Effect of neuraminidase on receptor-mediated adherence of Streptococcus pneumoniae to chinchilla tracheal epithelium

Streptococcus pneumoniae binds to host GAPDH on dying lung epithelial cells worsening secondary infection following influenza

Streptococcus pneumoniae (Spn) alone and during co-infection with influenza A virus (IAV) can result in severe pneumonia with mortality. Pneumococcal surface protein A (PspA) is an established virulence factor required for Spn evasion of lactoferricin and C-reactive protein-activated complement-mediated killing. Herein, we show that PspA functions as an adhesin to dying host cells. We demonstrate that PspA binds to host-derived glyceraldehyde-3-phosphate dehydrogenase (GAPDH) bound to outward-flipped phosphatidylserine residues on dying host cells. PspA-mediated adhesion was to apoptotic, pyroptotic, and necroptotic cells, but not healthy lung cells. Using isogenic mutants of Spn, we show that PspA-GAPDH-mediated binding to lung cells increases pneumococcal localization in the lower airway, and this is enhanced as a result of pneumolysin exposure or co-infection with IAV. PspA-mediated binding to GAPDH requires amino acids 230-281 in its α-helical domain with intratracheal inoculation of this PspA fragment alongside the bacteria reducing disease severity in an IAV/Spn pneumonia model.

Novel Virulence Role of Pneumococcal NanA in Host Inflammation and Cell Death Through the Activation of Inflammasome and the Caspase Pathway

Streptococcus pneumoniae is one of most deadly Gram-positive bacterium that causes significant mortality and morbidity worldwide. Intense inflammation and cytotoxicity is a hallmark of invasive pneumococcal disease. Pneumococcal NanA has been shown to exaggerate the production of inflammatory cytokines via unmasking of inhibitory Siglec-5 from its sialyl cis-ligands. To further investigate the mechanistic role of NanA and Siglec-5 in pneumococccal diseases, we systemically analyzed genes and signaling pathways differentially regulated in macrophages infected with wild type and NanA-deficient pneumococcus. We found that NanA-mediated desialylation impairs the Siglec-5-TLR-2 interaction and reduces the recruitment of phosphatase SHP-1 to Siglec-5. This dysregulated crosstalk between TLR-2 and inhibitory Siglec-5 exaggerated multiple inflammatory and death signaling pathways and consequently caused excessive inflammation and cytotoxicity in the infected macrophage. Collectively, our results reveal a novel virulence role of NanA in pneumococcal pathogenesis and suggest that targeting NanA activity may ameliorate the pneumococcus-mediated inflammation and cytotoxicity in severe invasive pneumococcal diseases.

Catalytic and lectin domains in neuraminidase A from Streptococcus pneumoniae are capable of an intermolecular assembly: Implications for biofilm formation

Neuraminidase A from Streptococcus pneumoniae (NanA) is a cell wall-bound modular enzyme containing one lectin and one catalytic domain. Unlike homologous NanB and NanC expressed by the same bacterium, the two domains within one NanA molecule do not form a stable interaction and are spatially separated by a 16-amino acid-long flexible linker. In this work, the ability of NanA to form intermolecular assemblies was characterized using the methods of molecular modeling and bioinformatic analysis based on crystallographic data and by bringing together previously published experimental data. It was concluded that two catalytic domains, as well as one catalytic and one lectin domain, originating from two cell wall-bound NanA molecules, can interact through a previously uncharacterized interdomain interface to form complexes stabilized by a network of intermolecular hydrogen bonds and salt bridges. Supercomputer modeling strongly indicated that artocarpin, an earlier experimentally discovered inhibitor of the pneumococcal biofilm formation, is able to bind to a site located in the catalytic domain of one NanA entity and prevent its interaction with the lectin or catalytic domain of another NanA entity, thus directly precluding the generation of intermolecular assemblies. The revealed structural adaptation is discussed as one plausible mechanism of noncatalytic participation of this potentially key pathogenicity enzyme in pneumococcal biofilm formation.

