Decoration of lipopolysaccharide with phosphorylcholine: a phase-variable characteristic of Haemophilus influenzae

Phosphocholine of pneumococcal teichoic acids: role in bacterial physiology and pneumococcal infection

Pneumococci have an absolute nutritional requirement for choline. Choline is incorporated as phosphocholine (PCho) into lipoteichoic (LTA) and teichoic acid (TA). The PCho residues are required for transformability, the activity of autolysins, the separation of daughter cells after cell division and for anchoring a family of surface proteins which play important roles in pneumococcal infection. The genes encoding the enzymes for PCho incorporation are described. Two strains that acquired the ability to grow in the absence of choline are discussed.

Non-typeable Haemophilus influenzae adhere to and invade human bronchial epithelial cells via an interaction of lipooligosaccharide with the PAF receptor

Adherence and invasion are thought to be key events in the pathogenesis of non-typeable Haemophilus influenzae (NTHi). The role of NTHi lipooligosaccharide (LOS) in adherence was examined using an LOS-coated polystyrene bead adherence assay. Beads coated with NTHi 2019 LOS adhered significantly more to 16HBE14 human bronchial epithelial cells than beads coated with truncated LOS isolated from an NTHi 2019 pgmB:ermr mutant (P = 0.037). Adherence was inhibited by preincubation of cell monolayers with NTHi 2019 LOS (P = 0.0009), but not by preincubation with NTHi 2019 pgmB:ermr LOS. Competitive inhibition studies with a panel of compounds containing structures found within NTHi LOS suggested that a phosphorylcholine (ChoP) moiety was involved in adherence. Further experiments revealed that mutations affecting the oligosaccharide region of LOS or the incorporation of ChoP therein caused significant decreases in the adherence to and invasion of bronchial cells by NTHi 2019 (P < 0.01). Analysis of infected monolayers by confocal microscopy showed that ChoP+ NTHi bacilli co-localized with the PAF receptor. Pretreatment of bronchial cells with a PAF receptor antagonist inhibited invasion by NTHi 2109 and two other NTHi strains expressing ChoP+ LOS glycoforms exhibiting high reactivity with an anti-ChoP antibody on colony immunoblots. These data suggest that a particular subset of ChoP+ LOS glycoforms could mediate NTHi invasion of bronchial cells by means of interaction with the PAF receptor.

Characterization of the phosphocholine-substituted oligosaccharide in lipopolysaccharides of type b Haemophilus influenzae

Haemophilus influenzae expresses heterogeneous populations of short-chain lipopolysaccharide (LPS) which exhibit extensive antigenic diversity among multiple oligosaccharide epitopes. These LPS oligosaccharide epitopes can carry phosphocholine (PCho) substituents, the expression of which is subject to high frequency phase variation mediated by genes in the lic1 genetic locus. The location and site of attachment of PCho substituents were determined by structural analysis of LPS from two type b H. influenzae strains, Eagan and RM7004. The lic2 locus is involved in phase variation of oligosaccharide expression. LPS obtained from the parent strains, from mutants generated by insertion of antibiotic resistance cassettes in the lic2 genetic locus, and from phase-variants showing high levels of PCho expression was characterized by electrospray ionization-mass spectrometry (ESI-MS) and 1H NMR spectroscopy of derived O-deacylated samples. ESI-MS of O-deacylated LPS from wild-type strains revealed mixtures of related glycoform structures differing in the number of hexose residues. Analysis of LPS from PCho-expressing phase-variants revealed similar mixtures of glycoforms, each containing a single PCho substituent. O-Deacylated LPS preparations from the lic2 mutants were much less complex than their respective parent strains, consisting only of Hex3 and/or Hex2 glycoforms, were examined in detail by high-field NMR techniques. It was found that the LPS samples contain the phosphoethanolamine (PEtn) substituted inner-core element, L-alpha-D-Hepp-(1-->2)-[PEtn-->6]-L-alpha-D-Hepp-(1--> 3)-L-alpha-D-He pp-(1-->5)-alpha-Kdo in which the major glycoforms carry a beta-D-Glcp or beta-D-Glcp-(1-->4)-beta-D-Glcp at the O-4 position of the 3-substituted heptose (HepI) and a beta-D-Galp at the O-2 position of the terminal heptose (HepIII). LPS from the lic2 mutants of both type b strains were found to carry PCho groups at the O-6 position of the terminal beta-D-Galp residue attached to HepIII. In the parent strains, the central heptose (HepII) of the LPS inner-core element is also substituted by hexose containing oligosaccharides. The expression of the galabiose epitope in LPS of H. influenzae type b strains has previously been linked to genes comprising the lic2 locus. The present study provides definitive evidence for the role of lic2 genes in initiating chain extension from HepII. From the analysis of core oligosaccharide samples, LPS from the lic2 mutant strain of RM7004 was also found to carry O-acetyl substituents. Mono-, di-, and tri-O-acetylated LPS oligosaccharides were identified. The major O-acetylated glycoforms were found to be substituted at the O-3 position of HepIII. A di-O-acetylated species was characterized which was also substituted at the O-6 postion of the terminal beta-D-Glc in the Hex3 glycoform. This is the first report pointing to the occurrence of O-acetyl groups in the inner-core region of H. influenzae LPS. We have previously shown that in H. influenzae strain Rd, a capsule-deficient type d strain, PCho groups are expressed in a different molecular environment, being attached at the O-6 position of a beta-D-Glcp, which is in turn attached to HepI.

