis did not influence uptake or sub-cellular localization, but inactive rNalP (rNalP^S426A) was unable to induce an increase in human brain microvascular endothelial cell metabolic activity provoked by proteolytically-active rNalP. Our data suggests a more complex and multifaceted role for NalP in meningococcal pathogenesis than was previously understood which includes novel intra-host cell functions.

A deeper mining on the protein composition of VA-MENGOC-BC®: An OMV-based vaccine against N. meningitidis serogroup B and C

The protein composition of an Outer Membrane Vesicle (OMV) preparation that constitutes the active pharmaceutical ingredient of VA-MENGOC-BC®, an effective vaccine against Neisseria meningitidis serogroups B, and C is presented. This preparation has a high lipid content and five abundant membrane proteins (FetA, PorA, PorB, RmpM, and Opc), constituting approximately 70% of the total protein mass. The protein composition was determined by combining the use of the Hexapeptide Ligand Library and an orthogonal tandem fractionation of tryptic peptides by reverse-phase chromatography at alkaline and acid pH. This approach equalizes the concentration of tryptic peptides derived from low- and high-abundance proteins as well as considerably simplifying the number of peptides analyzed by LC-MS/MS, enhancing the possibility of identifying low-abundance species. Fifty-one percent of the proteins originally annotated as membrane proteins in the genome of the MC58 strain were identified. One hundred and sixty-eight low-abundance cytosolic proteins presumably occluded within OMV were also identified. Four (NadA, NUbp, GNA2091, and fHbp), out of the five antigens constituting the Bexsero® vaccine, were detected in this OMV preparation. In particular, fHbp is also the active principle of the Trumenba® vaccine developed by Pfizer. The HpuA and HpuB gene products (not annotated in the MC58 genome) were identified in the CU385 strain, a clinical isolate that is used to produce this OMV. Considering the proteins identified here and previous work done by our group, the protein catalogue of this OMV preparation was extended to 266 different protein species.

Characterization and Distribution of the autB Gene in Neisseria meningitidis

We aimed to investigate and understand the characterization and distribution of the autB gene in Neisseria meningitidis in China. autB is flanked by two conservative genes, smpB and glcD, and it can be present in the majority of meningococcal isolates, but not in 053442 of clonal complex 4821 (CC4821) which contains a 968 bp sequence. In this study, we sequenced the intervenient region between smpB and glcD in 178 Chinese N. meningitidis strains isolated from both patients and carriers. There were 110 serogroupable strains, other 68 were non-groupable (NG). Ninety nine of the 178 strains were clustered into 13 CCs, the remaining 79 were unassigned (UA). CC4821 is one of the dominant CCs in China. Forty of the 42 CC4821 strains and 26 of the 79 UA strains were autB-null, while the remaining 12 CCs were autB-positive. According to the N-terminal sequence, most (97/112) of the autB-positive strains were clustered into AutB1 and the remaining 15 were AutB2. The autB gene and its flanking intergenic sequences was superseded by a perfectly conservative sequence of an identical 968 bp in all of the autB-null N. meningitidis strains which had no identity with the relatively conservative intergenic sequences that flanked the autB gene in autB-positive strains. There was a 10 bp DNA uptake sequence (DUS) at the beginning of the interval 968 bp sequence in the autB-null strains while there was a 9 bp Haemophilus-specific uptake sequence (hUS) at the beginning of the partial holB gene and at the end of the partial tmk gene in autB-positive strains, holB and tmk gene were flanking the autB gene in Haemophilus. In conclusion, not all pathogenic N. meningitidis strains especially CC4821 possess the autB gene in China and the corresponding spacer region of the autB-null strains was not homologous to that found in autB-positive strains. There's a hypothesis that the DUS and hUS are likely to play a key part in the mechanism of uptake or loss of the autB gene.

Biological Functions of the Secretome of Neisseria meningitidis

Neisseria meningitidis is a Gram-negative bacterial pathogen that normally resides as a commensal in the human nasopharynx but occasionally causes disease with high mortality and morbidity. To interact with its environment, it transports many proteins across the outer membrane to the bacterial cell surface and into the extracellular medium for which it deploys the common and well-characterized autotransporter, two-partner and type I secretion mechanisms, as well as a recently discovered pathway for the surface exposure of lipoproteins. The surface-exposed and secreted proteins serve roles in host-pathogen interactions, including adhesion to host cells and extracellular matrix proteins, evasion of nutritional immunity imposed by iron-binding proteins of the host, prevention of complement activation, neutralization of antimicrobial peptides, degradation of immunoglobulins, and permeabilization of epithelial layers. Furthermore, they have roles in interbacterial interactions, including the formation and dispersal of biofilms and the suppression of the growth of bacteria competing for the same niche. Here, we will review the protein secretion systems of N. meningitidis and focus on the functions of the secreted proteins.

