Development and evaluation of an improved mouse model of meningococcal colonization

Uncovering virulence factors in Cronobacter sakazakii: insights from genetic screening and proteomic profiling

The increasing problem of antibiotic resistance has driven the search for virulence factors in pathogenic bacteria, which can serve as targets for the development of new antibiotics. Although whole-genome Tn5 transposon mutagenesis combined with phenotypic assays has been a widely used approach, its efficiency remains low due to labor-intensive processes. In this study, we aimed to identify specific genes and proteins associated with the virulence of Cronobacter sakazakii, a pathogenic bacterium known for causing severe infections, particularly in infants and immunocompromised individuals. By employing a combination of genetic screening, comparative proteomics, and in vivo validation using zebrafish and rat models, we rapidly screened highly virulent strains and identified two genes, rcsA and treR, as potential regulators of C. sakazakii toxicity toward zebrafish and rats. Proteomic profiling revealed upregulated proteins upon knockout of rcsA and treR, including FabH, GshA, GppA, GcvH, IhfB, RfaC, MsyB, and three unknown proteins. Knockout of their genes significantly weakened bacterial virulence, confirming their role as potential virulence factors. Our findings contribute to understanding the pathogenicity of C. sakazakii and provide insights into the development of targeted interventions and therapies against this bacterium.IMPORTANCEThe emergence of antibiotic resistance in pathogenic bacteria has become a critical global health concern, necessitating the identification of virulence factors as potential targets for the development of new antibiotics. This study addresses the limitations of conventional approaches by employing a combination of genetic screening, comparative proteomics, and in vivo validation to rapidly identify specific genes and proteins associated with the virulence of Cronobacter sakazakii, a highly pathogenic bacterium responsible for severe infections in vulnerable populations. The identification of two genes, rcsA and treR, as potential regulators of C. sakazakii toxicity toward zebrafish and rats and the proteomic profiling upon knockout of rcsA and treR provides novel insights into the mechanisms underlying bacterial virulence. The findings contribute to our understanding of C. sakazakii's pathogenicity, shed light on the regulatory pathways involved in bacterial virulence, and offer potential targets for the development of novel interventions against this highly virulent bacterium.

Vertebrate and Invertebrate Animal and New In Vitro Models for Studying Neisseria Biology

The history of Neisseria research has involved the use of a wide variety of vertebrate and invertebrate animal models, from insects to humans. In this review, we itemise these models and describe how they have made significant contributions to understanding the pathophysiology of Neisseria infections and to the development and testing of vaccines and antimicrobials. We also look ahead, briefly, to their potential replacement by complex in vitro cellular models.

Et tu, Neisseria? Conflicts of Interest Between Neisseria Species

Neisseria meningitidis and Neisseria gonorrhoeae are two obligate human pathogens that have evolved to be uniquely adapted to their host. The meningococcus is frequently carried asymptomatically in the nasopharynx, while gonococcal infection of the urogenital tract usually elicits a marked local inflammatory response. Other members of the Neisseria genus are abundant in the upper airway where they could engage in co-operative or competitive interactions with both these pathogens. Here, we briefly outline the potential sites of contact between Neisseria spp. in the body, with emphasis on the upper airway, and describe the growing yet circumstantial evidence for antagonism from carriage studies and human volunteer challenge models with Neisseria lactamica. Recent laboratory studies have characterized antagonistic mechanisms that enable competition between Neisseria species. Several of these mechanisms, including Multiple Adhesin family (Mafs), Two Partner Secretion Systems, and Type VI secretion system, involve direct contact between bacteria; the genetic organisation of these systems, and the domain structure of their effector molecules have striking similarities. Additionally, DNA from one species of Neisseria can be toxic to another species, following uptake. More research is needed to define the full repertoire of antagonistic mechanisms in Neisseria spp., their distribution in strains, their range of activity, and contribution to survival in vivo. Understanding the targets of effectors could reveal how antagonistic relationships between close relatives shape subsequent interactions between pathogens and their hosts.