Challenge of investigating biologically relevant functions of virulence factors in bacterial pathogens

Recent innovations have increased enormously the opportunities for investigating the molecular basis of bacterial pathogenicity, including the availability of whole-genome sequences, techniques for identifying key virulence genes, and the use of microarrays and proteomics. These methods should provide powerful tools for analysing the patterns of gene expression and function required for investigating host-microbe interactions in vivo. But, the challenge is exacting. Pathogenicity is a complex phenotype and the reductionist approach does not adequately address the eclectic and variable outcomes of host-microbe interactions, including evolutionary dynamics and ecological factors. There are difficulties in distinguishing bacterial 'virulence' factors from the many determinants that are permissive for pathogenicity, for example those promoting general fitness. A further practical problem for some of the major bacterial pathogens is that there are no satisfactory animal models or experimental assays that adequately reflect the infection under investigation. In this review, we give a personal perspective on the challenge of characterizing how bacterial pathogens behave in vivo and discuss some of the methods that might be most relevant for understanding the molecular basis of the diseases for which they are responsible. Despite the powerful genomic, molecular, cellular and structural technologies available to us, we are still struggling to come to grips with the question of 'What is a pathogen?'

Characterization of a DNA region containing 5'-(CAAT)(n)-3' DNA sequences involved in lipooligosaccharide biosynthesis in Haemophilus somnus

Repetitive tetranucleotide sequences of 5'-(CAAT)(n)-3' have been identified at the 5' end of an open reading frame (ORF) named lob1 from Haemophilus somnus strain 738. Based on sequence analysis, lob1 has 59% DNA homology to lex2B, which is involved in lipooligosaccharide (LOS) biosynthesis in H. influenzae. We now report that the number of 5'-CAAT-3' repeats in lob1 varied from 31-35, but that 94% of colonies contained 33 repeats of 5'-CAAT-3' downstream of two potential start codons, as determined by DNA sequence analysis of the 5'-CAAT-3' region from individual colonies. If transcription began with the start codon closest to the 5'-CAAT-3' repeats, a protein of 34.5 kDa would be encoded when 33 repeats were present. However, we could not establish a correlation between the number of 5'-CAAT-3' repeats in lob1 with a specific LOS electrophoretic profile or reactivity with two LOS monoclonal antibodies, indicating multiple genes control LOS phase variation in H. somnus. Complementation of strain 129Pt with lob1 containing 33 5 '-CAAT-3' repeats in shuttle vector pLS88 resulted in transformants 129Pt(pLSlob1-33A) and 129Pt(pLSlob1-33B), both of which demonstrated the same altered LOS electrophoretic profile. Unlike strain 129Pt, both transformants underwent limited LOS phase variation, which correlated with variation in the number of 5'-CAAT-3' repeats in pLSlob1-33. Nanoelectrospray-mass spectrometry of O-deacylated LOS indicated that transformant 129Pt(pLSlob1-33A) LOS was composed of a different distribution of glycoforms than LOS of the parent strain. The ratio of glucose to galactose changed from 1:2 in strain 129Pt LOS to 2:1 in transformant 129Pt(pLSlob1-33A) LOS, as determined by gas chromatography-mass spectrometry. Nuclear magnetic resonance spectroscopy confirmed and extended these observations. Transformant 129Pt(pLSlob1-33A) was constitutively more reactive in colony immunoblotting to polyclonal antiserum made to purified strain 738 LOS, and was more susceptible to complement-mediated killing in the presence of anti-738 LOS serum than parent strain 129Pt. Based on these results, Lob1 appears to be a phase variable galactosyl transferase involved in LOS biosynthesis in H. somnus.

