Influence of the combination and phase variation status of the haemoglobin receptors HmbR and HpuAB on meningococcal virulence

Bacterial DNA methyltransferase: A key to the epigenetic world with lessons learned from proteobacteria

Epigenetics modulates expression levels of various important genes in both prokaryotes and eukaryotes. These epigenetic traits are heritable without any change in genetic DNA sequences. DNA methylation is a universal mechanism of epigenetic regulation in all kingdoms of life. In bacteria, DNA methylation is the main form of epigenetic regulation and plays important roles in affecting clinically relevant phenotypes, such as virulence, host colonization, sporulation, biofilm formation et al. In this review, we survey bacterial epigenomic studies and focus on the recent developments in the structure, function, and mechanism of several highly conserved bacterial DNA methylases. These methyltransferases are relatively common in bacteria and participate in the regulation of gene expression and chromosomal DNA replication and repair control. Recent advances in sequencing techniques capable of detecting methylation signals have enabled the characterization of genome-wide epigenetic regulation. With their involvement in critical cellular processes, these highly conserved DNA methyltransferases may emerge as promising targets for developing novel epigenetic inhibitors for biomedical applications.

Beyond Restriction Modification: Epigenomic Roles of DNA Methylation in Prokaryotes

The amount of bacterial and archaeal genome sequence and methylome data has greatly increased over the last decade, enabling new insights into the functional roles of DNA methylation in these organisms. Methyltransferases (MTases), the enzymes responsible for DNA methylation, are exchanged between prokaryotes through horizontal gene transfer and can function either as part of restriction-modification systems or in apparent isolation as single (orphan) genes. The patterns of DNA methylation they confer on the host chromosome can have significant effects on gene expression, DNA replication, and other cellular processes. Some processes require very stable patterns of methylation, resulting in conservation of persistent MTases in a particular lineage. Other processes require patterns that are more dynamic yet more predictable than what is afforded by horizontal gene transfer and gene loss, resulting in phase-variable or recombination-driven MTase alleles. In this review, we discuss what is currently known about the functions of DNA methylation in prokaryotes in light of these evolutionary patterns. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The surface lipoproteins of gram-negative bacteria: Protectors and foragers in harsh environments

Gram-negative pathogens are enveloped by an outer membrane that serves as a double-edged sword: On the one hand, it provides a layer of protection for the bacterium from environmental insults, including other bacteria and the host immune system. On the other hand, it restricts movement of vital nutrients into the cell and provides a plethora of antigens that can be detected by host immune systems. One strategy used to overcome these limitations is the decoration of the outer surface of gram-negative bacteria with proteins tethered to the outer membrane through a lipid anchor. These surface lipoproteins (SLPs) fulfill critical roles in immune evasion and nutrient acquisition, but as more bacterial genomes are sequenced, we are beginning to discover their prevalence and their different roles and mechanisms and importantly how we can exploit them as antimicrobial targets. This review will focus on representative SLPs that gram-negative bacteria use to overcome host innate immunity, specifically the areas of nutritional immunity and complement system evasion. We elaborate on the structures of some notable SLPs required for binding target molecules in hosts and how this information can be used alongside bioinformatics to understand mechanisms of binding and in the discovery of new SLPs. This information provides a foundation for the development of therapeutics and the design of vaccine antigens.

Molecular insights into meningococcal carriage isolates from Burkina Faso 7 years after introduction of a serogroup A meningococcal conjugate vaccine

In 2010, Burkina Faso completed the first nationwide mass-vaccination campaign of a meningococcal A conjugate vaccine, drastically reducing the incidence of disease caused by serogroup A meningococci. Since then, other strains, such as those belonging to serogroups W, X and C, have continued to cause outbreaks within the region. A carriage study was conducted in 2016 and 2017 in the country to characterize the meningococcal strains circulating among healthy individuals following the mass-vaccination campaign. Four cross-sectional carriage evaluation rounds were conducted in two districts of Burkina Faso, Kaya and Ouahigouya. Oropharyngeal swabs were collected for the detection of Neisseria meningitidis by culture. Confirmed N. meningitidis isolates underwent whole-genome sequencing for molecular characterization. Among 13 758 participants, 1035 (7.5 %) N. meningitidis isolates were recovered. Most isolates (934/1035; 90.2 %) were non-groupable and primarily belonged to clonal complex (CC) 192 (822/934; 88 %). Groupable isolates (101/1035; 9.8 %) primarily belonged to CCs associated with recent outbreaks in the region, such as CC11 (serogroup W) and CC10217 (serogroup C); carried serogroup A isolates were not detected. Phylogenetic analysis revealed several CC11 strains circulating within the country, several of which were closely related to invasive isolates. Three sequence types (STs) were identified among eleven CC10217 carriage isolates, two of which have caused recent outbreaks in the region (ST-10217 and ST-12446). Our results show the importance of carriage studies to track the outbreak-associated strains circulating within the population in order to inform future vaccination strategies and molecular surveillance programmes.

Transferrin Binding Protein B and Transferrin Binding Protein A2 Expand the Transferrin Recognition Range of Histophilus somni

The bacterial bipartite transferrin receptor is an iron acquisition system that several important human and animal pathogens require for survival. It consists of the TonB-dependent transporter transferrin binding protein A (TbpA) and the surface lipoprotein transferrin binding protein B (TbpB). Curiously, the Tbps are only found in host-specific pathogens and are themselves host specific, meaning that they will bind to the transferrin of their host species but not to the transferrins of other animal species. While this phenomenon has long been established, neither the steps in the evolutionary process that led to this exquisite adaptation for the host nor the steps that could alter it are known. We sought to gain insight into these processes by studying Tbp specificity in Histophilus somni, an economically important pathogen of cattle. A past study showed that whole cells of H. somni specifically bind bovine transferrin but not transferrin from sheep and goats, two bovids whose transferrins share 93% amino acid sequence identity with bovine transferrin. To our surprise, we found that H. somni can use sheep and goat transferrins as iron sources for growth and that HsTbpB, but not HsTbpA, has detectable affinity for sheep and goat transferrins. Furthermore, a third transferrin binding protein found in H. somni, HsTbpA2, also showed affinity for sheep and goat transferrins. Our results suggest that H. somni TbpB and TbpA2 may contribute to broadening the host transferrin recognition range of H. somni IMPORTANCE Host-restricted pathogens infect a single host species or a narrow range of host species. Histophilus somni, a pathogen that incurs severe economic losses for the cattle industry, infects cattle, sheep, and goats but not other mammals. The transferrin binding proteins, TbpA and TbpB, are thought to be a key iron acquisition system in H. somni; however, despite their importance, H. somni TbpA and TbpB were previously shown to be cattle transferrin specific. In our study, we find that H. somni TbpB and another little-studied Tbp, TbpA2, bind sheep and goat transferrins, as well as bovine transferrin. Our results suggest that TbpB and TbpA2 may allow for host range expansion and provide a mechanism for how host specificity in Tbp-encoding pathogens can be altered.