Glycan-metabolizing enzymes in microbe-host interactions: the Streptococcus pneumoniae paradigm

Streptococcus pneumoniae is a frequent colonizer of the upper airways; however, it is also an accomplished pathogen capable of causing life-threatening diseases. To colonize and cause invasive disease, this bacterium relies on a complex array of factors to mediate the host-bacterium interaction. The respiratory tract is rich in functionally important glycoconjugates that display a vast range of glycans, and, thus, a key component of the pneumococcus-host interaction involves an arsenal of bacterial carbohydrate-active enzymes to depolymerize these glycans and carbohydrate transporters to import the products. Through the destruction of host glycans, the glycan-specific metabolic machinery deployed by S. pneumoniae plays a variety of roles in the host-pathogen interaction. Here, we review the processing and metabolism of the major host-derived glycans, including N- and O-linked glycans, Lewis and blood group antigens, proteoglycans, and glycogen, as well as some dietary glycans. We discuss the role of these metabolic pathways in the S. pneumoniae-host interaction, speculate on the potential of key enzymes within these pathways as therapeutic targets, and relate S. pneumoniae as a model system to glycan processing in other microbial pathogens.

Pneumococcal neuraminidase activates TGF-β signalling

Neuraminidase A (NanA) is an important virulence factor that is anchored to the pneumococcal cell wall and cleaves sialic acid on host substrates. We noted that a secreted allele of NanA was over-represented in invasive pneumococcal isolates and promoted the development of meningitis when swapped into the genome of non-meningitis isolates replacing cell wall-anchored NanA. Both forms of recombinant NanA directly activated transforming growth factor (TGF)-β, increased SMAD signalling and promoted loss of endothelial tight junction ZO-1. However, in assays using whole bacteria, only the cell-bound NanA decreased expression of ZO-1 and showed NanA dependence of bacterial invasion of endothelial cells. We conclude that NanA secretion versus retention on the cell surface does not influence neurotropism of clinical isolates. However, we describe a new NanA-TGF-β signalling axis that leads to decreased blood-brain barrier integrity and enhances bacterial invasion.

Pneumococcal Neuraminidase A (NanA) Promotes Biofilm Formation and Synergizes with Influenza A Virus in Nasal Colonization and Middle Ear Infection

Even in the vaccine era, Streptococcus pneumoniae (the pneumococcus) remains a leading cause of otitis media, a significant public health burden, in large part because of the high prevalence of nasal colonization with the pneumococcus in children. The primary pneumococcal neuraminidase, NanA, which is a sialidase that catalyzes the cleavage of terminal sialic acids from host glycoconjugates, is involved in both of these processes. Coinfection with influenza A virus, which also expresses a neuraminidase, exacerbates nasal colonization and disease by S. pneumoniae, in part via the synergistic contributions of the viral neuraminidase. The specific role of its pneumococcal counterpart, NanA, in this interaction, however, is less well understood. We demonstrate in a mouse model that NanA-deficient pneumococci are impaired in their ability to cause both nasal colonization and middle ear infection. Coinfection with neuraminidase-expressing influenza virus and S. pneumoniae potentiates both colonization and infection but not to wild-type levels, suggesting an intrinsic role of NanA. Using in vitro models, we show that while NanA contributes to both epithelial adherence and biofilm viability, its effect on the latter is actually independent of its sialidase activity. These data indicate that NanA contributes both enzymatically and nonenzymatically to pneumococcal pathogenesis and, as such, suggest that it is not a redundant bystander during coinfection with influenza A virus. Rather, its expression is required for the full synergism between these two pathogens.

Neuraminidase A-Exposed Galactose Promotes Streptococcus pneumoniae Biofilm Formation during Colonization

