Interactions of pathogenic Neisseria with host cells. Is it possible to assemble the puzzle?

Galectins as potential therapeutic targets in STIs in the female genital tract

Every day, more than one million people worldwide acquire a sexually transmitted infection (STI). This public health problem has a direct effect on women's reproductive and sexual health as STIs can cause irreversible damage to fertility and can have negative consequences associated with discrimination and social exclusion. Infection with one sexually transmitted pathogen predisposes to co-infection with others, suggesting the existence of shared pathways that serve as molecular links between these diseases. Galectins, a family of β-galactoside-binding proteins, have emerged as endogenous mediators that facilitate cell-surface binding, internalization and cell invasion of many sexually transmitted pathogens, including Chlamydia trachomatis, Neisseria gonorrhoeae, Trichomonas vaginalis, Candida albicans, HIV and herpes simplex virus. The ability of certain galectins to dimerize or form multimeric complexes confers the capacity to interact simultaneously with glycosylated ligands on both the pathogen and the cervico-vaginal tissue on these proteins. Galectins can act as a bridge by engaging glycans from the pathogen surface and glycosylated receptors from host cells, which is a mechanism that has been shown to be shared by several sexually transmitted pathogens. In the case of viruses and obligate intracellular bacteria, binding to the cell surface promotes pathogen internalization and cell invasion. Inflammatory responses that occur in cervico-vaginal tissue might trigger secretion of galectins, which in turn control the establishment, evolution and severity of STIs. Thus, galectin-targeted therapies could potentially prevent or decrease STIs caused by a diverse array of pathogenic microorganisms; furthermore, anti-galectin agents might reduce treatment costs of STIs and reach the most vulnerable populations.

Inactivation of Genes Encoding MutL and MutS Proteins Influences Adhesion and Biofilm Formation by Neisseria gonorrhoeae

Neisseria gonorrhoeae is an etiological agent of gonorrhea, which remains a global health problem. This bacterium possesses MutL and MutS DNA repair proteins encoded by mutL and mutS genes, whose inactivation causes a mutator phenotype. We have demonstrated the differential gene expression in N. gonorrhoeae mutL and mutS mutants using DNA microarrays. A subset of differentially expressed genes encodes proteins that can influence adhesion and biofilm formation. Compared to the wild-type strain, N. gonorrhoeae mutL and mutS mutants formed denser biofilms with increased biofilm-associated biomass on the abiotic surface. The N. gonorrhoeae mutS::km, but not the mutL mutant, was also more adherent and invasive to human epithelial cells. Further, during infection of epithelial cells with N. gonorrhoeae mutS::km, the expression of some bacterial genes encoding proteins that can influence gonococcal adhesion was changed compared with their expression in cells infected with the wild-type gonococcus, as well as of human genes' encoding receptors utilized by N. gonorrhoeae (CD46, CEACAM 1, HSPG 2). Thus, deficiency in the mutS gene resulting in increased mutation frequency in singular organisms can be beneficial in populations because these mutants can be a source of features linked to microbial fitness.

Crossing the Blood-Brain Barrier by Neuroinvasive Pathogens

The penetration of the blood-brain barrier (BBB) and invasion of the central nervous system (CNS) are important steps for all neuroinvasive pathogens. All of the ways of pathogens passing through the BBB are still unclear. Among known pathways, pathogen traversal can occur paracellularly, transcellularly or using a “Trojan horse” mechanism. The first step of translocation across the BBB is the interactions of the pathogen’s ligands with the receptors of the host brain cells. Lyme disease, the most common vector-borne disease in the temperate zones of Europe and North America, are caused by Borreliella species (former Borrelia burgdorferi sensu lato) that affects the peripheral and the CNS. In this review, we have presented various pathogen interactions with endothelial cells, which allow the disruption of the BBB so that the pathogens can pass across the BBB.

Gonorrhea - an evolving disease of the new millennium

Etiology, transmission and protection: Neisseria gonorrhoeae (the gonococcus) is the etiological agent for the strictly human sexually transmitted disease gonorrhea. Infections lead to limited immunity, therefore individuals can become repeatedly infected. Pathology/symptomatology: Gonorrhea is generally a non-complicated mucosal infection with a pustular discharge. More severe sequellae include salpingitis and pelvic inflammatory disease which may lead to sterility and/or ectopic pregnancy. Occasionally, the organism can disseminate as a bloodstream infection. Epidemiology, incidence and prevalence: Gonorrhea is a global disease infecting approximately 60 million people annually. In the United States there are approximately 300, 000 cases each year, with an incidence of approximately 100 cases per 100,000 population. Treatment and curability: Gonorrhea is susceptible to an array of antibiotics. Antibiotic resistance is becoming a major problem and there are fears that the gonococcus will become the next "superbug" as the antibiotic arsenal diminishes. Currently, third generation extended-spectrum cephalosporins are being prescribed. Molecular mechanisms of infection: Gonococci elaborate numerous strategies to thwart the immune system. The organism engages in extensive phase (on/off switching) and antigenic variation of several surface antigens. The organism expresses IgA protease which cleaves mucosal antibody. The organism can become serum resistant due to its ability to sialylate lipooligosaccharide in conjunction with its ability to subvert complement activation. The gonococcus can survive within neutrophils as well as in several other lymphocytic cells. The organism manipulates the immune response such that no immune memory is generated which leads to a lack of protective immunity.

Novel anti-infective potential of salvianolic acid B against human serious pathogen Neisseria meningitidis

Background

Epidemics of meningococcal meningitis cause significant health problems especially in Sub-Saharan Africa. Novel anti-infective candidates are needed. In modern anti-adhesion therapy initial attachment of bacteria to host cells is prevented. Our unique studies have revealed anti-adhesive candidates from natural products, namely milk and berries, against Neisseria meningitidis adhesion. In the present study against N. meningitidis adhesion, a novel binding inhibitor was found; salvianolic acid B (SA-B), a polyphenol from the radix Salviae miltiorrhizae, an important part of Chinese folk medicine.

Methods

In order to test inhibition of meningococcal pili binding and anti-adhesion activity of SA-B, bovine thyroglobulin, a reference glycoprotein for meningococcal receptor was used in a microtiter plate assay. Inhibitory activity was tested by using serial dilutions of SA-B extracts of 98 and 70 % purity. Results were confirmed in a HEC-1B cell dot assay and antimicrobial activity was measured by using a microbroth dilution assay.

Results

Almost total (93 %) inhibition of pili binding, anti-adhesion, was achieved with the 70 % extract of SA-B at the concentration of 0.3 mg/mL in the bovine thyroglobulin reference model. 50 % binding inhibition activity was achieved with 0.6 µg/mL of the SA-B extract. Total inhibition of the pili binding to HEC-1B cells was found at the tested concentration of 0.5 mg/mL. The 98 % pure SA-B resulted in weaker inhibition. At the concentration of 0.3 mg/mL 78 % inhibition was achieved in the thyroglobulin model. For 50 % inhibition 2.4 μg/mL of pure SA-B was needed. The difference between the binding inhibition activities (70 and 98 % pure SA-B) was statistically significant (P = 0.03). Antimicrobial activity of 70 % SA-B, when investigated against N. meningitidis, was detected only in relatively high concentrations.

