Molecular Mimicry of Host Structures by Lipooligosaccharides of Neisseria Meningitidis: Characterization of Sialylated and Nonsialylated Lacto-N-Neotetraose (Galß1-4GlcNAcß1-3Galβ1-4Glc) Structures in Lipooligosaccharides Using Monoclonal Antibodies and Specific Lectins

Extraction and Electrophoretic Analysis of Bacterial Lipopolysaccharides and Outer Membrane Proteins

Lipopolysaccharides (LPS) (or lipooligosaccharides [LOS], which lack the O-antigen side chains characteristic of LPS), and outer membrane proteins (OMP) are major cell-surface molecules in the outer membrane (OM) of gram-negative bacteria. The LPS is responsible for causing endotoxic shock in infected hosts and, in conjunction with some OMPs, provides protection to the bacterium against host innate immune defenses and attachment to host cells. Electrophoretic analysis can provide valuable information regarding the size, number, and variability of LPS/LOS and OMP components between bacterial strains and mutants, which aids in understanding the basic biology and virulence factors of a particular species. Furthermore, highly purified extracts are normally not required if only electrophoretic analysis is to be done, and various methods have been established for such procedures. Here, we review ameliorated procedures for fast and convenient extraction of LPS/LOS and protein-enriched outer membranes (PEOM) for optimal electrophoretic resolution. Specifically, we will describe the phenol-based micro-method for LPS/LOS extraction, a differential extraction procedure with sodium lauryl sarcosinate for PEOM, and gel preparation for electrophoretic analysis of LPS/LOS samples in detail. Graphic abstract: Workflow for the preparation and analysis of LPS/LOS and PEOM.

More than a Pore: Nonlytic Antimicrobial Functions of Complement and Bacterial Strategies for Evasion

The complement system is an evolutionarily ancient defense mechanism against foreign substances. Consisting of three proteolytic activation pathways, complement converges on a common effector cascade terminating in the formation of a lytic pore on the target surface. The classical and lectin pathways are initiated by pattern recognition molecules binding to specific ligands, while the alternative pathway is constitutively active at low levels in circulation. Complement-mediated killing is essential for defense against many Gram-negative bacterial pathogens, and genetic deficiencies in complement can render individuals highly susceptible to infection, for example, invasive meningococcal disease. In contrast, Gram-positive bacteria are inherently resistant to the direct bactericidal activity of complement due to their thick layer of cell wall peptidoglycan. However, complement also serves diverse roles in immune defense against all bacteria by flagging them for opsonization and killing by professional phagocytes, synergizing with neutrophils, modulating inflammatory responses, regulating T cell development, and cross talk with coagulation cascades. In this review, we discuss newly appreciated roles for complement beyond direct membrane lysis, incorporate nonlytic roles of complement into immunological paradigms of host-pathogen interactions, and identify bacterial strategies for complement evasion.

Interactions of Bacteriophages and Bacteria at the Airway Mucosa: New Insights Into the Pathophysiology of Asthma

The airway epithelium is the primary site where inhaled and resident microbiota interacts between themselves and the host, potentially playing an important role on allergic asthma development and pathophysiology. With the advent of culture independent molecular techniques and high throughput technologies, the complex composition and diversity of bacterial communities of the airways has been well-documented and the notion of the lungs' sterility definitively rejected. Recent studies indicate that the microbial composition of the asthmatic airways across the spectrum of disease severity, differ significantly compared with healthy individuals. In parallel, a growing body of evidence suggests that bacterial viruses (bacteriophages or simply phages), regulating bacterial populations, are present in almost every niche of the human body and can also interact directly with the eukaryotic cells. The triptych of airway epithelial cells, bacterial symbionts and resident phages should be considered as a functional and interdependent unit with direct implications on the respiratory and overall homeostasis. While the role of epithelial cells in asthma pathophysiology is well-established, the tripartite interactions between epithelial cells, bacteria and phages should be scrutinized, both to better understand asthma as a system disorder and to explore potential interventions.