Streptococcus pneumoniae is an opportunistic pathogen that colonizes the nasopharynx. Herein we show that carbon availability is distinct between the nasopharynx and bloodstream of adult humans: glucose is absent from the nasopharynx, whereas galactose is abundant. We demonstrate that pneumococcal neuraminidase A (NanA), which cleaves terminal sialic acid residues from host glycoproteins, exposed galactose on the surface of septal epithelial cells, thereby increasing its availability during colonization. We observed that S. pneumoniae mutants deficient in NanA and β-galactosidase A (BgaA) failed to form biofilms in vivo despite normal biofilm-forming abilities in vitro Subsequently, we observed that glucose, sucrose, and fructose were inhibitory for biofilm formation, whereas galactose, lactose, and low concentrations of sialic acid were permissive. Together these findings suggested that the genes involved in biofilm formation were under some form of carbon catabolite repression (CCR), a regulatory network in which genes involved in the uptake and metabolism of less-preferred sugars are silenced during growth with preferred sugars. Supporting this notion, we observed that a mutant deficient in pyruvate oxidase, which converts pyruvate to acetyl-phosphate under non-CCR-inducing growth conditions, was unable to form biofilms. Subsequent comparative transcriptome sequencing (RNA-seq) analyses of planktonic and biofilm-grown pneumococci showed that metabolic pathways involving the conversion of pyruvate to acetyl-phosphate and subsequently leading to fatty acid biosynthesis were consistently upregulated during diverse biofilm growth conditions. We conclude that carbon availability in the nasopharynx impacts pneumococcal biofilm formation in vivo Additionally, biofilm formation involves metabolic pathways not previously appreciated to play an important role.

Streptococcus pneumoniae Senses a Human-like Sialic Acid Profile via the Response Regulator CiaR

Streptococcus pneumoniae is a human-adapted pathogen that encounters terminally sialylated glycoconjugates and free sialic acid (Sia) in the airways. Upon scavenging by the bacterial sialidase NanA, Sias serve as carbon sources for the bacteria. Unlike most animals in which cytidine-monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) converts Sia N-acetylneuraminic acid (Neu5Ac) into N-glycolylneuraminic acid (Neu5Gc), humans have an inactive CMAH, causing an absence of Neu5Gc and excess Neu5Ac. We find that pneumococcal challenge in Cmah(-/-) mice leads to heightened bacterial loads, virulence, and NanA expression. In vitro, NanA is upregulated in response to Neu5Ac compared with Neu5Gc, a process controlled by the two-component response regulator CiaR and requiring Sia uptake by the transporter SatABC. Additionally, compared with Neu5Gc, Neu5Ac increases pneumococcal resistance to antimicrobial reactive oxygen species in a CiaR-dependent manner. Thus, S. pneumoniae senses and responds to Neu5Ac, leading to CiaR activation and increased virulence and potentially explaining the greater susceptibility in humans.

Mechanisms of Bacterial Colonization of the Respiratory Tract

Respiratory tract infections are an important cause of morbidity and mortality worldwide. Chief among these are infections involving the lower airways. The opportunistic bacterial pathogens responsible for most cases of pneumonia can cause a range of local and invasive infections. However, bacterial colonization (or carriage) in the upper airway is the prerequisite of all these infections. Successful colonizers must attach to the epithelial lining, grow on the nutrient-limited mucosal surface, evade the host immune response, and transmit to a susceptible host. Here, we review the molecular mechanisms underlying these conserved stages of carriage. We also examine how the demands of colonization influence progression to disease. A range of bacteria can colonize the upper airway; nevertheless, we focus on strategies shared by many respiratory tract opportunistic pathogens. Understanding colonization opens a window to the evolutionary pressures these pathogens face within their animal hosts and that have selected for attributes that contribute to virulence and pathogenesis.

Anatomical site-specific contributions of pneumococcal virulence determinants

Streptococcus pneumoniae is an opportunistic pathogen globally associated with significant morbidity and mortality. It is capable of causing a wide range of diseases including sinusitis, conjunctivitis, otitis media, pneumonia, bacteraemia, sepsis, and meningitis. While its capsular polysaccharide is indispensible for invasive disease, and opsonising antibodies against the capsule are the basis for the current vaccines, a long history of biomedical research indicates that other components of this Gram-positive bacterium are also critical for virulence. Herein we review the contribution of pneumococcal virulence determinants to survival and persistence in the context of distinct anatomical sites. We discuss how these determinants allow the pneumococcus to evade mucociliary clearance during colonisation, establish lower respiratory tract infection, resist complement deposition and opsonophagocytosis in the bloodstream, and invade secondary tissues such as the central nervous system leading to meningitis. We do so in a manner that highlights both the critical role of the capsular polysaccharide and the accompanying and necessary protein determinants. Understanding the complex interplay between host and pathogen is necessary to find new ways to prevent pneumococcal infection. This review is an attempt to do so with consideration for the latest research findings.

