The phase-variable allele of the pilus glycosylation gene pglA is not strongly associated with strains of Neisseria gonorrhoeae isolated from patients with disseminated gonococcal infection

Sweet complexity: O-linked protein glycosylation in pathogenic Neisseria

The genus Neisseria, which colonizes mucosal surfaces, includes both commensal and pathogenic species that are exclusive to humans. The two pathogenic Neisseria species are closely related but cause quite different diseases, meningococcal sepsis and meningitis (Neisseria meningitidis) and sexually transmitted gonorrhea (Neisseria gonorrhoeae). Although obvious differences in bacterial niches and mechanisms for transmission exists, pathogenic Neisseria have high levels of conservation at the levels of nucleotide sequences, gene content and synteny. Species of Neisseria express broad-spectrum O-linked protein glycosylation where the glycoproteins are largely transmembrane proteins or lipoproteins localized on the cell surface or in the periplasm. There are diverse functions among the identified glycoproteins, for example type IV biogenesis proteins, proteins involved in antimicrobial resistance, as well as surface proteins that have been suggested as vaccine candidates. The most abundant glycoprotein, PilE, is the major subunit of pili which are an important colonization factor. The glycans attached can vary extensively due to phase variation of protein glycosylation (pgl) genes and polymorphic pgl gene content. The exact roles of glycosylation in Neisseria remains to be determined, but increasing evidence suggests that glycan variability can be a strategy to evade the human immune system. In addition, pathogenic and commensal Neisseria appear to have significant glycosylation differences. Here, the current knowledge and implications of protein glycosylation genes, glycan diversity, glycoproteins and immunogenicity in pathogenic Neisseria are summarized and discussed.

Molecular characterisation of Neisseria gonorrhoeae associated with disseminated gonococcal infections in Queensland, Australia: a retrospective surveillance study

OBJECTIVES

Gonorrhoea caused by Neisseria gonorrhoeae is the second most notified sexually transmitted infection (STI) in Australia and the case numbers for this STI have been increasing globally. Progressive gonococcal infection may lead to disseminated gonococcal infection (DGI), which causes significant morbidity among patients. This study aims to examine the genetic diversity of N. gonorrhoeae isolates collected in Queensland from January 2010 to August 2015 and to determine factors associated with DGI in Queensland.

DESIGN

Retrospective surveillance study for epidemiological purposes.

SETTING

All gonorrhoeae isolates referred by private and public pathology laboratories to the state of Queensland, Australia Neisseria reference laboratory.

METHODS

Between January 2010 and August 2015, 3953 N. gonorrhoeae isolates from both metropolitan and regional Queensland infections were typed with NG-MAST (N. gonorrhoeae multiantigen sequence typing) to assess the genetic diversity between strains. Whole-genome sequencing (WGS) was used to investigate strain-related factors associated with DGI.

RESULTS

ST6876 was the most common NG-MAST type, detected in 7.6% of the isolates. DGI was significantly more likely in females <30 years (OR 13.02, p<0.0001) and in older males >30 years (OR 6.04, p<0.0001), with most cases originating from North Queensland (OR 8.5, p<0.0001). Strains harbouring PIA class of porB type were associated with DGI (OR 33.23, p<0.0001).

CONCLUSION

Genotyping techniques, such as NG-MAST and WGS, are proving instrumental in providing an insight into the population structure of N. gonorrhoeae, and genetic mechanisms of pathogenesis, such as for DGI.

Complete Genome Sequences of Seven Neisseria gonorrhoeae Clinical Isolates from Mucosal and Disseminated Gonococcal Infections

Neisseria gonorrhoeae is a Gram-negative bacterium that causes the sexually transmitted infection gonorrhea. N. gonorrhoeae has progressively developed resistance to all currently prescribed antibiotics, and no vaccine is available. Here, we report the closed, completed, annotated genome sequences for seven N. gonorrhoeae strains obtained by single-molecule real-time (SMRT) long-read genome sequencing.