Interactions and Signal Transduction Pathways Involved during Central Nervous System Entry by Neisseria meningitidis across the Blood-Brain Barriers

The Gram-negative diplococcus Neisseria meningitidis, also called meningococcus, exclusively infects humans and can cause meningitis, a severe disease that can lead to the death of the afflicted individuals. To cause meningitis, the bacteria have to enter the central nervous system (CNS) by crossing one of the barriers protecting the CNS from entry by pathogens. These barriers are represented by the blood–brain barrier separating the blood from the brain parenchyma and the blood–cerebrospinal fluid (CSF) barriers at the choroid plexus and the meninges. During the course of meningococcal disease resulting in meningitis, the bacteria undergo several interactions with host cells, including the pharyngeal epithelium and the cells constituting the barriers between the blood and the CSF. These interactions are required to initiate signal transduction pathways that are involved during the crossing of the meningococci into the blood stream and CNS entry, as well as in the host cell response to infection. In this review we summarize the interactions and pathways involved in these processes, whose understanding could help to better understand the pathogenesis of meningococcal meningitis.

Predominant phosphorylation patterns in Neisseria meningitidis lipid A determined by top-down MS/MS

Among the virulence factors in Neisseria infections, a major inducer of inflammatory cytokines is the lipooligosaccharide (LOS). The activation of NF-κB via extracellular binding of LOS or lipopolysaccharide (LPS) to the toll-like receptor 4 and its coreceptor, MD-2, results in production of pro-inflammatory cytokines that initiate adaptive immune responses. LOS can also be absorbed by cells and activate intracellular inflammasomes, causing the release of inflammatory cytokines and pyroptosis. Studies of LOS and LPS have shown that their inflammatory potential is highly dependent on lipid A phosphorylation and acylation, but little is known on the location and pattern of these posttranslational modifications. Herein, we report on the localization of phosphoryl groups on phosphorylated meningococcal lipid A, which has two to three phosphate and zero to two phosphoethanolamine substituents. Intact LOS with symmetrical hexa-acylated and asymmetrical penta-acylated lipid A moieties was subjected to high-resolution ion mobility spectrometry MALDI-TOF MS. LOS molecular ions readily underwent in-source decay to give fragments of the oligosaccharide and lipid A formed by cleavage of the ketosidic linkage, which enabled performing MS/MS (pseudo-MS³). The resulting spectra revealed several patterns of phosphoryl substitution on lipid A, with certain species predominating. The extent of phosphoryl substitution, particularly phosphoethanolaminylation, on the 4'-hydroxyl was greater than that on the 1-hydroxyl. The heretofore unrecognized phosphorylation patterns of lipid A of meningococcal LOS that we detected are likely determinants of both pathogenicity and the ability of the bacteria to evade the innate immune system.

Fifteen-minute consultation: Symptoms and signs of meningococcal disease

Meningococcal disease remains a leading cause of meningitis, sepsis and death in children worldwide and in the UK. Successful vaccination programmes in the UK have, however, significantly reduced the burden of disease in children. Unfortunately, despite vaccination, a significant number of children are still diagnosed with invasive meningococcal disease each year.As the prevalence of meningococcal disease falls, it is important that we maintain awareness of the symptoms and signs of meningococcal disease because the prompt recognition of this life-threatening infection improves outcomes.In this article we discuss the pathology, epidemiology and recognition of invasive meningococcal disease in children. The aim is to maintain awareness of this rare but life-threatening infection.

Internalization of a polysialic acid-binding Escherichia coli bacteriophage into eukaryotic neuroblastoma cells

Eukaryotic organisms are continuously exposed to bacteriophages, which are efficient gene transfer agents in bacteria. However, bacteriophages are considered not to pass the eukaryotic cell membrane and enter nonphagocytic cells. Here we report the binding and penetration of Escherichia coli PK1A2 bacteriophage into live eukaryotic neuroblastoma cells in vitro. The phage interacts with cell surface polysialic acid, which shares structural similarity with the bacterial phage receptor. Using fluorescence and electron microscopy, we show that phages are internalized via the endolysosomal route and persist inside the human cells up to one day without affecting cell viability. Phage capsid integrity is lost in lysosomes, and the phage DNA is eventually degraded. We did not detect the entry of phage DNA into the nucleus; however, we speculate that this might occur as a rare event, and propose that this potential mechanism could explain prokaryote–eukaryote gene flow.

