Acylation of the lipooligosaccharide of Haemophilus influenzae and colonization: an htrB mutation diminishes the colonization of human airway epithelial cells

An outer membrane determinant for RNA phage genome entry in Pseudomonas aeruginosa

Host range of a phage is determined at the various life cycle stages during phage infection. We reported the limited phage-receptor interaction between the RNA phage, PP7 and its host Pseudomonas aeruginosa strains: PAO1 has susceptible type IV pilus (TFP) pilin, whereas PA14 has resistant pilin. Here, we have created a PA14 derivative (PA14P) with the PAO1 pilin gene and found that other determinants than TFP pilin could limit PP7 infectivity in PA14P. Transposon mutant screens revealed that PP7 infectivity was restored in the PA14P mutants (htrB2) lacking a secondary acyltransferase in lipid A biosynthesis. The lack of this enzyme increased the RNA phage entry, which is deemed attributed to the loosened lipopolysaccharide (LPS) structure. Polymyxin B treatment also selectively increased the RNA phage entry. These results demonstrated that LPS structures could limit the entry stage of RNA phages, providing another determinant for the host range in diverse P. aeruginosa strains.

The LpxL acyltransferase is required for normal growth and penta-acylation of lipid A in Burkholderia cenocepacia

Lipid A anchors the lipopolysaccharide (LPS) to the outer membrane and is usually composed of a hexa-acylated diglucosamine backbone. Burkholderia cenocepacia, an opportunistic pathogen, produces a mixture of tetra- and penta-acylated lipid A. "Late" acyltransferases add secondary acyl chains to lipid A after the incorporation of four primary acyl chains to the diglucosamine backbone. Here, we report that B. cenocepacia has only one late acyltransferase, LpxL (BCAL0508), which adds a myristoyl chain to the 2' position of lipid A resulting in penta-acylated lipid A. We also identified PagL (BCAL0788), which acts as an outer membrane lipase by removing the primary β-hydroxymyristate (3-OH-C14:0) chain at the 3 position, leading to tetra-acylated lipid A. Unlike PagL, LpxL depletion caused reduced cell growth and defects in cell morphology, both of which were suppressed by overexpressing the LPS flippase MsbA (BCAL2408), suggesting that lipid A molecules lacking the fifth acyl chain contributed by LpxL are not good substrates for the flippase. We also show that intracellular B. cenocepacia within macrophages produced more penta-acylated lipid A, suggesting lipid A penta-acylation in B. cenocepacia is required not only for bacterial growth and morphology but also for adaptation to intracellular lifestyle.

Lipid A structural modifications in extreme conditions and identification of unique modifying enzymes to define the Toll-like receptor 4 structure-activity relationship

Strategies utilizing Toll-like receptor 4 (TLR4) agonists for treatment of cancer, infectious diseases, and other targets report promising results. Potent TLR4 antagonists are also gaining attention as therapeutic leads. Though some principles for TLR4 modulation by lipid A have been described, a thorough understanding of the structure-activity relationship (SAR) is lacking. Only through a complete definition of lipid A-TLR4 SAR is it possible to predict TLR4 signaling effects of discrete lipid A structures, rendering them more pharmacologically relevant. A limited 'toolbox' of lipid A-modifying enzymes has been defined and is largely composed of enzymes from mesophile human and zoonotic pathogens. Expansion of this 'toolbox' will result from extending the search into lipid A biosynthesis and modification by bacteria living at the extremes. Here, we review the fundamentals of lipid A structure, advances in lipid A uses in TLR4 modulation, and the search for novel lipid A-modifying systems in extremophile bacteria. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

Site-specific activity of the acyltransferases HtrB1 and HtrB2 in Pseudomonas aeruginosa lipid A biosynthesis

Pseudomonas aeruginosa (PA) is an opportunistic Gram-negative pathogen associated with nosocomial infections, acute infections and chronic lung infections in patients with cystic fibrosis. The ability of PA to cause infection can be attributed to its ability to adapt to a multitude of environments. Modification of the lipid A portion of lipopolysaccharide (LPS) is a vital mechanism Gram-negative pathogens use to remodel the outer membrane in response to environmental stimuli. Lipid A, the endotoxic moiety of LPS, is the major component of the outer leaflet of the outer membrane of Gram-negative bacteria making it a critical factor for bacterial adaptation. One way PA modifies its lipid A is through the addition of laurate and 2-hydroxylaurate. This secondary or late acylation is carried out by the acyltransferase, HtrB (LpxL). Analysis of the PA genome revealed the presence of two htrB homologs, PA0011 (htrB1) and PA3242 (htrB2). In this study, we were able to show that each gene identified is responsible for site-specific modification of lipid A. Additionally, deletions of either gene altered resistance to specific classes of antibiotics, cationic antimicrobial peptides and increased membrane permeability suggesting a role for these enzymes in maintaining optimal membrane organization and integrity.

Use of recombinant human stromal cell-derived factor 1α-loaded fibrin/hyaluronic acid hydrogel networks to achieve functional repair of full-thickness bovine articular cartilage via homing of chondrogenic progenitor cells

OBJECTIVE

Articular cartilage damage after joint trauma seldom heals and often leads to osteoarthritis. We previously identified a migratory chondrogenic progenitor cell (CPC) population that responds chemotactically to cell death and rapidly repopulates the injured cartilage matrix, which suggests a potential approach for articular cartilage repair. This study was undertaken to determine whether recombinant human stromal cell-derived factor 1α (rhSDF-1α), a potent CPC chemoattractant, would improve the quality of cartilage regeneration, hypothesizing that increased recruitment of CPCs by rhSDF-1α would promote the formation of cartilage matrix upon chondrogenic induction.

