Capsule switching of Neisseria meningitidis sequence type 7 serogroup A to serogroup X

Microevolution and Its Impact on Hypervirulence, Antimicrobial Resistance, and Vaccine Escape in Neisseria meningitidis

Neisseria meningitidis is commensal of the human pharynx and occasionally invades the host, causing the life-threatening illness invasive meningococcal disease. The meningococcus is a highly diverse and adaptable organism thanks to natural competence, a propensity for recombination, and a highly repetitive genome. These mechanisms together result in a high level of antigenic variation to invade diverse human hosts and evade their innate and adaptive immune responses. This review explores the ways in which this diversity contributes to the evolutionary history and population structure of the meningococcus, with a particular focus on microevolution. It examines studies on meningococcal microevolution in the context of within-host evolution and persistent carriage; microevolution in the context of meningococcal outbreaks and epidemics; and the potential of microevolution to contribute to antimicrobial resistance and vaccine escape. A persistent theme is the idea that the process of microevolution contributes to the development of new hyperinvasive meningococcal variants. As such, microevolution in this species has significant potential to drive future public health threats in the form of hypervirulent, antibiotic-resistant, vaccine-escape variants. The implications of this on current vaccination strategies are explored.

The Epidemiology of Meningococcal Disease and Carriage, Genotypic Characteristics and Antibiotic Resistance of Neisseria meningitidis Isolates in Zhejiang Province, China, 2011–2021

Neisseria meningitidis (Nm) remains a worldwide leading cause of epidemic meningitis. During 2011–July 2021, 55 meningococcal disease (MD) cases were reported with a case fatality rate of 5.45% in Zhejiang Province, China. The median age was 7 years. The annual incidence was 0.0017–0.0183 per 100,000 population. The highest age-specific incidence was observed in the group younger than 1 year. Serogroup was identified in 30 laboratory-confirmed MD cases, and MenB was most predominant. MenB was mainly observed in two age groups: younger than 5 and older than 35 years. MenB incidence was significantly increasing from 0.0018 per 100,000 in 2013 to 0.0070 per 100,000 in 2019. During 2015–2020, 17 positive samples were detected from 2,827 throat swabs from healthy population, of which 70.59% was MenB. Twenty multilocus sequence typing sequence types (STs) containing eight newly assigned STs (ST15881–ST15888) were determined in all Nm isolates. Either in MD cases or in healthy population, MenB CC ST-4821 was the predominant ST. It was worth noting that two MenY CC ST-23 cases occurred in 2019 and 2021, respectively. MenY CC ST-23 MD cases increased gradually in China. Phylogeny results based on genome sequencing indicated that Chinese MenW CC ST-11 isolates were genetically linked and grouped together with Japanese isolates, separated from MenW CC ST-11 isolates from Saudi Arabia Hajj outbreak, Europe, South Africa, South America, North America, and Oceania. MenW CC ST-11 isolates from East Asia might have evolved locally. Antibiotic susceptibility tests revealed a relatively high resistance rate (22.86%) of Nm isolates to penicillin. This study provided valuable data for Chinese public health authorities to grasp the temporal epidemiological characteristics of MD and healthy carriage.

Immunobiology of Carbohydrates: Implications for Novel Vaccine and Adjuvant Design Against Infectious Diseases

Carbohydrates are ubiquitous molecules expressed on the surface of nearly all living cells, and their interaction with carbohydrate-binding proteins is critical to many immunobiological processes. Carbohydrates are utilized as antigens in many licensed vaccines against bacterial pathogens. More recently, they have also been considered as adjuvants. Interestingly, unlike other types of vaccines, adjuvants have improved immune response to carbohydrate-based vaccine in humans only in a few cases. Furthermore, despite the discovery of many new adjuvants in the last years, aluminum salts, when needed, remain the only authorized adjuvant for carbohydrate-based vaccines. In this review, we highlight historical and recent advances on the use of glycans either as vaccine antigens or adjuvants, and we review the use of currently available adjuvants to improve the efficacy of carbohydrate-based vaccines. A better understanding of the mechanism of carbohydrate interaction with innate and adaptive immune cells will benefit the design of a new generation of glycan-based vaccines and of immunomodulators to fight both longstanding and emerging diseases.