Eukaryotic organisms are continuously exposed to bacteriophages, but these are not thought to enter non-phagocytic cells. Here, Lehti et al. show that a bacteriophage can bind to a specific receptor on the surface of human neuroblastoma cells in vitro, and be internalized via the endolysosomal route.

Gonococcal lipooligosaccharide sialylation: virulence factor and target for novel immunotherapeutics

Gonorrhea has become resistant to most conventional antimicrobials used in clinical practice. The global spread of multidrug-resistant isolates of Neisseria gonorrhoeae could lead to an era of untreatable gonorrhea. New therapeutic modalities with novel mechanisms of action that do not lend themselves to the development of resistance are urgently needed. Gonococcal lipooligosaccharide (LOS) sialylation is critical for complement resistance and for establishing infection in humans and experimental mouse models. Here we describe two immunotherapeutic approaches that target LOS sialic acid: (i) a fusion protein that comprises the region in the complement inhibitor factor H (FH) that binds to sialylated gonococci and IgG Fc (FH/Fc fusion protein) and (ii) analogs of sialic acid that are incorporated into LOS but fail to protect the bacterium against killing. Both molecules showed efficacy in the mouse vaginal colonization model of gonorrhea and may represent promising immunotherapeutic approaches to target multidrug-resistant isolates. Disabling key gonococcal virulence mechanisms is an effective therapeutic strategy because the reduction of virulence is likely to be accompanied by a loss of fitness, rapid elimination by host immunity and consequently, decreased transmission.

Innate immune response to lipooligosaccharide: pivotal regulator of the pathobiology of invasive Neisseria meningitidis infections

Infections due to Neisseria meningitidis afflict more than one million people worldwide annually and cause death or disability in many survivors. The clinical course of invasive infections has been well studied, but our understanding of the cause of differences in patient outcomes has been limited because these are dependent on multiple factors including the response of the host, characteristics of the bacteria and interactions between the host and the bacteria. The meningococcus is a highly inflammatory organism, and the lipooligosaccharide (LOS) on the outer membrane is the most potent inflammatory molecule it expresses due to the interactions of the lipid A moiety of LOS with receptors of the innate immune system. We previously reported that increased phosphorylation of hexaacylated neisserial lipid A is correlated with greater inflammatory potential. Here we postulate that variability in lipid A phosphorylation can tip the balance of innate immune responses towards homeostatic tolerance or proinflammatory signaling that affects adaptive immune responses, causing disease with meningitis only, or septicemia with or without meningitis, respectively. Furthermore, we propose that studies of the relationship between bacterial virulence and gene expression should consider whether genetic variation could affect properties of biosynthetic enzymes resulting in LOS structural differences that alter disease pathobiology.

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.

Carbohydrate-mimetic peptides for pan anti-tumor responses

Molecular mimicry is fundamental to biology and transcends to many disciplines ranging from immune pathology to drug design. Structural characterization of molecular partners has provided insight into the origins and relative importance of complementarity in mimicry. Chemical complementarity is easy to understand; amino acid sequence similarity between peptides, for example, can lead to cross-reactivity triggering similar reactivity from their cognate receptors. However, conformational complementarity is difficult to decipher. Molecular mimicry of carbohydrates by peptides is often considered one of those. Extensive studies of innate and adaptive immune responses suggests the existence of carbohydrate mimicry, but the structural basis for this mimicry yields confounding details; peptides mimicking carbohydrates in some cases fail to exhibit both chemical and conformational mimicry. Deconvolution of these two types of complementarity in mimicry and its relationship to biological function can nevertheless lead to new therapeutics. Here, we discuss our experience examining the immunological aspects and implications of carbohydrate-peptide mimicry. Emphasis is placed on the rationale, the lessons learned from the methodologies to identify mimics, a perspective on the limitations of structural analysis, the biological consequences of mimicking tumor-associated carbohydrate antigens, and the notion of reverse engineering to develop carbohydrate-mimetic peptides in vaccine design strategies to induce responses to glycan antigens expressed on cancer cells.