Bacterial phosphorylcholine decreases susceptibility to the antimicrobial peptide LL-37/hCAP18 expressed in the upper respiratory tract

A number of pathogens of the upper respiratory tract express an unusual prokaryotic structure, phosphorylcholine (ChoP), on their cell surface. We tested the hypothesis that ChoP, also found on host membrane lipids in the form of phosphatidylcholine, acts so as to decrease killing by antimicrobial peptides that target differences between bacterial and host membranes. In Haemophilus influenzae, ChoP is a phase-variable structure on the oligosaccharide portion of the lipopolysaccharide (LPS). There was a bactericidal effect of the peptide LL-37/hCAP18 on a nontypeable H. influenzae strain, with an increasing selection for the ChoP(+) phase as the concentration of the peptide was raised from 0 to 10 microgram/ml. Moreover, constitutive ChoP-expressing mutants of unrelated strains showed up to 1,000-fold-greater survival compared to mutants without ChoP. The effect of ChoP on resistance to killing by LL-37/hCAP18 was dependent on the salt concentration and was observed only when bacteria were grown in the presence of environmental choline, a requirement for the expression of ChoP on the LPS. Further studies established that there is transcription of the LL-37/hCAP18 gene on the epithelial surface of the human nasopharynx in situ and inducible transcription in epithelial cells derived from the upper airway. The presence of highly variable amounts of LL-37/hCAP18 in normal nasal secretions (<1.2 to >80 microgram/ml) was demonstrated with an antibody against this peptide. It was concluded that ChoP alters the bacterial cell surface so as mimic host membrane lipids and decrease killing by LL-37/hCAP18, an antimicrobial peptide that may be expressed on the mucosal surface of the nasopharynx in bactericidal concentrations.

Phosphorylcholine decoration of lipopolysaccharide differentiates commensal Neisseriae from pathogenic strains: identification of licA-type genes in commensal Neisseriae

Phosphorylcholine (ChoP) is a potential candidate for a plurispecific vaccine, because it is present on surface components of many mucosal organisms, including Haemophilus influenzae, Streptococcus pneumoniae and Pseudomonas aeruginosa. In addition, ChoP has been detected on pili of Neisseria meningitidis and Neisseria gonorrhoeae. In this study, we demonstrate the presence of the phosphorylcholine epitope on the lipopolysaccharides (LPSs) of several species of commensal Neisseriae (Cn), a property that differentiates commensal from the pathogenic strains of Neisseriae. In an extended survey of 78 strains, we confirmed the exclusive expression of the ChoP epitope on pili of pathogenic Neisseriae. Despite the presence of pili on Cn, which are homologous to Class II pili of N. meningitidis, they did not react with anti-ChoP antibody. This observation was further supported by the fact that 14C-labelled choline was incorporated only in the LPSs of Cn. Analysis of the LPS of N. lactamica strain NL4 revealed two distinct and interconvertible molecular species of LPS with high and low levels of reactivity with anti-ChoP antibody. In addition, on/off phase variation gave rise to frequent modulation in the levels of antibody reactivity. A concurrent modulation was also observed in the binding of C-reactive protein, CRP, a ChoP-binding reactant that is implicated in bacterial clearance. Genetic analysis showed the presence of a gene in several Cn spp. with significant sequence identity to H. influenzae licA. This gene encodes choline kinase and is also involved in phase variation of the LPS-associated ChoP in H. influenzae. In contrast, licA-like genes were not identified in the pathogenic Neisseria strains tested. They are absent from N. meningitidis strain Z2491 genome database. These data suggest that the genetic basis for ChoP incorporation in Cn LPS resembles that in H. influenzae spp. and may be distinct from that generating the ChoP epitope on pili of pathogenic Neisseriae. Further, the modulation of ChoP expression on Cn LPS, and corresponding modulation of CRP binding, has the potential to confer the property of immune avoidance and thus of persistence on mucosa.