Neisseria meningitidis: using genomics to understand diversity, evolution and pathogenesis

Meningococcal disease remains an important cause of morbidity and death worldwide despite the development and increasing implementation of effective vaccines. Elimination of the disease is hampered by the enormous diversity and antigenic variability of the causative agent, Neisseria meningitidis, one of the most variable bacteria in nature. These features are attained mainly through high rates of horizontal gene transfer and alteration of protein expression through phase variation. The recent availability of whole-genome sequencing (WGS) of large-scale collections of N. meningitidis isolates from various origins, databases to facilitate storage and sharing of WGS data and the concomitant development of effective bioinformatics tools have led to a much more thorough understanding of the diversity of the species, its evolution and population structure and how virulent traits may emerge. Implementation of WGS is already contributing to enhanced epidemiological surveillance and is essential to ascertain the impact of vaccination strategies. This Review summarizes the recent advances provided by WGS studies in our understanding of the biology of N. meningitidis and the epidemiology of meningococcal disease.

Phase-variable bacterial loci: how bacteria gamble to maximise fitness in changing environments

Phase-variation of genes is defined as the rapid and reversible switching of expression - either ON-OFF switching or the expression of multiple allelic variants. Switching of expression can be achieved by a number of different mechanisms. Phase-variable genes typically encode bacterial surface structures, such as adhesins, pili, and lipooligosaccharide, and provide an extra contingency strategy in small-genome pathogens that may lack the plethora of 'sense-and-respond' gene regulation systems found in other organisms. Many bacterial pathogens also encode phase-variable DNA methyltransferases that control the expression of multiple genes in systems called phasevarions (phase-variable regulons). The presence of phase-variable genes allows a population of bacteria to generate a number of phenotypic variants, some of which may be better suited to either colonising certain host niches, surviving a particular environmental condition and/or evading an immune response. The presence of phase-variable genes complicates the determination of an organism's stably expressed antigenic repertoire; many phase-variable genes are highly immunogenic, and so would be ideal vaccine candidates, but unstable expression due to phase-variation may allow vaccine escape. This review will summarise our current understanding of phase-variable genes that switch expression by a variety of mechanisms, and describe their role in disease and pathobiology.

An Overview of Neisseria meningitidis

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

Differential expression of hemoglobin receptor, HmbR, between carriage and invasive isolates of Neisseria meningitidis contributes to virulence: lessons from a clonal outbreak

Carriage and invasion balance in the pathogenesis of Neisseria meningitidis was analyzed during a recent clonal outbreak of meningococcal B in Normandy, France, that offered the opportunity to compare six isolates undistinguable by conventional typing (B:14:P1.7,16:F3-3/ST-32) isolated from invasive disease or pharyngeal asymptomatic carriage. Data from animal model (transgenic mice rendered susceptible to N. meningitidis infection) showed an absence of virulence for two non-capsulated carriage isolates, an intermediate virulence for two capsulated carriage isolates and a marked virulence for two capsulated invasive isolates. This differential pathogenesis well correlated with whole genome sequencing analysis that clustered both isolates of each group together, forming their own arm within the Norman cluster. Gene-by-gene analysis specified that genes involved in iron acquisition were among the elements differentially represented in cluster of invasive isolates compared to cluster of capsulated carriage isolates. The hemoglobin receptor encoding gene hmbR was in an ON-phase in the capsulated invasive isolates while carriage capsulated isolates were in an OFF-phase. An ON-phase variant of a capsulated carriage isolate showed enhanced virulence. These data underline the role of phase variation (ON/OFF) of HmbR in the balance between disease isolates/carriage isolates.

The Diversity of Mammalian Hemoproteins and Microbial Heme Scavengers Is Shaped by an Arms Race for Iron Piracy

Iron is an essential micronutrient for most living species. In mammals, hemoglobin (Hb) stores more than two thirds of the body's iron content. In the bloodstream, haptoglobin (Hp) and hemopexin (Hpx) sequester free Hb or heme. Pathogenic microorganisms usually acquire iron from their hosts and have evolved complex systems of iron piracy to circumvent nutritional immunity. Herein, we performed an evolutionary analysis of genes coding for mammalian heme-binding proteins and heme-scavengers in pathogen species. The underlying hypothesis is that these molecules are engaged in a molecular arms race. We show that positive selection drove the evolution of mammalian Hb and Hpx. Positively selected sites in Hb are located at the interaction surface with Neisseria meningitidis heme scavenger HpuA and with Staphylococcus aureus iron-regulated surface determinant B (IsdB). In turn, positively selected sites in HpuA and IsdB are located in the flexible protein regions that contact Hb. A residue in Hb (S45H) was also selected on the Caprinae branch. This site stabilizes the interaction with Trypanosoma brucei hemoglobin-haptoglobin (HbHp) receptor (TbHpHbR), a molecule that also mediates trypanosome lytic factor (TLF) entry. In TbHpHbR, positive selection drove the evolution of a variant (L210S) which allows evasion from TLF but reduces affinity for HbHp. Finally, selected sites in Hpx are located at the interaction surface with the Haemophilus influenzae hemophore HxuA, which in turn displays fast evolving sites at the Hpx-binding interface. These results shed light into host-pathogens conflicts and establish the importance of nutritional immunity as an evolutionary force.