METHODS

Full-thickness bovine chondral defects were filled with hydrogel, composed of fibrin and hyaluronic acid and containing rhSDF-1α. Cell migration was monitored, followed by chondrogenic induction. Regenerated tissue was evaluated by histology, immunohistochemistry, and scanning electron microscopy. Push-out tests and unconfined compression tests were performed to assess the strength of tissue integration and the mechanical properties of the regenerated cartilage.

RESULTS

Use of rhSDF-1α dramatically improved CPC recruitment to the chondral defects at 12 days. After 6 weeks under chondrogenic conditions, cell morphology, proteoglycan density, and the ultrastructure of the repair tissue were all similar to that found in native cartilage. Compared with empty controls, neocartilage generated in rhSDF-1α-containing defects showed significantly greater interfacial strength, and acquired mechanical properties comparable to those of native cartilage.

CONCLUSION

This study showed that stimulating local CPC recruitment prior to treatment with chondrogenic factors significantly improves the biochemical and mechanical properties of the cartilage tissue formed in chondral defects. This simple approach may be implemented in vivo as a one-step procedure by staging the release of chemokine and chondrogenic factors from within the hydrogel, which can be achieved using smart drug-delivery systems.

Oxidation of thiols and modification of redox-sensitive signaling in human lung epithelial cells exposed to Pseudomonas pyocyanin

The aim of this study was to examine the effects of pyocyanin exposure on mitochondrial GSH, other cellular thiols (thioredoxin-1, Trx-1), and oxidant-sensitive signaling pathways hypoxia inducible factor (HIF-1α) and heme oxygenase (HO-1) in A549 and HBE cell lines. A549 human type II alveolar epithelial cells and human bronchial epithelial (HBE) cells were treated with varying concentrations of pyocyanin extracted from Pseudomonas aeruginosa bacteria. Cytoplasmic and mitochondrial thiols and oxidant sensitive signal transduction proteins (HIF-1α and HO-1) were measured. Exposure to pyocyanin generated reactive oxygen species (ROS) in cellular mitochondria and altered total cellular glutathione (GSH). Pyocyanin, at concentrations present in conditions in vivo, increased oxidized Trx-1 in A549 human type II alveolar epithelial cells and HBE cells by 184 and 74%, respectively. Oxidized mitochondrial glutathione (GSSG) was elevated more than twofold in both cell types. Pyocyanin also increased the cellular oxidant-sensitive proteins HIF-1α and HO-1. Data indicate that pyocyanin-induced alterations in mitochondrial and cytosolic thiols, as well as oxidant-sensitive proteins, may contribute to P. aeruginosa-mediated lung injury.

Effect of lipooligosaccharide mutations of Haemophilus influenzae on the middle and inner ears

OBJECTIVE

The purpose of this study was to determine the virulence of nontypeable Haemophilus influenzae 2019 (NTHi 2019) and its two lipooligosaccharide (LOS) mutant strains, B29 (gene htrB) and DK1 (gene rfaD), and compare their effect on the middle ear, round window membrane, and inner ear.

RESULTS

Fifteen chinchillas were divided into three equal groups and their bullas inoculated bilaterally with 0.5 ml of 10(2)CFU/ml of parent NTHi 2019, B29 or DK1 mutant strains. Two days after inoculation all animals had otitis media and inflamed middle ear mucosa. There was a trend of greater thickness and infiltration of the round window membrane in animals inoculated with the wild-type NTHi strain compared to the mutant strains and a significant increase in both inflammatory cell infiltration and bacteria presence in the scala tympani area of the inner ear. Strial edema was only observed in the wild-type-inoculated group.

CONCLUSIONS

LOS mutants of NTHi appear to have a reduced ability to pass through the round window membrane resulting in less inner ear inflammation and pathological changes.

Microwave-assisted sample preparation for rapid and sensitive analysis of H. pylori lipid A applicable to a single colony

The lipid A of Gram-negative bacteria plays a major role in the pathogenesis of bacterial infections. Lipid A diversity is observed both in the number and length of fatty-acid side chains and in the presence of terminal phosphate residues and associated modifications. In this report, we describe a new sample preparation method based on microwave-assisted enzymatic digestion and detergent-free mild hydrolysis, in conjunction with a MALDI-time-of-flight (TOF)/TOF analysis, to determine the structures of lipid A from Helicobacter pylori. The total time for sample preparation and mass spectrometric analysis is within 2 h and applicable to profiling the lipid A structures from dried bacterial cells on as little as 1 microg. The reliability of the technique was further demonstrated through the analysis of the lipid A from bacterial cells of different H. pylori strains. The phosphorylation and acylation patterns of lipid A could be elucidated using material from a single colony. Furthermore, we found unusual heptaacyl lipid A species present in H. pylori mutant that have not been previously reported, although the abundance was relatively low. The present study provides the first characterization of the lipid A component from a single bacterial colony sample by mass spectrometry.

Nontypeable Haemophilus influenzae as a pathogen in children

Nontypeable Haemophilus influenzae is a significant pathogen in children, causing otitis media, sinusitis, conjunctivitis, pneumonia, and occasionally invasive infections. H. influenzae type b conjugate vaccines have no effect on infections caused by nontypeable strains because nontypeable strains are nonencapsulated. Approximately, one-third of episodes of otitis media are caused by nontypeable H. influenzae and the bacterium is the most common cause of recurrent otitis media. Recent progress in elucidating molecular mechanisms of pathogenesis, understanding the role of biofilms in otitis media and an increasing understanding of immune responses have potential for development of novel strategies to improve prevention and treatment of otitis media caused by nontypeable H. influenzae. Feasibility of vaccination for prevention of otitis media due to nontypeable H. influenzae was recently demonstrated in a clinical trial with a vaccine that included the surface virulence factor, protein D.