The position of phosphorylcholine on the lipopolysaccharide of Haemophilus influenzae affects binding and sensitivity to C-reactive protein-mediated killing

The lic1 locus of Haemophilus influenzae controls the incorporation of environmental choline into lipopolysaccharide (LPS) as phosphorylcholine (ChoP) as well as the phase variation of this structure. ChoP is the target of an acute phase reactant in serum, C-reactive protein (CRP), which mediates killing through the activation of complement when bound to the organism. Structural analysis of the oligosaccharide region of the H. influenzae LPS showed that ChoP is linked to different hexose residues on different chain extensions in strains Rd and Eagan. Differences in the molecular environment of ChoP affect the epitope defined by monoclonal antibody 12D9 and were associated with polymorphisms within LicD, a putative diphosphonucleoside choline transferase. Exchanging the licD genes between the two strains with ChoP on different chain extensions was sufficient to switch its position. Allelic variants with ChoP on a hexose on heptose III rather than heptose I were sensitive to CRP-mediated serum bactericidal activity regardless of the genetic background. Differences in CRP-mediated killing correlated with differences in the binding of CRP from human serum to whole bacteria. This suggests that, in addition to the mechanism involving phase variation, the structural rearrangements within the oligosaccharide contribute to evasion of innate and acquired immunity.

Structural analysis of the lipopolysaccharide oligosaccharide epitopes expressed by Haemophilus influenzae strain RM.118-26

The structure of the lipopolysaccharide of Haemophilus influenzae mutant strain, RM.118-26, was investigated. Electrospray ionization-mass spectrometry on intact lipopolysaccharide, O-deacylated lipopolysaccharide and core oligosaccharides obtained from lipopolysaccharide after mild acid hydrolysis provided information on the composition and relative abundance of the glycoforms. Oligosaccharide samples were studied in detail using high-field NMR techniques. The structure of the major glycoform containing phosphocholine is identical to the Hex2 glycoform described for H. influenzae RM.118-28 [Risberg, A., Schweda, E.K.H. & Jansson, P.-E. (1997) Eur. J. Biochem. 243, 701-707]. A second major glycoform, containing three hexose residues (Hex3), in which a lactose unit, beta-D-Galp-(1-->4)-beta-D-Glcp, is attached at the O-2 position of the terminal heptose of the inner core element, L-alpha-D-Hepp-(1-->2)-L-alpha-D-Hepp-(1-->3)-[beta-D-Glcp-( 1-->4)-]- L-alpha-D-Hepp-(1-->5)-alpha-Kdo, carries no phosphocholine. Instead this lipopolysaccharide glycoform is partly (40%) substituted by an O-acetyl group linked to the 6-position of the glucose residue in the lactose unit and has the following structure:

Phase variation of lic1A, lic2A and lic3A in colonization of the nasopharynx, bloodstream and cerebrospinal fluid by Haemophilus influenzae type b

The role of phase variation of lic1A, lic2A and lic3A in the ability of Haemophilus influenzae type b to colonize the nasopharynx, bloodstream and cerebrospinal fluid (CSF) of infants was investigated. This was achieved by using PCR to determine the number of 5'-CAAT-3' repeats present in each gene, which is indicative of whether each ORF can be expressed. Multiple PCR products of different intensities were amplified from all three genes at each site sampled. This indicated that the nasopharynx, bloodstream and CSF were colonized by a heterogeneous population of organisms, expressing different combinations of lic genes. At each site however, a predominant PCR product was amplified from each gene, indicating that organisms with this genotype were the most abundant. The number of 5'-CAAT-3' repeats in this predominant product varied depending upon whether organisms were isolated from the nasopharynx, bloodstream or CSF. These observations suggest that the expression of different combinations of lic genes may influence the efficiency with which H. influenzae colonizes the nasopharynx, bloodstream and CSF of infant rats.

