Meningococcal biofilm formation: structure, development and phenotypes in a standardized continuous flow system

Distinct roles of the anaphylatoxin receptors C3aR, C5aR1 and C5aR2 in experimental meningococcal infections

The complement system is pivotal in the defense against invasive disease caused by Neisseria meningitidis (Nme, meningococcus), particularly via the membrane attack complex. Complement activation liberates the anaphylatoxins C3a and C5a, which activate three distinct G-protein coupled receptors, C3aR, C5aR1 and C5aR2 (anaphylatoxin receptors, ATRs). We recently discovered that C5aR1 exacerbates the course of the disease, revealing a downside of complement in Nme sepsis. Here, we compared the roles of all three ATRs during mouse nasal colonization, intraperitoneal infection and human whole blood infection with Nme. Deficiency of complement or ATRs did not alter nasal colonization, but significantly affected invasive disease: Compared to WT mice, the disease was aggravated in C3ar-/- mice, whereas C5ar1-/- and C5ar2-/- mice showed increased resistance to meningococcal sepsis. Surprisingly, deletion of either of the ATRs resulted in lower cytokine/chemokine responses, irrespective of the different susceptibilities of the mice. This was similar in ex vivo human whole blood infection using ATR inhibitors. Neutrophil responses to Nme were reduced in C5ar1-/- mouse blood. Upon stimulation with C5a plus Nme, mouse macrophages displayed reduced phosphorylation of ERK1/2, when C5aR1 or C5aR2 were ablated or inhibited, suggesting that both C5a-receptors prime an initial macrophage response to Nme. Finally, in vivo blockade of C5aR1 alone (PMX205) or along with C5aR2 (A8Δ71-73) resulted in ameliorated disease, whereas neither antagonizing C3aR (SB290157) nor its activation with a "super-agonist" peptide (WWGKKYRASKLGLAR) demonstrated a benefit. Thus, C5aR1 and C5aR2 augment disease pathology and are interesting targets for treatment, whereas C3aR is protective in experimental meningococcal sepsis.

Neisseria meningitidis Type IV Pili Trigger Ca2+-Dependent Lysosomal Trafficking of the Acid Sphingomyelinase To Enhance Surface Ceramide Levels

Acid sphingomyelinase (ASM) is a lipid hydrolase that converts sphingomyelin to ceramide and that can be activated by various cellular stress mechanisms, including bacterial pathogens. Vesicle transportation or trafficking of ASM from the lysosomal compartment to the cell membrane is a prerequisite for its activation in response to bacterial infections; however, the effectors and mechanisms of ASM translocation and activation are poorly defined. Our recent work documented the key importance of ASM for Neisseria meningitidis uptake into human brain microvascular endothelial cells (HBMEC). We clearly identified OpcA to be one bacterial effector promoting ASM translocation and activity, though it became clear that additional bacterial components were involved, as up to 80% of ASM activity and ceramide generation was retained in cells infected with an opcA-deficient mutant. We hypothesized that N. meningitidis might use pilus components to promote the translocation of ASM into HBMEC. Indeed, we found that both live, piliated N. meningitidis and pilus-enriched fractions trigger transient ASM surface display, followed by the formation of ceramide-rich platforms (CRPs). By using indirect immunocytochemistry and direct stochastic optical reconstruction microscopy, we show that the overall number of CRPs with a size of ∼80 nm in the plasma membrane is significantly increased after exposure to pilus-enriched fractions. Infection with live bacteria as well as exposure to pilus-enriched fractions transiently increased cytosolic Ca²⁺ levels in HBMEC, and this was found to be important for ASM surface display mediated by lysosomal exocytosis, as depletion of cytosolic Ca²⁺ resulted in a significant decrease in ASM surface levels, ASM activity, and CRP formation.

Deletion of D-Lactate Dehydrogenase A in Neisseria meningitidis Promotes Biofilm Formation Through Increased Autolysis and Extracellular DNA Release

Neisseria meningitidis is a Gram-negative bacterium that asymptomatically colonizes the human nasopharyngeal mucosa. Pilus-mediated initial adherence of N. meningitidis to the epithelial mucosa is followed by the formation of three-dimensional aggregates, called microcolonies. Dispersal from microcolonies contributes to the transmission of N. meningitidis across the epithelial mucosa. We have recently discovered that environmental concentrations of host cell-derived lactate influences N. meningitidis microcolony dispersal. Here, we examined the ability of N. meningitidis mutants deficient in lactate metabolism to form biofilms. A lactate dehydrogenease A (ldhA) mutant had an increased level of biofilm formation. Deletion of ldhA increased the N. meningitidis cell surface hydrophobicity and aggregation. In this study, we used FAM20, which belongs to clonal complex ST-11 that forms biofilms independently of extracellular DNA (eDNA). However, treatment with DNase I abolished the increased biofilm formation and aggregation of the ldhA-deficient mutant, suggesting a critical role for eDNA. Compared to wild-type, the ldhA-deficient mutant exhibited an increased autolytic rate, with significant increases in the eDNA concentrations in the culture supernatants and in biofilms. Within the ldhA mutant biofilm, the transcription levels of the capsule, pilus, and bacterial lysis genes were downregulated, while norB, which is associated with anaerobic respiration, was upregulated. These findings suggest that the absence of ldhA in N. meningitidis promotes biofilm formation and aggregation through autolysis-mediated DNA release.