Elucidating the Structural and Minimal Protective Epitope of the Serogroup X Meningococcal Capsular Polysaccharide

Despite the considerable progress toward the eradication of meningococcal disease with the introduction of glycoconjugate vaccines, previously unremarkable serogroup X has emerged in recent years, recording several outbreaks throughout the African continent. Different serogroup X polysaccharide-based vaccines have been tested in preclinical trials, establishing the principles for further improvement. To elucidate the antigenic determinants of the MenX capsular polysaccharide, we generated a monoclonal antibody, and its bactericidal nature was confirmed using the rabbit serum bactericidal assay. The antibody was tested by the inhibition enzyme-linked immunosorbent assay and surface plasmon resonance against a set of oligosaccharide fragments of different lengths. The epitope was shown to be contained within five to six α-(1–4) phosphodiester mannosamine repeating units. The molecular interactions between the protective monoclonal antibody and the MenX capsular polysaccharide fragment were further detailed at the atomic level by saturation transfer difference nuclear magnetic resonance (NMR) spectroscopy. The NMR results were used for validation of the in silico docking analysis between the X-ray crystal structure of the antibody (Fab fragment) and the modeled hexamer oligosaccharide. The antibody recognizes the MenX fragment by binding all six repeating units of the oligosaccharide via hydrogen bonding, salt bridges, and hydrophobic interactions. In vivo studies demonstrated that conjugates containing five to six repeating units can produce high functional antibody levels. These results provide an insight into the molecular basis of MenX vaccine-induced protection and highlight the requirements for the epitope-based vaccine design.

Nasopharyngeal Meningococcal Carriage among Children and Adolescents in Turkey in 2018: An Unexpected High Serogroup X Carriage

Meningococcal carriage studies and transmission modeling can predict IMD epidemiology and used to define invasive meningococcal disease (IMD) control strategies. In this multicenter study, we aimed to evaluate the prevalence of nasopharyngeal Neisseria meningitidis (Nm) carriage, serogroup distribution, and related risk factors in Turkey. Nasopharyngeal samples were collected from a total of 1267 children and adolescents and were tested with rt-PCR. Nm carriage was detected in 96 participants (7.5%, 95% CI 6.1–9.0), with the peak age at 13 years (12.5%). Regarding age groups, Nm carriage rate was 7% in the 0–5 age group, was 6.9%in the 6–10 age group, was 7.9% in the 11–14 age group, and was 9.3% in the 15–18 age group. There was no statistically significant difference between the groups (p > 0.05). The serogroup distribution was as follows: 25% MenX, 9.4% MenA, 9.4% MenB, 2.1% MenC, 3.1% MenW, 2.1% for MenY, and 48.9% for non-groupable. The Nm carriage rate was higher in children with previous upper respiratory tract infections and with a high number of household members, whereas it was lower in children with antibiotic use in the last month (p < 0.05 for all). In this study, MenX is the predominant carriage strain. The geographical distribution of Nm strains varies, but serogroup distribution in the same country might change in a matter of years. Adequate surveillance and/or a proper carriage study is paramount for accurate/dynamic serogroup distribution and the impact of the proposed vaccination.

Meningococcal disease surveillance in the Asia-Pacific region (2020): The global meningococcal initiative

The degree of surveillance data and control strategies for invasive meningococcal disease (IMD) varies across the Asia-Pacific region. IMD cases are often reported throughout the region, but the disease is not notifiable in some countries, including Myanmar, Bangladesh and Malaysia. Although there remains a paucity of data from many countries, specific nations have introduced additional surveillance measures. The incidence of IMD is low and similar across the represented countries (<0.2 cases per 100,000 persons per year), with the predominant serogroups of Neisseria meningitidis being B, W and Y, although serogroups A and X are present in some areas. Resistance to ciprofloxacin is also of concern, with the close monitoring of antibiotic-resistant clonal complexes (e.g., cc4821) being a priority. Meningococcal vaccination is only included in a few National Immunization Programs, but is recommended for high-risk groups, including travellers (such as pilgrims) and people with complement deficiencies or human immunodeficiency virus (HIV). Both polysaccharide and conjugate vaccines form part of recommendations. However, cost and misconceptions remain limiting factors in vaccine uptake, despite conjugate vaccines preventing the acquisition of carriage.

Neisseria gonorrhoeae 23S rRNA A2059G mutation is the only determinant necessary for high-level azithromycin resistance and improves in vivo biological fitness

Objectives

The global emergence of Neisseria gonorrhoeae isolates displaying high-level azithromycin resistance is a major concern for the currently recommended azithromycin/ceftriaxone dual therapy. N. gonorrhoeae high-level azithromycin resistance has been associated with an A2059G mutation in 23S rRNA. Here we investigated the specific contribution of this 23S rRNA A2059G mutation to high-level azithromycin resistance and its impact on biological fitness.