Neisseria genes required for persistence identified via in vivo screening of a transposon mutant library

The mechanisms used by human adapted commensal Neisseria to shape and maintain a niche in their host are poorly defined. These organisms are common members of the mucosal microbiota and share many putative host interaction factors with Neisseria meningitidis and Neisseria gonorrhoeae. Evaluating the role of these shared factors during host carriage may provide insight into bacterial mechanisms driving both commensalism and asymptomatic infection across the genus. We identified host interaction factors required for niche development and maintenance through in vivo screening of a transposon mutant library of Neisseria musculi, a commensal of wild-caught mice which persistently and asymptomatically colonizes the oral cavity and gut of CAST/EiJ and A/J mice. Approximately 500 candidate genes involved in long-term host interaction were identified. These included homologs of putative N. meningitidis and N. gonorrhoeae virulence factors which have been shown to modulate host interactions in vitro. Importantly, many candidate genes have no assigned function, illustrating how much remains to be learned about Neisseria persistence. Many genes of unknown function are conserved in human adapted Neisseria species; they are likely to provide a gateway for understanding the mechanisms allowing pathogenic and commensal Neisseria to establish and maintain a niche in their natural hosts. Validation of a subset of candidate genes confirmed a role for a polysaccharide capsule in N. musculi persistence but not colonization. Our findings highlight the potential utility of the Neisseria musculi-mouse model as a tool for studying the pathogenic Neisseria; our work represents a first step towards the identification of novel host interaction factors conserved across the genus.

The Neisseria genus contains many genetically related commensals of animals and humans, and two human pathogens, Neisseria gonorrhoeae and Neisseria meningitidis. The mechanisms allowing commensal Neisseria to maintain a niche in their host is little understood. To identify genes required for persistence, we screened a library of transposon mutants of Neisseria musculi, a commensal of wild-caught mice, in CAST/EiJ mice, which persistently and asymptomatically colonizes. Approximately 500 candidate host interaction genes were identified. A subset of these are homologs of N. meningitidis and N. gonorrhoeae genes known to modulate pathogen-host interactions in vitro. Many candidate genes have no known function, demonstrating how much remains to be learned about N. musculi niche maintenance. As many genes of unknown function are conserved in human adapted Neisseria, they provide a gateway for understanding Neisseria persistence mechanisms in general.

Past and Current Perspectives in Modeling Bacteria and Blood-Brain Barrier Interactions

The central nervous system (CNS) barriers are highly specialized cellular barriers that promote brain homeostasis while restricting pathogen and toxin entry. The primary cellular constituent regulating pathogen entry in most of these brain barriers is the brain endothelial cell (BEC) that exhibits properties that allow for tight regulation of CNS entry. Bacterial meningoencephalitis is a serious infection of the CNS and occurs when bacteria can cross specialized brain barriers and cause inflammation. Models have been developed to understand the bacterial - BEC interaction that lead to pathogen crossing into the CNS, however, these have been met with challenges due to these highly specialized BEC phenotypes. This perspective provides a brief overview and outlook of the in vivo and in vitro models currently being used to study bacterial brain penetration, and opinion on improved models for the future.

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.

Discordant Effects of Licensed Meningococcal Serogroup B Vaccination on Invasive Disease and Nasal Colonization in a Humanized Mouse Model

Background

The multicomponent meningococcal serogroup B vaccine (4CMenB) is an outer membrane vesicle and recombinant protein-based vaccine licensed to protect against serogroup B meningococcal disease. It remains unknown whether this vaccine will prevent carriage or transmission, key aspects in long-term vaccine success and disease eradication.

Methods

Using a "humanized" transgenic mouse model of nasal colonization, we took a systematic approach to estimate the potential for carriage prevention against antigenically diverse Neisseria meningitidis strains and to compare this protection to an invasive meningococcal disease challenge model.

Results

The 4CMenB vaccine prevented morbidity and mortality after lethal invasive doses of all meningococcal strains tested. Immunization effectively prevented carriage with only 1 of 4 single antigen-matched strains but reduced or prevented nasal colonization by all 4 isolates with multiple cross-reacting antigens. Each immunized mouse had substantial immunoglobulin G targeting the challenge strains, indicating that antibody correlates with protection against sepsis but not nasal carriage.

Conclusions

Immunization with the 4CMenB vaccine elicits a robust humoral response that correlates with protection against invasive challenge but not with prevention of asymptomatic colonization. This suggests that widespread use of this vaccine will reduce morbidity and mortality rates in immunized individuals, with the potential to contribute to herd protection against a subset of strains.