Streptococcus pneumoniae proteomics: determinants of pathogenesis and vaccine development

Streptococcus pneumoniae is a major pathogen that is responsible for a variety of invasive diseases. The bacteria gain entry initially by establishing a carriage state in the nasopharynx from where they migrate to other sites in the body. The worldwide distribution of the bacteria and the severity of the diseases have led to a significant level of interest in the development of vaccines against the bacteria. Current vaccines, based on the bacterial polysaccharide, have a number of limitations including poor immunogenicity and limited effectiveness against all pneumococcal serotypes. There are many challenges in developing vaccines that will be effective against the diverse range of isolates and serotypes for this highly variable bacterial pathogen. This review considers how proteomic technologies have extended our understanding of the pathogenic mechanisms of nasopharyngeal colonization and disease development as well as the critical areas in developing protein-based vaccines.

Structural characterization of the carbohydrate-binding module of NanA sialidase, a pneumococcal virulence factor

BACKGROUND

Streptococcus pneumoniae Neuraminidase A (NanA) is a multi-domain protein anchored to the bacterial surface. Upstream of the catalytic domain of NanA is a domain that conforms to the sialic acid-recognising CBM40 family of the CAZY (carbohydrate-active enzymes) database. This domain has been identified to play a critical role in allowing the bacterium to promote adhesion and invasion of human brain microvascular endothelial cells, and hence may play a key role in promoting bacterial meningitis. In addition, the CBM40 domain has also been reported to activate host chemokines and neutrophil recruitment during infection.

RESULTS

Crystal structures of both apo- and holo- forms of the NanA CBM40 domain (residues 121 to 305), have been determined to 1.8 Å resolution. The domain shares the fold of other CBM40 domains that are associated with sialidases. When in complex with α2,3- or α2,6-sialyllactose, the domain is shown to interact only with the terminal sialic acid. Significantly, a deep acidic pocket adjacent to the sialic acid-binding site is identified, which is occupied by a lysine from a symmetry-related molecule in the crystal. This pocket is adjacent to a region that is predicted to be involved in protein-protein interactions.

CONCLUSIONS

The structural data provide the details of linkage-independent sialyllactose binding by NanA CBM40 and reveal striking surface features that may hold the key to recognition of binding partners on the host cell surface. The structure also suggests that small molecules or sialic acid analogues could be developed to fill the acidic pocket and hence provide a new therapeutic avenue against meningitis caused by S. pneumoniae.

Inducing host protection in pneumococcal sepsis by preactivation of the Ashwell-Morell receptor

The endocytic Ashwell-Morell receptor (AMR) of hepatocytes detects pathogen remodeling of host glycoproteins by neuraminidase in the bloodstream and mitigates the lethal coagulopathy of sepsis. We have investigated the mechanism of host protection by the AMR during the onset of sepsis and in response to the desialylation of blood glycoproteins by the NanA neuraminidase of Streptococcus pneumoniae. We find that the AMR selects among potential glycoprotein ligands unmasked by microbial neuraminidase activity in pneumococcal sepsis to eliminate from blood circulation host factors that contribute to coagulation and thrombosis. This protection is attributable in large part to the rapid induction of a moderate thrombocytopenia by the AMR. We further show that neuraminidase activity in the blood can be manipulated to induce the clearance of AMR ligands including platelets, thereby preactivating a protective response in pneumococcal sepsis that moderates the severity of disseminated intravascular coagulation and enables host survival.

