Structural Analysis of the Lipopolysaccharide from Nontypeable Haemophilus influenzae Strain R2846

Panel 5: Microbiology and immunology panel

OBJECTIVE

The objective is to perform a comprehensive review of the literature from January 2007 through June 2011 on the virology, bacteriology, and immunology related to otitis media.

DATA SOURCES

PubMed database of the National Library of Medicine.

REVIEW METHODS

Three subpanels with co-chairs comprising experts in the virology, bacteriology, and immunology of otitis media were formed. Each of the panels reviewed the literature in their respective fields and wrote draft reviews. The reviews were shared with all panel members, and a second draft was created. The entire panel met at the 10th International Symposium on Recent Advances in Otitis Media in June 2011 and discussed the review and refined the content further. A final draft was created, circulated, and approved by the panel.

CONCLUSION

Excellent progress has been made in the past 4 years in advancing an understanding of the microbiology and immunology of otitis media. Advances include laboratory-based basic studies, cell-based assays, work in animal models, and clinical studies.

IMPLICATIONS FOR PRACTICE

The advances of the past 4 years formed the basis of a series of short-term and long-term research goals in an effort to guide the field. Accomplishing these goals will provide opportunities for the development of novel interventions, including new ways to better treat and prevent otitis media.

Evasion of killing by human antibody and complement through multiple variations in the surface oligosaccharide of Haemophilus influenzae

The lipopolysaccharide (LPS) of H. influenzae is highly variable. Much of the structural diversity is derived from phase variation, or high frequency on-off switching, of molecules attached during LPS biosynthesis. In this study, we examined the dynamics of LPS phase variation following exposure to human serum as a source of antibody and complement in multiple H. influenzae isolates. We show that lic2A, lgtC and lex2A switch from phase-off to phase-on following serial passage in human serum. These genes, which control attachment of a galα1-4gal di-galactoside structure (lic2A and lgtC phase-on) or an alternative glucose extension (lex2A phase-on) from the same hexose moiety, reduce binding of bactericidal antibody to conserved inner core LPS structures. The effects of the di-galactoside and alternative glucose extension were also examined in the context of the additional LPS phase variable structures phosphorylcholine (ChoP) and sialic acid. We found that di-galactoside, the alternative glucose extension, ChoP, and sialic acid each contribute independently to bacterial survival in the presence of human complement, and have an additive effect in combination. We propose that LPS phase variable extensions serve to shield conserved inner core structures from recognition by host immune components encountered during infection.

A novel branching pattern in the lipopolysaccharide expressed by non-typeable Haemophilus influenzae strain 1232

We report the novel branching pattern in lipopolysaccharide (LPS) expressed by non-typeable Haemophilus influenzae (NTHi) strain 1232. The strain expressed the β-d-Glcp-(1→4)-[α-d-Galp-(1→4)-β-d-Galp-(1→7)]-d-α-d-Hepp-(1→6)-β-d-Glcp chain linked to the proximal heptose (HepI) of the conserved triheptosyl inner-core moiety of NTHi LPS: l-α-d-HepIIIp-(1→2)-[PEtn→6]-l-α-d-HepIIp-(1→3)-l-α-d-HepIp-(1→5)-[PPEtn→4]-α-Kdop-(2→6)-lipid A. The structure has been elucidated using NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS) and capillary electrophoresis coupled to electrospray ionization tandem mass spectrometry (CE-ESI-MS(n)) on O-deacylated LPS and core oligosaccharide (OS) materials, as well as HPLC-ESI-MS(n) on permethylated, dephosphorylated OS. It was also found that a tetrasaccharide unit bearing sialic acid [α-Neu5Ac-(2→3)-β-d-Galp-(1→4)-β-d-GlcNAcp-(1→3)-β-d-Galp-(1→] could substitute O-4 of the β-d-Glcp linked to HepI. In addition, the distal heptose (HepIII) was substituted by PCho→6-β-d-Galp-(1→ at the O-2 position.