Regulation of capsule in Neisseria meningitidis

Neisseria meningitidis, a devastating pathogen exclusive to humans, expresses capsular polysaccharides that are the major meningococcal virulence determinants and the basis for successful meningococcal vaccines. With rare exceptions, the expression of capsule (serogroups A, B, C, W, X, Y) is required for systemic invasive meningococcal disease. Changes in capsule expression or structure (e.g. hypo- or hyper-encapsulation, capsule "switching", acetylation) can influence immunologic diagnostic assays or lead to immune escape. The loss or down-regulation of capsule is also critical in meningococcal biology facilitating meningococcal attachment, microcolony formation and the carriage state at human mucosal surfaces. Encapsulated meningococci contain a cps locus with promoters located in an intergenic region between the biosynthesis and the conserved capsule transport operons. The cps intergenic region is transcriptionally regulated (and thus the amount of capsule expressed) by IS element insertion, by a two-component system, MisR/MisS and through sequence changes that result in post-transcriptional RNA thermoregulation. Reversible on-off phase variation of capsule expression is controlled by slipped strand mispairing of homo-polymeric tracts and by precise insertion and excision of IS elements (e.g. IS1301) in the biosynthesis operon. Capsule structure can be altered by phase-variable expression of capsular polymer modification enzymes or "switched" through transformation and homologous recombination of different polymerases. Understanding the complex regulation of meningococcal capsule has important implications for meningococcal biology, pathogenesis, diagnostics, current and future vaccine development and vaccine strategies.

In vivo adaptation and persistence of Neisseria meningitidis within the nasopharyngeal mucosa

Neisseria meningitidis (Nme) asymptomatically colonizes the human nasopharynx, yet can initiate rapidly-progressing sepsis and meningitis in rare instances. Understanding the meningococcal lifestyle within the nasopharyngeal mucosa, a phase of infection that is prerequisite for disease, has been hampered by the lack of animal models. Herein, we compare mice expressing the four different human carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) that can bind the neisserial Opa protein adhesins, and find that expression of human CEACAM1 is necessary and sufficient to establish intranasal colonization. During infection, in vivo selection for phase variants expressing CEACAM1-specific Opa proteins occurs, allowing mucosal attachment and entry into the subepithelial space. Consistent with an essential role for Opa proteins in this process, Opa-deficient meningococci were unable to colonize the CEACAM1-humanized mice. While simple Opa-mediated attachment triggered an innate response regardless of meningococcal viability within the inoculum, persistence of viable Opa-expressing bacteria within the CEACAM1-humanized mice was required for a protective memory response to be achieved. Parenteral immunization with a capsule-based conjugate vaccine led to the accumulation of protective levels of Nme-specific IgG within the nasal mucus, yet the sterilizing immunity afforded by natural colonization was instead conferred by Nme-specific IgA without detectable IgG. Considered together, this study establishes that the availability of CEACAM1 helps define the exquisite host specificity of this human-restricted pathogen, displays a striking example of in vivo selection for the expression of desirable Opa variants, and provides a novel model in which to consider meningococcal infection and immunity within the nasopharyngeal mucosa.

Neisseria meningitidis (Nme), a common cause of bacterial meningitis, are carried asymptomatically in the nasopharynx by a substantial proportion of healthy individuals. Their strict adaptation to the human as host has so far impeded the development of animal models to study the meningococcal lifestyle in vivo. While several human CEACAMs are recognized by the neisserial Opa protein adhesins, we show here that the expression of human CEACAM1 in transgenic mice is necessary and sufficient to allow nasal colonization by Nme. The dependence on human CEACAM1 is attributable to the Opa proteins, since intranasal infection with Opa-negative colonies of Nme selects for bacteria expressing Opa proteins, and genetically Opa-deficient meningococci are unable to colonize these animals. We use this new mouse model to examine how innate immune factors such as neutrophils and complement limit colonization. Furthermore, we compare how adaptive responses elicited by colonization and those generated by parenteral vaccination differentially confer sterilizing immunity. Together, this work provides the first evidence of the critical nature of Opa-CEACAM1 binding in vivo, demonstrates that this is a major determinant of the host restriction by Nme, and reveals a clear disparity between immune correlates of sterilizing immunity conferred by natural colonization versus parenteral immunization.