Differential influences of complement on neutrophil responses to Neisseria meningitidis infection

The complement system is the primary innate immune determinant protecting against invasive diseases caused by the Gram-negative bacterium Neisseria meningitidis (Nme, meningococcus), as evidenced by the extreme susceptibility of individuals with complement deficiencies. In contrast, the role of phagocytes such as neutrophils is much less well understood, although they are recruited in great numbers to the cerebrospinal fluid during meningococcal meningitis. Here, we consider the interaction of Nme with primary human neutrophils using either purified cells or a whole blood model of infection. We found that neutrophils are capable of non-opsonic uptake and killing of different Nme strains. However, in the presence of immune serum featuring active complement, Nme association is strongly increased, whereas this is not the case in heat-inactivated immune serum. Blockade of complement at the level of C3 using the inhibitor compstatin Cp20 reduces the uptake dramatically. In addition, purified neutrophils did not mount an oxidative burst towards Nme unless complement was added and, vice versa, the oxidative burst was strongly reduced in whole blood upon complement inhibition. In contrast, there was no significant impact of complement on neutrophil degranulation or IL-8 secretion. Taken together, neutrophils require complement activation in order to mount a full response towards Nme.

Host-bacteria interaction and adhesin study for development of therapeutics

Host-pathogen interaction is one of the most important areas of study to understand the adhesion of the pathogen to the host organisms. To adhere on the host cell surface, bacteria assemble the diverse adhesive structures on its surface, which play a foremost role in targeting to the host cell. We have highlighted different bacterial adhesins which are either protein mediated or glycan mediated. The present article listed examples of different bacterial adhesin proteins involved in the interactions with their host, types and subtypes of the fimbriae and non-fimbriae bacterial adhesins. Different bacterial surface adhesin subunits interact with host via different host surface biomolecules. We have also discussed the interactome of some of the pathogens with their host. Therefore, the present study will help researchers to have a detailed understanding of different interacting bacterial adhesins and henceforth, develop new therapies, adhesin specific antibodies and vaccines, which can effectively control pathogenicity of the pathogens.

Complement C5a Receptor 1 Exacerbates the Pathophysiology of N. meningitidis Sepsis and Is a Potential Target for Disease Treatment

Sepsis caused by Neisseria meningitidis (meningococcus) is a rapidly progressing, life-threatening disease. Because its initial symptoms are rather unspecific, medical attention is often sought too late, i.e., when the systemic inflammatory response is already unleashed. This in turn limits the success of antibiotic treatment. The complement system is generally accepted as the most important innate immune determinant against invasive meningococcal disease since it protects the host through the bactericidal membrane attack complex. However, complement activation concomitantly liberates the C5a peptide, and it remains unclear whether this potent anaphylatoxin contributes to protection and/or drives the rapidly progressing immunopathogenesis associated with meningococcal disease. Here, we dissected the specific contribution of C5a receptor 1 (C5aR1), the canonical receptor for C5a, using a mouse model of meningococcal sepsis. Mice lacking C3 or C5 displayed susceptibility that was enhanced by >1,000-fold or 100-fold, respectively, consistent with the contribution of these components to protection. In clear contrast, C5ar1^−/− mice resisted invasive meningococcal infection and cleared N. meningitidis more rapidly than wild-type (WT) animals. This favorable outcome stemmed from an ameliorated inflammatory cytokine response to N. meningitidis in C5ar1^−/− mice in both in vivo and ex vivo whole-blood infections. In addition, inhibition of C5aR1 signaling without interference with the complement bactericidal activity reduced the inflammatory response also in human whole blood. Enticingly, pharmacologic C5aR1 blockade enhanced mouse survival and lowered meningococcal burden even when the treatment was administered after sepsis induction. Together, our findings demonstrate that C5aR1 drives the pathophysiology associated with meningococcal sepsis and provides a promising target for adjunctive therapy.

The devastating consequences of N. meningitidis sepsis arise due to the rapidly arising and self-propagating inflammatory response that mobilizes antibacterial defenses but also drives the immunopathology associated with meningococcemia. The complement cascade provides innate broad-spectrum protection against infection by directly damaging the envelope of pathogenic microbes through the membrane attack complex and triggers an inflammatory response via the C5a peptide and its receptor C5aR1 aimed at mobilizing cellular effectors of immunity. Here, we consider the potential of separating the bactericidal activities of the complement cascade from its immune activating function to improve outcome of N. meningitidis sepsis. Our findings demonstrate that the specific genetic or pharmacological disruption of C5aR1 rapidly ameliorates disease by suppressing the pathogenic inflammatory response and, surprisingly, allows faster clearance of the bacterial infection. This outcome provides a clear demonstration of the therapeutic benefit of the use of C5aR1-specific inhibitors to improve the outcome of invasive meningococcal disease.

Transcriptomic buffering of cryptic genetic variation contributes to meningococcal virulence

Background

Commensal bacteria like Neisseria meningitidis sometimes cause serious disease. However, genomic comparison of hyperinvasive and apathogenic lineages did not reveal unambiguous hints towards indispensable virulence factors. Here, in a systems biological approach we compared gene expression of the invasive strain MC58 and the carriage strain α522 under different ex vivo conditions mimicking commensal and virulence compartments to assess the strain-specific impact of gene regulation on meningococcal virulence.

Results

Despite indistinguishable ex vivo phenotypes, both strains differed in the expression of over 500 genes under infection mimicking conditions. These differences comprised in particular metabolic and information processing genes as well as genes known to be involved in host-damage such as the nitrite reductase and numerous LOS biosynthesis genes. A model based analysis of the transcriptomic differences in human blood suggested ensuing metabolic flux differences in energy, glutamine and cysteine metabolic pathways along with differences in the activation of the stringent response in both strains. In support of the computational findings, experimental analyses revealed differences in cysteine and glutamine auxotrophy in both strains as well as a strain and condition dependent essentiality of the (p)ppGpp synthetase gene relA and of a short non-coding AT-rich repeat element in its promoter region.

Conclusions

Our data suggest that meningococcal virulence is linked to transcriptional buffering of cryptic genetic variation in metabolic genes including global stress responses. They further highlight the role of regulatory elements for bacterial virulence and the limitations of model strain approaches when studying such genetically diverse species as N. meningitidis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3616-7) contains supplementary material, which is available to authorized users.