Targeting the Mucosal Barrier: How Pathogens Modulate the Cellular Polarity Network

The mucosal barrier is composed of polarized epithelial cells with distinct apical and basolateral surfaces separated by tight junctions and serves as both a physical and immunological barrier to incoming pathogens. Specialized polarity proteins are critical for establishment and maintenance of polarity. Many human pathogens have evolved virulence mechanisms that target the polarity network to enhance binding, create replication niches, move through the barrier by transcytosis, or bypass the barrier by disrupting cell-cell junctions. This review summarizes recent advances and compares and contrasts how three important human pathogens that colonize mucosal surfaces, Pseudomonas aeruginosa, Helicobacter pylori, and Neisseria meningitidis, subvert the host cell polarization machinery during infection.

Molecular mechanisms involved in the interaction of Neisseria meningitidis with cells of the human blood-cerebrospinal fluid barrier

Neisseria meningitidis is one of the most common aetiological agents of bacterial meningitis, affecting predominantly children and young adults. The interaction of N. meningitidis with human endothelial cells lining blood vessels of the blood-cerebrospinal fluid barrier (B-CSFB) is critical for meningitis development. In recent decades, there has been a significant increase in understanding of the molecular mechanisms involved in the interaction of N. meningitidis with brain vascular cells. In this review, we will describe how N. meningitidis adheres to the brain vasculature, may enter inside these cells, hijack receptor signalling pathways and alter host-cell responses in order to traverse the B-CSFB.

Expression of the Gene for Autotransporter AutB of Neisseria meningitidis Affects Biofilm Formation and Epithelial Transmigration

Neisseria meningitidis is a Gram-negative bacterium that resides as a commensal in the upper respiratory tract of humans, but occasionally, it invades the host and causes sepsis and/or meningitis. The bacterium can produce eight autotransporters, seven of which have been studied to some detail. The remaining one, AutB, has not been characterized yet. Here, we show that the autB gene is broadly distributed among pathogenic Neisseria spp. The gene is intact in most meningococcal strains. However, its expression is prone to phase variation due to slipped-strand mispairing at AAGC repeats located within the DNA encoding the signal sequence and is switched off in the vast majority of these strains. Moreover, various genetic disruptions prevent autB expression in most of the strains in which the gene is in phase indicating a strong selection against AutB synthesis. We observed that autB is expressed in two of the strains examined and that AutB is secreted and exposed at the cell surface. Functionality assays revealed that AutB synthesis promotes biofilm formation and delays the passage of epithelial cell layers in vitro. We hypothesize that this autotransporter is produced during the colonization process only in specific niches to facilitate microcolony formation, but its synthesis is switched off probably to evade the immune system and facilitate human tissue invasion.

Neisserial Heparin Binding Antigen (NHBA) Contributes to the Adhesion of Neisseria meningitidis to Human Epithelial Cells

Neisserial Heparin Binding Antigen (NHBA) is a surface-exposed lipoprotein ubiquitously expressed by Neisseria meningitidis strains and an antigen of the Bexsero^® vaccine. NHBA binds heparin through a conserved Arg-rich region that is the target of two proteases, the meningococcal NalP and human lactoferrin (hLf). In this work, in vitro studies showed that recombinant NHBA protein was able to bind epithelial cells and mutations of the Arg-rich tract abrogated this binding. All N-terminal and C-terminal fragments generated by NalP or hLf cleavage, regardless of the presence or absence of the Arg-rich region, did not bind to cells, indicating that a correct positioning of the Arg-rich region within the full length protein is crucial. Moreover, binding was abolished when cells were treated with heparinase III, suggesting that this interaction is mediated by heparan sulfate proteoglycans (HSPGs). N. meningitidis nhba knockout strains showed a significant reduction in adhesion to epithelial cells with respect to isogenic wild-type strains and adhesion of the wild-type strain was inhibited by anti-NHBA antibodies in a dose-dependent manner. Overall, the results demonstrate that NHBA contributes to meningococcal adhesion to epithelial cells through binding to HSPGs and suggest a possible role of anti-Bexsero^® antibodies in the prevention of colonization.