Human milk oligosaccharides inhibit rotavirus infectivity in vitro and in acutely infected piglets

Human milk (HM) is rich in oligosaccharides (HMO) that exert prebiotic and anti-infective activities. HM feeding reduces the incidence of rotavirus (RV) infection in infants. Herein, the anti-RV activity of oligosaccharides was tested in an established in vitro system for assessing cellular binding and viral infectivity/replication, and also tested in a newly developed, acute RV infection, in situ piglet model. For the in vitro work, crude HMO isolated from pooled HM, neutral HMO (lacto-N-neotetraose, LNnT; 2'-fucosyllactose) and acidic HMO (aHMO, '-sialyllactose, 3'-SL; -sialyllactose, -SL) were tested against the porcine OSU strain and human RV Wa strain. The RV Wa strain was not inhibited by any oligosaccharides. However, the RV OSU strain infectivity was dose-dependently inhibited by sialic acid (SA)-containing HMO. 3'-SL and 6'-SL concordantly inhibited (125)I-radiolabelled RV cellular binding and infectivity/replication. For the in situ study, a midline laparotomy was performed on 21-d-old formula-fed piglets and six 10 cm loops of ileum were isolated in situ. Briefly, 2 mg/ml of LNnT, aHMO mixture (40% 6'-SL/10 % 3'-SL/50 % SA) or media with or without the RV OSU strain (1 x 10(7) focus-forming units)were injected into the loops and maintained for 6 h. The loops treated with HMO treatments þ RV had lower RV replication, as assessed by non-structural protein-4 (NSP4) mRNA expression, than RV-treated loops alone. In conclusion, SA-containing HMO inhibited RV infectivity in vitro; however, both neutral HMO and SA with aHMO decreased NSP4 replication during acute RV infection in situ.

Neisseria meningitidis serogroup B lipooligosaccharide genotyping reveals high prevalence of L2 strains in Spain and unexpected relationship with factor H-binding protein expression

Neisseria meningitidis may be classified according to the lipooligosaccharide immunotype. We show that this classification can be achieved by PCR genotyping of the genes involved in the lipooligosaccharide inner-core biosynthesis, lpt3, lpt6, lgtG and lot3. Genotyping data correlated well (90-100%) with mass spectrometry data and was, therefore, applied to screen a random subset of recent N. meningitidis serogroup B isolates from Europe. Analysis of the proportion of the different lipooligosaccharide types highlighted the predominance of L3 strains. Surprisingly, high rates of L2 type strains were found in Spain (17%, versus 2.5% in Germany and 1.9% in the United Kingdom). Therefore, we also investigated further these Spanish L2 strains in an attempt to explain such prevalence despite the known sensitivity of L2 immunotype to complement. We explored the hypothesis that these strains express high amounts of factor H-binding protein (fHbp), but we found, on the contrary, that L2 strains express low or undetectable amounts of fHbp. Our findings suggest that, in addition to a genetic analysis, a multivalent approach may be necessary to estimate the effectiveness of a N. meningitidis serogroup B vaccine.

Characterization of the carbohydrate backbone of Vibrio parahaemolyticus O6 lipopolysaccharides