Diminished ICAM-1 expression and impaired pulmonary clearance of nontypeable Haemophilus influenzae in a mouse model of chronic obstructive pulmonary disease/emphysema

The airways of patients with chronic obstructive pulmonary disease (COPD) are continually colonized with bacterial opportunists like nontypeable Haemophilus influenzae (NTHi), and a wealth of evidence indicates that changes in bacterial populations within the lung can influence the severity of COPD. In this study, we used a murine model for COPD/emphysema to test the hypothesis that COPD affects pulmonary clearance. Mice were treated with a pulmonary bolus of elastase, and as reported previously, the lungs of these mice were pathologically similar to those with COPD/emphysema at approximately 1 month posttreatment. Pulmonary clearance of NTHi was significantly impaired in elastase-treated versus mock-treated mice. While histopathologic analysis revealed minimal differences in localized lung inflammation between the two groups, lower levels of intercellular adhesion molecule 1 (ICAM-1) were observed for the airway epithelial surface of elastase-treated mice than for those of control mice. Following infection, elastase-treated mice had lung pathology consistent with pneumonia for as long as 72 h postinfection, whereas at the same time point, mock-treated mice had cleared NTHi and showed little apparent pathology. Large aggregates of bacteria were observed within damaged lung tissue of the elastase-treated mice, whereas sparse individual bacteria were observed in lungs of mock-treated mice at the same time point postinfection. Additional infection studies showed that NTHi mutants with biofilm defects were less persistent in the elastase-treated mice than the parent strain. These findings establish a model for COPD-related infections and support the hypotheses that ICAM-1 promotes clearance of NTHi. Furthermore, the data indicate that NTHi may form biofilms within the context of COPD-related infections.

Identification of two late acyltransferase genes responsible for lipid A biosynthesis in Moraxella catarrhalis

Lipid A is a biological component of the lipo-oligosaccharide of a human pathogen, Moraxella catarrhalis. No other acyltransferases except for UDP-GlcNAc acyltransferase, responsible for lipid A biosynthesis in M. catarrhalis, have been identified. By bioinformatics, two late acyltransferase genes, lpxX and lpxL, responsible for lipid A biosynthesis were identified, and knockout mutants of each gene in M. catarrhalis strain O35E were constructed and named O35ElpxX and O35ElpxL. Structural analysis of lipid A from the parental strain and derived mutants showed that O35ElpxX lacked two decanoic acids (C10:0), whereas O35ElpxL lacked one dodecanoic (lauric) acid (C12:0), suggesting that lpxX encoded decanoyl transferase and lpxL encoded dodecanoyl transferase. Phenotypic analysis revealed that both mutants were similar to the parental strain in their toxicity in vitro. However, O35ElpxX was sensitive to the bactericidal activity of normal human serum and hydrophobic reagents. It had a reduced growth rate in broth and an accelerated bacterial clearance at 3 h (P < 0.01) or 6 h (P < 0.05) after an aerosol challenge in a murine model of bacterial pulmonary clearance. O35ElpxL presented similar patterns to those of the parental strain, except that it was slightly sensitive to the hydrophobic reagents. These results indicate that these two genes, particularly lpxX, encoding late acyltransferases responsible for incorporation of the acyloxyacyl-linked secondary acyl chains into lipid A, are important for the biological activities of M. catarrhalis.

Differential expression of cytokine genes and iNOS induced by nonviable nontypeable Haemophilus influenzae or its LOS mutants during acute otitis media in the rat

OBJECTIVE

We have previously demonstrated that the disruptions of nontypeable Haemophilus influenzae (NTHi) lipooligosaccharide (LOS) htrB and rfaD genes may play a role in the pathogenesis of otitis media (OM). The purpose of this study was to determine whether NTHi LOS gene disruptions influence the induction of gene expression for proinflammatory mediators in vivo using the rat model of acute OM.

METHODS

At 3, 6, 12, 24, 48 and 72 h after transbullar inoculation with nonviable NTHi, expression of genes for the cytokines and chemolines; tumor necrosis factor alpha (TNF-alpha), interleukin-lbeta (IL-1beta), and IL-6, IL-1alpha, IL-8, IL-10, and inducible nitric oxide synthase (iNOS) were quantitated by real-time PCR. Enzyme-linked immunosorbent assay was performed to confirm the gene expression data as determined by real-time PCR. The middle ear inflammatory responses were also evaluated.

RESULTS

The NTHi 2019 parent and its isogenic LOS htrB (B29) and rfaD (DK-1) mutant strains induced a significant up-regulation in gene expression for the cytokines examined compared to the sham-inoculated controls at 3, 6 and 12 h post-inoculation (P<0.05 in all cases). However, the NTHi 2019 cohort demonstrated a significant increase in gene expression for TNF-alpha (up to 6 h), IL-1alpha and IL-8 (up to 24 h), IL-1beta and IL-6 (up to 48 h), and IL-10 and iNOS (up to 72 h) relative to the animals inoculated with NTHi B29 (P<0.05, in all cases), Moreover, the concentrations of inflammatory cells in the middle ear lavage fluid samples from the NTHi 2019 cohort were 2.8-5.3-fold higher than those of the B29 cohort. There were no significant differences in mRNA expression of the cytokines between the NTHi 2019 and the DK-1-treated groups.

CONCLUSIONS

Data from this study indicate that the disruption of the NTHi htrB gene may impact the temporal mRNA expression of inflammatory mediators and inflammation within the middle ear.