Conclusions

Our results indicate that plant SA-B may prevent meningococcal infections by inhibiting meningococcal binding and may thus have an impact on the amount of nasopharyngeal carriers of N. meningitidis. This may prevent the spreading of meningococcal infections between humans. One could conclude that SA-B and its source dried radix S. miltiorrhizae, which is an important part of Chinese folk medicine, could be valuable candidates for further research in meningococcal disease prevention.

Meningococcal B Vaccination (4CMenB) in Infants and Toddlers

Neisseria meningitidis is a Gram-negative pathogen that actively invades its human host and leads to the development of life-threatening pathologies. One of the leading causes of death in the world, N. meningitidis can be responsible for nearly 1,000 new infections per 100,000 subjects during an epidemic period. The bacterial species are classified into 12 serogroups, five of which (A, B, C, W, and Y) cause the majority of meningitides. The three purified protein conjugate vaccines currently available target serogroups A, C, W, and Y. Serogroup B has long been a challenge but the discovery of the complete genome sequence of an MenB strain has allowed the development of a specific four-component vaccine (4CMenB). This review describes the pathogenetic role of N. meningitidis and the recent literature concerning the new meningococcal vaccine.

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.

Neisseria meningitidis serogroup B vaccine development

Neisseria meningitidis is an air-borne, gram-negative pathogen that actively invades its human host leading to the development of life-threatening pathologies. As one of the leading causes of death in the world, during an epidemic period N. meningitidis can be responsible for nearly 1000 new infections per 100,000 individuals. The bacterial species is further categorized into 13 serotypes, with five, A, B, C, W-135, and Y, being the most clinically relevant, causing the overwhelming majority of diseases. There are two contemporary, purified protein conjugate vaccines available that function by targeting serogroups A, C, W-135, and Y. Historically, serogroup B has posed a vaccination challenge; however, there are currently two vaccines in development able to target serotype B. This review will highlight N. meningitidis as a pathogen and explore the recent literature providing a current review of meningococcal vaccination development.

An insight into the ligand-receptor interactions involved in the translocation of pathogens across blood-brain barrier

Traversal of pathogen across the blood-brain barrier (BBB) is an essential step for central nervous system (CNS) invasion. Pathogen traversal can occur paracellularly, transcellularly, and/or in infected phagocytes (Trojan horse mechanism). To trigger the translocation processes, mainly through paracellular and transcellular ways, interactions between protein molecules of pathogen and BBB are inevitable. Simply, it takes two to tango: both host receptors and pathogen ligands. Underlying molecular basis of BBB translocation of various pathogens has been revealed in the last decade, and a plethora of experimental data on protein-protein interactions has been created. This review compiles these data and should give insights into the ligand-receptor interactions that occur during BBB translocation. Further, it sheds light on cell signaling events triggered in response to ligand-receptor interaction. Understanding of the molecular principles of pathogen-host interactions that are involved in traversal of the BBB should contribute to develop new vaccine and drug strategies to prevent CNS infections.

Structure of the PilM-PilN inner membrane type IV pilus biogenesis complex from Thermus thermophilus

Type IV pili are surface-exposed filaments, which extend from a variety of bacterial pathogens and play a major role in pathogenesis, motility, and DNA uptake. Here, we present the crystal structure of a complex between a cytoplasmic component of the type IV pilus biogenesis system from Thermus thermophilus, PilM, in complex with a peptide derived from the cytoplasmic portion of the inner membrane protein PilN. PilM also binds ATP, and its structure is most similar to the actin-like protein FtsA. PilN binds in a narrow channel between the 1A and 1C subdomains in PilM; the binding site is well conserved in other gram-negative bacteria, notably Neisseria meningitidis, Pseudomonas aeruginosa, and Vibrio cholerae. We find no evidence for the catalysis of ATP hydrolysis by PilM; fluorescence data indicate that the protein is likely to be saturated by ATP at physiological concentrations. In addition, binding of the PilN peptide appears to influence the environment of the ATP binding site. This is the first reported structure of a complex between two type IV pilus biogenesis proteins. We propose a model in which PilM binds ATP and then PilN as one of the first steps in the formation of the inner membrane platform of the type IV pilus biogenesis complex.

Proteome analysis of Neisseria meningitidis serogroup strains C associated with outbreaks in China

OBJECTIVE

During 2003-2005, an outbreak of meningitis due to Neisseria meningitidis serogroup C occurred in China. With the aim to find strain clues result in the final epidemics, the ancestral strain 053442, a clinical isolate, and a carrier strain 053426 with different gene type were analyzed.

METHODS

Clinical strain 053442 and carrier strain 053426 were cultured on GC agar plates under the same condition. Two-dimensional electrophoresis was performed using the pH 3-10 nonlinear IPG strips of 24 cm length, and all the protein spots were identified by matrix-assisted laser desorption/ionization time of flight spectrometry.

RESULTS

502 and 380 protein spots were identified in 053426 and 053442 respectively, relating to 266 and 202 different genes covering a wide range of cellular functions. The express volume and number of proteins involved in energy metabolism, protein synthesis and amino acid biosynthesis in 053426 were higher than in 053442. Virulence factor Opa, Opc and a series of proteins involved in pilus assembly and retraction were identified in 053442, which appear to be of primary importance in colonization and invasion of human cells. Compared to 053442, virulence protein species were less in 053426, with lower express volumes too. No Opa and Opc were detected in 053426.

CONCLUSIONS

The different protein expression profiles of the clinical strain 053442 and carrier strain 053426 in the present study provide some clues of the different pathogenicity of the two strains, which may account for result in the final epidemics.

Transcellular passage of Neisseria meningitidis across a polarized respiratory epithelium

Neisseria meningitidis is a major cause of sepsis and meningitis but is also a common commensal, present in the nasopharynx of between 8 and 20% of healthy individuals. During carriage, the bacterium is found on the surface of the nasopharyngeal epithelium and in deeper tissues, while to develop disease the meningococcus must spread across the respiratory epithelium and enter the systemic circulation. Therefore, investigating the pathways by which N. meningitidis crosses the epithelial barrier is relevant for understanding carriage and disease but has been hindered by the lack of appropriate models. Here, we have established a physiologically relevant model of the upper respiratory epithelial cell barrier to investigate the mechanisms responsible for traversal of N. meningitidis. Calu-3 human respiratory epithelial cells were grown on permeable cell culture membranes to form polarized monolayers of cells joined by tight junctions. We show that the meningococcus crosses the epithelial cell barrier by a transcellular route; traversal of the layer did not disrupt its integrity, and bacteria were detected within the cells of the monolayer. We demonstrate that successful traversal of the epithelial cell barrier by N. meningitidis requires expression of its type 4 pili (Tfp) and capsule and is dependent on the host cell microtubule network. The Calu-3 model should be suitable for dissecting the pathogenesis of infections caused by other respiratory pathogens, as well as the meningococcus.