Interstrain Cooperation in Meningococcal Biofilms: Role of Autotransporters NalP and AutA

Neisseria meningitidis (Nm) and Neisseria lactamica (Nl) are commensal bacteria that live in the human nasopharynx, where they form microcolonies. In contrast to Nl, Nm occasionally causes blood and/or meningitis infection with often fatal consequences. Here, we studied interactions between neisserial strains during biofilm formation. Fluorescent strains were engineered and analyzed for growth in single- and dual-strain biofilms with confocal laser-scanning microscopy. Different strains of diverse Neisseria species formed microcolonies of different sizes and morphologies. Pair-wise combinations of two invasive Nm strains and one Nm carrier isolate showed that these strains can coexist in spite of the fact that they produce toxins to combat congeners. This lack of competition was even observed when the biofilms were formed under nutrient limitation and can be explained by the observation that the separate microcolonies within mixed biofilms are mostly lineage specific. However, these microcolonies showed different levels of interaction. The coexistence of two strains was also observed in mixed biofilms of Nm and Nl strains. Inactivation of the autotransporter NalP, which prevents the release of the heparin-binding antigen NHBA and the α-peptide of IgA protease from the cell surface, and/or the production of autotransporter AutA increased interactions between microcolonies, as evidenced by close contacts between microcolonies on the substratum. Qualitative and quantitative analysis revealed an altered spatial distribution of each strain in mixed biofilms with consequences for the biomass, biofilm architecture and bacterial viability depending on the synthesis of NalP and AutA, the expression of which is prone to phase variation. Being in a consortium resulted in some cases in commensalism and cooperative behavior, which promoted attachment to the substratum or increased survival, possibly as result of the shared use of the biofilm matrix. We hypothesize that Nm strains can cooperate during host colonization, but, possibly, the different capacities of the microcolonies of each strain to resist the host's defenses limits the long-term coexistence of strains in the host.

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.

Impact of Moderate Temperature Changes on Neisseria meningitidis Adhesion Phenotypes and Proteome

Neisseria meningitidis, the meningococcus, bears the potential to cause life-threatening invasive diseases, but it usually colonizes the nasopharynx without causing any symptoms. Within the nasopharynx, Neisseria meningitidis must face temperature changes depending on the ambient air temperature. Indeed, the nasopharyngeal temperature can be substantially lower than 37°C, the temperature commonly used in experimental settings. Here, we compared the levels of meningococcal biofilm formation, autoaggregation, and cellular adherence at 32°C and 37°C and found a clear increase in all these phenotypes at 32°C suggestive of a stronger in vivo colonization capability at this temperature. A comparative proteome analysis approach revealed differential protein expression levels between 32°C and 37°C, predominantly affecting the bacterial envelope. A total of 375 proteins were detected. Use of database annotation or the PSORTb algorithm predicted 49 of those proteins to be localized in the outer membrane, 21 in either the inner or outer membrane, 35 in the periplasm, 56 in the inner membrane, and 208 in the cytosol; for 6 proteins, no annotation or prediction was available. Temperature-dependent regulation of protein expression was seen particularly in the periplasm as well as in the outer and inner membranes. Neisserial heparin binding antigen (NHBA), NMB1030, and adhesin complex protein (ACP) showed the strongest upregulation at 32°C and were partially responsible for the observed temperature-dependent phenotypes. Screening of different global regulators of Neisseria meningitidis suggested that the extracytoplasmic sigma factor σ^E might be involved in temperature-dependent biofilm formation. In conclusion, subtle temperature changes trigger adaptation events promoting mucosal colonization by meningococci.

Adhesins Involved in Attachment to Abiotic Surfaces by Gram-Negative Bacteria

During the first step of biofilm formation, initial attachment is dictated by physicochemical and electrostatic interactions between the surface and the bacterial envelope. Depending on the nature of these interactions, attachment can be transient or permanent. To achieve irreversible attachment, bacterial cells have developed a series of surface adhesins promoting specific or nonspecific adhesion under various environmental conditions. This article reviews the recent advances in our understanding of the secretion, assembly, and regulation of the bacterial adhesins during biofilm formation, with a particular emphasis on the fimbrial, nonfimbrial, and discrete polysaccharide adhesins in Gram-negative bacteria.

Characterization of motility and piliation in pathogenic Neisseria

Background

The type IV pili (Tfp) of pathogenic Neisseria (i.e., N. gonorrhoeae and N. meningitidis) are essential for twitching motility. Tfp retraction, which is dependent on the ATPase PilT, generates the forces that move bacteria over surfaces. Neisseria motility has mainly been studied in N. gonorrhoeae whereas the motility of N. meningitidis has not yet been characterized.

Results

In this work, we analyzed bacterial motility and monitored Tfp retraction using live-cell imaging of freely moving bacteria. We observed that N. meningitidis moved over surfaces at an approximate speed of 1.6 μm/s, whereas N. gonorrhoeae moved with a lower speed (1.0 μm/s). An alignment of the meningococcal and gonococcal pilT promoters revealed a conserved single base pair variation in the −10 promoter element that influence PilT expression. By tracking mutants with altered pilT expression or pilE sequence, we concluded that the difference in motility speed was independent of both. Live-cell imaging using total internal reflection fluorescence microscopy demonstrated that N. gonorrhoeae more often moved with fewer visible retracting filaments when compared to N. meningitidis. Correspondingly, meningococci also displayed a higher level of piliation in transmission electron microscopy. Nevertheless, motile gonococci that had the same number of filaments as N. meningitidis still moved with a lower speed.

Conclusions

These data reveal differences in both speed and piliation between the pathogenic Neisseria species during twitching motility, suggesting a difference in Tfp-dynamics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0424-6) contains supplementary material, which is available to authorized users.

