Nontypable Haemophilus influenzae displays a prevalent surface structure molecular pattern in clinical isolates

Epidemic Trends and Biofilm Formation Mechanisms of Haemophilus influenzae: Insights into Clinical Implications and Prevention Strategies

Haemophilus influenzae (H. influenzae) is a significant pathogen responsible for causing respiratory tract infections and invasive diseases, leading to a considerable disease burden. The Haemophilus influenzae type b (Hib) conjugate vaccine has notably decreased the incidence of severe infections caused by Hib strains, and other non-typable H. influenzae (NTHi) serotypes have emerged as epidemic strains worldwide. As a result, the global epidemic trends and antibiotic resistance characteristics of H. influenzae have been altered. Researches on the virulence factors of H. influenzae, particularly the mechanisms underlying biofilm formation, and the development of anti-biofilm strategies hold significant clinical value. This article provides a summary of the epidemic trends, typing methods, virulence factors, biofilm formation mechanisms, and prevention strategies of H. influenzae. The increasing prevalence of NTHi strains and antibiotic resistance among H. influenzae, especially the high β-lactamase positivity and the emergence of BLNAR strains have increased clinical difficulties. Understanding its virulence factors, especially the formation mechanism of biofilm, and formulating effective anti-biofilm strategies may help to reduce the clinical impact. Therefore, future research efforts should focus on developing new approaches to prevent and control H. influenzae infections.

Epithelial immune activation and intracellular invasion by non-typeable Haemophilus influenzae

Type-2 low asthma affects 30-50% of people with severe asthma and includes a phenotype characterized by sputum neutrophilia and resistance to corticosteroids. Airways inflammation in type-2 low asthma or COPD is potentially driven by persistent bacterial colonization of the lower airways by bacteria such as non-encapsulated Haemophilus influenzae (NTHi). Although pathogenic in the lower airways, NTHi is a commensal of the upper airways. It is not known to what extent these strains can invade airway epithelial cells, persist intracellularly and activate epithelial cell production of proinflammatory cytokines, and how this differs between the upper and lower airways. We studied NTHi infection of primary human bronchial epithelial cells (PBECs), primary nasal epithelial cells (NECs) and epithelial cell lines from upper and lower airways. NTHi strains differed in propensity for intracellular and paracellular invasion. We found NTHi was internalized within PBECs at 6 h, but live intracellular infection did not persist at 24 h. Confocal microscopy and flow cytometry showed NTHi infected secretory, ciliated and basal PBECs. Infection of PBECs led to induction of CXCL8, interleukin (IL)-1β, IL-6 and TNF. The magnitude of cytokine induction was independent of the degree of intracellular invasion, either by differing strains or by cytochalasin D inhibition of endocytosis, with the exception of the inflammasome-induced mediator IL-1β. NTHi-induced activation of TLR2/4, NOD1/2 and NLR inflammasome pathways was significantly stronger in NECs than in PBECs. These data suggest that NTHi is internalized transiently by airway epithelial cells and has capacity to drive inflammation in airway epithelial cells.

Complement evasion by the human respiratory tract pathogens Haemophilus influenzae and Moraxella catarrhalis

All infective bacterial species need to conquer the innate immune system in order to colonize and survive in their hosts. The human respiratory pathogens Haemophilus influenzae and Moraxella catarrhalis are no exceptions and have developed sophisticated mechanisms to evade complement-mediated killing. Both bacterial species carry lipooligosaccharides preventing complement attacks and attract and utilize host complement regulators C4b binding protein and factor H to inhibit the classical and alternative pathways of complement activation, respectively. In addition, the regulator of the terminal pathway of complement activation, vitronectin, is hijacked by both bacteria. An array of different outer membrane proteins (OMP) in H. influenzae and M. catarrhalis simultaneously binds complement regulators, but also plasminogen. Several of the bacterial complement-binding proteins are important adhesins and contain highly conserved regions for interactions with the host. Thus, some of the OMP are viable targets for new therapeutics, including vaccines aimed at preventing respiratory tract diseases such as otitis media in children and exacerbations in patients suffering from chronic obstructive pulmonary disease.