Structural characterization studies have been carried out on the carbohydrate backbone of Vibrio parahaemolyticus serotype O6 lipopolysaccharides (LPS). The carbohydrate backbone isolated from O6 LPS by sequential derivatization, that is, dephosphorylation, O-deacylation, pyridylamination, N-deacylation and N-acetylation, is a nonasaccharide consisting of 3 mol of D-glucosamine (GlcN) (of which one is pyridylaminated), 2 mol of L-glycero-D-manno-heptose (Hep), and 1 mol each of D-galactose (Gal), D-glucose (Glc), D-glucuronic acid (GlcA) and 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo). Structural analyses by nuclear magnetic resonance spectroscopy and fast-atom bombardment mass spectrometry demonstrated that the carbohydrate backbone is β-Galp-(1→2)-α-Hepp-(1→3)-α-Hepp-(1→5)-α-Kdop-(2→6)-β-GlcpNAc-(1→6)-GlcNAc-PA, in which the 3-substituted α-Hepp is further substituted by β-GlcpNAc-(1→4)-β-Glcp at position 4 and by β-GlcpA at position 2. In native O6 LPS, an additional 1 mol of D-galacturonic acid, which is liberated by dephosphorylation in hydrofluoric acid, is present at an unknown position. A previous study by the present authors reported that, of 13 O-serotype LPS of V. parahaemolyticus, the only LPS from which Kdo was detected was from O6 LPS after mild acid hydrolysis. In the present study, we have demonstrated that only 1 mol of Kdo is present at the lipid A proximal position, a component which is common to the LPS in all serotypes of the bacterium, and that there is no additional Kdo in the carbohydrate backbone of O6 LPS. ELISA and ELISA inhibition analysis using antisera against O6 and Salmonella enterica Minnesota R595 and LPS of both strains further revealed that Kdo is not involved as an antigenic determinant of O6 LPS.

Evolutionary forces shaping the Golgi glycosylation machinery: why cell surface glycans are universal to living cells

Despite more than 3 billion years since the origin of life on earth, the powerful forces of biological evolution seem to have failed to generate any living cell that is devoid of a dense and complex array of cell surface glycans. Thus, cell surface glycans seem to be as essential for life as having a DNA genetic code, diverse RNAs, structural/functional proteins, lipid-based membranes, and metabolites that mediate energy flux and signaling. The likely reasons for this apparently universal law of biology are considered here, and include the fact that glycans have the greatest potential for generating diversity, and thus evading recognition by pathogens. This may also explain why in striking contrast to the genetic code, glycans show widely divergent patterns between taxa. On the other hand, glycans have also been coopted for myriad intrinsic functions, which can vary in their importance for organismal survival. In keeping with these considerations, a significant percentage of the genes in the typical genome are dedicated to the generation and/or turnover of glycans. Among eukaryotes, the Golgi is the subcellular organelle that serves to generate much of the diversity of cell surface glycans, carrying out various glycan modifications of glycoconjugates that transit through the Golgi, en route to the cell surface or extracellular destinations. Here I present an overview of general considerations regarding the selective forces shaping evolution of the Golgi glycosylation machinery, and then briefly discuss the common types of variations seen in each major class of glycans, finally focusing on sialic acids as an extreme example of evolutionary glycan diversity generated by the Golgi. Future studies need to address both the phylogenetic diversity the Golgi and the molecular mechanisms for its rapid responses to intrinsic and environmental stimuli.

Infections of people with complement deficiencies and patients who have undergone splenectomy

The complement system comprises several fluid-phase and membrane-associated proteins. Under physiological conditions, activation of the fluid-phase components of complement is maintained under tight control and complement activation occurs primarily on surfaces recognized as "nonself" in an attempt to minimize damage to bystander host cells. Membrane complement components act to limit complement activation on host cells or to facilitate uptake of antigens or microbes "tagged" with complement fragments. While this review focuses on the role of complement in infectious diseases, work over the past couple of decades has defined several important functions of complement distinct from that of combating infections. Activation of complement in the fluid phase can occur through the classical, lectin, or alternative pathway. Deficiencies of components of the classical pathway lead to the development of autoimmune disorders and predispose individuals to recurrent respiratory infections and infections caused by encapsulated organisms, including Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae. While no individual with complete mannan-binding lectin (MBL) deficiency has been identified, low MBL levels have been linked to predisposition to, or severity of, several diseases. It appears that MBL may play an important role in children, who have a relatively immature adaptive immune response. C3 is the point at which all complement pathways converge, and complete deficiency of C3 invariably leads to severe infections, including those caused by meningococci and pneumococci. Deficiencies of the alternative and terminal complement pathways result in an almost exclusive predisposition to invasive meningococcal disease. The spleen plays an important role in antigen processing and the production of antibodies. Splenic macrophages are critical in clearing opsonized encapsulated bacteria (such as pneumococci, meningococci, and Escherichia coli) and intraerythrocytic parasites such as those causing malaria and babesiosis, which explains the fulminant nature of these infections in persons with anatomic or functional asplenia. Paramount to the management of patients with complement deficiencies and asplenia is educating patients about their predisposition to infection and the importance of preventive immunizations and seeking prompt medical attention.