Differential activation of acid sphingomyelinase and ceramide release determines invasiveness of Neisseria meningitidis into brain endothelial cells

The interaction with brain endothelial cells is central to the pathogenicity of Neisseria meningitidis infections. Here, we show that N. meningitidis causes transient activation of acid sphingomyelinase (ASM) followed by ceramide release in brain endothelial cells. In response to N. meningitidis infection, ASM and ceramide are displayed at the outer leaflet of the cell membrane and condense into large membrane platforms which also concentrate the ErbB2 receptor. The outer membrane protein Opc and phosphatidylcholine-specific phospholipase C that is activated upon binding of the pathogen to heparan sulfate proteoglycans, are required for N. meningitidis-mediated ASM activation. Pharmacologic or genetic ablation of ASM abrogated meningococcal internalization without affecting bacterial adherence. In accordance, the restricted invasiveness of a defined set of pathogenic isolates of the ST-11/ST-8 clonal complex into brain endothelial cells directly correlated with their restricted ability to induce ASM and ceramide release. In conclusion, ASM activation and ceramide release are essential for internalization of Opc-expressing meningococci into brain endothelial cells, and this segregates with invasiveness of N. meningitidis strains.

Neisseria meningitidis, an obligate human pathogen, is a causative agent of septicemia and meningitis worldwide. Meningococcal infection manifests in a variety of forms, including meningitis, meningococcemia with meningitis or meningococcemia without obvious meningitis. The interaction of N. meningitidis with human cells lining the blood vessels of the blood-cerebrospinal fluid barrier is a prerequisite for the development of meningitis. As a major pathogenicity factor, the meningococcal outer membrane protein Opc enhances bacterial entry into brain endothelial cells, however, mechanisms underlying trapping of receptors and signaling molecules following this interaction remained elusive. We now show that Opc-expressing meningococci activate acid sphingomyelinase (ASM) in brain endothelial cells, which hydrolyses sphingomyelin to cause ceramide release and formation of extended ceramide-enriched membrane platforms wherein ErbB2, an important receptor involved in bacterial uptake, clusters. Mechanistically, ASM activation relied on binding of N. meningitidis to its attachment receptor, HSPG, followed by activation of PC-PLC. Meningococcal isolates of the ST-11 clonal complex, which are reported to be more likely to cause severe sepsis, but rarely meningitis, barely invaded brain endothelial cells and revealed a highly restricted ability to induce ASM and ceramide release. Thus, our results unravel a differential activation of the ASM/ceramide system by the species N. meningitidis determining its invasiveness into brain endothelial cells.

Secreted single-stranded DNA is involved in the initial phase of biofilm formation by Neisseria gonorrhoeae

Neisseria gonorrhoeae is an obligate human pathogen that colonizes the genital tract and causes gonorrhoea. Neisseria gonorrhoeae can form biofilms during natural cervical infections, on glass and in continuous flow-chamber systems. These biofilms contain large amounts of extracellular DNA, which plays an important role in biofilm formation. Many clinical isolates contain a gonococcal genetic island that encodes a type IV secretion system (T4SS). The T4SS of N. gonorrhoeae strain MS11 secretes ssDNA directly into the medium. Biofilm formation, studied in continuous flow-chamber systems by confocal laser scanning microscopy (CLSM), was strongly reduced, especially in the initial phases of biofilm formation, in the presence of Exonuclease I, which specifically degrades ssDNA or in a ΔtraB strain that does not secrete ssDNA. To specifically detect ssDNA in biofilms using CLSM, a novel method was developed in which thermostable fluorescently labelled ssDNA- and ss/dsDNA-binding proteins were used to visualize ssDNA and total DNA in biofilms and planktonic cultures. Remarkably, mainly dsDNA was detected in biofilms of the ssDNA secreting strain. We conclude that the secreted ssDNA facilitates initial biofilm formation, but that the secreted ssDNA is not retained in mature biofilms.

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.

Comparative proteome analysis of spontaneous outer membrane vesicles and purified outer membranes of Neisseria meningitidis

Outer membrane vesicles (OMVs) of Gram-negative bacteria receive increasing attention because of various biological functions and their use as vaccines. However, the mechanisms of OMV release and selective sorting of proteins into OMVs remain unclear. Comprehensive quantitative proteome comparisons between spontaneous OMVs (SOMVs) and the outer membrane (OM) have not been conducted so far. Here, we established a protocol for metabolic labeling of neisserial proteins with (15)N. SOMV and OM proteins labeled with (15)N were used as an internal standard for proteomic comparison of the SOMVs and OMs of two different strains. This labeling approach, coupled with high-sensitivity mass spectrometry, allowed us to comprehensively unravel the proteome of the SOMVs and OMs. We quantified the relative distribution of 155 proteins between SOMVs and the OM. Complement regulatory proteins, autotransporters, proteins involved in iron and zinc acquisition, and a two-partner secretion system were enriched in SOMVs. The highly abundant porins PorA and PorB and proteins connecting the OM with peptidoglycan or the inner membrane, such as RmpM, MtrE, and PilQ, were depleted in SOMVs. Furthermore, the three lytic transglycosylases MltA, MltB, and Slt were less abundant in SOMVs. In conclusion, SOMVs are likely to be released from surface areas with a low local abundance of membrane-anchoring proteins and lytic transglycosylases. The enrichment of complement regulatory proteins, autotransporters, and trace metal binding and transport proteins needs to be explored in the context of the pathogenesis of meningococcal disease.

Identification of genes involved in Neisseria meningitidis colonization

Neisseria meningitidis is a worldwide cause of meningitis and septicemia leading at least to 50,000 deaths every year. Nevertheless, N. meningitidis is also a commensal bacterium that asymptomatically colonizes the epithelial cells of the nasopharynx of 10 to 30% of healthy individuals. Occasionally, N. meningitidis crosses the nasopharyngeal barrier and enters the bloodstream. During bacteremia, N. meningitidis may adhere to endothelial cells of brain vessels and invade meninges. To identify the genes required for meningococcal host colonization, we screened a signature-tagged transposon mutagenesis library using an innovative in vitro colonization model in order to identify mutants displaying decreased capacity to colonize human epithelial cells. Approximately 1,600 defined insertion mutants of invasive serogroup C strain NEM8013 were screened. Candidate mutants were tested individually for quantification of bacterial biomass with confocal microscope and COMSTAT software. Five mutants were demonstrated to exhibit significantly decreased colonization ability. The identified genes, including narP and estD, appeared to be involved in adaptation to hypoxic conditions and stress resistance. Interestingly, the genes fadD1, nnrS, and NMV_2034 (encoding a putative thioredoxin), prior to this study, had not been shown to be involved in colonization. Therefore, we provide here insights into the meningococcal functions necessary for the bacterium to adapt to growth on host cells.