Lung Surfactant Lipids Provide Immune Protection Against Haemophilus influenzae Respiratory Infection

Non-typeable Haemophilus influenzae (NTHi) causes persistent respiratory infections in patients with chronic obstructive pulmonary disease (COPD), probably linked to its capacity to invade and reside within pneumocytes. In the alveolar fluid, NTHi is in contact with pulmonary surfactant, a lipoprotein complex that protects the lung against alveolar collapse and constitutes the front line of defense against inhaled pathogens and toxins. Decreased levels of surfactant phospholipids have been reported in smokers and patients with COPD. The objective of this study was to investigate the effect of surfactant phospholipids on the host-pathogen interaction between NTHi and pneumocytes. For this purpose, we used two types of surfactant lipid vesicles present in the alveolar fluid: (i) multilamellar vesicles (MLVs, > 1 μm diameter), which constitute the tensioactive material of surfactant, and (ii) small unilamellar vesicles (SUVs, 0.1 μm diameter), which are generated after inspiration/expiration cycles, and are endocytosed by pneumocytes for their degradation and/or recycling. Results indicated that extracellular pulmonary surfactant binds to NTHi, preventing NTHi self-aggregation and inhibiting adhesion of NTHi to pneumocytes and, consequently, inhibiting NTHi invasion. In contrast, endocytosed surfactant lipids, mainly via the scavenger receptor SR-BI, did not affect NTHi adhesion but inhibited NTHi invasion by blocking bacterial uptake in pneumocytes. This blockade was made possible by inhibiting Akt phosphorylation and Rac1 GTPase activation, which are signaling pathways involved in NTHi internalization. Administration of the hydrophobic fraction of lung surfactant in vivo accelerated bacterial clearance in a mouse model of NTHi pulmonary infection, supporting the notion that the lipid component of lung surfactant protects against NTHi infection. These results suggest that alterations in surfactant lipid levels in COPD patients may increase susceptibility to infection by this pathogen.

Differential recognition of Haemophilus influenzae whole bacterial cells and isolated lipooligosaccharides by galactose-specific lectins

Bacterial surfaces are decorated with carbohydrate structures that may serve as ligands for host receptors. Based on their ability to recognize specific sugar epitopes, plant lectins are extensively used for bacteria typing. We previously observed that the galactose-specific agglutinins from Ricinus communis (RCA) and Viscum album (VAA) exhibited differential binding to nontypeable Haemophilus influenzae (NTHi) clinical isolates, their binding being distinctly affected by truncation of the lipooligosaccharide (LOS). Here, we examined their binding to the structurally similar LOS molecules isolated from strains NTHi375 and RdKW20, using microarray binding assays, saturation transfer difference NMR, and molecular dynamics simulations. RCA bound the LOS_RdKW20 glycoform displaying terminal Galβ(1,4)Glcβ, whereas VAA recognized the Galα(1,4)Galβ(1,4)Glcβ epitope in LOS_NTHi375 but not in LOS_RdKW20, unveiling a different presentation. Binding assays to whole bacterial cells were consistent with LOS_NTHi375 serving as ligand for VAA, and also suggested recognition of the glycoprotein HMW1. Regarding RCA, comparable binding to NTHi375 and RdKW20 cells was observed. Interestingly, an increase in LOS_NTHi375 abundance or expression of HMW1 in RdKW20 impaired RCA binding. Overall, the results revealed that, besides the LOS, other carbohydrate structures on the bacterial surface serve as lectin ligands, and highlighted the impact of the specific display of cell surface components on lectin binding.

Bacterial Surface Glycans: Microarray and QCM Strategies for Glycophenotyping and Exploration of Recognition by Host Receptors

Bacterial surfaces are decorated with a diversity of carbohydrate structures that play important roles in the bacteria-host relationships. They may offer protection against host defense mechanisms, elicit strong antigenic responses, or serve as ligands for host receptors, including lectins of the innate immune system. Binding by these lectins may trigger defense responses or, alternatively, promote attachment, thereby enhancing infection. The outcome will depend on the particular bacterial surface landscape, which may substantially differ among species and strains. In this chapter, we describe two novel methods for exploring interactions directly on the bacterial surface, based on the generation of bacterial microarrays and quartz crystal microbalance (QCM) sensor chips. Bacterial microarrays enable profiling of accessible carbohydrate structures and screening of their recognition by host receptors, also providing information on binding avidity, while the QCM approach allows determination of binding affinity and kinetics. In both cases, the chief element is the use of entire bacterial cells, so that recognition of the bacterial glycan epitopes is explored in their natural environment.