Antiphosphorylcholine antibody levels are elevated in humans with periodontal diseases

Human immunoglobulin G2 (IgG2) serum concentrations and the IgG2 antibody response to Actinobacillus actinomycetemcomitans can be influenced by genes, by environmental factors such as smoking, and by periodontal disease status. Examination of the IgG2 response to phosphorylcholine (PC), a response thought to be mainly induced by the C polysaccharide of Streptococcus pneumoniae, suggested that periodontal disease status was also associated with this response. This prompted the hypothesis that PC is an important oral antigen associated with organisms in the periodontal flora and that anti-PC antibody is elevated as a consequence of periodontal disease. Subjects in various periodontal disease diagnostic categories in which attachment loss is exhibited were tested for anti-PC in serum. Those with adult periodontitis, localized juvenile periodontitis, generalized early-onset periodontitis, and gingival recession all had similar levels of anti-PC IgG2 serum antibody which were significantly greater than in the group of subjects with no attachment loss. Analysis of plaque samples from subgingival and supragingival sites in all diseases categories for reactivity with the anti-PC specific monoclonal antibody TEPC-15 revealed that a substantial proportion of the bacteria in dental plaque (30 to 40%) bear PC antigen; this antigen was not restricted to morphotypes resembling only cocci but was also present on rods and branched filamentous organisms. We found that S. mitis, S. oralis, and S. sanguis, as well as oral actinomycetes, including A. viscosus, A. odontolyticus, and A. israelii, incorporated substantial amounts of [(3)H]choline from culture media. Further analysis of antigens derived from these organisms by Western blot indicated that S. oralis, S. sanguis, A. viscosus, A. odontolyticus, and A. israelii contained TEPC-15-reactive antigens. The data show that many commonly occurring bacterial species found in dental plaque contain PC antigen and that immunization with plaque-derived PC antigens as a consequence of inflammation and periodontal attachment loss may influence systemic anti-PC antibody concentrations.

Structural analysis of the lipopolysaccharide oligosaccharide epitopes expressed by a capsule-deficient strain of Haemophilus influenzae Rd

Structural elucidation of the lipopolysaccharide (LPS) of Haemophilus influenzae, strain Rd, a capsule-deficient type d strain, has been achieved by using high-field NMR techniques and electrospray ionization-mass spectrometry (ESI-MS) on delipidated LPS and core oligosaccharide samples. It was found that this organism expresses heterogeneous populations of LPS of which the oligosaccharide (OS) epitopes are subject to phase variation. ESI-MS of O-deacylated LPS revealed a series of related structures differing in the number of hexose residues linked to a conserved inner-core element, L-alpha-D-Hepp-(1-->2)-L-alpha-D-Hepp-(1-->3)-[beta-D-Glcp- (1-->4)-]- L-alpha-D-Hepp-(1-->5)-alpha-Kdo, and the degree of phosphorylation. The structures of the major LPS glycoforms containing three (two Glc and one Gal), four (two Glc and two Gal) and five (two Glc, two Gal and one GalNAc) hexoses were substituted by both phosphocholine (PCho) and phosphoethanolamine (PEtn) and were determined in detail. In the major glycoform, Hex3, a lactose unit, beta-D-Galp-(1-->4)-beta-D-Glcp, is attached at the O-2 position of the terminal heptose of the inner-core element. The Hex4 glycoform contains the PK epitope, alpha-D-Galp-(1-->4)-beta-D-Galp-(1-->4)-beta-D-Glcp while in the Hex5 glycoform, this OS is elongated by the addition of a terminal beta-D-GalpNAc residue, giving the P antigen, beta-D-GalpNAc-(1-->3)-alpha-D-Galp-(1-->4)-beta-D-Galp-(1-->4)-D-Glc p. The fully extended LPS glycoform (Hex5) has the following structure. [see text] The structural data provide the first definitive evidence demonstrating the expression of a globotetraose OS epitope, the P antigen, in LPS of H. influenzae. It is noteworthy that the molecular environment in which PCho units are found differs from that observed in an Rd- derived mutant strain (RM.118-28) [Risberg, A., Schweda, E. K. H. & Jansson, P-E. (1997) Eur. J. Biochem. 243, 701-707].