The Lst Sialyltransferase of Neisseria gonorrhoeae Can Transfer Keto-Deoxyoctanoate as the Terminal Sugar of Lipooligosaccharide: a Glyco-Achilles Heel That Provides a New Strategy for Vaccines to Prevent Gonorrhea

The lipooligosaccharide (LOS) of Neisseria gonorrhoeae plays key roles in pathogenesis and is composed of multiple possible glycoforms. These glycoforms are generated by the process of phase variation and by differences in the glycosyltransferase gene content of particular strains. LOS glycoforms of N. gonorrhoeae can be terminated with an N-acetylneuraminic acid (Neu5Ac), which imparts resistance to the bactericidal activity of serum. However, N. gonorrhoeae cannot synthesize the CMP-Neu5Ac required for LOS biosynthesis and must acquire it from the host. In contrast, Neisseria meningitidis can synthesize endogenous CMP-Neu5Ac, the donor molecule for Neu5Ac, which is a component of some meningococcal capsule structures. Both species have an almost identical LOS sialyltransferase, Lst, that transfers Neu5Ac from CMP-Neu5Ac to the terminus of LOS. Lst is homologous to the LsgB sialyltransferase of nontypeable Haemophilus influenzae (NTHi). Studies in NTHi have demonstrated that LsgB can transfer keto-deoxyoctanoate (KDO) from CMP-KDO to the terminus of LOS in place of Neu5Ac. Here, we show that Lst can also transfer KDO to LOS in place of Neu5Ac in both N. gonorrhoeae and N. meningitidis Consistent with access to the pool of CMP-KDO in the cytoplasm, we present data indicating that Lst is localized in the cytoplasm. Lst has previously been reported to be localized on the outer membrane. We also demonstrate that KDO is expressed as a terminal LOS structure in vivo in samples from infected women and further show that the anti-KDO monoclonal antibody 6E4 can mediate opsonophagocytic killing of N. gonorrhoeae Taken together, these studies indicate that KDO expressed on gonococcal LOS represents a new antigen for the development of vaccines against gonorrhea.IMPORTANCE The emergence of multidrug-resistant N. gonorrhoeae strains that are resistant to available antimicrobials is a current health emergency, and no vaccine is available to prevent gonococcal infection. Lipooligosaccharide (LOS) is one of the major virulence factors of N. gonorrhoeae The sialic acid N-acetylneuraminic acid (Neu5Ac) is present as the terminal glycan on LOS in N. gonorrhoeae In this study, we made an unexpected discovery that KDO can be incorporated as the terminal glycan on LOS of N. gonorrhoeae by the alpha-2,3-sialyltransferase Lst. We showed that N. gonorrhoeae express KDO on LOS in vivo and that the KDO-specific monoclonal antibody 6E4 can direct opsonophagocytic killing of N. gonorrhoeae These data support further development of KDO-LOS structures as vaccine antigens for the prevention of infection by N. gonorrhoeae.

The Neisseria gonorrhoeae Methionine Sulfoxide Reductase (MsrA/B) Is a Surface Exposed, Immunogenic, Vaccine Candidate

Control of the sexually transmitted infection gonorrhea is a major public health challenge, due to the recent emergence of multidrug resistant strains of Neisseria gonorrhoeae, and there is an urgent need for novel therapies or a vaccine to prevent gonococcal disease. In this study, we evaluated the methionine sulfoxide reductase (MsrA/B) of N. gonorrhoeae as a potential vaccine candidate, in terms of its expression, sequence conservation, localization, immunogenicity, and the functional activity of antibodies raised to it. Gonococcal MsrA/B has previously been shown to reduce methionine sulfoxide [Met(O)] to methionine (Met) in oxidized proteins and protect against oxidative stress. Here we have shown that the gene encoding MsrA/B is present, highly conserved, and expressed in all N. gonorrhoeae strains investigated, and we determined that MsrA/B is surface is exposed on N. gonorrhoeae. Recombinant MsrA/B is immunogenic, and mice immunized with MsrA/B and either aluminum hydroxide gel adjuvant or Freund's adjuvant generated a humoral immune response, with predominantly IgG1 antibodies. Higher titers of IgG2a, IgG2b, and IgG3 were detected in mice immunized with MsrA/B-Freund's adjuvant compared to MsrA/B-aluminum hydroxide adjuvant, while IgM titers were similar for both adjuvants. Antibodies generated by MsrA/B-Freund's in mice mediated bacterial killing via both serum bactericidal activity and opsonophagocytic activity. Anti-MsrA/B was also able to functionally block the activity of MsrA/B by inhibiting binding to its substrate, Met(O). We propose that recombinant MsrA/B is a promising vaccine antigen for N. gonorrhoeae.