Methods

A2059G/G2059A alleles were specifically cloned into all four genomic copies of 23S rDNA of an azithromycin-susceptible isolate and a high-level azithromycin-resistant isolate. WT and mutant strains were subsequently investigated for azithromycin susceptibility using the agar dilution method. In addition, their biological fitness was studied by comparative liquid growth in the presence of hydrophobic and amphipathic compounds, by competition assays in a mouse vaginal tract infection model and by competition assays for invasion and intracellular survival.

Results

Azithromycin susceptibility analyses showed that the 23S rRNA A2059G mutation is the only genetic determinant required for N. gonorrhoeae to display the high-level azithromycin resistance phenotype. Further analysis of biological fitness showed that strains containing 2059G outcompeted isogenic strains containing 2059A for colonization in the mouse vaginal tract infection model and for invasion of HeLa cervical epithelial cells. Furthermore, the A2059G mutation enhanced growth in the presence of lithocholic acid or Triton X-100.

Conclusions

Our findings that the 23S rRNA A2059G mutation is sufficient for high-level azithromycin resistance and that this mutation generally enhanced the biological fitness of N. gonorrhoeae have important implications for the currently recommended treatment policies and antimicrobial stewardship programmes.

Glycoconjugate vaccines: current approaches towards faster vaccine design

Introduction: Over the last decades, glycoconjugate vaccines have been proven to be a successful strategy to prevent infectious diseases. Many diseases remain to be controlled, especially in developing countries, and emerging antibiotic-resistant bacteria present an alarming public-health threat. The increasing complexity of future vaccines, and the need to accelerate development processes have triggered the development of faster approaches to glycoconjugate vaccines design. Areas covered: This review provides an overview of recent progress in glycoconjugation technologies toward faster vaccine design. Expert opinion: Among the different emerging approaches, glycoengineering has the potential to combine glycan assembly and conjugation to carrier systems (such as proteins or outer membrane vesicles) in one step, resulting in a simplified manufacturing process and fewer analytical controls. Chemical and enzymatic strategies, and their automation can facilitate glycoepitope identification for vaccine design. Other approaches, such as the liposomal encapsulation of polysaccharides, potentially enable fast and easy combination of numerous antigens in the same formulation. Additional progress is envisaged in the near future, and some of these systems still need to be further validated in humans. In parallel, new strategies are needed to accelerate the vaccine development process, including the associated clinical trials, up to vaccine release onto the market.

Development of a dual-antimicrobial counterselection method for markerless genetic engineering of bacterial genomes

Markerless genetic engineering of bacterial genomes is commonly performed by two-step homologous recombination methods using vectors carrying flanking regions of the target gene for site-specific vector integration and counterselection markers to provide positive selection pressure on the second recombination step resulting in vector excision. Here, we provide the proof-of-principle of a novel counterselection method that exploits antagonistic activities between bactericidal and bacteriostatic antibiotics and which can provide selection pressure on the second recombination step by selective killing of bacteria retaining the antibiotic selection marker. This method was optimized for the bacterial pathogens Listeria monocytogenes and Neisseria meningitidis by screening for antagonistic activities between the bactericidal aminoglycosides kanamycin, streptomycin, and gentamicin in combination with the bacteriostatic antibiotics chloramphenicol and erythromycin. The largest difference in selective killing of both L. monocytogenes and N. meningitidis containing an antibiotic selection marker versus wild-type bacteria was observed for the combination of erythromycin, gentamicin, and ermC as antibiotic selection marker. Therefore, this combination was used to generate two markerless deletion mutants for both L. monocytogenes and N. meningitidis. After applying the dual-antimicrobial selection pressure on cultures during the second recombination step, surviving colonies were replica plated on agar with and without erythromycin. On average, 12-13% of the randomly selected bacterial colonies had lost the selection marker due to a second recombination event and approximately half of these colonies were the desired markerless in-frame deletion mutants. Therefore, this method proved to be easy and fast and should be applicable to a wide variety of bacterial species.

Recent Advances in the Synthesis of Glycoconjugates for Vaccine Development

During the last decade there has been a growing interest in glycoimmunology, a relatively new research field dealing with the specific interactions of carbohydrates with the immune system. Pathogens’ cell surfaces are covered by a thick layer of oligo- and polysaccharides that are crucial virulence factors, as they mediate receptors binding on host cells for initial adhesion and organism invasion. Since in most cases these saccharide structures are uniquely exposed on the pathogen surface, they represent attractive targets for vaccine design. Polysaccharides isolated from cell walls of microorganisms and chemically conjugated to immunogenic proteins have been used as antigens for vaccine development for a range of infectious diseases. However, several challenges are associated with carbohydrate antigens purified from natural sources, such as their difficult characterization and heterogeneous composition. Consequently, glycoconjugates with chemically well-defined structures, that are able to confer highly reproducible biological properties and a better safety profile, are at the forefront of vaccine development. Following on from our previous review on the subject, in the present account we specifically focus on the most recent advances in the synthesis and preliminary immunological evaluation of next generation glycoconjugate vaccines designed to target bacterial and fungal infections that have been reported in the literature since 2011.