Pneumococcal licD2 gene is involved in phosphorylcholine metabolism

Phosphorylcholine is an important bioactive adduct to the teichoic acid (TA) and lipoteichoic acid (LTA) of the surface of Streptococcus pneumoniae. We have identified and characterized a genetic locus lic that is required for phosphorylcholine metabolism in S. pneumoniae. The pneumococcal lic locus consists of eight genes, licA, licB, licC and licD1, licD2 and three additional open reading frames. Pneumococcal licA, licB, licC, licD1 and licD2 have significant sequence similarity to licA, licB, licC and licD of Haemophilus influenzae. Mutation of licD2 led to decreased [3H]-choline uptake, aberrant migration of LTA chains in SDS-PAGE gels, loss of several surface proteins, and a phase-locked hypertransparent colony phenotype. Moreover, the licD2- mutant falled to undergo lysis after treatment with penicillin at high cell density and showed decreased transformation competence. Finally, the licD2- mutant demonstrated decreased adherence to the human type II alveolar cells, reduced nasopharyngeal colonization in infant rats, as well as significantly impaired virulence upon intraperitoneal challenge of CF1 mice. Identification of the lic genes in the pneumococcus will facilitate further characterization of the role of surface choline in microbial physiology and pathogenesis.

Phosphorylcholine: friend or foe of the immune system?

Phosphorylcholine (PC) is a structural component of a variety of prokaryotic and eukaryotic pathogens. In some cases, PC in infectious agents can benefit the infected host due to its targeting by both the innate and adaptive immune responses. However, as discussed here, PC exhibits a surprising range of immunomodulatory properties that might be to the detriment of the host.

Characterization of a WaaF (RfaF) homolog expressed by Haemophilus ducreyi

Haemophilus ducreyi lipooligosaccharide (LOS) is capable of inducing an inflammatory response in skin (A. A. Campagnari, L. M. Wild, G. Griffiths, R. J. Karalus, M. A. Wirth, and S. M. Spinola, Infect. Immun. 59:2601-2608, 1991) and likely contributes to the virulence of this sexually transmitted pathogen (B. A. Bauer, M. K. Stevens, and E. J. Hansen, Infect. Immun. 68:4290-4298, 1998). An open reading frame in H. ducreyi 35000 was found to encode a predicted protein that was 59% identical to the protein product of the rfaF (waaF) gene of Salmonella typhimurium. The H. ducreyi waaF gene was able to complement an S. typhimurium rfaF (waaF) mutant, a result which confirmed the identity of this gene. In contrast to the rfaF (waaF) gene of enteric bacteria, the H. ducreyi waaF gene was not located adjacent to other genes involved in lipopolysaccharide expression. Inactivation of the H. ducreyi waaF gene by insertion mutagenesis resulted in expression of a LOS that migrated much faster than wild-type LOS in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The LOS of this mutant also did not bind a monoclonal antibody directed against a cell surface-exposed epitope of wild-type H. ducreyi LOS. Testing of the wild-type H. ducreyi strain and its isogenic waaF mutant in the temperature-dependent rabbit model for dermal lesion production by H. ducreyi revealed that this waaF mutant was less virulent than the wild-type parent strain. Complementation of the H. ducreyi waaF mutant with the wild-type H. ducreyi waaF gene resulted in expression of both wild-type LOS and wild-type virulence by this mutant.

Molecular and cellular biology of pneumococcal infection

The complete pneumococcal genome sequence was released in November, 1997. This advance has been combined with new understanding of physiology and pathogenesis including characterization of a family of surface proteins adducted to choline, regulation of virulence, and the regulon for natural DNA transformation. These developments in our understanding of molecular and cellular biology of pneumococcal infection will allow the development of new vaccines and antibiotics against this community acquired pathogen.