An antagonist of the platelet-activating factor receptor inhibits adherence of both nontypeable Haemophilus influenzae and Streptococcus pneumoniae to cultured human bronchial epithelial cells exposed to cigarette smoke

BACKGROUND

COPD is emerging as the third largest cause of human mortality worldwide after heart disease and stroke. Tobacco smoking, the primary risk factor for the development of COPD, induces increased expression of platelet-activating factor receptor (PAFr) in the lung epithelium. Nontypeable Haemophilus influenzae (NTHi) and Streptococcus pneumoniae adhere to PAFr on the luminal surface of human respiratory tract epithelial cells.

OBJECTIVE

To investigate PAFr as a potential drug target for the prevention of infections caused by the main bacterial drivers of acute exacerbations in COPD patients, NTHi and S. pneumoniae.

METHODS

Human bronchial epithelial BEAS-2B cells were exposed to cigarette smoke extract (CSE). PAFr expression levels were determined using immunocytochemistry and quantitative polymerase chain reaction. The epithelial cells were challenged with either NTHi or S. pneumoniae labeled with fluorescein isothiocyanate, and bacterial adhesion was measured using immunofluorescence. The effect of a well-evaluated antagonist of PAFr, WEB-2086, on binding of the bacterial pathogens to BEAS-2B cells was then assessed. In silico studies of the tertiary structure of PAFr and the binding pocket for PAF and its antagonist WEB-2086 were undertaken.

RESULTS

PAFr expression by bronchial epithelial cells was upregulated by CSE, and significantly associated with increased bacterial adhesion. WEB-2086 reduced the epithelial adhesion by both NTHi and S. pneumoniae to levels observed for non-CSE-exposed cells. Furthermore, it was nontoxic toward the bronchial epithelial cells. In silico analyses identified a binding pocket for PAF/WEB-2086 in the predicted PAFr structure.

CONCLUSION

WEB-2086 represents an innovative class of candidate drugs for inhibiting PAFr-dependent lung infections caused by the main bacterial drivers of smoking-related COPD.

Combined Bacteria Microarray and Quartz Crystal Microbalance Approach for Exploring Glycosignatures of Nontypeable Haemophilus influenzae and Recognition by Host Lectins

Recognition of bacterial surface epitopes by host receptors plays an important role in the infectious process and is intimately associated with bacterial virulence. Delineation of bacteria-host interactions commonly relies on the detection of binding events between purified bacteria- and host-target molecules. In this work, we describe a combined microarray and quartz crystal microbalance (QCM) approach for the analysis of carbohydrate-mediated interactions directly on the bacterial surface, thus preserving the native environment of the bacterial targets. Nontypeable Haemophilus influenzae (NTHi) was selected as a model pathogenic species not displaying a polysaccharide capsule or O-antigen-containing lipopolysaccharide, a trait commonly found in several important respiratory pathogens. Here, we demonstrate the usefulness of NTHi microarrays for exploring the presence of carbohydrate structures on the bacterial surface. Furthermore, the microarray approach is shown to be efficient for detecting strain-selective binding of three innate immune lectins, namely, surfactant protein D, human galectin-8, and Siglec-14, to different NTHi clinical isolates. In parallel, QCM bacteria-chips were developed for the analysis of lectin-binding kinetics and affinity. This novel QCM approach involves capture of NTHi on lectin-derivatized chips followed by formaldehyde fixation, rendering the bacteria an integrated part of the sensor chip, and subsequent binding assays with label-free lectins. The binding parameters obtained for selected NTHi-lectin pairs provide further insights into the interactions occurring at the bacterial surface.

Nontypeable Haemophilus influenzae exploits the interaction between protein-E and vitronectin for the adherence and invasion to bronchial epithelial cells

Background

Nontypeable Haemophilus influenzae (NTHi) is one of the most common Gram-negative pathogens in otitis media and exacerbation of chronic obstructive pulmonary disease. NTHi has been reported to invade bronchial epithelial cells. This penetration enables NTHi to evade the host immune system and antibiotics, and it seems to be related to the intractable features of these diseases. However, the precise mechanism of the invasion has been unknown. We hypothesized that protein-E, an outer membrane protein of NTHi, plays a role in this penetration into bronchial epithelial cells.