Acyl chain specificity of the acyltransferases LpxA and LpxD and substrate availability contribute to lipid A fatty acid heterogeneity in Porphyromonas gingivalis

Porphyromonas gingivalis lipid A is heterogeneous with regard to the number, type, and placement of fatty acids. Analysis of lipid A by matrix-assisted laser desorption ionization-time of flight mass spectrometry reveals clusters of peaks differing by 14 mass units indicative of an altered distribution of the fatty acids generating different lipid A structures. To examine whether the transfer of hydroxy fatty acids with different chain lengths could account for the clustering of lipid A structures, P. gingivalis lpxA (lpxA(Pg)) and lpxD(Pg) were cloned and expressed in Escherichia coli strains in which the homologous gene was mutated. Lipid A from strains expressing either of the P. gingivalis transferases was found to contain 16-carbon hydroxy fatty acids in addition to the normal E. coli 14-carbon hydroxy fatty acids, demonstrating that these acyltransferases display a relaxed acyl chain length specificity. Both LpxA and LpxD, from either E. coli or P. gingivalis, were also able to incorporate odd-chain fatty acids into lipid A when grown in the presence of 1% propionic acid. This indicates that E. coli lipid A acyltransferases do not have an absolute specificity for 14-carbon hydroxy fatty acids but can transfer fatty acids differing by one carbon unit if the fatty acid substrates are available. We conclude that the relaxed specificity of the P. gingivalis lipid A acyltransferases and the substrate availability account for the lipid A structural clusters that differ by 14 mass units observed in P. gingivalis lipopolysaccharide preparations.

Lipooligosaccharides containing phosphorylcholine delay pulmonary clearance of nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) causes pulmonary infections in patients with chronic obstructive pulmonary disease and other mucociliary clearance defects. Like many bacteria inhabiting mucosal surfaces, NTHi produces lipooligosaccharide (LOS) endotoxins that lack the O side chain. Persistent NTHi populations express a discrete subset of LOS glycoforms, including those containing phosphorylcholine (PCho). In this study, we compared two NTHi strains with isogenic mutants lacking PCho for clearance from mice following pulmonary infection. Consistent with data from other model systems, populations of the strains NTHi 2019 and NTHi 86-028NP recovered from mouse lung contained an increased proportion of PCho+ variants compared to that in the inocula. PCho- mutants were more rapidly cleared. Serial passage of NTHi increased both PCho content and bacterial resistance to clearance, and no such increases were observed for PCho- mutants. Increased PCho content was also observed in NTHi populations within non-endotoxin-responsive C3H/HeJ and Toll-like receptor 4 null (TLR4-/-) mice, albeit at later times postinfection. Changes in bacterial subpopulations and clearance were unaffected in TLR2-/- mice compared to the subpopulations in and clearance from mice of the parental strain. The clearance of PCho- mutants occurred at earlier time points in both strain backgrounds and in all types of mice. Comparison of bacterial populations in lung tissue cryosections by immunofluorescent staining showed sparse bacteria within the air spaces of C57BL/6 mice and large bacterial aggregates within the lungs of MyD88-/- mice. These results indicate that PCho promotes bacterial resistance to pulmonary clearance early in infection in a manner that is at least partially independent of the TLR4 pathway.

Characterization of htrB and msbB mutants of the light organ symbiont Vibrio fischeri

Bacterial lipid A is an important mediator of bacterium-host interactions, and secondary acylations added by HtrB and MsbB can be critical for colonization and virulence in pathogenic infections. In contrast, Vibrio fischeri lipid A stimulates normal developmental processes in this bacterium's mutualistic host, Euprymna scolopes, although the importance of lipid A structure in this symbiosis is unknown. To further examine V. fischeri lipid A and its symbiotic function, we identified two paralogs of htrB (designated htrB1 and htrB2) and an msbB gene in V. fischeri ES114 and demonstrated that these genes encode lipid A secondary acyltransferases. htrB2 and msbB are found on the Vibrio "housekeeping" chromosome 1 and are conserved in other Vibrio species. Mutations in htrB2 and msbB did not impair symbiotic colonization but resulted in phenotypic alterations in culture, including reduced motility and increased luminescence. These mutations also affected sensitivity to sodium dodecyl sulfate, kanamycin, and polymyxin, consistent with changes in membrane permeability. Conversely, htrB1 is located on the smaller, more variable vibrio chromosome 2, and an htrB1 mutant was wild-type-like in culture but appeared attenuated in initiating the symbiosis and was outcompeted 2.7-fold during colonization when mixed with the parent. These data suggest that htrB2 and msbB play conserved general roles in vibrio biology, whereas htrB1 plays a more symbiosis-specific role in V. fischeri.

Role of different moieties from the lipooligosaccharide molecule in biological activities of the Moraxella catarrhalis outer membrane

Lipooligosaccharide (LOS), a major component of the outer membrane of Moraxella catarrhalis, consists of two major moieties: a lipid A and a core oligosaccharide (OS). The core OS can be dissected into a linker and three OS chains. To gain an insight into the biological activities of the LOS molecules of M. catarrhalis, we used a random transposon mutagenesis approach with an LOS specific monoclonal antibody to construct a serotype A O35Elgt3 LOS mutant. MALDI-TOF-MS of de-O-acylated LOS from the mutant and glycosyl composition, linkage, and NMR analysis of its OS indicated that the LOS contained a truncated core OS and consisted of a Glc-Kdo(2) (linker)-lipid A structure. Phenotypic analysis revealed that the mutant was similar to the wild-type strain in its growth rate, toxicity and susceptibility to hydrophobic reagents. However, the mutant was sensitive to bactericidal activity of normal human serum and had a reduced adherence to human epithelial cells. These data, combined with our previous data obtained from mutants which contained only lipid A or lacked LOS, suggest that the complete OS chain moiety of the LOS is important for serum resistance and adherence to epithelial cells, whereas the linker moiety is critical for maintenance of the outer membrane integrity and stability to preserve normal cell growth. Both the lipid A and linker moieties contribute to the LOS toxicity.