Bacterial neuraminidase inhibitory effects of prenylated isoflavones from roots of Flemingia philippinensis

Bacterial neuraminidase (NA) is one of the key enzymes involved in pathogenesis of inflammation during infection. The organic extract of the roots of Flemingia philippinensis showed high bacterial NA inhibitory activity with an IC50 of around 5μg/mL. Activity-guided separation of the methanol extract yielded nine prenylated isoflavones together with the novel species isoflavone (2) which was given the name flemingsin. Isolated prenylated isoflavones (1-9) were evaluated for NA inhibition and their IC50 values were determined to range between 0.30 and 56.8μM. The most potent inhibitor 4 (IC50=300nM, Ki=130nM) features a catechol motif in the B-ring and a furan in the A-ring. Structure-activity analysis also showed a 4-hydroxyl group within the B-ring was essential for NA inhibitory activity, because isoflavone (9) having protected 4-hydroxyl group was much less potent than its hydroxylated counterpart. All neuraminidase compounds screened were found to be reversible noncompetitive inhibitors. Furthermore, the most active NA inhibitors (1-9) were proven to be present in the native roots in high quantities by HPLC and LC-DAD-ESI/MS.

Comparative Genomic Analyses of Streptococcus pseudopneumoniae Provide Insight into Virulence and Commensalism Dynamics

Streptococcus pseudopneumoniae (SPPN) is a recently described species of the viridans group streptococci (VGS). Although the pathogenic potential of S. pseudopneumoniae remains uncertain, it is most commonly isolated from patients with underlying medical conditions, such as chronic obstructive pulmonary disease. S. pseudopneumoniae can be distinguished from the closely related species, S. pneumoniae and S. mitis, by phenotypic characteristics, including optochin resistance in the presence of 5% CO₂, bile insolubility, and the lack of the pneumococcal capsule. Previously, we reported the draft genome sequence of S. pseudopneumoniae IS7493, a clinical isolate obtained from an immunocompromised patient with documented pneumonia. Here, we use comparative genomics approaches to identify similarities and key differences between S. pseudopneumoniae IS7493, S. pneumoniae and S. mitis. The genome structure of S. pseudopneumoniae IS7493 is most closely related to that of S. pneumoniae R6, but several recombination events are evident. Analysis of gene content reveals numerous unique features that distinguish S. pseudopneumoniae from other streptococci. The presence of loci for competence, iron transport, pneumolysin production and antimicrobial resistance reinforce the phylogenetic position of S. pseudopneumoniae as an intermediate species between S. pneumoniae and S. mitis. Additionally, the presence of several virulence factors and antibiotic resistance mechanisms suggest the potential of this commensal species to become pathogenic or to contribute to increasing antibiotic resistance levels seen among the VGS.

Kinetics of coinfection with influenza A virus and Streptococcus pneumoniae

Secondary bacterial infections are a leading cause of illness and death during epidemic and pandemic influenza. Experimental studies suggest a lethal synergism between influenza and certain bacteria, particularly Streptococcus pneumoniae, but the precise processes involved are unclear. To address the mechanisms and determine the influences of pathogen dose and strain on disease, we infected groups of mice with either the H1N1 subtype influenza A virus A/Puerto Rico/8/34 (PR8) or a version expressing the 1918 PB1-F2 protein (PR8-PB1-F2(1918)), followed seven days later with one of two S. pneumoniae strains, type 2 D39 or type 3 A66.1. We determined that, following bacterial infection, viral titers initially rebound and then decline slowly. Bacterial titers rapidly rise to high levels and remain elevated. We used a kinetic model to explore the coupled interactions and study the dominant controlling mechanisms. We hypothesize that viral titers rebound in the presence of bacteria due to enhanced viral release from infected cells, and that bacterial titers increase due to alveolar macrophage impairment. Dynamics are affected by initial bacterial dose but not by the expression of the influenza 1918 PB1-F2 protein. Our model provides a framework to investigate pathogen interaction during coinfections and to uncover dynamical differences based on inoculum size and strain.

Influenza virus infected individuals often become coinfected with a bacterial pathogen and, consequently, morbidity and mortality are significantly increased. A better understanding of how these pathogens interact with each other and the host is of key importance. Here, we use data from infected mice together with mathematical modeling and quantitative analyses to understand how each pathogen influences the other, and how the 1918 influenza PB1-F2 protein and the bacterial strain and dose contribute to coinfection kinetics. We find that influenza viral titers increase when Streptococcus pneumoniae is present and that the bacteria establish and grow rapidly when influenza is present. Our model and analyses suggest that the influenza infection reduces the bacterial clearance ability of alveolar macrophages and that the subsequent S. pneumoniae infection enhances viral release from infected cells. These results provide new insights into the mechanisms of influenza coinfection and the differences in pathogenesis of influenza and S. pneumoniae strains.