Neisseria cinerea isolates can adhere to human epithelial cells by type IV pilus-independent mechanisms

In pathogenic Neisseria species the type IV pili (Tfp) are of primary importance in host-pathogen interactions. Tfp mediate initial bacterial attachment to cell surfaces and formation of microcolonies via pilus-pilus interactions. Based on genome analysis, many non-pathogenic Neisseria species are predicted to express Tfp, but aside from studies on Neisseria elongata, relatively little is known about the formation and function of pili in these organisms. Here, we have analysed pilin expression and the role of Tfp in Neisseria cinerea. This non-pathogenic species shares a close taxonomic relationship to the pathogen Neisseria meningitidis and also colonizes the human oropharyngeal cavity. Through analysis of non-pathogenic Neisseria genomes we identified two genes with homology to pilE, which encodes the major pilin of N. meningitidis. We show which of the two genes is required for Tfp expression in N. cinerea and that Tfp in this species are required for DNA competence, similar to other Neisseria. However, in contrast to the meningococcus, deletion of the pilin gene did not impact the association of N. cinerea to human epithelial cells, demonstrating that N. cinerea isolates can adhere to human epithelial cells by Tfp-independent mechanisms.

Multiple Functions of Glutamate Uptake via Meningococcal GltT-GltM L-Glutamate ABC Transporter in Neisseria meningitidis Internalization into Human Brain Microvascular Endothelial Cells

We previously reported that Neisseria meningitidis internalization into human brain microvasocular endothelial cells (HBMEC) was triggered by the influx of extracellular L-glutamate via the GltT-GltM L-glutamate ABC transporter, but the underlying mechanism remained unclear. We found that the ΔgltT ΔgltM invasion defect in assay medium (AM) was alleviated in AM without 10% fetal bovine serum (FBS) [AM(-S)]. The alleviation disappeared again in AM(-S) supplemented with 500 μM glutamate. Glutamate uptake by the ΔgltT ΔgltM mutant was less efficient than that by the wild-type strain, but only upon HBMEC infection. We also observed that both GltT-GltM-dependent invasion and accumulation of ezrin, a key membrane-cytoskeleton linker, were more pronounced when N. meningitidis formed larger colonies on HBMEC under physiological glutamate conditions. These results suggested that GltT-GltM-dependent meningococcal internalization into HBMEC might be induced by the reduced environmental glutamate concentration upon infection. Furthermore, we found that the amount of glutathione within the ΔgltT ΔgltM mutant was much lower than that within the wild-type N. meningitidis strain only upon HBMEC infection and was correlated with intracellular survival. Considering that the L-glutamate obtained via GltT-GltM is utilized as a nutrient in host cells, l-glutamate uptake via GltT-GltM plays multiple roles in N. meningitidis internalization into HBMEC.

TleA, a Tsh-like autotransporter identified in a human enterotoxigenic Escherichia coli strain

Enterotoxigenic Escherichia coli (ETEC), a leading cause of acute diarrhea, colonizes the intestine by means of adhesins. However, 15 to 50% of clinical isolates are negative for known adhesins, making it difficult to identify antigens for broad-coverage vaccines. The ETEC strain 1766a, obtained from a child with watery diarrhea in Chile, harbors the colonization factor CS23 but is negative for other known adhesins. One clone, derived from an ETEC 1766a genomic library (clone G10), did not produce CS23 yet was capable of adhering to Caco-2 cells. The goal of this study was to identify the gene responsible for this capacity. Random transposon-based mutagenesis allowed the identification of a 4,110-bp gene that codes for a homologue of the temperature-sensitive hemagglutinin (Tsh) autotransporter described in avian E. coli strains (97% identity, 90% coverage) and that is called TleA (Tsh-like ETEC autotransporter) herein. An isogenic ETEC 1766a strain with a tleA mutation showed an adhesion level similar to that of the wild-type strain, suggesting that the gene does not direct attachment to Caco-2 cells. However, expression of tleA conferred the capacity for adherence to nonadherent E. coli HB101. This effect coincided with the detection of TleA on the surface of nonpermeabilized bacteria, while, conversely, ETEC 1766a seems to secrete most of the produced autotransporter to the medium. On the other hand, TleA was capable of degrading bovine submaxillary mucin and leukocyte surface glycoproteins CD45 and P-selectin glycoprotein ligand 1 (PSGL-1). These results suggest that TleA promotes colonization of the intestinal epithelium and that it may modulate the host immune response.