Identification of LpxL, a late acyltransferase of Francisella tularensis

Lipopolysaccharide (LPS) is a major component of the outer membrane of gram-negative bacteria, and the lipid A region of LPS mediates stimulation of the immune system in a structure-dependent manner. Unlike the LPS of many other gram-negative bacteria, the LPS of Francisella tularensis isolated from in vitro cultures is not proinflammatory. This observed lack of proinflammatory prowess may reflect structural features of the lipid A, such as the number and length of the acyl chains and the single-phosphate group. To better understand this phenotype, we have begun to elucidate LPS biosynthesis in F. tularensis. We present complementation, mutational, and chemical data demonstrating that F. tularensis FTT0232c encodes a functional late acyltransferase enzyme with specificity similar to that of the Escherichia coli LpxL ortholog. Expression of this late acyltransferase complemented the temperature-sensitive and hypoacylated lipid A phenotypes of an E. coli lpxL mutant, expression of FTT0232c is increased during intracellular growth relative to that during in vitro growth, and finally, LPS obtained from a mutant of F. tularensis lacking FTT0232c showed an abundant triacyl lipid A species after mass spectrometric analysis, consistent with the loss of an LpxL late acyltransferase.

Nontypeable Haemophilus influenzae: understanding virulence and commensal behavior

Haemophilus influenzae is genetically diverse and exists as a near-ubiquitous human commensal or as a pathogen. Invasive type b disease has been almost eliminated in developed countries; however, unencapsulated strains - nontypeable H. influenzae (NTHi) - remain important as causes of respiratory infections. Respiratory tract disease occurs when NTHi adhere to or invade respiratory epithelial cells, initiating one or more of several proinflammatory pathways. Biofilm formation explains many of the observations seen in chronic otitis media and chronic bronchitis. However, NTHi biofilms seem to lack a biofilm-specific polysaccharide in the extracellular matrix, a source of controversy regarding their relevance. Successful commensalism requires dampening of the inflammatory response and evasion of host defenses, accomplished in part through phase variation.

Phosphorylcholine expression by nontypeable Haemophilus influenzae correlates with maturation of biofilm communities in vitro and in vivo

Nontypeable Haemophilus influenzae (NTHI) causes chronic infections that feature the formation of biofilm communities. NTHI variants within biofilms have on their surfaces lipooligosaccharides containing sialic acid (NeuAc) and phosphorylcholine (PCho). Our work showed that NeuAc promotes biofilm formation, but we observed no defect in the initial stages of biofilm formation for mutants lacking PCho. In this study, we asked if alterations in NTHI PCho content affect later stages of biofilm maturation. Biofilm communities were compared for NTHI 2019 and isogenic mutants that either lacked PCho (NTHI 2019 licD) or were constitutively locked in the PCho-positive phase (NTHI 2019 lic(ON)). Transformants expressing green fluorescent protein were cultured in continuous-flow biofilms and analyzed by confocal laser scanning microscopy. COMSTAT was used to quantify different biofilm parameters. PCho expression correlated significantly with increased biofilm thickness, surface coverage, and total biomass, as well as with a decrease in biofilm roughness. Comparable results were obtained by scanning electron microscopy. Analysis of thin sections of biofilms by transmission electron microscopy revealed shedding of outer membrane vesicles by NTHI bacteria within biofilms and staining of matrix material with ruthenium red in biofilms formed by NTHI 2019 lic(ON). The biofilms of all three strains were comparable in viability, the presence of extracellular DNA, and the presence of sialylated moieties on or between bacteria. In vivo infection studies using the chinchilla model of otitis media showed a direct correlation between PCho expression and biofilm formation within the middle-ear chamber and an inverse relationship between PCho and persistence in the planktonic phase in middle-ear effusions. Collectively, these data show that PCho correlates with, and may promote, the maturation of NTHI biofilms. Further, this structure may be disadvantageous in the planktonic phase.

Expression of the htrB gene is essential for responsiveness of Salmonella typhimurium and Campylobacter jejuni to harsh environments

In Campylobacter jejuni, an htrB homologous gene is located in the lipo-oligosaccharide synthesis gene cluster. This study examined the effects of htrB expression on the responsiveness of Salmonella typhimurium and C. jejuni to harsh environments. Complementation experiments showed that the C. jejuni htrB gene could restore the normal morphology of the Salmonella htrB mutant, and its ability to grow without inhibition under heat, acid and osmotic stresses, but not bile stress. This indicated that the htrB genes in C. jejuni and S. typhimurium exhibit similar pleiotropic effects. Moreover, quantitative real-time RT-PCR showed that expression of the C. jejuni htrB gene was upregulated under acid, heat, oxidative and osmotic stresses, but did not change under bile stress. This indicated that the C. jejuni htrB gene plays a role in regulating cell responses to various environmental changes. Furthermore, deletion mutation of the htrB gene in C. jejuni was lethal, indicating that the htrB gene is essential for C. jejuni survival. Therefore, these results showed that expression of the htrB gene is essential for the response of S. typhimurium and C. jejuni to environmental stresses.