Adhesion of Neisseria meningitidis to dermal vessels leads to local vascular damage and purpura in a humanized mouse model

Septic shock caused by Neisseria meningitidis is typically rapidly evolving and often fatal despite antibiotic therapy. Further understanding of the mechanisms underlying the disease is necessary to reduce fatality rates. Postmortem samples from the characteristic purpuric rashes of the infection show bacterial aggregates in close association with microvessel endothelium but the species specificity of N. meningitidis has previously hindered the development of an in vivo model to study the role of adhesion on disease progression. Here we introduced human dermal microvessels into SCID/Beige mice by xenografting human skin. Bacteria injected intravenously exclusively associated with the human vessel endothelium in the skin graft. Infection was accompanied by a potent inflammatory response with the secretion of human inflammatory cytokines and recruitment of inflammatory cells. Importantly, infection also led to local vascular damage with hemostasis, thrombosis, vascular leakage and finally purpura in the grafted skin, replicating the clinical presentation for the first time in an animal model. The adhesive properties of the type IV pili of N. meningitidis were found to be the main mediator of association with the dermal microvessels in vivo. Bacterial mutants with altered type IV pili function also did not trigger inflammation or lead to vascular damage. This work demonstrates that local type IV pili mediated adhesion of N. meningitidis to the vascular wall, as opposed to circulating bacteria, determines vascular dysfunction in meningococcemia.

Bacterial resistance mechanisms against host defense peptides

Host defense peptides and proteins are important components of the innate host defense against pathogenic microorganisms. They target negatively charged bacterial surfaces and disrupt microbial cytoplasmic membranes, which ultimately leads to bacterial destruction. Throughout evolution, pathogens devised several mechanisms to protect themselves from deleterious damage of host defense peptides. These strategies include (a) inactivation and cleavage of host defense peptides by production of host defense binding proteins and proteases, (b) repulsion of the peptides by alteration of pathogen's surface charge employing modifications by amino acids or amino sugars of anionic molecules (e.g., teichoic acids, lipid A and phospholipids), (c) alteration of bacterial membrane fluidity, and (d) expulsion of the peptides using multi drug pumps. Together with bacterial regulatory network(s) that regulate expression and activity of these mechanisms, they represent attractive targets for development of novel antibacterials.

Olfactory nerve--a novel invasion route of Neisseria meningitidis to reach the meninges

Neisseria meningitidis is a human-specific pathogen with capacity to cause septic shock and meningitis. It has been hypothesized that invasion of the central nervous system (CNS) is a complication of a bacteremic condition. In this study, we aimed to characterize the invasion route of N. meningitidis to the CNS. Using an intranasally challenged mouse disease model, we found that twenty percent of the mice developed lethal meningitis even though no bacteria could be detected in blood. Upon bacterial infection, epithelial lesions and redistribution of intracellular junction protein N-cadherin were observed at the nasal epithelial mucosa, especially at the olfactory epithelium, which is functionally and anatomically connected to the CNS. Bacteria were detected in the submucosa of the olfactory epithelium, along olfactory nerves in the cribriform plate, at the olfactory bulb and subsequently at the meninges and subarachnoid space. Furthermore, our data suggest that a threshold level of bacteremia is required for the development of meningococcal sepsis. Taken together, N. meningitidis is able to pass directly from nasopharynx to meninges through the olfactory nerve system. This study enhances our understanding how N. meningitidis invades the meninges. The nasal olfactory nerve system may be a novel target for disease prevention that can improve outcome and survival.