Pneumococcal neuraminidase A: an essential upper airway colonization factor for Streptococcus pneumoniae

Streptococcus pneumoniae colonizes the upper respiratory tract from where the organisms may disseminate systemically to cause life threatening infections. The mechanisms by which pneumococci colonize epithelia are not understood, but neuraminidase A (NanA) has a major role in promoting growth and survival in the upper respiratory tract. In this article we show that mutants of S. pneumoniae D39 deficient in NanA or neuraminidase B (NanB) are abrogated in adherence to three epithelial cell lines, and to primary nasopharyngeal cells. Adherence levels were partly restored by nanA complementation in trans. Enzymic activity of NanA was shown to be necessary for pneumococcal adherence to epithelial cells, and adherence of the nanA mutant was restored to wild-type level by pre-incubation of epithelial cells with Lactococcus lactis cells expressing NanA. Pneumococcal nanA or nanB mutants were deficient in biofilm formation, while expression of NanA on L. lactis or Streptococcus gordonii promoted biofilm formation by these heterologous host organisms. The results suggest that NanA is an enzymic factor mediating adherence to epithelial cells by decrypting receptors for adhesion, and functions at least in part as an adhesin in biofilm formation. Neuraminidase A thus appears to play multiple temporal roles in pneumococcal infection, from adherence to host tissues, colonization, and community development, to systemic spread and crossing of the blood-brain barrier.

Viral-bacterial interactions in acute otitis media

Acute otitis media (AOM) is a polymicrobial disease, which usually occurs as a complication of viral upper respiratory tract infection (URI). While respiratory viruses alone may cause viral AOM, they increase the risk of bacterial middle ear infection and worsen clinical outcomes of bacterial AOM. URI viruses alter Eustachian tube (ET) function via decreased mucociliary action, altered mucus secretion and increased expression of inflammatory mediators among other mechanisms. Transient reduction in protective functions of the ET allows colonizing bacteria of the nasopharynx to ascend into the middle ear and cause AOM. Advances in research help us to better understand the host responses to viral URI, the mechanisms of viral-bacterial interactions in the nasopharynx and the development of AOM. In this review, we present current knowledge regarding viral-bacterial interactions in the pathogenesis and clinical course of AOM. We focus on the common respiratory viruses and their established role in AOM.

Leukocyte inflammatory responses provoked by pneumococcal sialidase

Cell surface expression of sialic acid has been reported to decrease during immune cell activation, but the significance and regulation of this phenomenon are still being investigated. The major human bacterial pathogen Streptococcus pneumoniae causes pneumonia, sepsis and meningitis, often accompanied by strong inflammatory responses. S. pneumoniae expresses a sialidase (NanA) that contributes to mucosal colonization, platelet clearance, and blood-brain barrier penetration. Using wild-type and isogenic NanA-deficient mutant strains, we showed that S. pneumoniae NanA can desialylate the surface of human THP-1 monocytes, leading to increased ERK phosphorylation, NF-κB activation, and proinflammatory cytokine release. S. pneumoniae NanA expression also stimulates interleukin-8 release and extracellular trap formation from human neutrophils. A mechanistic contribution of unmasking of inhibitory Siglec-5 from cis sialic acid interactions to the proinflammatory effect of NanA is suggested by decreased SHP-2 recruitment to the Siglec-5 intracellular domain and RNA interference studies. Finally, NanA increased production of proinflammatory cytokines in a murine intranasal challenge model of S. pneumoniae pneumonia.