Subversion of nutritional immunity by the pathogenic Neisseriae

The pathogenic Neisseria species, including Neisseria meningitidis and Neisseria gonorrhoeae, are obligate human pathogens that cause significant morbidity and mortality. The success of these pathogens, with regard to causing disease in humans, is inextricably linked to their ability to acquire necessary nutrients in the hostile environment of the host. Humans deploy a significant arsenal of weaponry to defend against bacterial pathogens, not least of which are the metal-sequestering proteins that entrap and withhold transition metals, including iron, zinc and manganese, from invaders. This review will discuss the general strategies that bacteria employ to overcome these metal-sequestering attempts by the host, and then will focus on the relatively uncommon 'metal piracy' approaches utilized by the pathogenic Neisseria for this purpose. Because acquiring metals from the environment is critical to microbial survival, interfering with this process could impede growth and therefore disease initiation or progression. This review will also discuss how interfering with metal uptake by the pathogenic Neisseriae could be deployed in the development of novel or improved preventative or therapeutic measures against these important pathogens.

Evaluation of three recombinant outer membrane proteins, OmpA1, Tdr, and TbpA, as potential vaccine antigens against virulent Aeromonas hydrophila infection in channel catfish (Ictalurus punctatus)

A virulent clonal population of Aeromonas hydrophila (VAh) is recognized as the etiological agent in outbreaks of motile aeromonas septicemia (MAS) in catfish aquaculture in the southeastern United States since 2009. Genomic subtraction revealed three outer membrane proteins present in VAh strain ML09-119 but not in low virulence reference A. hydrophila strains: major outer membrane protein OmpA1, TonB-dependent receptor (Tdr), and transferrin-binding protein A (TbpA). Here, the genes encoding ompA1, tdr, and tbpA were cloned from A. hydrophila ML09-119 and expressed in Escherichia coli. The purified recombinant OmpA1, Tdr, and TbpA proteins had estimated molecular weights of 37.26, 78.55, and 41.67 kDa, respectively. Catfish fingerlings vaccinated with OmpA1, Tdr, and TbpA emulsified with non-mineral oil adjuvant were protected against subsequent VAh strain ML09-119 infection with 98.59%, 95.59%, and 47.89% relative percent survival (RPS), respectively. Furthermore, the mean liver, spleen, and anterior kidney bacterial concentrations were significantly lower in catfish vaccinated with the OmpA1 and Tdr than the sham-vaccinated control group. ELISA demonstrated that catfish immunized with OmpA1, Tdr, and TbpA produce significant antibody response by 21 days post-immunization. Therefore, OmpA1 and Tdr proteins could be used as potential candidates for vaccine development against virulent A. hydrophila infection. However, TbpA protein failed to provide strong protection.

Whole genome sequencing reveals within-host genetic changes in paired meningococcal carriage isolates from Ethiopia

Background

Meningococcal colonization is a prerequisite for transmission and disease, but the bacterium only very infrequently causes disease while asymptomatic carriage is common. Carriage is highly dynamic, showing a great variety across time and space within and across populations, but also within individuals. The understanding of genetic changes in the meningococcus during carriage, when the bacteria resides in its natural niche, is important for understanding not only the carriage state, but the dynamics of the entire meningococcal population.

Results

Paired meningococcal isolates, obtained from 50 asymptomatic carriers about 2 months apart were analyzed with whole genome sequencing (WGS). Phylogenetic analysis revealed that most paired isolates from the same individual were closely related, and the average and median number of allelic differences between paired isolates defined as the same strain was 35. About twice as many differences were seen between isolates from different individuals within the same sequence type (ST). In 8%, different strains were detected at different time points. A difference in ST was observed in 6%, including an individual who was found to carry three different STs over the course of 9 weeks. One individual carried different strains from the same ST.

In total, 566 of 1605 cgMLST genes had undergone within-host genetic changes in one or more pairs. The most frequently changed cgMLST gene was relA that was changed in 47% of pairs. Across the whole genome, pilE, differed mostly, in 85% of the pairs. The most frequent mechanisms of genetic difference between paired isolates were phase variation and recombination, including gene conversion. Different STs showed variation with regard to which genes that were most frequently changed, mostly due to absence/presence of phase variation.

Conclusions

This study revealed within-host genetic differences in meningococcal isolates during short-term asymptomatic carriage. The most frequently changed genes were genes belonging to the pilin family, the restriction/modification system, opacity proteins and genes involved in glycosylation. Higher resolution genome-wide sequence typing is necessary to resolve the diversity of isolates and reveals genetic differences not discovered by traditional typing schemes, and would be the preferred choice of technology.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3806-3) contains supplementary material, which is available to authorized users.

An international invasive meningococcal disease outbreak due to a novel and rapidly expanding serogroup W strain, Scotland and Sweden, July to August 2015

The 23rd World Scout Jamboree in 2015 took place in Japan and included over 33,000 scouts from 162 countries. Within nine days of the meeting ending, six cases of laboratory-confirmed invasive serogroup W meningococcal disease occurred among scouts and their close contacts in Scotland and Sweden. The isolates responsible were identical to one-another by routine typing and, where known (4 isolates), belonged to the ST-11 clonal complex (cc11) which is associated with large outbreaks and high case fatality rates. Recent studies have demonstrated the need for high-resolution genomic typing schemes to assign serogroup W cc11 isolates to several distinct strains circulating globally over the past two decades. Here we used such schemes to confirm that the Jamboree-associated cases constituted a genuine outbreak and that this was due to a novel and rapidly expanding strain descended from the strain that has recently expanded in South America and the United Kingdom. We also identify the genetic differences that define the novel strain including four point mutations and three putative recombination events involving the horizontal exchange of 17, six and two genes, respectively. Noteworthy outcomes of these changes were antigenic shifts and the disruption of a transcriptional regulator.

Characterization of the role in adherence of Mycoplasma hyorhinis variable lipoproteins containing different repeat unit copy numbers

Mycoplasma hyorhinis (M. hyorhinis) is an important pathogen of pigs. In previous studies, the variable lipoprotein (Vlp) family has been shown to play a role in mediating M. hyorhinis cytoadhesion. Herein, we performed several experiments to study the function of each Vlp family member in detail, especially examining the cytoadhesion functional domain and how the repeat unit copy number impacts on function. Recombinant proteins rVlpII, composed of region II from all seven Vlp members; rVlpIII, composed of repeat peptides from region III of all of Vlp members; as well as a series of recombinant rVlp proteins for each member containing different repeat unit copy numbers were constructed. All of the proteins were expressed in Escherichia coli and purified by affinity chromatography. The recombinant proteins, as well as seven keyhole limpet hemocyanin-conjugated Vlp peptides containing two copies of the repeat unit, were analyzed for their adherence to swine tracheal epithelial cells using a microtiter plate adherence assay. Both rVlpII and rVlpIII proteins were able to bind to cell membrane proteins. Among the repeat unit peptides, only PepVlpB and PepVlpG were able to bind to cell membrane proteins. All of the Vlp members had cytoadhesion capability. The adhesion abilities of the proteins containing 0 or 3 copies of the repeat unit were stronger than those of the proteins containing 12 copies. For rVlpA, rVlpB, rVlpD, rVlpF and rVlpG, the proteins containing no copies bound stronger than the proteins containing 3 copies. In contrast, the adherence of rVlpC3 was stronger than that of rVlpC0. There was no significant difference between the adherence of rVlpE3 and that of rVlpE0. Our results suggest that the major cytoadhesion sites of Vlps are mainly contained in region II, the function of which would be blocked by region III when region III is longer.