A Novel Sialylation Site on Neisseria gonorrhoeae Lipooligosaccharide Links Heptose II Lactose Expression with Pathogenicity

Sialylation of lacto-N-neotetraose (LNnT) extending from heptose I (HepI) of gonococcal lipooligosaccharide (LOS) contributes to pathogenesis. Previously, gonococcal LOS sialyltransterase (Lst) was shown to sialylate LOS in Triton X-100 extracts of strain 15253, which expresses lactose from both HepI and HepII, the minimal structure required for monoclonal antibody (MAb) 2C7 binding. Ongoing work has shown that growth of 15253 in cytidine monophospho-N-acetylneuraminic acid (CMP-Neu5Ac)-containing medium enables binding to CD33/Siglec-3, a cell surface receptor that binds sialic acid, suggesting that lactose termini on LOSs of intact gonococci can be sialylated. Neu5Ac was detected on LOSs of strains 15253 and an MS11 mutant with lactose only from HepI and HepII by mass spectrometry; deleting HepII lactose rendered Neu5Ac undetectable. Resistance of HepII lactose Neu5Ac to desialylation by α2-3-specific neuraminidase suggested an α2-6 linkage. Although not associated with increased factor H binding, HepII lactose sialylation inhibited complement C3 deposition on gonococci. Strain 15253 mutants that lacked Lst or HepII lactose were significantly attenuated in mice, confirming the importance of HepII Neu5Ac in virulence. All 75 minimally passaged clinical isolates from Nanjing, China, expressed HepII lactose, evidenced by reactivity with MAb 2C7; MAb 2C7 was bactericidal against the first 62 (of 75) isolates that had been collected sequentially and were sialylated before testing. MAb 2C7 effectively attenuated 15253 vaginal colonization in mice. In conclusion, this novel sialylation site could explain the ubiquity of gonococcal HepII lactose in vivo Our findings reinforce the candidacy of the 2C7 epitope as a vaccine antigen and MAb 2C7 as an immunotherapeutic antibody.

Development of a novel engineered antibody targeting Neisseria species

OBJECTIVES

A Neissaria bacterial pilus sugar, bacillosamine, was synthesized and, for the first time, used as a probe to screen a single-chain variable fragment (scFv).

RESULTS

Four Neisseria, Neisseria gonorrhoeae, Neisseria meningitidis, Neisseria sicca and Neisseria subflava, and two negative controls, Streptococcus pneumoniae and Escherichia coli, were tested through ELISA, immunostaining and gold nanoparticle immunological assay. All results indicated that the selected scFv is feasible for the specific detection of Neisseria species via the recognition of bacillosamine.

CONCLUSIONS

The recombinant scFv could detect Neisseria strains at 10⁶ CFU/ml.

De novo synthesis of D- and L-fucosamine containing disaccharides

The availability of rare monosaccharides that cannot be isolated from natural sources is currently limiting the access to the synthesis and the biological evaluation of complex bacterial cell-surface glycans. Here, we report the synthesis of D- and L-fucosamine building blocks by a de novo approach from L- and D-Garner aldehydes. These differentially protected monosaccharide building blocks were utilized to prepare disaccharides present on the surface of Pseudomonas aeruginosa bacteria.