Opacity proteins increase Neisseria gonorrhoeae fitness in the female genital tract due to a factor under ovarian control

The neisserial opacity (Opa) proteins are a family of antigenically distinct outer membrane proteins that undergo phase-variable expression. Opa(+) variants of Neisseria gonorrhoeae strain FA1090 are selected in a cyclical pattern from the lower genital tract of estradiol-treated mice. Here we show that cyclical recovery of Opa(+) gonococci does not occur in ovariectomized mice; therefore, the reproductive cycle plays a role in the selection kinetics in vivo. As predicted by the selection pattern shown by wild-type gonococci, we demonstrated that a constitutive Opa-expressing strain was more fit than an Opa-deficient mutant in the early and late phases of infection. We found no evidence that Opa-mediated colonization selects for Opa(+) variants during murine infection based on adherence assays with cultured murine epithelial cells. We also tested the hypothesis that complement selects for Opa protein expression during infection. Although some Opa(+) variants of a serum-sensitive derivative of strain FA1090 were more resistant to the bactericidal activity of normal human serum, selection for Opa expression was not abrogated in C3-depleted mice. Finally, as previously reported, Opa(+) gonococci were more sensitive to serine proteases. Thus, proteases or protease inhibitors may contribute to the observed in vivo selection pattern. We concluded that Opa proteins promote persistence of N. gonorrhoeae in the female genital tract and that opa gene phase variation allows gonococci to evade or capitalize upon unidentified host factors of the mammalian reproductive cycle. This work revealed an intimate interaction between pathogen and host and provides evidence that hormonally related factors shape bacterial adaptation.

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.

Expression of phosphofructokinase in Neisseria meningitidis

Neisseria meningitidis serogroup B is a pathogen that can infect diverse sites within the human host. According to the N. meningitidis genomic information and experimental observations, glucose can be completely catabolized through the Entner-Doudoroff pathway and the pentose phosphate pathway. The Embden-Meyerhof-Parnas pathway is not functional, because the gene for phosphofructokinase (PFK) is not present. The phylogenetic distribution of PFK indicates that in most obligate aerobic organisms, PFK is lacking. We conclude that this is because of the limited contribution of PFK to the energy supply in aerobically grown organisms in comparison with the energy generated through oxidative phosphorylation. Under anaerobic or microaerobic conditions, the available energy is limiting, and PFK provides an advantage, which explains the presence of PFK in many (facultatively) anaerobic organisms. In accordance with this, in silico flux balance analysis predicted an increase of biomass yield as a result of PFK expression. However, analysis of a genetically engineered N. meningitidis strain that expressed a heterologous PFK showed that the yield of biomass on substrate decreased in comparison with a pfkA-deficient control strain, which was associated mainly with an increase in CO(2) production, whereas production of by-products was similar in the two strains. This might explain why the pfkA gene has not been obtained by horizontal gene transfer, since it is initially unfavourable for biomass yield. No large effects related to heterologous expression of pfkA were observed in the transcriptome. Although our results suggest that introduction of PFK does not contribute to a more efficient strain in terms of biomass yield, achievement of a robust, optimal metabolic network that enables a higher growth rate or a higher biomass yield might be possible after adaptive evolution of the strain, which remains to be investigated.

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.

The effect of hfq on global gene expression and virulence in Neisseria gonorrhoeae

Hfq is an RNA chaperone that functions as a pleiotropic regulator for RNA metabolism in bacteria. In several pathogenic bacteria, Hfq contributes indirectly to virulence by binding to riboregulators that modulate the stability or translation efficiency of RNA transcripts. To characterize the role of Hfq in the pathogenicity of Neisseria gonorrhoeae, we generated an N. gonorrhoeae hfq mutant. Infectivity and global changes in gene expression caused by the hfq mutation in N. gonorrhoeae strain MS11 were analyzed. Transcriptional analysis using a custom-made N. gonorrhoeae microarray revealed that 369 ORFs were differentially regulated in the hfq mutant, MS11hfq, in comparison with the wild-type strain (202 were upregulated, and 167 were downregulated). The loss-of-function mutation in hfq led to pleiotropic phenotypic effects, including an altered bacterial growth rate and reduced adherence to epithelial cells. Twitching motility and microcolony formation were not affected. Hfq also appears to play a minor role in inducing the inflammatory response of infected human epithelial cells. Interleukin-8 production was slightly decreased, and activation of c-Jun N-terminal kinase, a mitogen-activated protein kinase, was reduced in MS11hfq-infected epithelial cells in comparison with wild type-infected cells. However, activation of nuclear factor kappa B, extracellular signal-regulated kinase 1/2 and p38 remained unchanged. The data presented suggest that Hfq plays an important role as a post-transcriptional regulator in N. gonorrhoeae strain MS11 but does not contribute significantly to its virulence in cell culture models.

Identification of meningococcal genes necessary for colonization of human upper airway tissue

Neisseria meningitidis is an exclusively human pathogen that has evolved primarily to colonize the nasopharynx rather than to cause systemic disease. Colonization is the most frequent outcome following meningococcal infection and a prerequisite for invasive disease. The mechanism of colonization involves attachment of the organism to epithelial cells via bacterial type IV pili (Tfp), but subsequent events during colonization remain largely unknown. We analyzed 576 N. meningitidis mutants for their capacity to colonize human nasopharyngeal tissue in an organ culture model to identify bacterial genes required for colonization. Eight colonization-defective mutants were isolated. Two mutants were unable to express Tfp and were defective for adhesion to epithelial cells, which is likely to be the basis of their attenuation in nasopharyngeal tissue. Three other mutants are predicted to have lost previously uncharacterized surface molecules, while the remaining mutants have transposon insertions in genes of unknown function. We have identified novel meningococcal colonization factors, and this should provide insights into the survival of this important pathogen in its natural habitat.

Modification of lipooligosaccharide with phosphoethanolamine by LptA in Neisseria meningitidis enhances meningococcal adhesion to human endothelial and epithelial cells

The lipooligosaccharide (LOS) of Neisseria meningitidis can be decorated with phosphoethanolamine (PEA) at the 4' position of lipid A and at the O-3 and O-6 positions of the inner core of the heptose II residue. The biological role of PEA modification in N. meningitidis remains unclear. During the course of our studies to elucidate the pathogenicity of the ST-2032 (invasive) meningococcal clonal group, disruption of lptA, the gene that encodes the PEA transferase for 4' lipid A, led to a approximately 10-fold decrease in N. meningitidis adhesion to four kinds of human endothelial and epithelial cell lines at an multiplicity of infection of 5,000. Complementation of the lptA gene in a Delta lptA mutant restored wild-type adherence. By matrix-assisted laser desorption ionization-time-of-flight mass spectrometry analysis, PEA was lost from the lipid A of the Delta lptA mutant compared to that of the wild-type strain. The effect of LptA on meningococcal adhesion was independent of other adhesins such as pili, Opc, Opa, and PilC but was inhibited by the presence of capsule. These results indicate that modification of LOS with PEA by LptA enhances meningococcal adhesion to human endothelial and epithelial cells in unencapsulated N. meningitidis.

Mechanisms of microbial traversal of the blood-brain barrier

Central nervous system (CNS) infections continue to be an important cause of morbidity and mortality. Microbial invasion and traversal of the blood-brain barrier is a prerequisite for CNS infections. Pathogens can cross the blood-brain barrier transcellularly, paracellularly and/or in infected phagocytes (the so-called Trojan-horse mechanism). Consequently, pathogens can cause blood-brain barrier dysfunction, including increased permeability, pleocytosis and encephalopathy. A more complete understanding of the microbial-host interactions that are involved in microbial traversal of the blood-brain barrier and the associated barrier dysfunction should help to develop new strategies to prevent CNS infections.