Metagenomics Reveals Dysbiosis and a Potentially Pathogenic N. flavescens Strain in Duodenum of Adult Celiac Patients

OBJECTIVES

Celiac disease (CD)-associated duodenal dysbiosis has not yet been clearly defined, and the mechanisms by which CD-associated dysbiosis could concur to CD development or exacerbation are unknown. In this study, we analyzed the duodenal microbiome of CD patients.

METHODS

The microbiome was evaluated in duodenal biopsy samples of 20 adult patients with active CD, 6 CD patients on a gluten-free diet, and 15 controls by DNA sequencing of 16S ribosomal RNA libraries. Bacterial species were cultured, isolated and identified by mass spectrometry. Isolated bacterial species were used to infect CaCo-2 cells, and to stimulate normal duodenal explants and cultured human and murine dendritic cells (DCs). Inflammatory markers and cytokines were evaluated by immunofluorescence and ELISA, respectively.

RESULTS

Proteobacteria was the most abundant and Firmicutes and Actinobacteria the least abundant phyla in the microbiome profiles of active CD patients. Members of the Neisseria genus (Betaproteobacteria class) were significantly more abundant in active CD patients than in the other two groups (P=0.03). Neisseria flavescens (CD-Nf) was the most abundant Neisseria species in active CD duodenum. Whole-genome sequencing of CD-Nf and control-Nf showed genetic diversity of the iron acquisition systems and of some hemoglobin-related genes. CD-Nf was able to escape the lysosomal compartment in CaCo-2 cells and to induce an inflammatory response in DCs and in ex-vivo mucosal explants.

CONCLUSIONS

Marked dysbiosis and an abundance of a peculiar CD-Nf strain characterize the duodenal microbiome in active CD patients thus suggesting that the CD-associated microbiota could contribute to the many inflammatory signals in this disorder.

A TonB-dependent receptor regulates antifungal HSAF biosynthesis in Lysobacter

Lysobacter species are Gram-negative bacteria that are emerging as new sources of antibiotics, including HSAF (Heat Stable Antifungal Factor), which was identified from L. enzymogenes with a new mode of action. LesR, a LuxR solo, was recently shown to regulate the HSAF biosynthesis via an unidentified mechanism in L. enzymogenes OH11. Here, we used a comparative proteomic approach to identify the LesR targets and found that LesR influenced the expression of 33 proteins belonging to 10 functional groups, with 9 proteins belonging to the TBDR (TonB-Dependent Receptor) family. The fundamental role of bacterial TBDR in nutrient uptake motivates us to explore their potential regulation on HSAF biosynthesis which is also modulated by nutrient condition. Six out of 9 TBDR coding genes were individually in-frame deleted. Phenotypic and gene-expression assays showed that TBDR7, whose level was lower in a strain overexpressing lesR, was involved in regulating HSAF yield. TBDR7 was not involved in the growth, but played a vital role in transcribing the key HSAF biosynthetic gene. Taken together, the current lesR-based proteomic study provides the first report that TBDR7 plays a key role in regulating antibiotic (HSAF) biosynthesis, a function which has never been found for TBDRs in bacteria.

Genome-wide mining of potential virulence-associated genes in Riemerella anatipestifer using random transposon mutagenesis

Riemerella anatipestifer infection is a severe disease confronting the duck industry worldwide. However, little is known about the molecular basis of R. anatipestifer pathogenesis. In this study, we screened 3580 transposon Tn4351 insertion mutagenesis mutants of the highly virulent strain YZb1 in a duckling infection experiment and found 29 of them to be attenuated and 28 potential virulence-associated genes were identified. Molecular characterization of transposon insertion sites showed that of the 28 screened genes, two were predicted to encode TonB-dependent outer membrane receptor (plugs), sixteen encoded enzymes, and seven encoded hypothetical proteins. In addition, of the 28 affected genes, 19 were only found in bacteria belonging to the phylum Bacteroidetes and 10 were only found in the family Flavobacteriaceae. The median lethal dose of the mutants M11 and M29, which was affected in Riean_0060 and Riean_1537 respectively, were about 1700-fold and 210-fold higher than that of the wild-type strain YZb1, and those of the complemented strains M11(pRES-Riean_0060) and M29(pRES-Riean_1537) were decreased by 25- and 3-fold respectively compared to those of the mutants M11 and M29. Additional analysis indicated that the blood bacterial loading of ducklings infected with M11 or M29 was decreased significantly, as compared with that in ducklings infected with the wild-type strain YZb1. Thus, our results indicate that Riean_0060 and Riean_1537 were involved in R. anatipestifer pathogenesis.

The functions of the variable lipoprotein family of Mycoplasma hyorhinis in adherence to host cells

Mycoplasma hyorhinis (M. hyorhinis) is a swine pathogen that is associated with various human cancers and contamination in cell cultures. However, no studies on the adhesion molecules of this pathogen have yet been reported. The variable lipoprotein (Vlp) family is an important surface component of M. hyorhinis. Herein, we performed several experiments to identify the function of the Vlp family in adherence to host cells. Seven recombinant Vlp (rVlp) proteins were expressed in Escherichia coli and purified by affinity chromatography. The potential role of rVlp adherence to pig kidney (PK-15) and swine tracheal epithelial (STEC) cells was then studied by indirect immunofluorescence assay and microtiter plate adherence assay. Adhesion of M. hyorhinis to PK-15 and STEC cells was specifically inhibited by the addition of a cocktail of rVlp proteins. The rVlp protein mixture was shown to bind to both PK-15 and STEC cells. The binding increased in a dose-dependent manner and could be blocked by antisera against the rVlp proteins. Most of the rVlp proteins could bind individually to both PK-15 and STEC cells except for rVlpD and rVlpF, which bound only to STEC cells. Because Vlp members vary in size among different strains and generations, they may vary in their cytoadhesion capabilities in various strains. In summary, the present results indicate that the Vlp family functions as adhesins of M. hyorhinis.