Phosphorylcholine decreases early inflammation and promotes the establishment of stable biofilm communities of nontypeable Haemophilus influenzae strain 86-028NP in a chinchilla model of otitis media

Nontypeable Haemophilus influenzae (NTHi) is a leading causative agent of otitis media. Much of the inflammation occurring during NTHi disease is initiated by lipooligosaccharides (LOS) on the bacterial surface. Phosphorylcholine (PCho) is added to some LOS forms in a phase-variable manner, and these PCho(+) variants predominate in vivo. Thus, we asked whether this modification confers some advantage during infection. Virulence of an otitis media isolate (NTHi strain 86-028NP) was compared with that of an isogenic PCho transferase (licD) mutant using a chinchilla (Chinchilla lanigera) model of otitis media. Animals infected with NTHi 86-028NP licD demonstrated increased early inflammation and a delayed increase in bacterial counts compared to animals infected with NTHi 86-028NP. LOS purified from chinchilla-passed NTHi 86-028NP had increased PCho content compared to LOS purified from the inoculum. Both strains were recovered from middle ear fluids as long as 14 days postinfection. Biofilms were macroscopically visible in the middle ears of euthanized animals infected with NTHi 86-028NP 7 days and 14 days postchallenge. Conversely, less dense biofilms were observed in animals infected with NTHi 86-028NP licD 7 days postinfection, and none of the animals infected with NTHi 86-028NP licD had a visible biofilm by 14 days. Fluorescent antibody staining revealed PCho(+) variants within biofilms, similar to our prior results with tissue culture cells in vitro (S. L. West-Barnette, A. Rockel, and W. E. Swords, Infect. Immun. 74:1828-1836, 2006). Animals coinfected with equal proportions of both strains had equal persistence of each strain and somewhat greater severity of disease. We thus conclude that PCho promotes NTHi infection and persistence by reducing the host inflammatory response and by promoting formation of stable biofilm communities.

Biofilm growth increases phosphorylcholine content and decreases potency of nontypeable Haemophilus influenzae endotoxins

Nontypeable Haemophilus influenzae (NTHI) is a common respiratory commensal and opportunistic pathogen. NTHI is normally contained within the airways by host innate defenses that include recognition of bacterial endotoxins by Toll-like receptor 4 (TLR4). NTHI produces lipooligosaccharide (LOS) endotoxins which lack polymeric O side chains and which may contain host glycolipids. We recently showed that NTHI biofilms contain variants with sialylated LOS glycoforms that are essential to biofilm formation. In this study, we show that NTHI forms biofilms on epithelial cell layers. Confocal analysis revealed that sialylated variants were distributed throughout the biofilm, while variants expressing phosphorylcholine (PCho) were found within the biofilm. Consistent with this observation, PCho content of LOS purified from NTHI biofilms was increased compared to LOS from planktonic cultures. Hypothesizing that the observed changes in endotoxin composition could affect bioactivity, we compared inflammatory responses to NTHI LOS purified from biofilm and planktonic cultures. Our results show that endotoxins from biofilms induced weaker host innate responses. While we observed a minimal effect of sialylation on LOS bioactivity, there was a significant decrease in bioactivity associated with PCho substitutions. We thus conclude that biofilm growth increases the proportion of PCho+ variants in an NTHI population, resulting in a net decrease in LOS bioactivity. Thus, in addition to their well-documented resistance phenotypes, our data show that biofilm communities of NTHI bacteria contain variants that evoke less potent host responses.

Expression of a Porphyromonas gingivalis lipid A palmitylacyltransferase in Escherichia coli yields a chimeric lipid A with altered ability to stimulate interleukin-8 secretion

In Escherichia coli the gene htrB codes for an acyltransferase that catalyses the incorporation of laurate into lipopolysaccharide (LPS) as a lipid A substituent. We describe the cloning, expression and characterization of a Porphyromonas gingivalis htrB homologue. When the htrB homologue was expressed in wild-type E. coli or a mutant strain deficient in htrB, a chimeric LPS with altered lipid A structure was produced. Compared with wild-type E. coli lipid A, the new lipid A species contained a palmitate (C16) in the position normally occupied by laurate (C12) suggesting that the cloned gene performs the same function as E. coli htrB but preferentially transfers the longer-chain palmitic acid that is known to be present in P. gingivalis LPS. LPS was purified from wild-type E. coli, the E. coli htrB mutant strain and the htrB mutant strain expressing the P. gingivalis acyltransferase. LPS from the palmitate bearing chimeric LPS as well as the htrB mutant exhibited a reduced ability to activate human embryonic kidney 293 (HEK293) cells transfected with TLR4/MD2. LPS from the htrB mutant also had a greatly reduced ability to stimulate interleukin-8 (IL-8) secretion in both endothelial cells and monocytes. In contrast, the activity of LPS from the htrB mutant bacteria expressing the P. gingivalis gene displayed wild-type activity to stimulate IL-8 production from endothelial cells but a reduced ability to stimulate IL-8 secretion from monocytes. The intermediate activation observed in monocytes for the chimeric LPS was similar to the pattern seen in HEK293 cells expressing TLR4/MD2 and CD14. Thus, the presence of a longer-chain fatty acid on E. coli lipid A altered the activity of the LPS in monocytes but not endothelial cell assays and the difference in recognition does not appear to be related to differences in Toll-like receptor utilization.

Characterization of a cluster of three glycosyltransferase enzymes essential for Moraxella catarrhalis lipooligosaccharide assembly

Moraxella catarrhalis isolates express lipooligosaccharide (LOS) molecules on their surface, which share epitopes similar to that of the Neisseria and Haemophilus species. These common LOS epitopes have been implicated in various steps of pathogenesis for the different organisms. In this study, a cluster of three LOS glycosyltransferase genes (lgt) were identified in M. catarrhalis 7169, a strain that produces a serotype B LOS. Mutants in these glycosyltransferase genes were constructed, and the resulting LOS phenotypes were consistent with varying degrees of truncation compared to wild-type LOS. The LOS structures of each lgt mutant were no longer detected by a monoclonal antibody (MAb 4G5) specific to a highly conserved terminal epitope nor by a monoclonal antibody (MAb 3F7) specific to the serotype B LOS side chain. Mass spectrometry of the LOS glycoforms assembled by two of these lgt mutants indicated that lgt1 encodes an alpha(1-2) glucosyltransferase and the lgt2 encodes a beta(1-4) galactosyltransferase. However, these structural studies could not delineate the function for lgt3. Therefore, M. catarrhalis lgt3 was introduced into a defined beta(1-4) glucosyltransferase Haemophilus ducreyi 35000glu- mutant in trans, and monoclonal antibody analysis confirmed that Lgt3 complemented the LOS defect. These data suggest that lgt3 encodes a glucosyltransferase involved in the addition of a beta(1-4)-linked glucose to the inner core. Furthermore, we conclude that this enzymatic step is essential for the assembly of the complete LOS glycoform expressed by M. catarrhalis 7169.