In vitro resistance mechanisms of Neisseria meningitidis against neutrophil extracellular traps

Neisseria meningitidis (Nm) is a leading cause of septicemia in childhood. Nm septicemia is unique with respect to very quick disease progression, high in vivo bacterial replication rate and its considerable mortality. Nm circumvents major mechanisms of innate immunity such as complement system and phagocytosis. Neutrophil extracellular traps (NETs) are formed from neutrophils during systemic infection and are suggested to contain invading microorganisms. Here, we investigated the interaction of Nm with NETs. Both, meningococci and spontaneously released outer membrane vesicles (SOMVs) were potent NET inducers. NETs were unable to kill NET bound meningococci, but slowed down their proliferation rate. Using Nm as model organism we identified three novel mechanisms how bacteria can evade NET-mediated killing: (i) modification of lipid A of meningococcal LPS with phosphoethanolamine protected Nm from NET-bound cathepsin G; (ii) expression of the high-affinity zinc uptake receptor ZnuD allowed Nm to escape NET-mediated nutritional immunity; (iii) binding of SOMVs to NETs saved Nm from NET binding and the consequent bacteriostatic effect. Escape from NETs may contribute to the most rapid progression of meningococcal disease. The induction of NET formation by Nm in vivo might aggravate thrombosis in vessels ultimately directing to disseminated intravascular coagulation (DIC).

ZnuD, a potential candidate for a simple and universal Neisseria meningitidis vaccine

Neisseria meningitidis serogroup B (MenB) is a major cause of bacterial sepsis and meningitis, with the highest disease burden in young children. Available vaccines are based on outer membrane vesicles (OMVs) obtained from wild-type strains. However, particularly in toddlers and infants, they confer protection mostly against strains expressing the homologous protein PorA, a major and variable outer membrane protein. In the quest for alternative vaccine antigens able to provide broad MenB strain coverage in younger populations, but potentially also across all age groups, ZnuD, a protein expressed under zinc-limiting conditions, may be considered a promising candidate. Here, we have investigated the potential value of ZnuD and show that it is a conserved antigen expressed by all MenB strains tested except for some strains of clonal complex ST-8. In mice and guinea pigs immunized with ZnuD-expressing OMVs, antibodies were elicited that were able to trigger complement-mediated killing of all the MenB strains and serogroup A, C, and Y strains tested when grown under conditions of zinc limitation. ZnuD is also expressed during infection, since anti-ZnuD antibodies were detected in sera from patients. In conclusion, we confirm the potential of ZnuD-bearing OMVs as a component of an effective MenB vaccine.

PilT2 enhances the speed of gonococcal type IV pilus retraction and of twitching motility

Type IV pilus (T4P) dynamics is important for various bacterial functions including host cell interaction, surface motility, and horizontal gene transfer. T4P retract rapidly by depolymerization, generating large mechanical force. The gene that encodes the pilus retraction ATPase PilT has multiple paralogues, whose number varies between different bacterial species, but their role in regulating physical parameters of T4P dynamics remains unclear. Here, we address this question in the human pathogen Neisseria gonorrhoeae, which possesses two pilT paralogues, namely pilT2 and pilU. We show that the speed of twitching motility is strongly reduced in a pilT2 deletion mutant, while directional persistence time and sensitivity of speed to oxygen are unaffected. Using laser tweezers, we found that the speed of single T4P retraction was reduced by a factor of ≈ 2 in a pilT2 deletion strain, whereas pilU deletion showed a minor effect. The maximum force and the probability for switching from retraction to elongation under application of high force were not significantly affected. We conclude that the physical parameters of T4P are fine-tuned through PilT2.

Opc expression, LPS immunotype switch and pilin conversion contribute to serum resistance of unencapsulated meningococci

Neisseria meningitidis employs polysaccharides and outer membrane proteins to cope with human serum complement attack. To screen for factors influencing serum resistance, an assay was developed based on a colorimetric serum bactericidal assay. The screening used a genetically modified sequence type (ST)-41/44 clonal complex (cc) strain lacking LPS sialylation, polysaccharide capsule, the factor H binding protein (fHbp) and MutS, a protein of the DNA repair mechanism. After killing of >99.9% of the bacterial cells by serum treatment, the colorimetric assay was used to screen 1000 colonies, of which 35 showed enhanced serum resistance. Three mutant classes were identified. In the first class of mutants, enhanced expression of Opc was identified. Opc expression was associated with vitronectin binding and reduced membrane attack complex deposition confirming recent observations. Lipopolysaccharide (LPS) immunotype switch from immunotype L3 to L8/L1 by lgtA and lgtC phase variation represented the second class. Isogenic mutant analysis demonstrated that in ST-41/44 cc strains the L8/L1 immunotype was more serum resistant than the L3 immunotype. Consecutive analysis revealed that the immunotypes L8 and L1 were frequently observed in ST-41/44 cc isolates from both carriage and disease. Immunotype switch to L8/L1 is therefore suggested to contribute to the adaptive capacity of this meningococcal lineage. The third mutant class displayed a pilE allelic exchange associated with enhanced autoaggregation. The mutation of the C terminal hypervariable region D of PilE included a residue previously associated with increased pilus bundle formation. We suggest that autoaggregation reduced the surface area accessible to serum complement and protected from killing. The study highlights the ability of meningococci to adapt to environmental stress by phase variation and intrachromosomal recombination affecting subcapsular antigens.