Nuclear trafficking, histone cleavage and induction of apoptosis by the meningococcal App and MspA autotransporters

Neisseria meningitidis, a major cause of bacterial meningitis and septicaemia, secretes multiple virulence factors, including the adhesion and penetration protein (App) and meningococcal serine protease A (MspA). Both are conserved, immunogenic, type Va autotransporters harbouring S6‐family serine endopeptidase domains. Previous work suggested that both could mediate adherence to human cells, but their precise contribution to meningococcal pathogenesis was unclear. Here, we confirm that App and MspA are in vivo virulence factors since human CD46‐expressing transgenic mice infected with meningococcal mutants lacking App, MspA or both had improved survival rates compared with mice infected with wild type. Confocal imaging showed that App and MspA were internalized by human cells and trafficked to the nucleus. Cross‐linking and enzyme‐linked immuno assay (ELISA) confirmed that mannose receptor (MR), transferrin receptor 1 (TfR1) and histones interact with MspA and App. Dendritic cell (DC) uptake could be blocked using mannan and transferrin, the specific physiological ligands for MR and TfR1, whereas in vitro clipping assays confirmed the ability of both proteins to proteolytically cleave the core histone H3. Finally, we show that App and MspA induce a dose‐dependent increase in DC death via caspase‐dependent apoptosis. Our data provide novel insights into the roles of App and MspA in meningococcal infection.

Disease-associated Neisseria meningitidis isolates inhibit wound repair in respiratory epithelial cells in a type IV pilus-independent manner

Neisseria meningitidis is the causative agent of meningococcal disease. Onset of meningococcal disease can be extremely rapid and can kill within a matter of hours. However, although a much-feared pathogen, Neisseria meningitidis is frequently found in the nasopharyngeal mucosae of healthy carriers. The bacterial factors that distinguish disease- from carriage-associated meningococci are incompletely understood. Evidence suggesting that disruptions to the nasopharynx may increase the risk of acquiring meningococcal disease led us to evaluate the ability of disease- and carriage-associated meningococcal isolates to inhibit cell migration, using an in vitro assay for wound repair. We found that disease-associated isolates in our collection inhibited wound closure, while carriage-associated isolates were more variable, with many isolates not inhibiting wound repair at all. For isolates selected for further study, we found that actin morphology, such as presence of lamellipodia, correlated with cell migration. We demonstrated that multiple meningococcal virulence factors, including the type IV pili, are dispensable for inhibition of wound repair. Inhibition of wound repair was also shown to be an active process, i.e., requiring live bacteria undergoing active protein synthesis.

Neisseria meningitidis: pathogenesis and immunity

The recent advances in cellular microbiology, genomics, and immunology has opened new horizons in the understanding of meningococcal pathogenesis and in the definition of new prophylactic intervention. It is now clear that Neissera meningitidis has evolved a number of surface structures to mediate interaction with host cells and a number of mechanisms to subvert the immune system and escape complement-mediated killing. In this review we report the more recent findings on meningococcal adhesion and on the bacteria-complement interaction highlighting the redundancy of these mechanisms. An effective vaccine against meningococcus B, based on multiple antigens with different function, has been recently licensed. The antibodies induced by the 4CMenB vaccine could mediate bacterial killing by activating directly the classical complement pathway or, indirectly, by preventing binding of fH on the bacterial surface and interfering with colonization.

The meningococcal autotransporter AutA is implicated in autoaggregation and biofilm formation

Autotransporters (ATs) are proteins secreted by Gram-negative bacteria that often play a role in virulence. Eight different ATs have been identified in Neisseria meningitidis, but only six of them have been characterized. AutA is one of the remaining ATs. Its expression remains controversial. Here, we show that the autA gene is present in many neisserial species, but its expression is often disrupted by various genetic features; however, it is expressed in certain strains of N. meningitidis. By sequencing the autA gene in large panels of disease isolates and Western blot analysis, we demonstrated that AutA expression is prone to phase variation at AAGC nucleotide repeats located within the DNA encoding the signal sequence. AutA is not secreted into the extracellular medium, but remains associated with the bacterial cell surface. We further demonstrate that AutA expression induces autoaggregation in a process that, dependent on the particular strain, may require extracellular DNA (eDNA). This property influences the organization of bacterial communities like lattices and biofilms. In vitro assays evidenced that AutA is a self-associating AT that binds DNA. We suggest that AutA-mediated autoaggregation might be particularly important for colonization and persistence of the pathogen in the nasopharynx of the host.