Genetic and molecular analyses reveal an evolutionary trajectory for glycan synthesis in a bacterial protein glycosylation system

Although protein glycosylation systems are becoming widely recognized in bacteria, little is known about the mechanisms and evolutionary forces shaping glycan composition. Species within the genus Neisseria display remarkable glycoform variability associated with their O-linked protein glycosylation (pgl) systems and provide a well developed model system to study these phenomena. By examining the potential influence of two ORFs linked to the core pgl gene locus, we discovered that one of these, previously designated as pglH, encodes a glucosyltransferase that generates unique disaccharide products by using polyprenyl diphosphate-linked monosaccharide substrates. By defining the function of PglH in the glycosylation pathway, we identified a metabolic conflict related to competition for a shared substrate between the opposing glycosyltransferases PglA and PglH. Accordingly, we propose that the presence of a stereotypic, conserved deletion mutation inactivating pglH in strains of Neisseria gonorrhoeae, Neisseria meningitidis, and related commensals, reflects a resolution of this conflict with the consequence of reduced glycan diversity. This model of genetic détente is supported by the characterization of pglH "missense" alleles encoding proteins devoid of activity or reduced in activity such that they cannot exert their effect in the presence of PglA. Thus, glucose-containing glycans appear to be a trait undergoing regression at the genus level. Together, these findings document a role for intrinsic genetic interactions in shaping glycan evolution in protein glycosylation systems.

Biochemical characterization of the O-linked glycosylation pathway in Neisseria gonorrhoeae responsible for biosynthesis of protein glycans containing N,N'-diacetylbacillosamine

The O-linked protein glycosylation pathway in Neisseria gonorrhoeae is responsible for the synthesis of a complex oligosaccharide on undecaprenyl diphosphate and subsequent en bloc transfer of the glycan to serine residues of select periplasmic proteins. Protein glycosylation (pgl) genes have been annotated on the basis of bioinformatics and top-down mass spectrometry analysis of protein modifications in pgl-null strains [Aas, F. E., et al. (2007) Mol. Microbiol. 65, 607-624; Vik, A., et al. (2009) Proc. Natl. Acad. Sci. U.S.A. 106, 4447-4452], but relatively little biochemical analysis has been performed to date. In this report, we present the expression, purification, and functional characterization of seven Pgl enzymes. Specifically, the enzymes studied are responsible for synthesis of an uncommon uridine diphosphate (UDP)-sugar (PglD, PglC, and PglB-acetyltransferase domain), glycan assembly (PglB-phospho-glycosyltransferase domain, PglA, PglE, and PglH), and final oligosaccharide transfer (PglO). UDP-2,4-diacetamido-2,4,6-trideoxy-α-d-hexose (DATDH), which is the first sugar in glycan biosynthesis, was produced enzymatically, and the stereochemistry was assigned as uridine diphosphate N'-diacetylbacillosamine (UDP-diNAcBac) by nuclear magnetic resonance characterization. In addition, the substrate specificities of the phospho-glycosyltransferase, glycosyltransferases, and oligosaccharyltransferase (OTase) were analyzed in vitro, and in most cases, these enzymes exhibited strong preferences for the native substrates relative to closely related glycans. In particular, PglO, the O-linked OTase, and PglB(Cj), the N-linked OTase from Campylobacter jejuni, preferred the native N. gonorrhoeae and C. jejuni substrates, respectively. This study represents the first comprehensive biochemical characterization of this important O-linked glycosylation pathway and provides the basis for further investigations of these enzymes as antibacterial targets.

Neisseria gonorrhoeae pilin glycan contributes to CR3 activation during challenge of primary cervical epithelial cells

Expression of type IV pili by Neisseria gonorrhoeae plays a critical role in mediating adherence to human epithelial cells. Gonococcal pilin is modified with an O-linked glycan, which may be present as a di- or monosaccharide because of phase variation of select pilin glycosylation genes. It is accepted that bacterial proteins may be glycosylated; less clear is how the protein glycan may mediate virulence. Using primary, human, cervical epithelial (i.e. pex) cells, we now provide evidence to indicate that the pilin glycan mediates productive cervical infection. In this regard, pilin glycan-deficient mutant gonococci exhibited an early hyper-adhesive phenotype but were attenuated in their ability to invade pex cells. Our data further indicate that the pilin glycan was required for gonococci to bind to the I-domain region of complement receptor 3, which is naturally expressed by pex cells. Comparative, quantitative, infection assays revealed that mutant gonococci lacking the pilin glycan did not bind to the I-domain when it is in a closed, low-affinity conformation and cannot induce an active conformation to complement receptor 3 during pex cell challenge. To our knowledge, these are the first data to directly demonstrate how a protein-associated bacterial glycan may contribute to pathogenesis.