Structural analysis of haemoglobin binding by HpuA from the Neisseriaceae family

The Neisseriaceae family of bacteria causes a range of diseases including meningitis, septicaemia, gonorrhoea and endocarditis, and extracts haem from haemoglobin as an important iron source within the iron-limited environment of its human host. Herein we report crystal structures of apo- and haemoglobin-bound HpuA, an essential component of this haem import system. The interface involves long loops on the bacterial receptor that present hydrophobic side chains for packing against the surface of haemoglobin. Interestingly, our structural and biochemical analyses of Kingella denitrificans and Neisseria gonorrhoeae HpuA mutants, although validating the interactions observed in the crystal structure, show how Neisseriaceae have the fascinating ability to diversify functional sequences and yet retain the haemoglobin binding function. Our results present the first description of HpuA's role in direct binding of haemoglobin.

The Neisseriaceae bacteria family extract heame from the haemoglobin of its host, the HpuA protein is part of this system. Here, the authors report crystal structures of apo- and haemoglobin-bound HpuA and analyse mutants to examine the interaction between HpuA and haemoglobin.

How the Knowledge of Interactions between Meningococcus and the Human Immune System Has Been Used to Prepare Effective Neisseria meningitidis Vaccines

In the last decades, tremendous advancement in dissecting the mechanisms of pathogenicity of Neisseria meningitidis at a molecular level has been achieved, exploiting converging approaches of different disciplines, ranging from pathology to microbiology, immunology, and omics sciences (such as genomics and proteomics). Here, we review the molecular biology of the infectious agent and, in particular, its interactions with the immune system, focusing on both the innate and the adaptive responses. Meningococci exploit different mechanisms and complex machineries in order to subvert the immune system and to avoid being killed. Capsular polysaccharide and lipooligosaccharide glycan composition, in particular, play a major role in circumventing immune response. The understanding of these mechanisms has opened new horizons in the field of vaccinology. Nowadays different licensed meningococcal vaccines are available and used: conjugate meningococcal C vaccines, tetravalent conjugate vaccines, an affordable conjugate vaccine against the N. menigitidis serogroup A, and universal vaccines based on multiple antigens each one with a different and peculiar function against meningococcal group B strains.

Investigation into the Antigenic Properties and Contributions to Growth in Blood of the Meningococcal Haemoglobin Receptors, HpuAB and HmbR

Acquisition of iron from host complexes is mediated by four surface-located receptors of Neisseria meningitidis. The HmbR protein and heterodimeric HpuAB complex bind to haemoglobin whilst TbpBA and LbpBA bind iron-loaded transferrin and lactoferrin complexes, respectively. The haemoglobin receptors are unevenly distributed; disease-causing meningococcal isolates encode HmbR or both receptors while strains with only HpuAB are rarely-associated with disease. Both these receptors are subject to phase variation and 70–90% of disease isolates have one or both of these receptors in an ON expression state. The surface-expression, ubiquity and association with disease indicate that these receptors could be potential virulence factors and vaccine targets. To test for a requirement during disease, an hmbR deletion mutant was constructed in a strain (MC58) lacking HpuAB and in both a wild-type and TbpBA deletion background. The hmbR mutant exhibited an identical growth pattern to wild-type in whole blood from healthy human donors whereas growth of the tbpBA mutant was impaired. These results suggest that transferrin is the major source of iron for N. meningitidis during replication in healthy human blood. To examine immune responses, polyclonal antisera were raised against His-tagged purified-recombinant variants of HmbR, HpuA and HpuB in mice using monolipopolysaccharide as an adjuvant. Additionally, monoclonal antibodies were raised against outer membrane loops of HmbR presented on the surface of EspA, an E. coli fimbrial protein. All antisera exhibited specific reactivity in Western blots but HmbR and HpuA polyclonal sera were reactive against intact meningococcal cells. None of the sera exhibited bactericidal activity against iron-induced wild-type meningococci. These findings suggest that the HmbR protein is not required during the early stages of disease and that immune responses against these receptors may not be protective.

Genomic Analysis of the Evolution and Global Spread of Hyper-invasive Meningococcal Lineage 5

Background

The predominant model for bacterial pandemics is the emergence of a virulent variant that diversifies as it spreads in human populations. We investigated a 40-year meningococcal disease pandemic caused by the hyper-invasive ET-5/ST-32 complex.

Methods

A global collection of Neisseria meningitidis isolates dating from 1969 to 2008 was whole genome sequenced (WGS) and analysed using a gene-by-gene approach at http://pubmlst.org/neisseria.

Findings

Analysis of WGS data identified a ‘Lineage 5 pan genome’ of 1940 genes, 1752 (92%) of which were present in all isolates (Lineage 5 ‘core genome’). Genetic diversity, which was mostly generated by horizontal gene transfer, was unevenly distributed in the genome; however, genealogical analysis of diverse and conserved core genes, accessory genes, and antigen encoding genes, robustly identified a star phylogeny with a number of sub-lineages. Most European and American isolates belonged to one of two closely related sub-lineages, which had diversified before the identification of the pandemic in the 1970s. A third, genetically more diverse sub-lineage, was associated with Asian isolates. Several isolates had acquired DNA from the related gonococcus.

Interpretation

These data were inconsistent with a single point of origin followed by pandemic spread, rather suggesting that the sub-lineages had diversified and spread by asymptomatic transmission, with multiple distinct strains causing localised hyperendemic outbreaks.

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Genomic analysis of hyper-invasive N. meningitidis lineage 5 distinguishing sub-lineages and localised outbreaks.

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Identification of a gonococcal conjugative plasmid consistent with horizontal genetic transfer.

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Discovery of a distinct type 4 secretion system previously unidentified in Neisseria.

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Gene-by-gene analysis of conserved and diverse loci providing essential tools in serogroup B vaccine development.