The hypervariable region of meningococcal major pilin PilE controls the host cell response via antigenic variation

Type IV pili (Tfp) are expressed by many Gram-negative bacteria to promote aggregation, adhesion, internalization, twitching motility, or natural transformation. Tfp of Neisseria meningitidis, the causative agent of cerebrospinal meningitis, are involved in the colonization of human nasopharynx. After invasion of the bloodstream, Tfp allow adhesion of N. meningitidis to human endothelial cells, which leads to the opening of the blood-brain barrier and meningitis. To achieve firm adhesion, N. meningitidis induces a host cell response that results in elongation of microvilli surrounding the meningococcal colony. Here we study the role of the major pilin subunit PilE during host cell response using human dermal microvascular endothelial cells and the pharynx carcinoma-derived FaDu epithelial cell line. We first show that some PilE variants are unable to induce a host cell response. By engineering PilE mutants, we observed that the PilE C-terminus domain, which contains a disulfide bonded region (D-region), is critical for the host cell response and that hypervariable regions confer different host cell specificities. Moreover, the study of point mutants of the pilin D-region combined with structural modeling of PilE revealed that the D-region contains two independent regions involved in signaling to human dermal microvascular endothelial cells (HDMECs) or FaDu cells. Our results indicate that the diversity of the PilE D-region sequence allows the induction of the host cell response via several receptors. This suggests that Neisseria meningitidis has evolved a powerful tool to adapt easily to many niches by modifying its ability to interact with host cells.

Type IV pili (Tfp) are long appendages expressed by many Gram-negative bacteria, including Neisseria meningitidis, the causative agent of cerebrospinal meningitis. These pili are involved in many aspects of pathogenesis: natural competence, aggregation, adhesion, and twitching motility. More specifically, Neisseria meningitidis, which is devoid of a secretion system to manipulate its host, has evolved its Tfp to signal to brain endothelial cells and open the blood-brain barrier. In this report, we investigate, at the molecular level, the involvement of the major pilin subunit PilE in host cell response. Our results indicate that the PilE C-terminal domain, which contains a disulfide bonded region (D-region), is critical for the host cell response and contains two independent regions involved in host cell signaling.

Identification of the immunoproteome of the meningococcus by cell surface immunoprecipitation and MS

Most healthy adults are protected from meningococcal disease by the presence of naturally acquired anti-meningococcal antibodies; however, the identity of the target antigens of this protective immunity remains unclear, particularly for protection against serogroup B disease. To identify the protein targets of natural protective immunity we developed an immunoprecipitation and proteomics approach to define the immunoproteome of the meningococcus. Sera from 10 healthy individuals showing serum bactericidal activity against both a meningococcal C strain (L91543) and the B strain MC58, together with commercially available pooled human sera, were used as probe antisera. Immunoprecipitation was performed with each serum sample and live cells from both meningococcal strains. Immunoprecipitated proteins were identified by MS. Analysis of the immunoproteome from each serum demonstrated both pan-reactive antigens that were recognized by most sera as well as subject-specific antigens. Most antigens were found in both meningococcal strains, but a few were strain-specific. Many of the immunoprecipitated proteins have been characterized previously as surface antigens, including adhesins and proteases, several of which have been recognized as vaccine candidate antigens, e.g. factor H-binding protein, NadA and neisserial heparin-binding antigen. The data demonstrate clearly the presence of meningococcal antibodies in healthy individuals with no history of meningococcal infection and a wide diversity of immune responses. The identification of the immunoreactive proteins of the meningococcus provides a basis for understanding the role of each antigen in the natural immunity associated with carriage and may help to design vaccination strategies.