Uptake and incorporation of choline and ethanolamine into lipoteichoic acid and teichoic acid by the choline-independent mutant JY2190 of Streptococcus pneumoniae

Lipoteichoic acid- and teichoic acid-containing muropeptides were isolated from choline- or ethanolamine-grown cells of the choline-independent mutant JY2190 of Streptococcus pneumoniae. Choline was taken up and incorporated into lipoteichoic acid and teichoic acid with 81% efficiency, compared with the parent strain Rx1. With similar efficiency, ethanolamine was incorporated. Accordingly, the mutant is a valuable tool for identifying the individual genes encoding the enzymes of choline utilisation, because any of these genes can be deleted without affecting viability and growth rate.

Adaptation of Haemophilus influenzae to acquired and innate humoral immunity based on phase variation of lipopolysaccharide

Phase variation in colony morphology has been associated with the pathogenesis of infection caused by Haemophilus influenzae. This study shows that differences in colony opacity in non-typeable H. influenzae (NTHi) strain H233 involve phase changes in the lipopolysaccharide (LPS) and depend on the expression of licl and lic2, which contain translational switches based on intragenic tandem repeats of 5'-CAAT-3'. Genetic analysis showed that opaque organisms have an out-of-frame number of repeats in both licl, required for the expression of phosphorylcholine (ChoP), and lic2, a putative galactosyl transferase that adds the terminal galactose on Galalpha1-4Gal. Defined variants in these loci were used to examine the contribution of individual LPS structures to resistance to serum bactericidal activity mediated by antibody and C-reactive protein (CRP). The addition of ChoP by licl was the only factor in serum killing involving CRP and complement. The terminal galactose moiety, in contrast, conferred resistance to killing by naturally acquired antibody and complement present in human serum. As Galalpha1-4Gal is also found on human glycolipids, it appears that decoration of the cell surface with this host-like antigen blocks antibody-mediated serum bactericidal activity. Genetic analysis of NTHi within the human respiratory tract demonstrated that Galalpha1-4Gal may not be expressed during carriage but may be advantageous for the organism in inflammatory states such as pneumonia.

The phosphorylcholine epitope undergoes phase variation on a 43-kilodalton protein in Pseudomonas aeruginosa and on pili of Neisseria meningitidis and Neisseria gonorrhoeae

Phosphorylcholine (ChoP) is a component of the teichoic acids of Streptococcus pneumoniae and has been recently identified on the lipopolysaccharide of Haemophilus influenzae, also a major pathogen of the human respiratory tract. Other gram-negative pathogens that frequently infect the human respiratory tract were surveyed for the presence of the ChoP epitope as indicated by binding to monoclonal antibodies (MAbs) recognizing this structure. The ChoP epitope was found on a 43-kDa protein on all clinical isolates of Pseudomonas aeruginosa examined and on several class I and II pili of Neisseria meningitidis. The specificity of the anti-ChoP MAb was demonstrated by the inhibition of binding in the presence of ChoP but not structural analogs. As in the case of H. influenzae, the expression of this epitope was phase variable on these species. In P. aeruginosa, this epitope was expressed at detectable levels only at lower growth temperatures. Expression of the ChoP epitope on piliated neisseriae displayed phase variation, both linked to pilus expression and independently of fully piliated bacteria.

Nontypeable Haemophilus influenzae: pathogenesis and prevention

In this paper, we describe the ability of nontypeable Haemophilus influenzae (NTHi) to coexist with the human host and the devastating results associated with disruption of the delicate state of balanced pathogenesis, resulting in both acute and chronic respiratory tract infections. It has been seen that the strains of NTHi causing disease show a marked genetic and phenotypic diversity but that changes in the lipooligosaccharide (LOS) and protein size and antigenicity in chronically infected individuals indicate that individual strains of NTHi can remain and adapt themselves to avoid expulsion from their infective niche. The lack of reliance of NTHi on a single mechanism of attachment and its ability to interact with the host with rapid responses to its environment confirmed the success of this organism as both a colonizer and a pathogen. In vitro experiments on cell and organ cultures, combined with otitis media and pulmonary models in chinchillas, rats, and mice, have allowed investigations into individual interactions between NTHi and the mammalian host. The host-organism interaction appears to be a two-way process, with NTHi using cell surface structures to directly interact with the mammalian host and using secreted proteins and LOS to change the mammalian host in order to pave the way for colonization and invasion. Many experiments have also noted that immune system evasion through antigenic variation, secretion of enzymes and epithelial cell invasion allowed NTHi to survive for longer periods despite a specific immune response being mounted to infection. Several outer membrane proteins and LOS derivatives are discussed in relation to their efficacy in preventing pulmonary infections and otitis media in animals. General host responses with respect to age, genetic makeup, and vaccine delivery routes are considered, and a mucosal vaccine strategy is suggested.