Mechanisms in Neisseria meningitidis for resistance against complement-mediated killing

Bacterial meningitis and septicaemia is a global health problem often caused by Neisseria meningitidis. The complement system is the most important aspect of host defence against this pathogen, and the critical interaction between the two is influenced by genetic polymorphisms on both the bacterial and the host side; variations of the meningococcus may lead to increased survival in human sera, whereas humans with complement deficiencies are more susceptible to meningococcal infections. Here we discuss the mechanisms of meningococcal resistance against complement-mediated killing and the influence of both bacterial and host genetic factors.

Carbohydrate composition of meningococcal lipopolysaccharide modulates the interaction of Neisseria meningitidis with human dendritic cells

Meningococcal lipopolysaccharide (LPS) is of crucial importance for the pathogenesis of invasive infection. We show that sialylation and elongation of the alpha-chain effectively shields viable unencapsulated Neisseria meningitidis from recognition by human dendritic cells (DC). In contrast, beta- and gamma- chain of the LPS carbohydrate moiety play only a minor role in the interaction with DC. The protective function of the LPS for the bacteria can be counteracted in vivo by phase variation of the lgtA gene encoding LPS glycosyltransferase A. Capsule expression protects N. meningitidis efficiently from recognition and phagocytosis by DC independent of the LPS structure. Despite the significant impact of LPS composition on the adhesion and phagocytosis of N. meningitidis no differences were found in terms of cytokine levels secreted by DC for IL1-beta, IL-6, IL-8, TNF-alpha, IFN-gamma and GM-CSF. However, significantly lower levels of the regulatory mediator IL-10 were induced by encapsulated strains in comparison to isogenic unencapsulated derivatives. IL-10 secretion was shown to depend on phagocytosis because poly alpha-2,8 sialic acid did not influence IL-10 secretion. The use of truncated LPS isoforms in vaccine preparations can therefore not only result in attenuation but also in more efficient targeting of DC.

Enhanced factor H binding to sialylated Gonococci is restricted to the sialylated lacto-N-neotetraose lipooligosaccharide species: implications for serum resistance and evidence for a bifunctional lipooligosaccharide sialyltransferase in Gonococci

We isolated serologically identical (by serovar determination and porin variable region [VR] typing) strains of Neisseria gonorrhoeae from an infected male and two of his monogamous female sex partners. One strain (termed 398078) expressed the L1 (Galalpha1 --> 4 [corrected] Galbeta1 --> 4Glcbeta1 --> 4HepI) lipooligosaccharide (LOS) structure exclusively; the other (termed 398079) expressed the lacto-N-neotetraose (LNT; Galbeta1 --> 4GlcNAcbeta1 --> 3Galbeta1 --> 4Glcbeta1 --> 4HepI) LOS structure. The strain from the male index case expressed both glycoforms and exhibited both immunotypes. Nuclear magnetic resonance analysis revealed that sialic acid linked to the terminal Gal of L1 LOS via an alpha2 --> 6 linkage and, as expected, to the terminal Gal of LNT LOS via an alpha2--> 3 linkage. Insertional inactivation of the sialyltransferase gene (known to sialylate LNT LOS) abrogated both L1 LOS sialylation and LNT LOS sialylation, suggesting a bifunctional nature of this enzyme in gonococci. Akin to our previous observations, sialylation of the LNT LOS of strain 398079 enhanced the binding of the complement regulatory molecule, factor H. Rather surprisingly, factor H did not bind to sialylated strain 398078. LOS sialylation conferred the LNT LOS-bearing strain complete (100%) resistance to killing by even 50% nonimmune normal human serum (NHS), whereas sialylation of L1 LOS conferred resistance only to 10% NHS. The ability of gonococcal sialylated LNT to bind factor H confers high-level serum resistance, which is not seen with sialylated L1 LOS. Thus, serum resistance mediated by sialylation of gonococcal L1 and LNT LOS occurs by different mechanisms, and specificity of factor H binding to sialylated gonococci is restricted to the LNT LOS species.