Oxygen depletion triggers switching between discrete speed modes of gonococcal type IV pili

Type IV pili are polymeric bacterial appendages that affect host cell interaction, motility, biofilm formation, and horizontal gene transfer. These force-generating motors work in at least three distinct velocity modes-elongation, and retraction at two distinct speeds, high and low. Yet it is unclear which regulatory inputs control their speeds. Here, we addressed this question for the human pathogen Neisseria gonorrhoeae. Using a combination of image analysis and surface analytics, we simultaneously monitored the speed of twitching motility and the concentration of oxygen. While oxygen was detectable, bacteria moved in the high-speed mode (1.5 μm/s). Upon full depletion of oxygen, gonococci simultaneously switched into the low-speed mode (0.5 μm/s). Speed switching was complete within seconds, independent of transcription, and reversible upon oxygen restoration. Using laser tweezers, we found that oxygen depletion triggered speed switching of the pilus motor at the single-molecule level. In the transition regime, single pili switched between both modes, indicating bistability. Switching is well described by a two-state model whereby the oxygen level controls the occupancy of the states.

A snapshot of a pathogenic bacterium mid-evolution: Neisseria meningitidis is becoming a nitric oxide-tolerant aerobe

Members of the Neisseria genus typically display the ability to carry out denitrification of nitrite to nitrous oxide as an alternative to oxygen respiration when oxygen is depleted. The key enzymes nitrite and nitric oxide reductase are found across the Neisseria genus. Within Neisseria meningitidis, however, a number of research groups have found that a significant proportion of strains lack a functional nitrite reductase. It appears that N. meningitidis is on an evolutionary trajectory towards loss of the capacity to reduce nitrite. In the present paper, I propose that N. meningitidis is evolving to become a nitric oxide-tolerant aerobe in order to occupy an oxygen-rich niche close to host tissue (and hence oxygen perfusion). Other features of the genomic and functional specialization of N. meningitidis, such as possession of a polysaccharide capsule and various acquired reactive oxygen species-resistance mechanisms, support this proposition. The importance of oxygen availability more generally is discussed with reference to recent findings with other mucosal pathogens.

Polar invasion and translocation of Neisseria meningitidis and Streptococcus suis in a novel human model of the blood-cerebrospinal fluid barrier

Acute bacterial meningitis is a life-threatening disease in humans. Discussed as entry sites for pathogens into the brain are the blood-brain and the blood-cerebrospinal fluid barrier (BCSFB). Although human brain microvascular endothelial cells (HBMEC) constitute a well established human in vitro model for the blood-brain barrier, until now no reliable human system presenting the BCSFB has been developed. Here, we describe for the first time a functional human BCSFB model based on human choroid plexus papilloma cells (HIBCPP), which display typical hallmarks of a BCSFB as the expression of junctional proteins and formation of tight junctions, a high electrical resistance and minimal levels of macromolecular flux when grown on transwell filters. Importantly, when challenged with the zoonotic pathogen Streptococcus suis or the human pathogenic bacterium Neisseria meningitidis the HIBCPP show polar bacterial invasion only from the physiologically relevant basolateral side. Meningococcal invasion is attenuated by the presence of a capsule and translocated N. meningitidis form microcolonies on the apical side of HIBCPP opposite of sites of entry. As a functionally relevant human model of the BCSFB the HIBCPP offer a wide range of options for analysis of disease-related mechanisms at the choroid plexus epithelium, especially involving human pathogens.

Mechanisms of meningococcal colonisation

Despite advances against infectious diseases over the past century, Neisseria meningitidis remains a major causative agent of meningitis and septicaemia worldwide. Its adaptation for survival in the human nasopharynx makes the meningococcus a highly successful commensal bacterium. Recent progress has been made in understanding the mechanisms that enable neisserial colonisation, in terms of the role of type IV pili, the impact of other adhesins, biofilm formation, nutrient acquisition and resistance to host immune defences. Refinements in cell-based and in vivo models will lead to improved understanding of the colonisation process, and hopefully to more effective vaccines and therapeutic strategies.

Characterization of FarR as a highly specialized, growth phase-dependent transcriptional regulator in Neisseria meningitidis

Transcriptional regulators play an important role for the survival of Neisseria meningitidis within its human host. We have recently shown that FarR acts as transcriptional repressor of the adhesin nadA in N. meningitidis. Here, we examined the FarR regulon by microarray analyses, qRT-PCR, and electrophoretic mobility shift assays, revealing that FarR is a highly specific repressor of nadA. We demonstrate by reporter gene fusion assays that alterations of the FarR binding site within the nadA promoter are sufficient to induce transcription of nadA. Furthermore, farR expression is growth phase-dependent. The highest transcription rate was observed in the late-exponential growth phase of meningococci. Upon contact with active components of the complement system in normal human serum, expression of farR is slightly downregulated. Concluding, we present FarR as an exquisitely specialized, growth phase-dependent, possibly complement-responsive transcriptional regulator in N. meningitidis.

Comparative proteomic analysis of biofilm and planktonic cells of Neisseria meningitidis

Neisseria meningitidis is a commensal of the human nasopharynx occasionally causing invasive disease. In vitro biofilms have been employed to model meningococcal carriage. A proteomic analysis of meningococcal biofilms was conducted and metabolic changes related to oxygen and nutrient limitation and upregulation of proteins involved in ROS defense were observed. The upregulated MntC which protects against ROS was shown to be required for meningococcal biofilm formation, but not for planktonic growth. ROS-induced proteomic changes might train the biofilm to cope with immune effectors.

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.

Biofilm formation by the human pathogen Neisseria meningitidis

The past decade has seen an increasing interest in biofilm formation by Neisseria meningitidis, a human facultative pathogen causing life-threatening childhood disease commencing from asymptomatic nasopharyngeal colonization. Studying the biology of in vitro biofilm formation improves the understanding of inter-bacterial processes in asymptomatic carriage, of bacterial aggregate formation on host cells, and of meningococcal population biology. This paper reviews publications referring to meningococcal biofilm formation with an emphasis on the role of motility and of extracellular DNA. The theory of sub-dividing the meningococcal population in settler and spreader lineages is discussed, which provides a mechanistic framework for the assumed balance of colonization efficacy and transmission frequency.