Phosphorylcholine on the lipopolysaccharide of Haemophilus influenzae contributes to persistence in the respiratory tract and sensitivity to serum killing mediated by C-reactive protein

Haemophilus influenzae undergoes phase variation in expression of the phosphorylcholine (ChoP) epitope, a structure present on several invasive pathogens residing in the human respiratory tract. In this study, structural analysis comparing organisms with and without this epitope confirmed that variants differ in the presence of ChoP on the cell surface-exposed outer core of the lipopolysaccharide. During nasopharyngeal carriage in infant rats, there was a gradual selection for H. influenzae variants that express ChoP. In addition, genotypic analysis of the molecular switch that controls phase variation predicted that the ChoP+ phenotype was predominant in H. influenzae in human respiratory tract secretions. However, ChoP+ variants of nontypable H. influenzae were more sensitive to the bactericidal activity of human serum unrelated to the presence of naturally acquired antibody to ChoP. Serum bactericidal activity required the binding of C-reactive protein (CRP) with subsequent activation of complement through the classical pathway. Results of this study suggested that the ability of H. influenzae to vary expression of this unusual bacterial structure may correlate with its ability both to persist on the mucosal surface (ChoP+ phenotype) and to cause invasive infection by evading innate immunity mediated by CRP (ChoP- phenotype).

A genomic perspective on protein families

In order to extract the maximum amount of information from the rapidly accumulating genome sequences, all conserved genes need to be classified according to their homologous relationships. Comparison of proteins encoded in seven complete genomes from five major phylogenetic lineages and elucidation of consistent patterns of sequence similarities allowed the delineation of 720 clusters of orthologous groups (COGs). Each COG consists of individual orthologous proteins or orthologous sets of paralogs from at least three lineages. Orthologs typically have the same function, allowing transfer of functional information from one member to an entire COG. This relation automatically yields a number of functional predictions for poorly characterized genomes. The COGs comprise a framework for functional and evolutionary genome analysis.

Primary structure of a new phosphocholine-containing glycoglycerolipid of Mycoplasma fermentans

The chemical structure of a novel phosphocholine-containing glycoglycerolipid, the major polar lipid in the cell membrane of Mycoplasma fermentans PG18, was investigated by chemical analyses, gas-liquid chromatography-mass spectrometry, matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, as well as one- and two-dimensional homo- and heteronuclear NMR spectroscopy and identified as 6'-O-(3"-phosphocholine-2"-amino-1"-phospho-1", 3"-propanediol)-alpha-D-glucopyranosyl-(1'-->3)-1,2-diacyl-glycerol (MfGL-II). Palmitate (16:0) and stearate (18:0), in a 3.6:1 molar ratio, constitute the major fatty acids present. MALDI-TOF mass spectrometry revealed two major pseudomolecular ions at m/z 1049.5 [MI + H]+ and 1077.3 [MII + H]+ representing a dipalmitoyl as the major component and a palmitoyl-stearoyl structure as a minor component. This is the first report of 2-amino-1,3-propanediol-1,3-bisphosphate present in a natural product. This glycoglycerolipid is the second phosphocholine-containing glycoglycerolipid found in M. fermentans.

The biology of pneumococcal infection

For 100 y, the study of the molecular mechanism of pneumococcal infection has richly rewarded biomedical science and pediatrics. More recently, a framework has emerged for how the pathogen engineers colonization, invasion of the lung and bloodstream, and finally, entry into the brain. This trafficking is then followed by a separate set of events to generate the symptoms of disease. Understanding the ligand receptor interactions that dictate these events has suggested new concepts for how to control the course of an infectious process and improve the morbidity and mortality of encounters with this prevalent pathogen of children.