In meso crystal structure and docking simulations suggest an alternative proteoglycan binding site in the OpcA outer membrane adhesin

OpcA is an integral outer membrane adhesin protein from Neisseria meningitidis, the causative agent of meningococcal meningitis and septicemia. It binds to sialic acid (SA)-containing polysaccharides on the surface of epithelial cells. The crystal structure of OpcA showed that the protein adopts a 10-stranded beta-barrel structure, with five extensive loop regions on the extracellular side of the membrane. These form a crevice structure, lined with basic residues, which was hypothesized to act as the binding site for polysaccharide ligands. In the current study, a distinctly different OpcA structure has been obtained using crystals grown from a lipidic mesophase. Comparison of the two structures shows that the largest loop (L2), which closes over the end of the beta-barrel in the original crystal form, adopts a much more extended structure by reaching outward and away from the protein. The difference in conformation may be attributable to the absence of zinc ions from the crystallization conditions for the in meso crystal form: in the original structure, two zinc ions were bound to the external loops. Molecular dynamics (MD) simulations performed on the two OpcA models in a lipid bilayer environment demonstrated pronounced loop mobility. These observations support the view that the loop regions of OpcA are capable of a high degree of conformational flexibility. The original binding site for polysaccharide is not present in the in meso crystal form, and is disrupted during MD simulations. Docking analysis suggests a putative alternative location for the SA ligand in the new crystal form of OpcA.

Structure refinement of the OpcA adhesin using molecular dynamics

OpcA from Neisseria meningitidis, the causative agent of meningococcal meningitis and septicemia, is an integral outer membrane protein that facilitates meningococcal adhesion through binding the proteoglycan receptors of susceptible cells. Two structures of OpcA have been determined by x-ray diffraction to 2 A resolution, revealing dramatically different conformations in the extracellular loops--the protein domain implicated in proteoglycan binding. In the first structure, a positively charged crevice formed by loops 1 and 2 was identified as the site for binding proteoglycans, whereas in the second structure the crevice was not evident as loops 1 and 2 adopted different conformations. To reconcile these results, molecular-dynamics simulations were carried out on both structures embedded in a solvated lipid bilayer membrane. Free of crystal contacts and crystallization agents, the loops were observed to undergo large structural transformations, suggesting that the conformation of the loops in either x-ray structure is affected by crystallization. Subsequent simulations of both structures in their crystal lattices confirmed this conclusion. Based on our molecular-dynamics trajectories, we propose a model for OpcA that combines stable structural features of the available x-ray structures. In this model, all five extracellular loops of OpcA have stable secondary structures. The loops form a funnel that leads to the base of the beta-barrel and that includes Tyr-169 on its exposed surface, which has been implicated in proteoglycan binding.

mgpB and mgpC sequence diversity in Mycoplasma genitalium is generated by segmental reciprocal recombination with repetitive chromosomal sequences

Mycoplasma genitalium is associated with sexually transmitted infections in men and women that, if untreated, can persist, suggesting that mechanism(s) exist to facilitate immune evasion. Approximately 4% of the limited M. genitalium genome contains repeat sequences termed MgPar regions that have homology to mgpB and mgpC, which encode antigenic proteins associated with attachment. We have previously shown that mgpB sequences vary within a single strain of M. genitalium in a pattern consistent with recombination between mgpB and MgPar sequences (Iverson-Cabral et al.). In the current study, we show that mgpC heterogeneity similarly occurs within the type strain, G-37(T), cultured in vitro and among cervical specimens collected from a persistently infected woman. In all cases, alternative mgpC sequences are indicative of recombination with MgPar regions. Additionally, the isolation of single-colony M. genitalium clonal variants containing alternative mgpB or mgpC sequences allowed us to demonstrate that mgpB and mgpC heterogeneity is associated with corresponding changes within donor MgPar regions, consistent with reciprocal recombination. Better-defined systems of antigenic variation are typically mediated by unidirectional gene conversion, so the generation of genetic diversity observed in M. genitalium by the mutual exchange of sequences makes this organism unique among bacterial pathogens.

Differential in vitro infectious abilities of two common Japan-specific sequence-type (ST) clones of disease-associated ST-2032 and carrier-associated ST-2046 Neisseria meningitidis strains in human endothelial and epithelial cell lines

The Japan-specific sequence type (ST) clones, as well as several major epidemic-prone clones such as ST-32, have been identified previously among Neisseria meningitidis isolates in Japan. In this study, the infectious properties of various ST clones, including the two common Japan-specific ones, were examined and compared by in vitro infection assays using human endothelial and epithelial cell lines. The known invasive clones, as well as the Japan-specific ST-2032 strains that were frequently isolated from patients, exhibited high infectious abilities in adherence and invasion. In contrast, the Japan-specific ST-2046 and ST-198 strains, both of which were frequently isolated from carriers in Japan, were less efficient in adherence and invasion. The expression of the bacterial surface molecules such as pilin, Opc, Opa and PilC, and the lipooligosaccharide structure, did not differ between disease-associated and carrier-associated isolates. These results suggest that in vitro infection assays may discriminate between disease-associated (patient-dominant) and carrier-associated (carrier-dominant) meningococcal ST clones. The ST-2032 clone showed the highest infectious activity in vitro, suggesting that it may possess some unidentified factors necessary for the infectious ability that were not present in the ST-2046 clone with the lowest infectious ability.

Purification and three-dimensional electron microscopy structure of the Neisseria meningitidis type IV pilus biogenesis protein PilG

Type IV pili are surface-exposed retractable fibers which play a key role in the pathogenesis of Neisseria meningitidis and other gram-negative pathogens. PilG is an integral inner membrane protein and a component of the type IV pilus biogenesis system. It is related by sequence to the extensive GspF family of secretory proteins, which are involved in type II secretion processes. PilG was overexpressed and purified from Escherichia coli membranes by detergent extraction and metal ion affinity chromatography. Analysis of the purified protein by perfluoro-octanoic acid polyacrylamide gel electrophoresis showed that PilG formed dimers and tetramers. A three-dimensional (3-D) electron microscopy structure of the PilG multimer was determined using single-particle averaging applied to samples visualized by negative staining. Symmetry analysis of the unsymmetrized 3-D volume provided further evidence that the PilG multimer is a tetramer. The reconstruction also revealed an asymmetric bilobed structure approximately 125 A in length and 80 A in width. The larger lobe within the structure was identified as the N terminus by location of Ni-nitrilotriacetic acid nanogold particles to the N-terminal polyhistidine tag. We propose that the smaller lobe corresponds to the periplasmic domain of the protein, with the narrower "waist" region being the transmembrane section. This constitutes the first report of a 3-D structure of a member of the GspF family and suggests a physical basis for the role of the protein in linking cytoplasmic and periplasmic protein components of the type II secretion and type IV pilus biogenesis systems.

Two ABC transporter operons and the antimicrobial resistance gene mtrF are pilT responsive in Neisseria gonorrhoeae

Retraction of type IV pili is mediated by PilT. We show that loss of pilT function leads to upregulation of mtrF (multiple transferable resistance) and two operons encoding putative ABC transporters in Neisseria gonorrhoeae MS11. This effect occurs indirectly through the transcriptional regulator FarR, which until now has been shown to regulate only farAB. L-Glutamine can reverse pilT downregulation of the ABC transporter operons and mtrF.