Pyrophosphate-mediated iron acquisition from transferrin in Neisseria meningitidis does not require TonB activity

The ability to acquire iron from various sources has been demonstrated to be a major determinant in the pathogenesis of Neisseria meningitidis. Outside the cells, iron is bound to transferrin in serum, or to lactoferrin in mucosal secretions. Meningococci can extract iron from iron-loaded human transferrin by the TbpA/TbpB outer membrane complex. Moreover, N. meningitidis expresses the LbpA/LbpB outer membrane complex, which can extract iron from iron-loaded human lactoferrin. Iron transport through the outer membrane requires energy provided by the ExbB-ExbD-TonB complex. After transportation through the outer membrane, iron is bound by periplasmic protein FbpA and is addressed to the FbpBC inner membrane transporter. Iron-complexing compounds like citrate and pyrophosphate have been shown to support meningococcal growth ex vivo. The use of iron pyrophosphate as an iron source by N. meningitidis was previously described, but has not been investigated. Pyrophosphate was shown to participate in iron transfer from transferrin to ferritin. In this report, we investigated the use of ferric pyrophosphate as an iron source by N. meningitidis both ex vivo and in a mouse model. We showed that pyrophosphate was able to sustain N. meningitidis growth when desferal was used as an iron chelator. Addition of a pyrophosphate analogue to bacterial suspension at millimolar concentrations supported N. meningitidis survival in the mouse model. Finally, we show that pyrophosphate enabled TonB-independent ex vivo use of iron-loaded human or bovine transferrin as an iron source by N. meningitidis. Our data suggest that, in addition to acquiring iron through sophisticated systems, N. meningitidis is able to use simple strategies to acquire iron from a wide range of sources so as to sustain bacterial survival.

Identification and analysis of three virulence-associated TonB-dependent outer membrane receptors of Pseudomonas fluorescens

Pseudomonas fluorescens is a Gram-negative bacterium that can infect a wide range of farmed fish. However, very little is known about the virulence mechanism of P. fluorescens as a fish pathogen. In this study, we identified and analyzed 3 TonB-dependent outer membrane receptors (TDRs) from a pathogenic P. fluorescens strain isolated from fish. In silico analysis revealed that all 3 proteins (named Tdr1 to 3) possess structural domains typical of TDRs. Quantitative real time RT-PCR analysis showed that tdr1, tdr2, and tdr3 expressions were upregulated under iron-depleted conditions. Compared to the wild type, mutants defective in tdr1, tdr2, and tdr3 were retarded in growth to different extents. Infection in a turbot Scophthalmus maximus model showed that all 3 mutants were impaired in their ability to desseminate into and colonize host tissues. In addition, the tdr1 and tdr3 mutants exhibited significantly reduced virulence. When used as subunit vaccines, purified recombinant proteins of Tdr1, Tdr2, and, in particular, Tdr3 elicited significant protection in turbot against lethal P. fluorescens challenge. The vaccinated fish produced specific serum antibodies, which, when incubated with P. fluorescens, blocked infection of P. fluorescens in fish cells. Together these results indicate that Tdr1, Tdr2, and Tdr3 are iron-regulated factors that participate in bacterial virulence and induce protective immunity as subunit vaccines.

Mathematical and live meningococcal models for simple sequence repeat dynamics - coherent predictions and observations

Evolvability by means of simple sequence repeat (SSR) instability is a feature under the constant influence of opposing selective pressures to expand and compress the repeat tract and is mechanistically influenced by factors that affect genetic instability. In addition to direct selection for protein expression and structural integrity, other factors that influence tract length evolution were studied. The genetic instability of SSRs that switch the expression of antibiotic resistance ON and OFF was modelled mathematically and monitored in a panel of live meningococcal strains. The mathematical model showed that the SSR length of a theoretical locus in an evolving population may be shaped by direct selection of expression status (ON or OFF), tract length dependent (α) and tract length independent factors (β). According to the model an increase in α drives the evolution towards shorter tracts. An increase in β drives the evolution towards a normal distribution of tract lengths given that an upper and a lower limit are set. Insertion and deletion biases were shown to skew allelic distributions in both directions. The meningococcal SSR model was tested in vivo by monitoring the frequency of spectinomycin resistance OFF→ON switching in a designed locus. The instability of a comprehensive panel of the homopolymeric SSRs, constituted of a range of 5-13 guanine nucleotides, was monitored in wildtype and mismatch repair deficient backgrounds. Both the repeat length itself and mismatch repair deficiency were shown to influence the genetic instability of the homopolymeric tracts. A possible insertion bias was observed in tracts ≤G10. Finally, an inverse correlation between the number of tract-encoded amino acids and growth in the presence of ON-selection illustrated a limitation to SSR expansion in an essential gene associated with the designed model locus and the protein function mediating antibiotic resistance.

Phase variation mediates reductions in expression of surface proteins during persistent meningococcal carriage

Asymptomatic and persistent colonization of the upper respiratory tract by Neisseria meningitidis occurs despite elicitation of adaptive immune responses against surface antigens. A putative mechanism for facilitating host persistence of this bacterial commensal and pathogen is alterations in expression of surface antigens by simple sequence repeat (SSR)-mediated phase variation. We investigated how often phase variation occurs during persistent carriage by analyzing the SSRs of eight loci in multiple isolates from 21 carriers representative of 1 to 6 months carriage. Alterations in repeat number were detected by a GeneScan analysis and occurred at 0.06 mutations/gene/month of carriage. The expression states were determined by Western blotting and two genes, fetA and nadA, exhibited trends toward low expression states. A critical finding from our unique examination of combinatorial expression states, "phasotypes," was for significant reductions in expression of multiple phase-variable surface proteins during persistent carriage of some strains. The immune responses in these carriers were examined by measuring variant-specific PorA IgG antibodies, capsular group Y IgG antibodies and serum bactericidal activity in concomitant serum samples. Persistent carriage was associated with high levels of specific IgG antibodies and serum bactericidal activity while recent strain acquisition correlated with a significant induction of antibodies. We conclude that phase-variable genes are driven into lower expression states during long-term persistent meningococcal carriage, in part due to continuous exposure to antibody-mediated selection, suggesting localized hypermutation has evolved to facilitate host persistence.