Differential involvement of toll-like receptors 2 and 4 in the host response to acute respiratory infections with wild-type and mutant Haemophilus influenzae strains

We used a mouse model of acute respiratory infections to investigate the role of Toll-like receptor 2 (TLR2) and TLR4 in the host response to Haemophilus influenzae. Acute aerosol exposures to wild-type strains of H. influenzae showed that TLR4 function was essential for TNF-alpha induction, neutrophil influx, and bacterial clearance. To determine how lipooligosaccharide (LOS) modifications would affect the role of TLR4 in inducing the host response, we used acute infections with an H. influenzae strain expressing a mutation in the htrB gene. This mutant strain expresses an LOS subunit with decreased acylation. In response to H. influenzae htrB infection, tumor necrosis factor alpha (TNF-alpha) secretion remained TLR4 dependent. But the decrease in LOS acylation made the neutrophil influx and the bacterial clearance also dependent on TLR2, as shown by the decreased host response elicited in TLR2 knockout mice compared to C57BL/6 mice. A subsequent analysis of TLR2 and TLR4 gene expression by quantitative PCR indicated that TLR4 function induces TLR2 expression and vice versa. These results indicate that some changes in the LOS subunit of H. influenzae can favor signaling through non-TLR4 receptors, such as TLR2. The results also indicate a close interaction between TLR4 and TLR2 that tightly regulates the expression of both receptors.

Biosynthesis, transport, and modification of lipid A

Lipopolysaccharide (LPS) is the major surface molecule of Gram-negative bacteria and consists of three distinct structural domains: O-antigen, core, and lipid A. The lipid A (endotoxin) domain of LPS is a unique, glucosamine-based phospholipid that serves as the hydrophobic anchor of LPS and is the bioactive component of the molecule that is associated with Gram-negative septic shock. The structural genes encoding the enzymes required for the biosynthesis of Escherchia coli lipid A have been identified and characterized. Lipid A is often viewed as a constitutively synthesized structural molecule. However, determination of the exact chemical structures of lipid A from diverse Gram-negative bacteria shows that the molecule can be further modified in response to environmental stimuli. These modifications have been implicated in virulence of pathogenic Gram-negative bacteria and represent one of the molecular mechanisms of microbial surface remodeling used by bacteria to help evade the innate immune response. The intent of this review is to discuss the enzymatic machinery involved in the biosynthesis of lipid A, transport of the molecule, and finally, those enzymes involved in the modification of its structure in response to environmental stimuli.

Prevalence of the hifBC, hmw1A, hmw2A, hmwC, and hia Genes in Haemophilus influenzae Isolates

Adherence of Haemophilus influenzae to respiratory epithelial cells is the first step in the pathogenesis of H. influenzae infection and is facilitated by the action of several adhesins located on the surface of the bacteria. In this study, prevalences of hifBC, which represent the pilus gene cluster; hmw1A, hmw2A, and hmwC, which represent high-molecular-weight (HMW) adhesin genes; and hia, which represents H. influenzae adhesin (Hia) genes were determined among clinical isolates of encapsulated type b (Hib) and nonencapsulated (NTHi) H. influenzae. hifBC genes were detected in 109 of 170 (64%) Hib strains and in 46 of 162 (28%) NTHi isolates (P = 0.0001) and were more prevalent among the invasive type b strains than invasive NTHi strains (P = 0.00003). Furthermore, hifBC genes were significantly more prevalent (P = 0.0398) among NTHi throat isolates than NTHi middle ear isolates. hmw1A, hmw2A, hmwC, and hia genes were not detected in Hib strains. Among NTHi isolates, the prevalence of hmw1A was 51%, the prevalence of hmw2A was 23%, the prevalence of hmwC was 48%, and the prevalence of hia was 33%. The hmw genes were significantly more prevalent among middle ear than throat isolates, while hia did not segregate with a respiratory tract site. These results show the variability of the presence of adhesin genes among clinical H. influenzae isolates and suggest that hemagglutinating pili may play a larger role in H. influenzae nasopharyngeal colonization than in acute otitis media whereas the HMW adhesins may be virulence factors for acute otitis media.