Results

We utilized two NTHi strains. NTHi efficiently attached to plate-bound vitronectin (254–309 / field at 1,000× magnification) and this attachment was blocked by pretreatment with protein-E peptide (PE^84–108). The blockade of adhesion was dependent on the concentration of PE^84–108. NTHi strains invaded bronchial epithelial cells and the intracellular bacteria were localized in early endosomes. Furthermore, intracellular invasion of NTHi was also blocked by PE^84–108, but not by Arg-Gly-Asp (RGD) peptide. Pretreatment with PE^84–108 significantly prevented cells from being invaded by both NTHi strains, which was confirmed by fluorescent microscope observation. In addition, pretreatment with PE^84–108 significantly reduced percentages of CFU after gentamicin treatment of cells per input CFU.

Conclusions

These results suggest that NTHi does not directly bind to the cell surface, but binds to host vitronectin that is bound to the cell surface, via bacterial protein-E. Bacterial protein-E and host vitronectin play a role in the attachment to bronchial epithelial cells and is also involved in the subsequent intracellular invasion of NTHi. A novel vaccine or treatment strategy targeting the protein-E-vitronectin axis may prevent respiratory intracellular infection of NTHi and may lead to better clinical outcomes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0600-8) contains supplementary material, which is available to authorized users.

Relative Contribution of P5 and Hap Surface Proteins to Nontypable Haemophilus influenzae Interplay with the Host Upper and Lower Airways

Nontypable Haemophilus influenzae (NTHi) is a major cause of opportunistic respiratory tract disease, and initiates infection by colonizing the nasopharynx. Bacterial surface proteins play determining roles in the NTHi-airways interplay, but their specific and relative contribution to colonization and infection of the respiratory tract has not been addressed comprehensively. In this study, we focused on the ompP5 and hap genes, present in all H. influenzae genome sequenced isolates, and encoding the P5 and Hap surface proteins, respectively. We employed isogenic single and double mutants of the ompP5 and hap genes generated in the pathogenic strain NTHi375 to evaluate P5 and Hap contribution to biofilm growth under continuous flow, to NTHi adhesion, and invasion/phagocytosis on nasal, pharyngeal, bronchial, alveolar cultured epithelial cells and alveolar macrophages, and to NTHi murine pulmonary infection. We show that P5 is not required for bacterial biofilm growth, but it is involved in NTHi interplay with respiratory cells and in mouse lung infection. Mechanistically, P5_NTHi375 is not a ligand for CEACAM1 or α5 integrin receptors. Hap involvement in NTHi375-host interaction was shown to be limited, despite promoting bacterial cell adhesion when expressed in H. influenzae RdKW20. We also show that Hap does not contribute to bacterial biofilm growth, and that its absence partially restores the deficiency in lung infection observed for the ΔompP5 mutant. Altogether, this work frames the relative importance of the P5 and Hap surface proteins in NTHi virulence.

Characterization of nontypable Haemophilus influenzae isolates recovered from adult patients with underlying chronic lung disease reveals genotypic and phenotypic traits associated with persistent infection

Nontypable Haemophilus influenzae (NTHi) has emerged as an important opportunistic pathogen causing infection in adults suffering obstructive lung diseases. Existing evidence associates chronic infection by NTHi to the progression of the chronic respiratory disease, but specific features of NTHi associated with persistence have not been comprehensively addressed. To provide clues about adaptive strategies adopted by NTHi during persistent infection, we compared sequential persistent isolates with newly acquired isolates in sputa from six patients with chronic obstructive lung disease. Pulse field gel electrophoresis (PFGE) identified three patients with consecutive persistent strains and three with new strains. Phenotypic characterisation included infection of respiratory epithelial cells, bacterial self-aggregation, biofilm formation and resistance to antimicrobial peptides (AMP). Persistent isolates differed from new strains in showing low epithelial adhesion and inability to form biofilms when grown under continuous-flow culture conditions in microfermenters. Self-aggregation clustered the strains by patient, not by persistence. Increasing resistance to AMPs was observed for each series of persistent isolates; this was not associated with lipooligosaccharide decoration with phosphorylcholine or with lipid A acylation. Variation was further analyzed for the series of three persistent isolates recovered from patient 1. These isolates displayed comparable growth rate, natural transformation frequency and murine pulmonary infection. Genome sequencing of these three isolates revealed sequential acquisition of single-nucleotide variants in the AMP permease sapC, the heme acquisition systems hgpB, hgpC, hup and hxuC, the 3-deoxy-D-manno-octulosonic acid kinase kdkA, the long-chain fatty acid transporter ompP1, and the phosphoribosylamine glycine ligase purD. Collectively, we frame a range of pathogenic traits and a repertoire of genetic variants in the context of persistent infection by NTHi.