Analogies and homologies in lipopolysaccharide and glycoprotein biosynthesis in bacteria

Bacteria generate and attach countless glycan structures to diverse macromolecules. Despite this diversity, the mechanisms of glycoconjugate biosynthesis are often surprisingly similar. The focus of this review is on the commonalities between lipopolysaccharide (LPS) and glycoprotein assembly pathways and their evolutionary relationship. Three steps that are essential for both pathways are completed by membrane proteins. These include the initiation of glycan assembly through the attachment of a first sugar residue onto the lipid carrier undecaprenyl pyrophosphate, the translocation across the plasma membrane and the final transfer onto proteins or lipid A-core. Two families of initiating enzymes have been described: the polyprenyl-P N-acetylhexosamine-1-P transferases and the polyprenyl-P hexosamine-1-P transferases, represented by Escherichia coli WecA and Salmonella enterica WbaP, respectively. Translocases are either Wzx-like flippases or adenosine triphosphate (ATP)-binding cassette transporters (ABC transporters). The latter can consist either of two polypeptides, Wzt and Wzm, or of a single polypeptide homolog to the Campylobacter jejuni PglK. Finally, there are two families of conjugating enzymes, the N-oligosaccharyltransferases (N-OTase), best represented by C. jejuni PglB, and the O-OTases, including Neisseria meningitidis PglL and the O antigen ligases involved in LPS biosynthesis. With the exception of the N-OTases, probably restricted to glycoprotein synthesis, members of all these transmembrane protein families can be involved in the synthesis of both glycoproteins and LPS. Because many translocation and conjugation enzymes display relaxed substrate specificity, these bacterial enzymes could be exploited in engineered living bacteria for customized glycoconjugate production, generating potential vaccines and therapeutics.

Genetic, structural, and antigenic analyses of glycan diversity in the O-linked protein glycosylation systems of human Neisseria species

Bacterial capsular polysaccharides and lipopolysaccharides are well-established ligands of innate and adaptive immune effectors and often exhibit structural and antigenic variability. Although many surface-localized glycoproteins have been identified in bacterial pathogens and symbionts, it not clear if and how selection impacts associated glycoform structure. Here, a systematic approach was devised to correlate gene repertoire with protein-associated glycoform structure in Neisseria species important to human health and disease. By manipulating the protein glycosylation (pgl) gene content and assessing the glycan structure by mass spectrometry and reactivity with monoclonal antibodies, it was established that protein-associated glycans are antigenically variable and that at least nine distinct glycoforms can be expressed in vitro. These studies also revealed that in addition to Neisseria gonorrhoeae strain N400, one other gonococcal strain and isolates of Neisseria meningitidis and Neisseria lactamica exhibit broad-spectrum O-linked protein glycosylation. Although a strong correlation between pgl gene content, glycoform expression, and serological profile was observed, there were significant exceptions, particularly with regard to levels of microheterogeneity. This work provides a technological platform for molecular serotyping of neisserial protein glycans and for elucidating pgl gene evolution.