Phosphorylcholine allows for evasion of bactericidal antibody by Haemophilus influenzae

The human pathogen Haemophilus influenzae has the ability to quickly adapt to different host environments through phase variation of multiple structures on its lipooligosaccharide (LPS), including phosphorylcholine (ChoP). During colonization with H. influenzae, there is a selection for ChoP+ phase variants. In a murine model of nasopharyngeal colonization, this selection is lost in the absence of adaptive immunity. Based on previous data highlighting the importance of natural antibody in limiting H. influenzae colonization, the effect of ChoP expression on antibody binding and its bactericidal activity was investigated. Flow cytometric analysis revealed that ChoP+ phase variants had decreased binding of antibody to LPS epitopes compared to ChoP- phase variants. This difference in antibody binding correlated with increased survival of ChoP+ phase variants in the presence of antibody-dependent, complement-mediated killing. ChoP+ phase variants were also more resistant to trypsin digestion, suggesting a general effect on the physical properties of the outer membrane. Moreover, ChoP-mediated protection against antibody binding correlated with increased resilience of outer membrane integrity. Collectively, these data suggest that ChoP expression provides a selective advantage during colonization through ChoP-mediated effects on the accessibility of bactericidal antibody to the cell surface.

Genes required for the synthesis of heptose-containing oligosaccharide outer core extensions in Haemophilus influenzae lipopolysaccharide

Heptose-containing oligosaccharides (OSs) are found in the outer core of the lipopolysaccharide (LPS) of a subset of non-typable Haemophilus influenzae (NTHi) strains. Candidate genes for the addition of either l-glycero-d-manno-heptose (ld-Hep) or d-glycero-d-manno-heptose (dd-Hep) and subsequent hexose sugars to these OSs have been identified from the recently completed genome sequences available for NTHi strains. losA1/losB1 and losA2/losB2 are two sets of related genes in which losA has homology to genes encoding glycosyltransferases and losB to genes encoding heptosyltransferases. Each set of genes is variably present across NTHi strains and is located in a region of the genome with an alternative gene organization between strains that contributes to LPS heterogeneity. Dependent upon the strain background, the LPS phenotype, structure and serum resistance of strains mutated in these genes were altered when compared with the relevant parent strain. Our studies confirm that losB1 and losB2 usually encode dd-heptosyl- and ld-heptosyl transferases, respectively, and that losA1 and losA2 encode glycosyltransferases that play a role in OS extensions of NTHi LPS.

The diversity of the core oligosaccharide in lipopolysaccharides

Bacterial lipopolysaccharides (LPSs) are the major component of the outer membrane of Gram-negative bacteria. They have a structural role since they contribute to the cellular rigidity by increasing the strength of cell wall and mediating contacts with the external environment that can induce structural changes to allow life in different conditions. Furthermore, the low permeability of the outer membrane acts as a barrier to protect bacteria from host-derived antimicrobial compounds. Lipopolysaccharides are amphiphilic macromolecules generally comprising three defined regions distinguished by their genetics, structures and function: the lipid A, the core oligosaccharide and a polysaccharide portion, the O-chain. In some Gram-negative bacteria LPS can terminate with the core portion to form rough type LPS (R-LPS, LOS). The core oligosaccharide is an often branched and phosphorylated heterooligosaccharide with less than fifteen sugars, more conserved in the inner region, proximal to the lipid A, and often carrying non-stoichiometric substitutions leading to variation and micro-heterogeneity. The core oligosaccharide contributes to the bacterial viability and stability of the outer membrane, can assure the serological specificity and possesses antigenic properties.