Outer membrane protein P5 is required for resistance of nontypeable Haemophilus influenzae to both the classical and alternative complement pathways

The complement system is an important first line of defense against the human pathogen Haemophilus influenzae. To survive and propagate in vivo, H. influenzae has evolved mechanisms for subverting this host defense, most of which have been shown to involve outer surface structures, including lipooligosaccharide glycans and outer surface proteins. Bacterial defense against complement acts at multiple steps in the pathway by mechanisms that are not fully understood. Here we identify outer membrane protein P5 as an essential factor in serum resistance of both H. influenzae strain Rd and nontypeable H. influenzae (NTHi) clinical isolate NT127. P5 was essential for resistance of Rd and NT127 to complement in pooled human serum. Further investigation determined that P5 expression decreased cell surface binding of IgM, a potent activator of the classical pathway of complement, to both Rd and NT127. Additionally, P5 expression was required for NT127 to bind factor H (fH), an important inhibitor of alternative pathway (AP) activation. Collectively, the results obtained in this work highlight the ability of H. influenzae to utilize a single protein to perform multiple protective functions for evading host immunity.

Relative contributions of lipooligosaccharide inner and outer core modifications to nontypeable Haemophilus influenzae pathogenesis

Nontypeable Haemophilus influenzae (NTHi) is a frequent commensal of the human nasopharynx that causes opportunistic infection in immunocompromised individuals. Existing evidence associates lipooligosaccharide (LOS) with disease, but the specific and relative contributions of NTHi LOS modifications to virulence properties of the bacterium have not been comprehensively addressed. Using NTHi strain 375, an isolate for which the detailed LOS structure has been determined, we compared systematically a set of isogenic mutant strains expressing sequentially truncated LOS. The relative contributions of 2-keto-3-deoxyoctulosonic acid, the triheptose inner core, oligosaccharide extensions on heptoses I and III, phosphorylcholine, digalactose, and sialic acid to NTHi resistance to antimicrobial peptides (AMP), self-aggregation, biofilm formation, cultured human respiratory epithelial infection, and murine pulmonary infection were assessed. We show that opsX, lgtF, lpsA, lic1, and lic2A contribute to bacterial resistance to AMP; lic1 is related to NTHi self-aggregation; lgtF, lic1, and siaB are involved in biofilm growth; opsX and lgtF participate in epithelial infection; and opsX, lgtF, and lpsA contribute to lung infection. Depending on the phenotype, the involvement of these LOS modifications occurs at different extents, independently or having an additive effect in combination. We discuss the relative contribution of LOS epitopes to NTHi virulence and frame a range of pathogenic traits in the context of infection.

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.

Microbial modulation of host immunity with the small molecule phosphorylcholine

All microorganisms dependent on persistence in a host for survival rely on either hiding from or modulating host responses to infection. The small molecule phosphorylcholine, or choline phosphate (ChoP), is used for both of these purposes by a wide array of bacterial and parasitic microbes. While the mechanisms underlying ChoP acquisition and expression are diverse, a unifying theme is the use of ChoP to reduce the immune response to infection, creating an advantage for ChoP-expressing microorganisms. In this minireview, we discuss several benefits of ChoP expression during infection as well as how the immune system fights back against ChoP-expressing pathogens.

Modified lipooligosaccharide structure protects nontypeable Haemophilus influenzae from IgM-mediated complement killing in experimental otitis media

Nontypeable Haemophilus influenzae (NTHi) is a Gram-negative, human-restricted pathogen. Although this bacterium typically colonizes the nasopharynx in the absence of clinical symptoms, it is also one of the major pathogens causing otitis media (OM) in children. Complement represents an important aspect of the host defense against NTHi. In general, NTHi is efficiently killed by complement-mediated killing; however, various resistance mechanisms have also evolved. We measured the complement resistance of NTHi isolates isolated from the nasopharynx and the middle ear fluids of OM patients. Furthermore, we determined the molecular mechanism of NTHi complement resistance. Complement resistance was strongly increased in isolates from the middle ear, which correlated with decreased binding of IgM. We identified a crucial role for the R2866_0112 gene in complement resistance. Deletion of this gene altered the lipooligosaccharide (LOS) composition of the bacterium, which increased IgM binding and complement-mediated lysis. In a novel mouse model of coinfection with influenza virus, we demonstrate decreased virulence for the R2866_0112 deletion mutant. These findings identify a mechanism by which NTHi modifies its LOS structure to prevent recognition by IgM and activation of complement. Importantly, this mechanism plays a crucial role in the ability of NTHi to cause OM.