Comparative genomics of Neisseria meningitidis: core genome, islands of horizontal transfer and pathogen-specific genes

To better understand Neisseria meningitidis genomes and virulence, microarray comparative genome hybridization (mCGH) data were collected from one Neisseria cinerea, two Neisseria lactamica, two Neisseria gonorrhoeae and 48 Neisseria meningitidis isolates. For N. meningitidis, these isolates are from diverse clonal complexes, invasive and carriage strains, and all major serogroups. The microarray platform represented N. meningitidis strains MC58, Z2491 and FAM18, and N. gonorrhoeae FA1090. By comparing hybridization data to genome sequences, the core N. meningitidis genome and insertions/deletions (e.g. capsule locus, type I secretion system) related to pathogenicity were identified, including further characterization of the capsule locus, bioinformatics analysis of a type I secretion system, and identification of some metabolic pathways associated with intracellular survival in pathogens. Hybridization data clustered meningococcal isolates from similar clonal complexes that were distinguished by the differential presence of six distinct islands of horizontal transfer. Several of these islands contained prophage or other mobile elements, including a novel prophage and a transposon carrying portions of a type I secretion system. Acquisition of some genetic islands appears to have occurred in multiple lineages, including transfer between N. lactamica and N. meningitidis. However, island acquisition occurs infrequently, such that the genomic-level relationship is not obscured within clonal complexes. The N. meningitidis genome is characterized by the horizontal acquisition of multiple genetic islands; the study of these islands reveals important sets of genes varying between isolates and likely to be related to pathogenicity.

The Solution Structure of a Domain from the Neisseria meningitidis Lipoprotein PilP Reveals a New β-Sandwich Fold

Type IV pili are long, thin fibres, which extend from the surface of the bacterial pathogen Neisseria meningitidis; they play a key role in adhesion and colonisation of host cells. PilP is a lipoprotein, suggested to be involved in the assembly and stabilization of an outer membrane protein, PilQ, which is required for pilus formation. Here we describe the expression of a recombinant fragment of PilP, spanning residues 20 to 181, and determination of the solution structure of a folded domain, spanning residues 85 to 163, by NMR. The N-terminal third of the protein, from residues 20 to 84, is apparently unfolded. Protease digestion yielded a 113 residue fragment that contained the folded domain. The domain adopts a simple beta-sandwich type fold, consisting of a three-stranded beta-sheet packed against a four-stranded beta-sheet. There is also a short segment of 3(10) helix at the N-terminal part of the folded domain. We were unable to identify any other proteins that are closely related in structure to the PilP domain, although the fold appears to be distantly related to the lipocalin family. Over 40 homologues of PilP have been identified in Gram-negative bacteria and the majority of conserved residues lie within the folded domain. The fourth beta-strand and adjacent loop regions contain a high proportion of conserved residues, including three glycine residues, which seem to play a role in linking the two beta-sheets. The two beta-sheets pack together to form a crevice, lined with conserved hydrophobic residues: we suggest that this feature could act as a binding site for a small ligand. The results show that PilP and its homologues have a conserved, folded domain at the C-terminal end of the protein that may be involved in mediating binding to hydrophobic ligands.

Type IV pili function upstream of the Dif chemotaxis pathway in Myxococcus xanthus EPS regulation

The developmental bacterium Myxococcus xanthus utilizes gliding motility to aggregate during the formation of multicellular fruiting bodies. The social (S) component of M. xanthus gliding motility requires at least two extracellular surface structures, type IV pili (Tfp) and the fibril polysaccharide or exopolysaccharide (EPS). Retraction of Tfp is proposed to power S motility and EPS from neighbouring cells is suggested to provide an anchor and trigger for Tfp retraction. The production of EPS in M. xanthus is regulated in part by the Dif chemosensory pathway; however, the input signal for the Dif pathway in EPS regulation remains to be uncovered. Using a genetic approach combined with quantitative and qualitative analysis, we demonstrate here that Tfp function upstream of the Dif proteins in regulating EPS production. The requirement of Tfp for the production of EPS was verified using various classes of Tfp mutants. Construction and examination of double and triple mutants indicated that mutations in dif are epistatic to those in pil. Furthermore, extracellular complementation between various Tfp and dif mutants suggests that Tfp, instead of being signals, may constitute the sensor or part of the sensor responsible for mediating signal input into the Dif pathway. We propose that S motility involves a regulatory loop in which EPS triggers Tfp retraction and Tfp provide proximity signals to the Dif pathway to modulate EPS production.

Immunization with live Neisseria lactamica protects mice against meningococcal challenge and can elicit serum bactericidal antibodies

Natural immunity against Neisseria meningitidis is thought to develop following nasopharyngeal colonization with this bacterium or other microbes expressing cross-reactive antigens. Neisseria lactamica is a commensal of the upper respiratory tract which is often carried by infants and young children; epidemiological evidence indicates that colonization with this bacterium can elicit serum bactericidal activity (SBA) against Neisseria meningitidis, the most validated correlate of protective immunity. Here we demonstrate experimentally that immunization of mice with live N. lactamica protects animals against lethal meningococcal challenge and that some, but not all, strains of N. lactamica elicit detectable SBA in immunized animals regardless of the serogroup of N. meningitidis. While it is unlikely that immunization with live N. lactamica will be implemented as a vaccine against meningococcal disease, understanding the basis for the induction of cross-protective immunity and SBA should be valuable in the design of subunit vaccines for the prevention of this important human infection.

Low-phosphate-dependent invasion resembles a general way for Neisseria gonorrhoeae to enter host cells

Obligate human-pathogenic Neisseria gonorrhoeae expresses numerous variant surface proteins mediating adherence to and invasion of target cells. The invariant major outer membrane porin PorB of serotype A (P.IA) gonococci triggers invasion into Chang cells only if the medium is devoid of phosphate. Since gonococci expressing PorB(IA) are frequently isolated from patients with severe disseminating infections, the interaction initiated by the porin may be of major relevance for the development of this serious disease. Here, we investigated the low-phosphate-dependent invasion and compared it to the well-known pathways of entry initiated by Opa proteins. P.IA-triggered invasion requires clathrin-coated pit formation and the action of actin and Rho GTPases. However, in contrast to Opa-initiated invasion via heparan sulfate proteoglycans, microtubules, acidic sphingomyelinase, phosphatidylinositol 3-kinase, and myosin light chain kinase are not involved in this entry pathway. Nor are Src kinases required, as they are in invasion, e.g., via the CEACAM3 receptor. Invasion by PorB(IA) occurs in a wide spectrum of cell types, such as primary human epithelial and endothelial cells and in cancer cells of human and animal origin. Low-phosphate-dependent invasion is thus a pathway of gonococcal entry distinct from Opa-mediated invasion.

Intrastrain heterogeneity of the mgpB gene in Mycoplasma genitalium is extensive in vitro and in vivo and suggests that variation is generated via recombination with repetitive chromosomal sequences

Mycoplasma genitalium is associated with reproductive tract disease in women and may persist in the lower genital tract for months, potentially increasing the risk of upper tract infection and transmission to uninfected partners. Despite its exceptionally small genome (580 kb), approximately 4% is composed of repeated elements known as MgPar sequences (MgPa repeats) based on their homology to the mgpB gene that encodes the immunodominant MgPa adhesin protein. The presence of these MgPar sequences, as well as mgpB variability between M. genitalium strains, suggests that mgpB and MgPar sequences recombine to produce variant MgPa proteins. To examine the extent and generation of diversity within single strains of the organism, we examined mgpB variation within M. genitalium strain G-37 and observed sequence heterogeneity that could be explained by recombination between the mgpB expression site and putative donor MgPar sequences. Similarly, we analyzed mgpB sequences from cervical specimens from a persistently infected woman (21 months) and identified 17 different mgpB variants within a single infecting M. genitalium strain, confirming that mgpB heterogeneity occurs over the course of a natural infection. These observations support the hypothesis that recombination occurs between the mgpB gene and MgPar sequences and that the resulting antigenically distinct MgPa variants may contribute to immune evasion and persistence of infection.