Phasevarions mediate random switching of gene expression in pathogenic Neisseria

Many host-adapted bacterial pathogens contain DNA methyltransferases (mod genes) that are subject to phase-variable expression (high-frequency reversible ON/OFF switching of gene expression). In Haemophilus influenzae, the random switching of the modA gene controls expression of a phase-variable regulon of genes (a “phasevarion”), via differential methylation of the genome in the modA ON and OFF states. Phase-variable mod genes are also present in Neisseria meningitidis and Neisseria gonorrhoeae, suggesting that phasevarions may occur in these important human pathogens. Phylogenetic studies on phase-variable mod genes associated with type III restriction modification (R-M) systems revealed that these organisms have two distinct mod genes—modA and modB. There are also distinct alleles of modA (abundant: modA11, 12, 13; minor: modA4, 15, 18) and modB (modB1, 2). These alleles differ only in their DNA recognition domain. ModA11 was only found in N. meningitidis and modA13 only in N. gonorrhoeae. The recognition site for the modA13 methyltransferase in N. gonorrhoeae strain FA1090 was identified as 5′-AGAAA-3′. Mutant strains lacking the modA11, 12 or 13 genes were made in N. meningitidis and N. gonorrhoeae and their phenotype analyzed in comparison to a corresponding mod ON wild-type strain. Microarray analysis revealed that in all three modA alleles multiple genes were either upregulated or downregulated, some of which were virulence-associated. For example, in N. meningitidis MC58 (modA11), differentially expressed genes included those encoding the candidate vaccine antigens lactoferrin binding proteins A and B. Functional studies using N. gonorrhoeae FA1090 and the clinical isolate O1G1370 confirmed that modA13 ON and OFF strains have distinct phenotypes in antimicrobial resistance, in a primary human cervical epithelial cell model of infection, and in biofilm formation. This study, in conjunction with our previous work in H. influenzae, indicates that phasevarions may be a common strategy used by host-adapted bacterial pathogens to randomly switch between “differentiated” cell types.

The pathogenic Neisseria are bacterial pathogens that cause meningitis and gonorrhoea. They have adapted to life exclusively in humans and have developed unique strategies to colonize the host and to evade the immune response. Central among these strategies are genetic switches that randomly turn genes on and off. In most cases, the genes controlled by these switches, contingency genes, are required for making bacterial surface structures. Recently we described a new class of contingency gene that methylates DNA. Rather than affecting the synthesis of a single surface structure, on/off switching of this DNA-methyltransferase gene leads to random switching of multiple genes. In this study, we have shown that this mechanism exists in all pathogenic Neisseria, and alters expression of multiple genes in all cases we examined. The two distinct populations of bacteria generated by this process had different behavior in model systems of colonization and infection. Understanding this process is key to understanding these human pathogens, and to developing strategies for treatment and prevention of the diseases they cause.

Diversity of penA alterations and subtypes in Neisseria gonorrhoeae strains from Sydney, Australia, that are less susceptible to ceftriaxone

Increasing numbers of Neisseria gonorrhoeae strains with decreased susceptibilities to ceftriaxone and other oral cephalosporins widely used for the treatment of gonorrhea have been isolated in Sydney, Australia, over several years. In this study, we examined the complete penicillin-binding protein 2 (PBP 2) amino acid sequences of 109 gonococci, selected on the basis of their diverse temporal and geographic origins and because they exhibited a range of ceftriaxone MICs: < OR =0.03 microg/ml (n = 59), 0.06 microg/ml (n = 43), and 0.125 microg/ml (n = 7). Auxotyping, serotyping, and genotyping by N. gonorrhoeae multiantigen sequence typing sequence-based analysis was also performed. In total, 20 different amino acid sequence patterns were identified, indicating considerable variation in the PBP 2 sequences in this study sample. Only some of the N. gonorrhoeae isolates with significantly higher ceftriaxone MICs contained a mosaic PBP 2 pattern, while more isolates exhibited a nonmosaic PBP 2 pattern containing an A501V substitution. Although particular N. gonorrhoeae genotypes in our sample were shown to be less susceptible to ceftriaxone, the reduced susceptibility to ceftriaxone was not specific to any particular genotype and was observed in a broad range of auxotypes, serotypes, and genotypes. Overall, the results of our study show that N. gonorrhoeae strains exhibiting reduced sensitivity to ceftriaxone are not of a particular subtype and that a number of different mutations in PBP 2 may contribute to this phenomenon.