Bacterial adhesins in host-microbe interactions

Most commensal and pathogenic bacteria interacting with eukaryotic hosts express adhesive molecules on their surfaces that promote interaction with host cell receptors or with soluble macromolecules. Even though bacterial attachment to epithelial cells may be beneficial for bacterial colonization, adhesion may come at a cost because bacterial attachment to immune cells can facilitate phagocytosis and clearing. Many pathogenic bacteria have solved this dilemma by producing an antiphagocytic surface layer usually consisting of polysaccharide and by expressing their adhesins on polymeric structures that extend out from the cell surface. In this review, we will focus on the interaction between bacterial adhesins and the host, with an emphasis on pilus-like structures.

Clinical and laboratory evidence for Neisseria meningitidis biofilms

Neisseria meningitidis is the etiologic agent of meningococcal meningitis. Carriage of the organism is approximately 10% while active disease occurs at a rate of 1:100,000. Recent publications demonstrate that N. meningitidis has the ability to form biofilms on glass, plastic or cultured human bronchial epithelial cells. Microcolony-like structures are also observed in histological sections from patients with active meningococcal disease. This review investigates the possible role of meningococcal biofilms in carriage and active disease, based on the laboratory and clinical aspects of the disease.

Meningococcal biofilm growth on an abiotic surface - a model for epithelial colonization?

Neisseria meningitidis colonizes the human nasopharynx asymptomatically, often for prolonged periods, but occasionally invades from this site to cause life-threatening infection. In the nasopharynx aggregated organisms are closely attached to the epithelial surface, in a state in which the expression of components of the bacterial envelope differs significantly from that found in organisms multiplying exponentially in liquid phase culture or in the blood. We and others have hypothesized that here they are in the biofilm state, and to explore this we have investigated biofilm formation by the serogroup B strain MC58 on an abiotic surface, in a sorbarod system. Transcriptional changes were analysed, focusing on alteration in gene expression relevant to polysaccharide capsulation, lipooligosaccharide and outer-membrane protein synthesis - all phenotypes of importance in epithelial colonization. We report downregulation of genes controlling capsulation and the production of core oligosaccharide, and upregulation of genes encoding a range of outer-membrane components, reflecting phenotypic changes that have been established to occur in the colonizing state. A limited comparison with organisms recovered from an extended period of co-cultivation with epithelial cells suggests that this model system may better mirror natural colonization than do short-term meningococcal/epithelial cell co-cultivation systems. Modelling prolonged meningococcal colonization with a sorbarod system offers insight into gene expression during this important, but experimentally relatively inaccessible, phase of human infection.

Expression of the meningococcal adhesin NadA is controlled by a transcriptional regulator of the MarR family

Two closely related pathogenic species have evolved in the genus Neisseria: N. meningitidis and N. gonorrhoeae, which occupy different host niches and cause different clinical entities. In contrast to the pathogen N. gonorrhoeae, N. meningitidis is a commensal and only rarely becomes invasive. Little is known about the genetic background of the entirely different lifestyles in these closely related species. Meningococcal NMB1843 encodes a transcriptional regulator of the MarR family. The gonococcal homologue FarR regulates expression of farAB, mediating fatty acid resistance. We show that NmFarR also directly interacts with NmfarAB. Yet, by contrast to N. gonorrhoeae, no significant sensitivity to fatty acids was observed in a DeltafarR mutant due to intrinsic resistance of meningococci. Further analyses identified an NmFarR-repressed protein absent from N. gonorrhoeae. This protein is the meningococcus-specific adhesin and vaccine component NadA that has most likely been acquired by horizontal gene transfer. NmFarR binds to a 16 base pair palindromic repeat within the nadA promoter. De-repression of nadA resulted in significantly higher association of a DeltafarR strain with epithelial cells. Hence NmFarR has gained control over a meningococcus-specific gene involved in host colonization and thus contributed to divergent niche adaptation in pathogenic Neisseriae.

Role of HrpA in biofilm formation of Neisseria meningitidis and regulation of the hrpBAS transcripts

Two-partner secretion systems of gram-negative organisms are utilized in adherence, invasion, and biofilm formation. The HrpAB proteins of Neisseria meningitidis are members of a two-partner secretion system, and HrpA is established as being important to adherence and intracellular escape. This study set out to determine the expression pattern of members of the hrpBAS putative operon and to find a functional role for the HrpA protein. The upregulation of these genes was found in situations of anaerobiosis and cell contact. These observations prompted the study of the function of HrpA in biofilms on human bronchial epithelial cells. HrpA mutants in encapsulated and unencapsulated NMB strains demonstrated biofilm growth equivalent to that of the wild-type strain at 6 h but a decreased ability to form biofilms at 48 h. Biofilms formed by hrpA mutants for 48 h on collagen-coated coverslips demonstrated significant reductions compared to those of wild-type strains. Taken together, these observations imply a role for HrpA in the biofilm structure. Further analysis demonstrated the presence of HrpA on the surface of the bacterium.

Biofilm formation on human airway epithelia by encapsulated Neisseria meningitidis serogroup B

Neisseria meningitidis is the etiologic agent of meningococcal meningitis. We compared 48-h biofilm formation by N. meningitidis serogroup B strains NMB, MC58, C311 and isogenic mutants defective in capsule formation on SV-40 transformed human bronchial epithelial (HBE) cells in a flow cell. We demonstrated that strains NMB and NMB siaA-D were defective in biofilm formation over glass, and there was a partial rescue of biofilm growth for strain NMB on collagen-coated coverslips at 48 h. We demonstrated all three serogroup B strains form biofilms of statistically equivalent average height on HBE cells as their isogenic capsular mutants. Strain NMB also formed a biofilm of statistically equivalent biomass as the NMB siaA-D mutant on HBE cells at 6 and 48 h. These biofilms are significantly larger than biofilms formed over glass or collagen. Verification that strain NMB expressed capsule in biofilms on HBE cells was demonstrated by staining with 2.2.B, a monoclonal antibody with specificity for the serogroup B capsule. ELISA analysis demonstrated that strains MC58 and C311 also produced capsules during biofilm growth. These findings suggest that encapsulated meningococci can form biofilms on epithelial cells suggesting that biofilm formation may play a role in nasopharyngeal colonization.