Profiling LPS glycoforms of non-typeable Haemophilus influenzae by multiple-stage tandem mass spectrometry

Non-typeable (acapsular) Haemophilus influenzae (NTHi) is a major cause of otitis media accounting for 25-30% of all cases of the disease. Lipopolysaccharide (LPS) is an essential and exposed component of the H. influenzae cell wall. A characteristic feature of H. influenzae LPS is the extensive inter-strain and intra-strain heterogeneity of glycoform structure which is key to the role of the molecule in both commensal and disease-causing behavior of the bacterium. However, to characterize LPS structure unambiguously is a major challenge due to the extreme heterogeneity of glycoforms that certain strains express. A powerful tool for obtaining sequence and branching information is multiple-stage tandem ESI-MS (ESI-MS( n )) performed on dephosphorylated and permethylated oligosaccharide material using an ESI-quadrupole ion trap mass spectrometer. In general, permethylation increases the MS response by several orders of magnitude and sequence information is readily obtained since methyl tagging allows the distinction between fragment ions generated by cleavage of a single glycosidic bond and inner fragments resulting from the rupture of two glycosidic linkages. Using this approach we are now able to identify all isomeric glycoforms in very heterogeneous LPS preparations.

Review: Lipopolysaccharide biosynthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa causes serious nosocomial infections, and an important virulence factor produced by this organism is lipopolysaccharide (LPS). This review summarizes knowledge about biosynthesis of all three structural domains of LPS - lipid A, core oligosaccharide, and O polysaccharides. In addition, based on similarities with other bacterial species, this review proposes new hypothetical pathways for unstudied steps in the biosynthesis of P. aeruginosa LPS. Lipid A biosynthesis is discussed in relation to Escherichia coli and Salmonella, and the biosyntheses of core sugar precursors and core oligosaccharide are summarised. Pseudomonas aeruginosa attaches a Common Polysaccharide Antigen and O-Specific Antigen polysaccharides to lipid A-core. Both forms of O polysaccharide are discussed with respect to their independent synthesis mechanisms. Recent advances in understanding O-polysaccharide biosynthesis since the last major review on this subject, published nearly a decade ago, are highlighted. Since P. aeruginosa O polysaccharides contain unusual sugars, sugar-nucleotide biosynthesis pathways are reviewed in detail. Knowledge derived from detailed studies in the O5, O6 and O11 serotypes is applied to predict biosynthesis pathways of sugars in poorly-studied serotypes, especially O1, O4, and O13/O14. Although further work is required, a full understanding of LPS biosynthesis in P. aeruginosa is almost within reach.

Structural analysis of the core region of O-lipopolysaccharide of Porphyromonas gingivalis from mutants defective in O-antigen ligase and O-antigen polymerase

Porphyromonas gingivalis synthesizes two lipopolysaccharides (LPSs), O-LPS and A-LPS. Here, we elucidate the structure of the core oligosaccharide (OS) of O-LPS from two mutants of P. gingivalis W50, Delta PG1051 (WaaL, O-antigen ligase) and Delta PG1142 (O-antigen polymerase), which synthesize R-type LPS (core devoid of O antigen) and SR-type LPS (core plus one repeating unit of O antigen), respectively. Structural analyses were performed using one-dimensional and two-dimensional nuclear magnetic resonance spectroscopy in combination with composition and methylation analysis. The outer core OS of O-LPS occurs in two glycoforms: an "uncapped core," which is devoid of O polysaccharide (O-PS), and a "capped core," which contains the site of O-PS attachment. The inner core region lacks L(D)-glycero-D(l)-manno-heptosyl residues and is linked to the outer core via 3-deoxy-D-manno-octulosonic acid, which is attached to a glycerol residue in the outer core via a monophosphodiester bridge. The outer region of the "uncapped core" is attached to the glycerol and is composed of a linear alpha-(1-->3)-linked d-Man OS containing four or five mannopyranosyl residues, one-half of which are modified by phosphoethanolamine at position 6. An amino sugar, alpha-D-allosamine, is attached to the glycerol at position 3. In the "capped core," there is a three- to five-residue extension of alpha-(1-->3)-linked Man residues glycosylating the outer core at the nonreducing terminal residue. beta-D-GalNAc from the O-PS repeating unit is attached to the nonreducing terminal Man at position 3. The core OS of P. gingivalis O-LPS is therefore a highly unusual structure, and it is the basis for further investigation of the mechanism of assembly of the outer membrane of this important periodontal bacterium.