Role of FNR and FNR-regulated, sugar fermentation genes in Neisseria meningitidis infection

While it is generally accepted that anaerobic metabolism is required during infection, supporting experimental data have only been described in a limited number of studies. To provide additional evidence on the role of anaerobic metabolism in bacterial pathogens while invading mammalian hosts, we analysed the effect of the inactivation of FNR, the major regulatory protein involved in the adaptation to oxygen restrictive conditions, and of two of the FNR-regulated genes on the survival of Neisseria meningitidis serogroup B (MenB) in vivo. We found that fnr deletion resulted in more than 1 log reduction in the meningococcal capacity to proliferate both in infant rats and in mice. To identify which of the FNR-regulated genes were responsible for this attenuated phenotype, we defined the FNR regulon by combining DNA microarray analysis and FNR-DNA binding studies. Under oxygen-restricted conditions, FNR positively controlled the transcription of nine transcriptional units, the most upregulated of which were the two operons NMB0388-galM and mapA-pgmbeta implicated in sugar metabolism and fermentation. When galM and mapA were knocked out, the mutants were attenuated by 2 and 3 logs respectively. As the operons are controlled by FNR, from these data we conclude that MenB survival in the host anatomical sites where oxygen is limiting is supported by sugar fermentation.

Co-ordinate action of bacterial adhesins and human carcinoembryonic antigen receptors in enhanced cellular invasion by capsulate serum resistant Neisseria meningitidis

Neisseria meningitidis (Nm) is a human specific opportunistic pathogen that occasionally penetrates mucosal barriers via the action of adhesins and invasins and evades host immune mechanisms during further dissemination via capsule expression. From in vitro studies, the primary adhesion of capsulate bacteria is believed to be mediated by polymeric pili, followed by invasion via outer membrane adhesins such as Opa proteins. As the latter requires the surface capsule to be down-modulated, invading bacteria would be serum sensitive and thus avirulent. However, there is recent evidence that capsulate bacteria may interact via Opa proteins when host cells express high levels of carcinoembryonic antigen-related cell adhesion molecules (CEACAMs), their target receptors. Such a situation may arise following increased circulation of inflammatory cytokines that upregulate certain adhesion molecules on host cells. In this study, using a tetracycline controlled expression system, we have developed cell lines with inducible CEACAM expression to mimic post-inflammation state of target tissues and analysed the interplay between the three surface components capsule, pili and Opa proteins in cellular interactions. With two distinct cell lines, not only the level but also the rate of adhesion of capsulate Opa-expressing Nm increased concurrently with CEACAM density. Moreover, when threshold levels of receptor were reached, cellular invasion ensued in an Opa-dependent manner. In studies with cell lines intrinsically expressing pilus receptors, notable synergism in cellular interactions between pili and Opa of several meningococcal strains was observed and was independent of capsule type. A number of internalized bacteria were shown to express capsule and when directly isolated from host cells, these bacteria were as serum resistant as the inoculated phenotype. Furthermore, we observed that agents that block Opa-CEACAM binding substantially reduced cellular invasion, while maintaining a low level of cellular adhesion. These studies highlight some of the factors that may determine increased host susceptibility to infection by serum resistant phenotypes; and demonstrate the potential of selective inhibition of key interactions in preventing target tissue penetration while maintaining a level of colonization.

Pilus-mediated adhesion of Neisseria meningitidis is negatively controlled by the pilus-retraction machinery

The type IV pili (Tfp) of Neisseria meningitidis play an essential role in meningococcal virulence by mediating the initial interaction of bacteria with host cells. Tfp are also subject to retraction, which relies on the PilT protein. Among the other components of the Tfp machinery, PilC1, a pilus-associated protein, is important for Tfp biogenesis and adhesion. Adhesion of N. meningitidis to living epithelial cells was previously shown to rely on the upregulation of the pilC1 gene. On the other hand the lack of induction of pilC1 is believed to be responsible for the low adhesion of N. meningitidis onto fixed dead cells. Surprisingly, a pilT mutant, unable to retract its pili, has been shown to adhere very efficiently onto both living and fixed epithelial cells. To elucidate the mechanisms by which the pilus retraction machinery mediates meningococcal adhesion onto fixed cells, an analysis of gene expression levels in wild-type and pilT meningococci was performed using DNA microarrays. One of the upregulated genes in the pilT strain was pilC1. This result was confirmed using quantitative real-time reverse-transcription polymerase chain reaction (RT-PCR) and immunoblot analysis. The transcription starting point responsible for the upregulation of pilC1 in a pilT background was shown to be different from those controlling the induction of pilC1 upon contact with living host cells. Subsequent work using a strain hyperproducing PilT confirmed that PilT downregulates the production of PilC1. Furthermore using a pilC1 allele under the control of IPTG, we demonstrated that the upregulation of pilC1 in a pilT background was responsible for the adhesive phenotype onto fixed dead cells. Taken together our results demonstrate that the pilus retraction machinery negatively controlled the adhesiveness of the Tfp via the expression of pilC1.

Evidence for cross-regulated cytokine response in human peripheral blood mononuclear cells exposed to whole gonococcal bacteria in vitro

Neisseria gonorrhoeae is an obligate human pathogen that represents a significant health concern, particularly in the developing world. Although generally associated with an acute inflammatory infection of urogenital epithelia cells, infections have been noted in multiple tissues and many infected individuals can become asymptomatic carriers. Few studies of immune response to N. gonorrhoeae infection in peripheral blood have evaluated the production of T helper cytokines (TH1/TH2) induced early after infection. We developed a quantitative realtime PCR assay based on the gonococcal rmpIII gene to monitor dose-response effects of infection on cytokine release from peripheral blood mononuclear cells (PBMCs). We observed upregulation of CD69 transcription and surface CD25 expression on lymphocytes, consistent with early T-cell activation. We observed dosage-dependent transcription of the chemotactic factor IL-8 and previously unreported activation of the chemoattractant MCP-2. Multiplex analysis of broad cytokine protein production revealed a differential increase in the TH1 and TH2 associated cytokines: IL-2, IL-4, IL-8, IL-10, IL-12, IFN-gamma, TNF-alpha and MCP-1. Markov models of protein accumulation implicated a cross-regulated response to infection, notably for IL-8, IL-10 and IL-12. Taken together, the cytokine profile we observed early in response to whole gonococcal bacteria was broader than has been previously described and may have relevance for the contribution of antagonistic signaling events early in infection and in understanding peripheral immune mechanisms engaged to control infection.