Genomic and global approaches to unravelling how hypermutable sequences influence bacterial pathogenesis

Rapid adaptation to fluctuations in the host milieu contributes to the host persistence and virulence of bacterial pathogens. Adaptation is frequently mediated by hypermutable sequences in bacterial pathogens. Early bacterial genomic studies identified the multiplicity and virulence-associated functions of these hypermutable sequences. Thus, simple sequence repeat tracts (SSRs) and site-specific recombination were found to control capsular type, lipopolysaccharide structure, pilin diversity and the expression of outer membrane proteins. We review how the population diversity inherent in the SSR-mediated mechanism of localised hypermutation is being unlocked by the investigation of whole genome sequences of disease isolates, analysis of clinical samples and use of model systems. A contrast is presented between the problematical nature of analysing simple sequence repeats in next generation sequencing data and in simpler, pragmatic PCR-based approaches. Specific examples are presented of the potential relevance of this localized hypermutation to meningococcal pathogenesis. This leads us to speculate on the future prospects for unravelling how hypermutable mechanisms may contribute to the transmission, spread and persistence of bacterial pathogens.

Shared and distinct mechanisms of iron acquisition by bacterial and fungal pathogens of humans

Iron is the most abundant transition metal in the human body and its bioavailability is stringently controlled. In particular, iron is tightly bound to host proteins such as transferrin to maintain homeostasis, to limit potential damage caused by iron toxicity under physiological conditions and to restrict access by pathogens. Therefore, iron acquisition during infection of a human host is a challenge that must be surmounted by every successful pathogenic microorganism. Iron is essential for bacterial and fungal physiological processes such as DNA replication, transcription, metabolism, and energy generation via respiration. Hence, pathogenic bacteria and fungi have developed sophisticated strategies to gain access to iron from host sources. Indeed, siderophore production and transport, iron acquisition from heme and host iron-containing proteins such as hemoglobin and transferrin, and reduction of ferric to ferrous iron with subsequent transport are all strategies found in bacterial and fungal pathogens of humans. This review focuses on a comparison of these strategies between bacterial and fungal pathogens in the context of virulence and the iron limitation that occurs in the human body as a mechanism of innate nutritional defense.

The distribution and 'in vivo' phase variation status of haemoglobin receptors in invasive meningococcal serogroup B disease: genotypic and phenotypic analysis

Two haemoglobin-binding proteins, HmbR and HpuAB, contribute to iron acquisition by Neisseria meningitidis. These receptors are subject to high frequency, reversible switches in gene expression--phase variation (PV)--due to mutations in homopolymeric (poly-G) repeats present in the open reading frame. The distribution and PV state of these receptors was assessed for a representative collection of isolates from invasive meningococcal disease patients of England, Wales and Northern Ireland. Most of the major clonal complexes had only the HmbR receptor whilst the recently expanding ST-275-centred cluster of the ST-269 clonal complex had both receptors. At least one of the receptors was in an 'ON' configuration in 76.3% of the isolates, a finding that was largely consistent with phenotypic analyses. As PV status may change during isolation and culture of meningococci, a PCR-based protocol was utilised to confirm the expression status of the receptors within contemporaneously acquired clinical specimens (blood/cerebrospinal fluid) from the respective patients. The expression state was confirmed for all isolate/specimen pairs with <15 tract repeats indicating that the PV status of these receptors is stable during isolation. This study therefore establishes a protocol for determining in vivo PV status to aid in determining the contributions of phase variable genes to invasive meningococcal disease. Furthermore, the results of the study support a putative but non-essential role of the meningococcal haemoglobin receptors as virulence factors whilst further highlighting their vaccine candidacy.

The role of TonB-dependent receptor TbdR1 in Riemerella anatipestifer in iron acquisition and virulence

Riemerella anatipestifer is an important duck pathogen and causes serious economic losses to the duck industry worldwide. To date, four full R. anatipestifer genomic sequences have been submitted to the GenBank database and 31 TonB-dependent outer membrane receptors, which may play critical roles in host-bacteria interactions, were predicted for R. anatipestifer strain GSM15868. In our previous study, we reported that the TonB-dependent receptor TbdR1 was a cross immunogenic antigen among R. anatipestifer serotypes 1, 2, and 10. However, the biological functions of TbdR1 in R. anatipestifer remain unclear. In the present study, a tbdR1 (Riean_1607) deletion mutant CH3ΔtbdR1 of R. anatipestifer strain CH3 was constructed and characterized for iron-limited growth, biofilm formation, and pathogenicity to ducklings. Our results showed that TbdR1 was involved in hemin iron acquisition and the tbdR1 deletion significantly reduced biofilm formation and adhesion to and invasion of Vero cells. Animal experiments indicated that the median lethal dose of the CH3ΔtbdR1 mutant in ducklings was about 45-fold higher than that of the wild-type CH3 strain. Additional analysis indicated that bacterial loads in blood, liver, and brain tissues in CH3ΔtbdR1-infected ducklings were decreased significantly compared to those in wild-type CH3-infected ducklings. Thus, our results demonstrated that TbdR1 was involved in hemin iron acquisition and necessary for optimal bacterial virulence.

The role of variable DNA tandem repeats in bacterial adaptation

DNA tandem repeats (TRs), also designated as satellite DNA, are inter- or intragenic nucleotide sequences that are repeated two or more times in a head-to-tail manner. Because TR tracts are prone to strand-slippage replication and recombination events that cause the TR copy number to increase or decrease, loci containing TRs are hypermutable. An increasing number of examples illustrate that bacteria can exploit this instability of TRs to reversibly shut down or modulate the function of specific genes, allowing them to adapt to changing environments on short evolutionary time scales without an increased overall mutation rate. In this review, we discuss the prevalence and distribution of inter- and intragenic TRs in bacteria and the mechanisms of their instability. In addition, we review evidence demonstrating a role of TR variations in bacterial adaptation strategies, ranging from immune evasion and tissue tropism to the modulation of environmental stress tolerance. Nevertheless, while bioinformatic analysis reveals that most bacterial genomes contain a few up to several dozens of intra- and intergenic TRs, only a small fraction of these have been functionally studied to date.