Proteinaceous determinants of surface colonization in bacteria: bacterial adhesion and biofilm formation from a protein secretion perspective

Bacterial colonization of biotic or abiotic surfaces results from two quite distinct physiological processes, namely bacterial adhesion and biofilm formation. Broadly speaking, a biofilm is defined as the sessile development of microbial cells. Biofilm formation arises following bacterial adhesion but not all single bacterial cells adhering reversibly or irreversibly engage inexorably into a sessile mode of growth. Among molecular determinants promoting bacterial colonization, surface proteins are the most functionally diverse active components. To be present on the bacterial cell surface, though, a protein must be secreted in the first place. Considering the close association of secreted proteins with their cognate secretion systems, the secretome (which refers both to the secretion systems and their protein substrates) is a key concept to apprehend the protein secretion and related physiological functions. The protein secretion systems are here considered in light of the differences in the cell-envelope architecture between diderm-LPS (archetypal Gram-negative), monoderm (archetypal Gram-positive) and diderm-mycolate (archetypal acid-fast) bacteria. Besides, their cognate secreted proteins engaged in the bacterial colonization process are regarded from single protein to supramolecular protein structure as well as the non-classical protein secretion. This state-of-the-art on the complement of the secretome (the secretion systems and their cognate effectors) involved in the surface colonization process in diderm-LPS and monoderm bacteria paves the way for future research directions in the field.

The biology of Neisseria adhesins

Members of the genus Neisseria include pathogens causing important human diseases such as meningitis, septicaemia, gonorrhoea and pelvic inflammatory disease syndrome. Neisseriae are found on the exposed epithelia of the upper respiratory tract and the urogenital tract. Colonisation of these exposed epithelia is dependent on a repertoire of diverse bacterial molecules, extending not only from the surface of the bacteria but also found within the outer membrane. During invasive disease, pathogenic Neisseriae also interact with immune effector cells, vascular endothelia and the meninges. Neisseria adhesion involves the interplay of these multiple surface factors and in this review we discuss the structure and function of these important molecules and the nature of the host cell receptors and mechanisms involved in their recognition. We also describe the current status for recently identified Neisseria adhesins. Understanding the biology of Neisseria adhesins has an impact not only on the development of new vaccines but also in revealing fundamental knowledge about human biology.

Autotransporter secretion: varying on a theme

Autotransporters are widely distributed among Gram-negative bacteria. They can have a large variety of functions and many of them have a role in virulence. They are synthesized as large precursors with an N-terminal signal sequence that mediates transport across the inner membrane via the Sec machinery and a translocator domain that mediates the transport of the connected passenger domain across the outer membrane to the bacterial cell surface. Like integral outer membrane proteins, the translocator domain folds in a β-barrel structure and requires the Bam machinery for its insertion into the outer membrane. After transport across the outer membrane, the passenger may stay connected via the translocator domain to the bacterial cell surface or it is proteolytically released into the extracellular milieu. Based on the size of the translocator domain and its position relative to the passenger in the precursor, autotransporters are divided into four sub-categories. We review here the current knowledge of the biogenesis, structure and function of various autotransporters.

The adhesin complex protein (ACP) of Neisseria meningitidis is a new adhesin with vaccine potential

The acp gene encoding the 13-kDa adhesin complex protein (ACP) from Neisseria meningitidis serogroup B strain MC58 was cloned and expressed in Escherichia coli, and the purified recombinant ACP (rACP) was used for immunization studies. Analysis of the ACP amino acid sequences from 13 meningococcal strains, isolated from patients and colonized individuals, and 178 strains in the Bacterial Isolate Genome Sequence (BIGS) database showed the presence of only three distinct sequence types (I, II, and III) with high similarity (>98%). Immunization of mice with type I rACP in detergent micelles and liposomes and in saline solution alone induced high levels of serum bactericidal activity (SBA; titers of 1/512) against the homologous strain MC58 and killed strains of heterologous sequence types II and III with similar SBA titers (1/128 to 1/512). Levels of expression of type I, II, or III ACP by different meningococcal strains were similar. ACP functioned as an adhesin, as demonstrated by reduced adherence of acp knockout (MC58 ΔACP) meningococci to human cells in vitro and the direct surface binding of rACP and by the ability of anti-rACP sera to inhibit adherence of wild-type bacteria. ACP also mediated the invasion of noncapsular meningococci into human epithelial cells, but it was not a particularly impressive invasin, as the internalized bacterial numbers were low. In summary, the newly identified ACP protein is an adhesin that induces cross-strain bactericidal activity and is therefore an attractive target antigen for incorporation into the next generation of serogroup B meningococcal vaccines.