Biosafety level 2 model of pneumonic plague and protection studies with F1 and Psa

Attenuated Yersinia pestis pgm strains, such as KIM5, lack the siderophore yersiniabactin. Strain KIM5 does not induce significant pneumonia when delivered intranasally. In this study, mice were found to develop pneumonia after intranasal challenge with strain KIM5 when they were injected intraperitoneally with iron dextran, though not with iron sulfate. KIM5-infected mice treated daily with 4 mg iron dextran died in 3 days with severe pneumonia. Pneumonia was less severe if 4 mg iron dextran was administered only once before infection. The best-studied experimental vaccine against plague currently consists of the Yersinia pestis capsular antigen F1 and the type 3 secreted protein LcrV. The F1 antigen was shown to be protective against KIM5 infections in mice administered iron dextran doses leading to light or severe pneumonia, supporting the use of an iron dextran-treated model of pneumonic plague. Since F1 has been reported to be incompletely protective in some primates, and bacterial isolates lacking F1 are still virulent, there has been considerable interest in identifying additional protective subunit immunogens. Here we showed that the highly conserved Psa fimbriae of Y. pestis (also called pH 6 antigen) are expressed in murine organs after infection through the respiratory tract. Studies with iron dextran-treated mice showed that vaccination with the Psa fimbrial protein together with an adjuvant afforded incomplete but significant protection in the mouse model described. Therefore, further investigations to fully characterize the protective properties of the Psa fimbriae are warranted.

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.

Transgenic mice expressing human transferrin as a model for meningococcal infection

The pathogenesis of meningococcal disease is poorly understood due to the lack of a relevant animal model. Moreover, the use of animal models is not optimal as most meningococcal virulence determinants recognize receptors that are specifically expressed in human tissues. One major element of the host specificity is the system of meningococcal iron uptake by transferrin-binding proteins that bind specifically human transferrin but not murine transferrin. We developed a new mouse model for experimental meningococcal infection using transgenic mice expressing human transferrin. Intraperitoneal challenge of transgenic mice induced bacteremia for at least 48 h with an early stage of multiplication, whereas the initial inoculum was rapidly cleared from blood in wild-type mice. Inflammation in the subarachnoidal space with a high influx of polymorphonuclear cells was observed only in transgenic mice. Meningococcal mutants that were unable to use transferrin as a source of iron were rapidly cleared from both wild-type and transgenic mice. Thus, transgenic mice expressing human transferrin may represent an important advance as a new mouse model for in vivo studies of meningococcal virulence and immunogenicity factors.

Determination of human transferrin concentrations in mouse models of neisserial infection

Transferrin constitutes the major protein involved in the transport of iron from the sites of absorption to the sites of storage and utilization. Despite the high affinity of transferrin for iron, most bacterial pathogens, such as the human restricted Neisseria meningitidis, have developed iron acquisition mechanisms. Several animal models of bacterial infection that include the exogenous supply of human transferrin have been implemented, and tests using transgenic mouse models are underway. Here we describe an ELISA sandwich procedure based on two monoclonal antibodies with negligible cross-reactivity to murine transferrin, to estimate human transferrin concentrations in mouse sera. The assay can detect as little as 10 ng/ml of human transferrin with coefficients of variation ranging from 1.6% to 4.4% (intra-assay) and 3.8% to 5% (inter-assay). The recovery values range from 90% to 110% in the assay working range (25-400 ng/ml). Human transferrin concentrations estimated in sera from 41 human transferrin transgenic mice ranged from 2 to 14 microg/ml. Further estimations of human transferrin levels in mouse sera of a previously described mouse model of N. meningitidis were also carried out. The intraperitoneal injection of 8 mg of human transferrin achieved a sustained value of human transferrin in mouse sera in the range of 1-2mg/ml over the first 24h, indicating that bacteria reaching the blood stream during this time would be exposed to levels of hTf found in normal human serum.

Evaluation of recombinant lipidated P2086 protein as a vaccine candidate for group B Neisseria meningitidis in a murine nasal challenge model