Prevalence and phase variable expression status of two autotransporters, NalP and MspA, in carriage and disease isolates of Neisseria meningitidis

Neisseria meningitidis is a human nasopharyngeal commensal capable of causing life-threatening septicemia and meningitis. Many meningococcal surface structures, including the autotransporter proteins NalP and MspA, are subject to phase variation (PV) due to the presence of homopolymeric tracts within their coding sequences. The functions of MspA are unknown. NalP proteolytically cleaves several surface-located virulence factors including the 4CMenB antigen NhbA. Therefore, NalP is a phase-variable regulator of the meningococcal outer membrane and secretome whose expression may reduce isolate susceptibility to 4CMenB-induced immune responses. To improve our understanding of the contributions of MspA and NalP to meningococcal-host interactions, their distribution and phase-variable expression status was studied in epidemiologically relevant samples, including 127 carriage and 514 invasive isolates representative of multiple clonal complexes and serogroups. Prevalence estimates of >98% and >88% were obtained for mspA and nalP, respectively, with no significant differences in their frequencies in disease versus carriage isolates. 16% of serogroup B (MenB) invasive isolates, predominately from clonal complexes ST-269 and ST-461, lacked nalP. Deletion of nalP often resulted from recombination events between flanking repetitive elements. PolyC tract lengths ranged from 6–15 bp in nalP and 6–14 bp in mspA. In an examination of PV status, 58.8% of carriage, and 40.1% of invasive nalP-positive MenB isolates were nalP phase ON. The frequency of this phenotype was not significantly different in serogroup Y (MenY) carriage strains, but was significantly higher in invasive MenY strains (86.3%; p<0.0001). Approximately 90% of MenB carriage and invasive isolates were mspA phase ON; significantly more than MenY carriage (32.7%) or invasive (13.7%) isolates. This differential expression resulted from different mode mspA tract lengths between the serogroups. Our data indicates a differential requirement for NalP and MspA expression in MenB and MenY strains and is a step towards understanding the contributions of phase-variable loci to meningococcal biology.

Distribution and diversity of the haemoglobin-haptoglobin iron-acquisition systems in pathogenic and non-pathogenic Neisseria

A new generation of vaccines containing multiple protein components that aim to provide broad protection against serogroup B meningococci has been developed. One candidate, 4CMenB (4 Component MenB), has been approved by the European Medicines Agency, but is predicted to provide at most 70–80 % strain coverage; hence there is a need for second-generation vaccines that achieve higher levels of coverage. Prior knowledge of the diversity of potential protein vaccine components is a key step in vaccine design. A number of iron import systems have been targeted in meningococcal vaccine development, including the HmbR and HpuAB outer-membrane proteins, which mediate the utilization of haemoglobin or haemoglobin–haptoglobin complexes as iron sources. While the genetic diversity of HmbR has been described, little is known of the diversity of HpuAB. Using whole genome sequences deposited in a Bacterial Isolate Genome Sequence Database (BIGSDB), the prevalence and diversity of HpuAB among Neisseria were investigated. HpuAB was widely present in a range of Neisseria species whereas HmbR was mainly limited to the pathogenic species Neisseria meningitidis and Neisseria gonorrhoeae. Patterns of sequence variation in sequences from HpuAB proteins were suggestive of recombination and diversifying selection consistent with strong immune selection. HpuAB was subject to repeat-mediated phase variation in pathogenic Neisseria and the closely related non-pathogenic Neisseria species Neisseria lactamica and Neisseria polysaccharea but not in the majority of other commensal Neisseria species. These findings are consistent with HpuAB being subject to frequent genetic transfer potentially limiting the efficacy of this receptor as a vaccine candidate.

Comprehensive antigen screening identifies Moraxella catarrhalis proteins that induce protection in a mouse pulmonary clearance model

Moraxella catarrhalis is one of the three most common causative bacterial pathogens of otitis media, however no effective vaccine against M. catarrhalis has been developed so far. To identify M. catarrhalis vaccine candidate antigens, we used carefully selected sera from children with otitis media and healthy individuals to screen small-fragment genomic libraries that are expressed to display frame-selected peptides on a bacterial cell surface. This ANTIGENome technology led to the identification of 214 antigens, 23 of which were selected by in vitro or in vivo studies for additional characterization. Eight of the 23 candidates were tested in a Moraxella mouse pulmonary clearance model, and 3 of these antigens induced significantly faster bacterial clearance compared to adjuvant or to the previously characterized antigen OmpCD. The most significant protection data were obtained with the antigen MCR_1416 (Msp22), which was further investigated for its biological function by in vitro studies suggesting that Msp22 is a heme binding protein. This study comprises one of the most exhaustive studies to identify potential vaccine candidate antigens against the bacterial pathogen M. catarrhalis.

Analysis of the regulated transcriptome of Neisseria meningitidis in human blood using a tiling array

Neisseria meningitidis is the major cause of septicemia and meningococcal meningitis. During the course of infection, the bacterium must adapt to different host environments as a crucial factor for survival and dissemination; in particular, one of the crucial factors in N. meningitidis pathogenesis is the ability to grow and survive in human blood. We recently showed that N. meningitidis alters the expression of 30% of the open reading frames (ORFs) of the genome during incubation in human whole blood and suggested the presence of fine regulation at the gene expression level in order to control this step of pathogenesis. In this work, we used a customized tiling oligonucleotide microarray to define the changes in the whole transcriptional profile of N. meningitidis in a time course experiment of ex vivo bacteremia by incubating bacteria in human whole blood and then recovering RNA at different time points. The application of a newly developed bioinformatic tool to the tiling array data set allowed the identification of new transcripts--small intergenic RNAs, cis-encoded antisense RNAs, mRNAs with extended 5' and 3' untranslated regions (UTRs), and operons--differentially expressed in human blood. Here, we report a panel of expressed small RNAs, some of which can potentially regulate genes involved in bacterial metabolism, and we show, for the first time in N. meningitidis, extensive antisense transcription activity. This analysis suggests the presence of a circuit of regulatory RNA elements used by N. meningitidis to adapt to proliferate in human blood that is worthy of further investigation.

Evolutionary and genomic insights into meningococcal biology

Epidemic disease caused by Neisseria meningitidis, the meningococcus, has been recognized for two centuries, but remains incompletely controlled and understood. There have been dramatic reductions in serogroup A and C meningococcal disease following the introduction of protein-polysaccharide conjugate vaccines, but there is currently no comprehensive vaccine against serogroup B meningococci. Genetic analyses of meningococcal populations have provided many insights into the biology, evolution and pathogenesis of this important pathogen. The meningococcus, and its close relative the gonococcus, are the only pathogenic members of the genus Neisseria, and the invasive propensity of meningococci varies widely, with approximately a dozen 'hyperinvasive lineages' responsible for most disease. Despite this, attempts to identify a 'pathogenome', a subset of genes associated with the invasive phenotypes, have failed; however, genome-wide studies of representative meningococcal isolates using high-throughput sequencing are beginning to provide details on the relationship of invasive phenotype and genotype in this fascinating organism and how this relationship has evolved.