Infections caused by Neisseria meningitidis serogroup B are still significant causes of mortality and morbidity worldwide, and broadly protective vaccines of defined antigen composition are not yet licensed. Here, we describe the properties of the adhesin complex protein (ACP), which we demonstrate is a newly recognized molecule that is highly conserved and expressed to similar levels in meningococci and facilitates meningococcal interactions with human cells. We also report that a recombinant ACP protein vaccine induces murine antibodies that significantly kill meningococci expressing different ACP. Taken together, these properties demonstrate that ACP merits serious consideration as a component of a broadly protective vaccine against serogroup B meningococci.

Attachment and invasion of Neisseria meningitidis to host cells is related to surface hydrophobicity, bacterial cell size and capsule

We compared exemplar strains from two hypervirulent clonal complexes, strain NMB-CDC from ST-8/11 cc and strain MC58 from ST-32/269 cc, in host cell attachment and invasion. Strain NMB-CDC attached to and invaded host cells at a significantly greater frequency than strain MC58. Type IV pili retained the primary role for initial attachment to host cells for both isolates regardless of pilin class and glycosylation pattern. In strain MC58, the serogroup B capsule was the major inhibitory determinant affecting both bacterial attachment to and invasion of host cells. Removal of terminal sialylation of lipooligosaccharide (LOS) in the presence of capsule did not influence rates of attachment or invasion for strain MC58. However, removal of either serogroup B capsule or LOS sialylation in strain NMB-CDC increased bacterial attachment to host cells to the same extent. Although the level of inhibition of attachment by capsule was different between these strains, the regulation of the capsule synthesis locus by the two-component response regulator MisR, and the level of surface capsule determined by flow cytometry were not significantly different. However, the diplococci of strain NMB-CDC were shown to have a 1.89-fold greater surface area than strain MC58 by flow cytometry. It was proposed that the increase in surface area without changing the amount of anchored glycolipid capsule in the outer membrane would result in a sparser capsule and increase surface hydrophobicity. Strain NMB-CDC was shown to be more hydrophobic than strain MC58 using hydrophobicity interaction chromatography and microbial adhesion-to-solvents assays. In conclusion, improved levels of adherence of strain NMB-CDC to cell lines was associated with increased bacterial cell surface and surface hydrophobicity. This study shows that there is diversity in bacterial cell surface area and surface hydrophobicity within N. meningitidis which influence steps in meningococcal pathogenesis.

From self sufficiency to dependence: mechanisms and factors important for autotransporter biogenesis

Autotransporters are a superfamily of proteins that use the type V secretion pathway for their delivery to the surface of Gram-negative bacteria. At first glance, autotransporters look to contain all the functional elements required to promote their own secretion: an amino-terminal signal peptide to mediate translocation across the inner membrane, a central passenger domain that is the secreted functional moiety, and a channel-forming carboxyl terminus that facilitates passenger domain translocation across the outer membrane. However, recent discoveries of common structural themes, translocation intermediates and accessory interactions have challenged the perceived simplicity of autotransporter secretion. Here, we discuss how these studies have led to an improved understanding of the mechanisms responsible for autotransporter biogenesis.

Meningococcal ligands and molecular targets of the host

Meningococcal mechanisms of adhesion are complex, involving multiple adhesins and their respective target receptors on host cells. Three major surface structures--pili, Opa, and Opc--have been known for some time to mediate meningococcal adhesion to target human cells. More recently, several other relatively minor adhesins have also come to light. The literature on bacterial adhesion mechanisms provides numerous examples of various adhesins acting cooperatively in an apparently hierarchical and sequential manner; in other instances, adhesins may act in concert leading to high avidity interactions, often a prelude to cellular invasion and tissue penetration. Such examples are also present in the case of meningococci, although our knowledge of adhesin cooperation and synergy is far from complete. Meningococcal mechanisms used to target the host, which are often specific for the host or a tissue within the host, include both lectin-like interactions and protein-protein interactions; the latter tend to determine specificity in general. Understanding (a) what determines specificity (i.e. molecular features of adhesins and receptors), (b) encourages cellular penetration (i.e. adhesin pairs, which act in concert or synergistically to deliver effective signals for invasion and induce other cellular responses), (c) level of redundancy (more than one mechanisms of targeting host receptors), (d) host situations that encourage tissue penetration (inflammatory situations during which circulating cytokines upregulate target cell receptors, effectively encouraging greater adhesion/invasion), and (e) down-stream effects on host functions in general are all clearly important in our future strategies of controlling meningococcal pathogenesis.