Neisseria meningitidis is a major causative agent of bacterial meningitis in human beings, especially among young children (</=2 years of age). Prevention of group B meningococcal disease represents a particularly difficult challenge in vaccine development, due to the inadequate immune response elicited against type B capsular polysaccharide. We have established an adult mouse intranasal challenge model for group B N. meningitidis to evaluate potential vaccine candidates through active immunization. Swiss Webster mice were inoculated intranasally with meningococci, and bacteria were recovered from the noses for at least 3 days postchallenge. Iron dextran was required in the bacterial inoculum to ensure sufficient meningococcal recovery from nasal tissue postchallenge. This model has been utilized to evaluate the potential of a recombinant lipidated group B meningococcal outer membrane protein P2086 (rLP2086) as a vaccine candidate. In this study, mice were immunized subcutaneously with purified rLP2086 formulated with or without an attenuated cholera toxin as an adjuvant. The mice were then challenged intranasally with N. meningitidis strain H355 or M982, and the colonization of nasal tissue was determined by quantitative culture 24 h postchallenge. We demonstrated that immunization with rLP2086 significantly reduced nasal colonization of mice challenged with the two different strains of group B N. meningitidis. Mice immunized with rLP2086 produced a strong systemic immunoglobulin G response, and the serum antibodies were cross-reactive with heterologous strains of group B N. meningitidis. The antibodies have functional activity against heterologous N. meningitidis strain, as demonstrated via bactericidal and infant rat protection assays. These results suggest that rLP2086 is a potential vaccine candidate for group B N. meningitidis.

Characterization of a novel Neisseria meningitidis Fur and iron-regulated operon required for protection from oxidative stress: utility of DNA microarray in the assignment of the biological role of hypothetical genes

We have previously shown that in the human pathogen Neisseria meningitidis group B (MenB) more than 200 genes are regulated in response to growth with iron. Among the Fur-dependent, upregulated genes identified by microarray analysis was a putative operon constituted by three genes, annotated as NMB1436, NMB1437 and NMB1438 and encoding proteins with so far unknown function. The operon was remarkably upregulated in the presence of iron and, on the basis of gel retardation analysis, its regulation was Fur dependent. In this study, we have further characterized the role of iron and Fur in the regulation of the NMB1436-38 operon and we have mapped the promoter and the Fur binding site. We also demonstrate by mutant analysis that the NMB1436-38 operon is required for protection of MenB to hydrogen peroxide-mediated killing. By using both microarray analysis and S1 mapping, we demonstrate that the operon is not regulated by oxidative stress signals. We also show that the deletion of the NMB1436-38 operon results in an impaired capacity of MenB to survive in the blood of mice using an adult mouse model of MenB infection. Finally, we show that the NMB1436-38 deletion mutant exhibits increased susceptibility to the killing activity of polymorphonuclears (PMNs), suggesting that the 'attenuated' phenotype is mediated in part by the increased sensitivity to reactive oxygen species-producing cells. This study represents one of the first examples of the use of DNA microarray to assign a biological role to hypothetical genes in bacteria.

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.

The Neisseria meningitidis serogroup A capsular polysaccharide O-3 and O-4 acetyltransferase

Neisseria meningitidis serogroup A capsular polysaccharide (CPS) is composed of a homopolymer of O-acetylated, alpha1-->6-linked ManNAc 1-phosphate that is distinct from the capsule structures of the other meningococcal disease-causing serogroups, B, C, Y, and W-135. The serogroup A capsule biosynthetic genetic cassette consists of four open reading frames, mynA-D (sacA-D), that are specific to serogroup A, but the functions of these genes have not been well characterized. mynC was found to encode an inner membrane-associated acetyltransferase that is responsible for the O-acetylation of the CPS of serogroup A. The wild-type CPS as revealed by 1H NMR had 60-70% O-acetylated ManNAc residues that contained acetyl groups at O-3, with some species acetylated at O-4 and at both O-3 and O-4. A non-polar mynC mutant generated by introducing an aphA-3 kanamycin resistance cassette produced CPS with no O-acetylation. A serogroup A capsule-specific monoclonal antibody was shown to recognize the wild-type O-acetylated CPS, but not the CPS of the mynC mutant, which lacked O-acetylation. MynC was C-terminally His-tagged and overexpressed in Escherichia coli to obtain the predicted approximately 26-kDa protein. The acetyltransferase activity of purified MynC was demonstrated in vitro using [14C]acetyl-CoA. MynC O-acetylated the O-acetylated CPS of the mynC mutant and further acetylated the wild-type CPS of serogroup A meningococci, but not the CPS of serogroup B or C meningococci. Genetic complementation of the mynC mutant confirmed the function of MynC as the serogroup A CPS O-3 and O-4 acetyltransferase. MynC represents a new subclass of O-acetyltransferases that utilize acetyl-CoA to decorate the D-mannosamine capsule of N. meningitidis serogroup A.

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.