3D structure/function analysis of PilX reveals how minor pilins can modulate the virulence properties of type IV pili

Type IV pili (Tfp) are widespread filamentous bacterial organelles that mediate multiple virulence-related phenotypes. They are composed mainly of pilin subunits, which are processed before filament assembly by dedicated prepilin peptidases. Other proteins processed by these peptidases, whose molecular nature and mode of action remain enigmatic, play critical roles in Tfp biology. We have performed a detailed structure/function analysis of one such protein, PilX from Neisseria meningitidis, which is crucial for formation of bacterial aggregates and adhesion to human cells. The x-ray crystal structure of PilX reveals the alpha/beta roll fold shared by all pilins, and we show that this protein colocalizes with Tfp. These observations suggest that PilX is a minor, or low abundance, pilin that assembles within the filaments in a similar way to pilin. Deletion of a PilX distinctive structural element, which is predicted to be exposed on the filament surface, abolishes aggregation and adhesion. Our results support a model in which surface-exposed motifs in PilX subunits stabilize bacterial aggregates against the disruptive force of pilus retraction and illustrate how a minor pilus component can enhance the functional properties of pili of rather simple composition and structure.

Biochemical characterization of a Neisseria meningitidis polysialyltransferase reveals novel functional motifs in bacterial sialyltransferases

The extracellular polysaccharide capsule is an essential virulence factor of Neisseria meningitidis, a leading cause of severe bacterial meningitis and sepsis. Serogroup B strains, the primary disease causing isolates in Europe and America, are encapsulated in α-2,8 polysialic acid (polySia). The capsular polymer is synthesized from activated sialic acid by action of a membrane-associated polysialyltransferase (NmB-polyST). Here we present a comprehensive characterization of NmB-polyST. Different from earlier studies, we show that membrane association is not essential for enzyme functionality. Recombinant NmB-polyST was expressed, purified and shown to synthesize long polySia chains in a non-processive manner in vitro. Subsequent structure–function analyses of NmB-polyST based on refined sequence alignments allowed the identification of two functional motifs in bacterial sialyltransferases. Both (D/E-D/E-G and HP motif) are highly conserved among different sialyltransferase families with otherwise little or no sequence identity. Their functional importance for enzyme catalysis and CMP-Neu5Ac binding was demonstrated by mutational analysis of NmB-polyST and is emphasized by structural data available for the Pasteurella multocida sialyltransferase PmST1. Together our data are the first description of conserved functional elements in the highly diverse families of bacterial (poly)sialyltransferases and thus provide an advanced basis for understanding structure–function relations and for phylogenetic sorting of these important enzymes.

Living in a changing environment: insights into host adaptation in Neisseria meningitidis from comparative genomics

Neisseria meningitidis (the meningococcus) colonizes the human nasopharynx of about 10% of the human population. However, for reasons that are still mostly unknown meningococci occasionally enter the cerebrospinal fluid leading to often fatal bacterial meningitis especially in children and young adults. The genetic basis for the observed differences in the pathogenic potential of different strains has only partially been unravelled so far. With the advent of whole genome sequencing technologies, complete genome sequences from three pathogenic meningococcal strains have become available and allow for a comprehensive analysis of the genomic and genetic differences occurring within this species. In this review, the general properties of the meningococcal genomes so far sequenced is given with an emphasis on the chromosomal rearrangements that have occurred, and the genomic islands and prophages that have been identified. The concomitant development of microarray technology for comparative genome hybridization studies of a large set of different meningococcal isolates as well as strains from other Neisseria species has extended our understanding of meningococcal population genetics on a genome-wide scale thus bridging the gap between meningococcal epidemiology and genomics. Finally, we briefly discuss the potential impact of meningococcal life style on its genome architecture and how in turn this genomic make-up might lead to a virulent phenotype making N. meningitidis an accidental pathogen. The overall properties of the meningococcal genome are characterized by genomic variability and instability, resulting in increased functional flexibility within this species.

Biofilms formed by nontypeable Haemophilus influenzae in vivo contain both double-stranded DNA and type IV pilin protein

Nontypeable Haemophilus influenzae (NTHI) strains are members of the normal human nasopharyngeal flora, as well as frequent opportunistic pathogens of both the upper and lower respiratory tracts. Recently, it has been shown that NTHI can form biofilms both in vitro and in vivo. NTHI strains within in vitro-formed biofilms differentially express both epitopes of lipooligosaccharide (LOS) and the outer membrane proteins P2, P5, and P6, whereas those generated either in a 96-well plate assay in vitro or in a mammalian host have been shown to incorporate a specific glycoform of sialylated LOS within the biofilm matrix. While DNA has been identified as a key component of the biofilm matrix formed in vitro by several bacterial pathogens, here we demonstrate for the first time that in addition to sialylated LOS, the biofilm formed by NTHI in vivo contains both type IV pilin protein and a significant amount of double-stranded DNA. The DNA appeared to be arranged in a dense interlaced meshwork of fine strands as well as in individual thicker "ropes" that span water channels, suggesting that DNA could be imparting structural stability to the biofilm produced by NTHI in vivo. The presence of type IV pilin protein both appearing as small aggregates within the biofilm matrix and tracking along DNA strands supports our observations which showed that type IV pili are expressed by NTHI during experimental otitis media when these bacteria form a biofilm in the middle ear space.