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.

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.

Opa proteins and CEACAMs: pathways of immune engagement for pathogenic Neisseria

Neisseria meningitidis and Neisseria gonorrhoeae are globally important pathogens, which in part owe their success to their ability to successfully evade human immune responses over long periods. The phase-variable opacity-associated (Opa) adhesin proteins are a major surface component of these organisms, and are responsible for bacterial adherence and entry into host cells and interactions with the immune system. Most immune interactions are mediated via binding to members of the carcinoembryonic antigen cell adhesion molecule (CEACAM) family. These Opa variants are able to bind to different receptors of the CEACAM family on epithelial cells, neutrophils, and T and B lymphocytes, influencing the innate and adaptive immune responses. Increased epithelial cell adhesion creates the potential for prolonged infection, invasion and dissemination. Furthermore, Opa proteins may inhibit T-lymphocyte activation and proliferation, B-cell antibody production, and innate inflammatory responses by infected epithelia, in addition to conferring increased resistance to antibody-dependent, complement-mediated killing. While vaccines containing Opa proteins could induce adhesion-blocking and bactericidal antibodies, the consequence of CEACAM binding by a candidate Opa-containing vaccine requires further investigation. This review summarizes current knowledge of the immunological consequences of the interaction between meningococcal and gonococcal Opa proteins and human CEACAMs, considering the implications for pathogenesis and vaccine development.

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.

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.

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.

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.

Meningococcal interactions with the host

Neisseria meningitidis interacts with host tissues through hierarchical, concerted and co-ordinated actions of a number of adhesins; many of which undergo antigenic and phase variation, a strategy that helps immune evasion. Three major structures, pili, Opa and Opc predominantly influence bacterial adhesion to host cells. Pili and Opa proteins also determine host and tissue specificity while Opa and Opc facilitate efficient cellular invasion. Recent studies have also implied a role of certain adhesin-receptor pairs in determining increased host susceptibility to infection. This chapter examines our current knowledge of meningococcal adhesion and invasion mechanisms particularly related to human epithelial and endothelial cells which are of primary importance in the disease process.

Meningococcal protein antigens and vaccines

The development of a comprehensive vaccine against meningococcal disease has been challenging. Recent developments in molecular genetics have provided both explanations for these challenges and possible solutions. Since genome sequence data became available there has been a marked increase in number of protein antigens that have been suggested as prospective vaccine components. This review catalogues the proposed vaccine candidates and examines the evidence for their inclusion in potential protein vaccine formulations.

Identification, characterization, and molecular application of a virulence-associated autotransporter from a pathogenic Pseudomonas fluorescens strain

A gene, pfa1, encoding an autotransporter was cloned from a pathogenic Pseudomonas fluorescens strain, TSS, isolated from diseased fish. The expression of pfa1 is enhanced during infection and is regulated by growth phase and growth conditions. Mutation of pfa1 significantly attenuates the overall bacterial virulence of TSS and impairs the abilities of TSS in biofilm production, interaction with host cells, modulation of host immune responses, and dissemination in host blood. The putative protein encoded by pfa1 is 1,242 amino acids in length and characterized by the presence of three functional domains that are typical for autotransporters. The passenger domain of PfaI contains a putative serine protease (Pap) that exhibits apparent proteolytic activity when expressed in and purified from Escherichia coli as a recombinant protein. Consistent with the important role played by PfaI in bacterial virulence, purified recombinant Pap has a profound cytotoxic effect on cultured fish cells. Enzymatic analysis showed that recombinant Pap is relatively heat stable and has an optimal temperature and pH of 50 degrees C and pH 8.0. The domains of PfaI that are essential to autotransporting activity were localized, and on the basis of this, a PfaI-based autodisplay system (named AT1) was engineered to facilitate the insertion and transport of heterologous proteins. When expressed in E. coli, AT1 was able to deliver an integrated Edwardsiella tarda immunogen (Et18) onto the surface of bacterial cells. Compared to purified recombinant Et18, Et18 displayed by E. coli via AT1 induced significantly enhanced immunoprotection.