Identification of a novel sialic acid transporter in Haemophilus ducreyi

Haemophilus ducreyi, the causative agent of chancroid, produces a lipooligosaccharide (LOS) which terminates in N-acetyllactosamine. This glycoform can be further extended by the addition of a single sialic acid residue to the terminal galactose moiety. H. ducreyi does not synthesize sialic acid, which must be acquired from the host during infection or from the culture medium when the bacteria are grown in vitro. However, H. ducreyi does not have genes that are highly homologous to the genes encoding known bacterial sialic acid transporters. In this study, we identified the sialic acid transporter by screening strains in a library of random transposon mutants for those mutants that were unable to add sialic acid to N-acetyllactosamine-containing LOS. Mutants that reacted with the monoclonal antibody 3F11, which recognizes the terminal lactosamine structure, and lacked reactivity with the lectin Maackia amurensis agglutinin, which recognizes alpha2,3-linked sialic acid, were further characterized to demonstrate that they produced a N-acetyllactosamine-containing LOS by silver-stained sodium dodecyl sulfate-polyacrylamide gel electrophoresis and mass spectrometric analyses. The genes interrupted in these mutants were mapped to a four-gene cluster with similarity to genes encoding bacterial ABC transporters. Uptake assays using radiolabeled sialic acid confirmed that the mutants were unable to transport sialic acid. This study is the first report of bacteria using an ABC transporter for sialic acid uptake.

Sialylation of group B streptococcal capsular polysaccharide is mediated by cpsK and is required for optimal capsule polymerization and expression

Several bacterial pathogens have evolved the means to escape immune detection by mimicking host cell surface carbohydrates that are crucial for self/non-self recognition. Sialic acid, a terminal residue on these carbohydrates, inhibits activation of the alternate pathway of complement by recruiting the immune modulating molecule factors H, I, and iC3b. Sialylation of capsular polysaccharide (CPS) is important for virulence of group B streptococci (GBS), a significant human pathogen. We previously reported that cpsK, a gene within the cps locus of type III GBS, could complement a sialyltransferase deficient lst mutant of Haemophilus ducreyi, implicating its role in sialylation of the GBS capsule. To explore the function of cpsK in GBS capsule production, we created a mutant in cpsK. Immunoblot analysis and enzyme-linked immunosorbent assay using anti-type III CPS antisera demonstrated that the mutant CPS did not contain sialic acid. This was confirmed by high-performance liquid chromatography after mild acid hydrolysis of the CPS. Although increased CPS chain length was seen for this strain, CPS production was <20% of the parental isolate. An episomal cpsK copy restored synthesis of sialo-CPS to wild-type levels. These data support our hypothesis that cpsK encodes the GBS CPS sialyltransferase and provide further evidence that lack of CPS oligosaccharide sialylation reduces the amount of CPS expressed on the cell surface. These observations also imply that one or more of the components involved in synthesis or transport of oligosaccharide repeating units requires a sialo-oligosaccharide for complete activity.

Influence of the length of the lipooligosaccharide alpha chain on its sialylation in Neisseria meningitidis

The sialylation of lipooligosaccharide (LOS) in Neisseria meningitidis plays a role in the resistance of the organism to killing by normal human serum. The length of the alpha chain extending out from the heptose I [Hep (I)] moiety of LOS influenced sialylation of N. meningitidis LOS in vitro and in vivo. The alpha chain required a terminal Gal and a trisaccharide or longer oligosaccharide to serve as an acceptor for sialylation. The disaccharide lactose (Galbeta1-4Glc) in the alpha chain of immunotype L8 LOS could not function as an acceptor for the sialyltransferase, probably due to steric hindrance imposed by the neighboring Hep (II) with phosphorylethanolamine and another group attached.