Critical determinants of the interactions of capsule-expressing Neisseria meningitidis with host cells: the role of receptor density in increased cellular targeting via the outer membrane Opa proteins

Neisseria meningitidis capsule is an important virulence determinant required for survival in the blood but is reportedly involved in inhibiting cellular interactions mediated by meningococcal outer membrane adhesins. However, evidence from our previous studies suggested that target receptor density on host cells may determine whether or not capsulate bacteria can adhere via outer membrane proteins such as Opa. To confirm this and evaluate the impact of capsulation on bacterial interactions, we used Opa(+) and Opa(-) derivatives of capsulate and acapsulate meningococcal isolates and transfected cell lines expressing CEACAM1, a receptor targeted by Opa proteins. To assess the extent and rate of cell association, subpopulations of stably transfected Chinese hamster ovary cells with different receptor levels were derived. A quantitative correlation of CEACAM1 levels and Opa-dependent binding of both capsulate and acapsulate bacteria was demonstrated, which was accelerated at high receptor densities. However, it appears that invasion by Opa(+) capsulate bacteria only occurs when a threshold level of CEACAM density has been reached. Target cells expressing high levels of CEACAM1 (MFI c. 400) bound threefold more, but internalized 20-fold more Opa(+) capsulate bacteria than those with intermediate expression (MFI c. 100). No overall selection of acapsulate phenotype was observed in the internalized population. These observations confirm that capsule may not be an adequate barrier for cellular interactions and demonstrate the role of a host factor that may determine capsulate bacterial invasion potential. Upregulation of CEACAMs, which can occur in response to inflammatory cytokines, could lead to translocation of a small number of fully capsulate bacteria across mucosal epithelium into the bloodstream sufficient to cause a rapid onset of disseminated disease. Thus the data also suggest a novel rationale for the epidemiological observations that individuals with prior infectious/inflammatory conditions carry a high risk of invasive meningococcal disease.

A gonococcal homologue of meningococcal gamma-glutamyl transpeptidase gene is a new type of bacterial pseudogene that is transcriptionally active but phenotypically silent

Background

It has been speculated that the γ-glutamyl transpeptidase (ggt) gene is present only in Neisseria meningitidis and not among related species such as Neisseria gonorrhoeae and Neisseria lactamica, because N. meningitidis is the only bacterium with GGT activity. However, nucleotide sequences highly homologous to the meningococcal ggt gene were found in the genomes of N. gonorrhoeae isolates.

Results

The gonococcal homologue (ggt gonococcal homologue; ggh) was analyzed. The nucleotide sequence of the ggh gene was approximately 95 % identical to that of the meningococcal ggt gene. An open reading frame in the ggh gene was disrupted by an ochre mutation and frameshift mutations induced by a 7-base deletion, but the amino acid sequences deduced from the artificially corrected ggh nucleotide sequences were approximately 97 % identical to that of the meningococcal ggt gene. The analyses of the sequences flanking the ggt and ggh genes revealed that both genes were localized in a common DNA region containing the fbp-ggt (or ggh)-glyA-opcA-dedA-abcZ gene cluster. The expression of the ggh RNA could be detected by dot blot, RT-PCR and primer extension analyses. Moreover, the truncated form of ggh-translational product was also found in some of the gonococcal isolates.

Conclusion

This study has shown that the gonococcal ggh gene is a pseudogene of the meningococcal ggt gene, which can also be designated as Ψggt. The gonococcal ggh (Ψggt) gene is the first identified bacterial pseudogene that is transcriptionally active but phenotypically silent.

Recognition of saccharides by the OpcA, OpaD, and OpaB outer membrane proteins from Neisseria meningitidis

The adhesion of the pathogen Neisseria meningitidis to host cell surface proteoglycan, mediated by the integral outer membrane proteins OpcA and Opa, plays an important part in the processes of colonization and invasion by the bacterium. The precise specificities of the OpcA and Opa proteins are, however, unknown. Here we use a fluorescence-based binding assay to show that both proteins bind to mono- and disaccharides with high affinity. Binding of saccharides caused a quench in the intrinsic fluorescence emission of both proteins, and mutation of selected Tyr residues within the external loop regions caused a substantial decrease in fluorescence. We suggest that the intrinsic fluorescence arises from resonance energy transfer from Tyr to Trp residues in the beta-barrel portion of the structure. OpcA bound sialic acid with a Kd of 0.31 microM and was shown to be specific for pyranose saccharides. The binding specificities of two different Opa proteins were compared; unlike OpcA, neither protein bound to monosaccharides, but both bound to maltose, lactose, and sialic acid-containing oligosaccharides, with Kd values in the micromolar range. OpaB had a 10-fold higher affinity for sialic acid-containing ligands than OpaD as a result of the mutation Y165V, which was shown to restore this specificity to OpaD. Finally, the OpcA- and Opa-dependent adhesion of meningococci to epithelial cells was shown to be partially inhibited by exogenously added sialic acid and maltose. The results show that OpcA and the Opa proteins can be thought of as outer membrane lectins and that simple saccharides can modulate their recognition of complex proteoglycan receptors.

Evidence of positive Darwinian selection in putative meningococcal vaccine antigens

Meningococcal meningitidis is a life-threatening disease. In Europe and the United States the majority of cases are caused by virulent meningococcal strains belonging to serogroup B. Presently there is no effective vaccine against serogroup B strains, as traditional vaccine antigens such as polysaccharide capsules are unusable as they lead to autoimmunity. The year 2000 saw the publication of the complete genome of Neisseria meningitidis MC58, a virulent serogroup B bacterium. Working in conjunction with the sequencing project, researchers endeavored to locate highly conserved membrane-associated proteins that elicit an immune response. It is hoped that these proteins will provide a basis for novel vaccines against serogroup B strains. A number of potential vaccine antigens have been located and are presently in phase I clinical trials. Recently many reports pertaining to the evidence of positive Darwinian selection in membrane proteins of pathogens have been reported. This study utilized in silico methods to test for evidence of historical positive Darwinian selection in seven such vaccine candidates. We found that two of these proteins show signatures of adaptive evolution, while the remaining proteins show evidence of strong purifying selection. This has significant implications for the design of a vaccine against serogroup B strains, as it has been shown that vaccines that target epitopes that are under strong purifying selection are better than those that target variable epitopes.

Carcinoembryonic antigen-related cell adhesion molecule (CEACAM)-binding recombinant polypeptide confers protection against infection by respiratory and urogenital pathogens

The human-specific pathogens Neisseria meningitidis, N. gonorrhoea, Haemophilus influenzae and Moraxella catarrhalis share the property of targeting the carcinoembryonic antigen (CEA)-related cell adhesion molecules (CEACAMs) expressed on human epithelia. CEACAMs are signalling receptors implicated in cell adhesion and regulation of several physiological functions. Their targeting by pathogens can lead to tissue invasion. Although the CEACAM-binding ligands of the bacteria are structurally diverse, they target a common site on the receptor. We have generated a recombinant polypeptide that blocks the interactions of the mucosal pathogens with human epithelial cells and antibodies against it inhibit M. catarrhalis interactions with the receptor. As such, it is a potential antimicrobial agent to prevent infection via a strategy unlikely to promote bacterial resistance and a vaccine candidate against M. catarrhalis. In addition, it could serve more widely as a novel research tool and as a potential therapeutic agent in CEACAM-based physiological disorders.