Nontypeable Haemophilus influenzae (NTHi) colonizes the nasopharynx of especially young children without any obvious symptoms. However, NTHi is also a major pathogen in otitis media (OM), one of the most common childhood infections. Although this pathogen is often associated with OM, the mechanism by which this bacterium is able to cause OM is largely unknown. Our study addresses a key biological question that is highly relevant for child health: what is the molecular mechanism that enables NTHi to cause OM? We show that isolates collected from the middle ear fluid exhibit increased complement resistance and that the lipooligosaccharide (LOS) structure determines IgM binding and complement activation. Modification of the LOS structure decreased NTHi virulence in a novel NTHi-influenza A virus coinfection OM mouse model. Our findings may also have important implications for other Gram-negative pathogens harboring LOS, such as Neisseria meningitidis, Moraxella catarrhalis, and Bordetella pertussis.

Host cell kinases, α5 and β1 integrins, and Rac1 signalling on the microtubule cytoskeleton are important for non-typable Haemophilus influenzae invasion of respiratory epithelial cells

Non-typable Haemophilus influenzae (NTHi) is a common commensal of the human nasopharynx, but causes opportunistic infection when the respiratory tract is compromised by infection or disease. The ability of NTHi to invade epithelial cells has been described, but the underlying molecular mechanisms are poorly characterized. We previously determined that NTHi promotes phosphorylation of the serine-threonine kinase Akt in A549 human lung epithelial cells, and that Akt phosphorylation and NTHi cell invasion are prevented by inhibition of phosphoinositide 3-kinase (PI3K). Because PI3K-Akt signalling is associated with several host cell networks, the purpose of the current study was to identify eukaryotic molecules important for NTHi epithelial invasion. We found that inhibition of Akt activity reduced NTHi internalization; differently, bacterial entry was increased by phospholipase Cγ1 inhibition but was not affected by protein kinase inhibition. We also found that α5 and β1 integrins, and the tyrosine kinases focal adhesion kinase and Src, are important for NTHi A549 cell invasion. NTHi internalization was shown to be favoured by activation of Rac1 guanosine triphosphatase (GTPase), together with the guanine nucleotide exchange factor Vav2 and the effector Pak1. Also, Pak1 might be associated with inactivation of the microtubule destabilizing agent Op18/stathmin, to facilitate microtubule polymerization and NTHi entry. Conversely, inhibition of RhoA GTPase and its effector ROCK increased the number of internalized bacteria. Src and Rac1 were found to be important for NTHi-triggered Akt phosphorylation. An increase in host cyclic AMP reduced bacterial entry, which was linked to protein kinase A. These findings suggest that NTHi finely manipulates host signalling molecules to invade respiratory epithelial cells.

Distribution of genes involved in sialic acid utilization in strains of Haemophilus parasuis

Haemophilus parasuis is a porcine respiratory pathogen, well known as the aetiological agent of Glässer's disease. H. parasuis comprises strains of different virulence, but the virulence factors of this bacterium are not well defined. A neuraminidase activity has been previously detected in H. parasuis, but the role of sialylation in the virulence of this bacterium has not been studied. To explore the relationship between sialic acid (Neu5Ac) and virulence, we assessed the distribution of genes involved in sialic acid metabolism in 21 H. parasuis strains from different clinical origins (including nasal and systemic isolates). The neuraminidase gene nanH, together with CMP-Neu5Ac synthetase and sialyltransferase genes neuA, siaB and lsgB, were included in the study. Neuraminidase activity was found to be common in H. parasuis isolates, and the nanH gene from 12 isolates was expressed in Escherichia coli and further characterized. Sequence analysis showed that the NanH predicted protein contained the motifs characteristic of the catalytic site of sialidases. While an association between the presence of nanH and the different origins of the strains was not detected, the lsgB gene was predominantly present in the systemic isolates, and was not amplified from any of the nasal isolates tested. Analysis of the lipooligosaccharide (LOS) from reference strains Nagasaki (virulent, lsgB(+)) and SW114 (non-virulent, lsgB(-)) showed the presence of sialic acid in the LOS from the Nagasaki strain, supporting the role of sialylation in the virulence of this bacterial pathogen. Further studies are needed to clarify the role of sialic acid in the pathogenicity of H. parasuis.

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.