Gonococcal MtrE and its surface-expressed Loop 2 are immunogenic and elicit bactericidal antibodies

OBJECTIVES

The rise in multidrug resistant Neisseria gonorrhoeae poses a threat to healthcare, while the development of an effective vaccine has remained elusive due to antigenic and phase variability of surface-expressed proteins. In the current study, we identified a fully conserved surface expressed protein and characterized its suitability as a vaccine antigen.

METHODS

An in silico approach was used to predict surface-expressed proteins and analyze sequence conservation and phase variability. The most conserved protein and its surface-exposed Loop 2, which was displayed as both a structural and linear epitope on the oligomerization domain of C4b binding protein, were used to immunize mice. Immunogenicity was subsequently analyzed by determination of antibody titers and serum bactericidal activity.

RESULTS

MtrE was identified as one of the most conserved surface-expressed proteins. Furthermore, MtrE and both Loop 2-containing fusion proteins elicited high protein-specific antibody titers and particularly the two Loop 2 fusion proteins showed high anti-Loop 2 titers. In addition, antibodies raised against all three proteins were able to recognize MtrE expressed on the surface of N. gonorrhoeae and showed high MtrE-dependent bactericidal activity.

CONCLUSIONS

Our results show that MtrE and Loop 2 are promising novel conserved surface-expressed antigens for vaccine development against N. gonorrhoeae.

Identification and Characterization of the Neisseria gonorrhoeae MscS-Like Mechanosensitive Channel

Mechanosensitive channels are ubiquitous in bacteria and provide an essential mechanism to survive sudden exposure to a hypo-osmotic environment by the sensing and release of increased turgor pressure. No mechanosensitive channels have thus far been identified and characterized for the human-specific bacterial pathogen Neisseria gonorrhoeae In this study, we identified and characterized the N. gonorrhoeae MscS-like mechanosensitive channel (Ng-MscS). Electrophysiological analyses by the patch clamp method showed that Ng-MscS is stretch activated and contains pressure-dependent gating properties. Further mutagenesis studies of critical residues forming the hydrophobic vapor lock showed that gain-of-function mutations in Ng-MscS inhibited bacterial growth. Subsequent analysis of the function of Ng-MscS in N. gonorrhoeae by osmotic down-shock assays revealed that the survival of Ng-mscS deletion mutants was significantly reduced compared with that of wild-type strains, while down-shock survival was restored upon the ectopic complementation of mscS Finally, to investigate whether Ng-MscS is important for N. gonorrhoeae during infections, competition assays were performed by using a murine vaginal tract infection model. Ng-mscS deletion mutants were outcompeted by N. gonorrhoeae wild-type strains for colonization and survival in this infection model, highlighting that Ng-MscS contributes to in vivo colonization and survival. Therefore, Ng-MscS might be a promising target for the future development of novel antimicrobials.

Combined Chemical Synthesis and Tailored Enzymatic Elongation Provide Fully Synthetic and Conjugation-Ready Neisseria meningitidis Serogroup X Vaccine Antigens

Studies on the polymerization mode of Neisseria meningitidis serogroup X capsular polymerase CsxA recently identified a truncated construct that can be immobilized and used for length controlled on-column production of oligosaccharides. Here, we combined the use of a synthetic acceptor bearing an appendix for carrier protein conjugation and the on-column process to a novel chemo-enzymatic strategy. After protein coupling of the size optimized oligosaccharide produced by the one-pot elongation procedure, we obtained a more homogeneous glycoconjugate compared to the one previously described starting from the natural polysaccharide. Mice immunized with the conjugated fully synthetic oligomer elicited functional antibodies comparable to controls immunized with the current benchmark MenX glycoconjugates prepared from the natural capsule polymer or from fragments of it enzymatically elongated. This pathogen-free technology allows the fast total in vitro construction of predefined bacterial polysaccharide fragments. Compared to conventional synthetic protocols, the procedure is more expeditious and drastically reduces the number of purification steps to achieve the oligomers. Furthermore, the presence of a linker for conjugation in the synthetic acceptor minimizes manipulations on the enzymatically produced glycan prior to protein conjugation. This approach enriches the methods for fast construction of complex bacterial carbohydrates.