CD46-independent binding of neisserial type IV pili and the major pilus adhesin, PilC, to human epithelial cells

Neisseria gonorrhoeae is a gram-negative bacterial pathogen which infects the human mucosal epithelium. An early critical event in neisserial infection is the type IV pilus-mediated adherence to the host cell. The PilC protein, located on the pilus tip, has earlier been identified as the major pilus adhesin. Previous studies suggested that the cell surface protein CD46 is a pilus receptor for Neisseria. We investigated the role of CD46 in pilus-mediated gonococcal infection of epithelial cells. Differences in binding efficiencies of piliated gonococci as well as purified pilus adhesin PilC2 on human epithelial cell lines did not correlate to the level of surface-expressed CD46. Additionally, no binding of piliated gonococci or PilC2 protein was observed on CD46-transfected CHO and MDCK cells. Furthermore, specific down-regulation of CD46 expression in human epithelial cell lines by RNA interference did not alter the binding efficiency of piliated gonococci or purified PilC2 protein, although other CD46-dependent processes, such as measles virus infection and C3b cleavage, were significantly reduced. These data support the notion that pilus-mediated gonococcal infection of epithelial cells can occur in a CD46-independent manner, thus questioning the function of CD46 as an essential pilus receptor for pathogenic neisseriae.

Neisseria meningitidis NadA is a new invasin which promotes bacterial adhesion to and penetration into human epithelial cells

Neisseria meningitidis is a human pathogen, which is a major cause of sepsis and meningitis. The bacterium colonizes the upper respiratory tract of approximately 10% of humans where it lives as a commensal. On rare occasions, it crosses the epithelium and reaches the bloodstream causing sepsis. From the bloodstream it translocates the blood-brain barrier, causing meningitis. Although all strains have the potential to cause disease, a subset of them, which belongs to hypervirulent lineages, causes disease more frequently than others. Recently, we described NadA, a novel antigen of N. meningitidis, present in three of the four known hypervirulent lineages. Here we show that NadA is a novel bacterial invasin which, when expressed on the surface of Escherichia coli, promotes adhesion to and invasion into Chang epithelial cells. Deletion of the N-terminal globular domain of recombinant NadA or pronase treatment of human cells abrogated the adhesive phenotype. A hypervirulent strain of N. meningitidis where the nad A gene was inactivated had a reduced ability to adhere to and invade into epithelial cells in vitro. NadA is likely to improve the fitness of N. meningitidis contributing to the increased virulence of strains that belong to the hypervirulent lineages.

Biofilm formation by Neisseria meningitidis

Biofilm formation by the human pathogen Neisseria meningitidis was analyzed. Biofilm-forming meningococcal strains were identified and quantitated by crystal violet staining. Laser scanning confocal microscopy of the meningococcal biofilm revealed variable layers up to 90 microm in thickness. A total of 39 meningococcal isolates were studied; 23 were nasopharyngeal-carriage isolates, and 16 were invasive-disease isolates. Thirty percent of carriage isolates and 12.5% of invasive-disease isolates formed biofilms proficiently on a polystyrene surface. Generally, the strains that formed biofilms showed high-level cell surface hydrophobicity, characteristic of strains lacking a capsule. The inhibitory role of capsule in biofilm formation was further confirmed by comparing the biofilm-forming capabilities of a serogroup B wild-type strain of a disease-associated isolate to those of its capsule-deficient mutant (ctrA). Some strains of meningococci form biofilms, and this process is likely important in menigococcal colonization.

Mannose-binding lectin binds to two major outer membrane proteins, opacity protein and porin, of Neisseria meningitidis

Human mannose-binding lectin (MBL) provides a first line of defense against microorganisms by complement activation and/or opsonization in the absence of specific Ab. This serum collectin has been shown to activate complement when bound to repeating sugar moieties on several microorganisms, including encapsulated serogroup B and C meningococci, which leads to increased bacterial killing. In the present study, we sought to identify the meningococcal cell surface components to which MBL bound and to characterize such binding. Outer membrane complex containing both lipooligosaccharide (LOS) and proteins and LOS from Neisseria meningitidis were examined for MBL binding by dot blot and ELISA. MBL bound outer membrane complex but not LOS. The binding to bacteria by whole-cell ELISA did not require calcium and was not inhibited by N-acetyl-glucosamine or mannose. With the use of SDS-PAGE, immunoblot analysis, and mAbs specific for meningococcal opacity (Opa) proteins and porin proteins, we determined that MBL bound to Opa and porin protein B (porB). The N-terminal amino acid sequences of the two MBL binding proteins confirmed Opa and PorB. Purified PorB inhibited the binding of MBL to meningococci. Escherichia coli with surface-expressed gonococcal Opa bound significantly more MBL than did the control strain. The binding of human factor H to purified PorB was markedly inhibited by MBL in a dose-dependent manner. Meningococci incubated with human serum bound MBL as detected by ELISA. We conclude that MBL binds to meningococci by a novel target recognition of two nonglycosylated outer membrane proteins, Opa and PorB.

Phase and antigenic variation in bacteria

Phase and antigenic variation result in a heterogenic phenotype of a clonal bacterial population, in which individual cells either express the phase-variable protein(s) or not, or express one of multiple antigenic forms of the protein, respectively. This form of regulation has been identified mainly, but by no means exclusively, for a wide variety of surface structures in animal pathogens and is implicated as a virulence strategy. This review provides an overview of the many bacterial proteins and structures that are under the control of phase or antigenic variation. The context is mainly within the role of the proteins and variation for pathogenesis, which reflects the main body of literature. The occurrence of phase variation in expression of genes not readily recognizable as virulence factors is highlighted as well, to illustrate that our current knowledge is incomplete. From recent genome sequence analysis, it has become clear that phase variation may be more widespread than is currently recognized, and a brief discussion is included to show how genome sequence analysis can provide novel information, as well as its limitations. The current state of knowledge of the molecular mechanisms leading to phase variation and antigenic variation are reviewed, and the way in which these mechanisms form part of the general regulatory network of the cell is addressed. Arguments both for and against a role of phase and antigenic variation in immune evasion are presented and put into new perspective by distinguishing between a role in bacterial persistence in a host and a role in facilitating evasion of cross-immunity. Finally, examples are presented to illustrate that phase-variable gene expression should be taken into account in the development of diagnostic assays and in the interpretation of experimental results and epidemiological studies.

Pathogenicity islands in bacterial pathogenesis

In this review, we focus on a group of mobile genetic elements designated pathogenicity islands (PAI). These elements play a pivotal role in the virulence of bacterial pathogens of humans and are also essential for virulence in pathogens of animals and plants. Characteristic molecular features of PAI of important human pathogens and their role in pathogenesis are described. The availability of a large number of genome sequences of pathogenic bacteria and their benign relatives currently offers a unique opportunity for the identification of novel pathogen-specific genomic islands. However, this knowledge has to be complemented by improved model systems for the analysis of virulence functions of bacterial pathogens. PAI apparently have been acquired during the speciation of pathogens from their nonpathogenic or environmental ancestors. The acquisition of PAI not only is an ancient evolutionary event that led to the appearance of bacterial pathogens on a timescale of millions of years but also may represent a mechanism that contributes to the appearance of new pathogens within a human life span. The acquisition of knowledge about PAI, their structure, their mobility, and the pathogenicity factors they encode not only is helpful in gaining a better understanding of bacterial evolution and interactions of pathogens with eukaryotic host cells but also may have important practical implications such as providing delivery systems for vaccination, tools for cell biology, and tools for the development of new strategies for therapy of bacterial infections.