Pseudomonas aeruginosa pyocyanin directly oxidizes glutathione and decreases its levels in airway epithelial cells

Production of pyocyanin enhances Pseudomonas aeruginosa virulence. Many of pyocyanin's in vitro and in vivo cytotoxic effects on human cells appear to result from its ability to redox cycle. Pyocyanin directly accepts electrons from NADH or NADPH with subsequent electron transfer to oxygen, generating reactive oxygen species. Reduced glutathione (GSH) is an important cellular antioxidant, and it contributes to the regulation of redox-sensitive signaling systems. Using the human bronchial epithelial (HBE) and the A549 human type II alveolar epithelial cell lines, we tested the hypothesis that pyocyanin can deplete airway epithelial cells of GSH. Incubation of both cell types with pyocyanin led to a concentration-dependent loss of cellular GSH (up to 50%) and an increase in oxidized GSH (GSSG) in the HBE, but not A549 cells, at 24 h. An increase in total GSH, mostly as GSSG, was detected in the culture media, suggesting export of GSH or GSSG from the pyocyanin-exposed cells. Loss of GSH could be due to pyocyanin-induced H(2)O(2) formation. However, overexpression of catalase only partially prevented the pyocyanin-mediated decline in cellular GSH. Cell-free electron paramagnetic resonance studies revealed that pyocyanin directly oxidizes GSH, forming pyocyanin free radical and O(2)(-). Pyocyanin oxidized other thiol-containing compounds, cysteine and N-acetyl-cysteine, but not methionine. Thus GSH may enhance pyocyanin-induced cytotoxicity by functioning as an alternative source of reducing equivalents for pyocyanin redox cycling. Pyocyanin-mediated alterations in cellular GSH may alter epithelial cell functions by modulating redox sensitive signaling events.

Sialylation of lipooligosaccharides promotes biofilm formation by nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) is a major cause of opportunistic respiratory tract infections, including otitis media and bronchitis. The persistence of NTHi in vivo is thought to involve bacterial persistence in a biofilm community. Therefore, there is a need for further definition of bacterial factors contributing to biofilm formation by NTHi. Like other bacteria inhabiting host mucosal surfaces, NTHi has on its surface a diverse array of lipooligosaccharides (LOS) that influence host-bacterial interactions. In this study, we show that LOS containing sialic (N-acetyl-neuraminic) acid promotes biofilm formation by NTHi in vitro and bacterial persistence within the middle ear or lung in vivo. LOS from NTHi in biofilms was sialylated, as determined by comparison of electrophoretic mobilities and immunochemical reactivities before and after neuraminidase treatment. Biofilm formation was significantly reduced in media lacking sialic acid, and a siaB (CMP-sialic acid synthetase) mutant was deficient in biofilm formation in three different in vitro model systems. The persistence of an asialylated siaB mutant was attenuated in a gerbil middle ear infection model system, as well as in a rat pulmonary challenge model system. These data show that sialylated LOS glycoforms promote biofilm formation by NTHi and persistence in vivo.

Antimicrobial peptides in defence of the oral and respiratory tracts

Antimicrobial peptides (AMPs) are components of complex host secretions, acting synergistically with other innate defence molecules to combat infection and control resident microbial populations throughout the oral cavity and respiratory tract. AMPs are directly antimicrobial, bind lipopolysaccharide (LPS) and lipoteichoic acid, and are immunomodulatory signals. Pathogenic and commensal organisms display a variety of resistance mechanisms, which are related to structure of cell wall components (e.g. LPS) and cytoplasmic membranes, and peptide breakdown mechanisms. For example, LPS of the AMP-resistant cystic fibrosis pathogen Burkholderia cepacia is under-phosphorylated and highly substituted with charge-neutralising 4-deoxy-4-aminoarabinose. Additionally, host mimicry by addition of phosphorylcholine contributes to resistance in oral and respiratory organisms. Porphyromonas gingivalis, Pseudomonas aeruginosa and other pathogens produce extracellular and membrane-bound proteases that degrade AMPs. Many of these bacterial properties are environmentally regulated. Their modulation in response to host defences and inflammation can result in altered sensitivity to AMPs, and may additionally change other host-microbe interactions, e.g. binding to Toll-like receptors. The diversity and breadth of antimicrobial cover and immunomodulatory function provided by AMPs is central to the ability of a host to respond to the diverse and highly adaptable organisms colonising oral and respiratory mucosa.

Respiratory infections caused by non-typeable Haemophilus influenzae

PURPOSE OF REVIEW

This review will consider recent developments in the clinical aspects of infections due to non-typeable Haemophilus influenzae. In addition, newer developments in the areas of mechanisms of pathogenesis, host pathogen interaction, immune responses and efforts toward vaccine development will be reviewed briefly.

RECENT FINDINGS

Non-typeable H. influenzae continues to be a common cause of otitis media in infants and children, sinusitis in children and adults, pneumonia in adults, and lower respiratory tract infection in adults with chronic obstructive pulmonary disease. While the rate of beta-lactamase production by isolates of H. influenzae varies geographically, most regions show a rate of 20-35% of isolates producing beta-lactamase. Recent studies have highlighted the possible role of bacterial biofilms formed by H. influenzae as a cause of otitis media. Several lines of evidence indicate that H. influenzae causes intracellular infection in the lower respiratory tract in chronic obstructive pulmonary disease and this observation has important implications in understanding the human immune response to the bacterium. Lipooligosaccharide is an important virulence factor for H. influenzae and research is generating new information on the complex role of this molecule in colonization and infection of the respiratory tract. Several surface molecules are under active evaluation as vaccine antigens.

SUMMARY

Non-typeable H. influenzae is an important cause of respiratory tract infections in children and adults. Most strains are susceptible to amoxicillin/clavulanate, fluoroquinolones and the newer macrolides. Research in the next decade promises substantial progress in the challenge of developing vaccines for nontypeable H. influenzae.

Susceptibility of nontypeable Haemophilus influenzae to human beta-defensins is influenced by lipooligosaccharide acylation

Nontypeable Haemophilus influenzae (NTHI) lipooligosaccharide htrB mutants exhibited greater than 45-fold-increased sensitivity to human beta-defensin 2 (HBD-2) compared to the wild type. Complementation by htrB in trans to acylation competence reversed this increased sensitivity. In contrast, NTHI was more susceptible to HBD-3 and showed no changes in sensitivity as a result of lipooligosaccharide mutations in oligosaccharide and lipid A biosynthesis genes.