Importance Sialic acids decorate the surface of all mammalian cells and play important roles in physiology, development, and evolution. Siglecs are sialic acid-binding receptors on the surface of immune cells, many of which engage in cis interactions with host sialoglycan ligands and dampen inflammatory responses through transduction of inhibitory signals. Recently, certain bacterial pathogens have been shown to suppress leukocyte innate immune responses by molecular mimicry of host sialic acid structures and engagement of inhibitory Siglecs. Our present work shows that the converse can be true, i.e., that a microbial sialic acid-cleaving enzyme can induce proinflammatory responses, which are in part mediated by unmasking of an inhibitory Siglec. We conclude that host leukocytes are poised to detect and respond to microbial sialidase activity with exaggerated inflammatory responses, which could be beneficial or detrimental to the host depending on the site, stage and magnitude of infection.

Sialic acids decorate the surface of all mammalian cells and play important roles in physiology, development, and evolution. Siglecs are sialic acid-binding receptors on the surface of immune cells, many of which engage in cis interactions with host sialoglycan ligands and dampen inflammatory responses through transduction of inhibitory signals. Recently, certain bacterial pathogens have been shown to suppress leukocyte innate immune responses by molecular mimicry of host sialic acid structures and engagement of inhibitory Siglecs. Our present work shows that the converse can be true, i.e., that a microbial sialic acid-cleaving enzyme can induce proinflammatory responses, which are in part mediated by unmasking of an inhibitory Siglec. We conclude that host leukocytes are poised to detect and respond to microbial sialidase activity with exaggerated inflammatory responses, which could be beneficial or detrimental to the host depending on the site, stage and magnitude of infection.

Exposure of Thomsen-Friedenreich antigen in Streptococcus pneumoniae infection is dependent on pneumococcal neuraminidase A

Pneumococcal hemolytic uremic syndrome is recognized in a small portion of otherwise healthy children who have or have recently had Streptococcus pneumoniae infections, including severe pneumonia, meningitis, and bacteremia. As in other types of hemolytic uremic syndrome (HUS), pneumococcal HUS is characterized by microangiopathic hemolytic anemia, and thrombocytopenia, usually with extensive kidney damage. Although not demonstrated in vivo, the pathogenesis of pneumococcal HUS has been attributed to the action pneumococcal neuraminidase exposing the usually cryptic Thomsen-Friedenreich antigen (T-antigen) on red blood cells (RBC), and kidney glomeruli. We evaluated the effect of pneumococcal infection on desialylation of RBC and glomeruli during pneumococcal infections in mice. Following intravenous infection with capsular type 19F pneumococci, CFU levels exceeding 1000 CFU/mL blood by the third day were significantly more likely to result in exposed T-antigen on RBC than lower levels of bacteremia. In a pneumonia model, significantly more T-antigen was exposed on RBC in mice treated with penicillin than in those receiving mock treatment. Utilizing mutant pneumococci, we demonstrated that neuraminidase A but not neuraminidase B was necessary for exposure of T-antigen on RBC in vivo. Thus, pneumococcal neuraminidase A is necessary for the exposure of T-antigen in vivo and treatment with penicillin increases this effect. Interestingly, NanA(-) pneumococci were found in the blood in higher numbers and caused more deaths than wild type, NanB(-), or the NanA(-)/NanB(-) pneumococci.

Three surface exoglycosidases from Streptococcus pneumoniae, NanA, BgaA, and StrH, promote resistance to opsonophagocytic killing by human neutrophils

Streptococcus pneumoniae (the pneumococcus) is a major human pathogen and a leading cause of inflammatory infections such as pneumonia and otitis media. An important mechanism for host defense against S. pneumoniae is opsonophagocytic killing by neutrophils. To persist in the human host, the pneumococcus has developed strategies to evade opsonization and subsequent neutrophil-mediated killing. Utilizing a genomic approach, we identified NanA, the major pneumococcal neuraminidase, as a factor important for resistance to opsonophagocytic killing in ex vivo killing assays using human neutrophils. The effect of NanA was shown using both type 4 (TIGR4) and type 6A clinical isolates. NanA promotes this resistance by acting in conjunction with two other surface-associated exoglycosidases, BgaA, a beta-galactosidase, and StrH, an N-acetylglucosaminidase. Experiments using human serum showed that these exoglycosidases reduced deposition of complement component C3 on the pneumococcal surface, providing a mechanism for this resistance. Additionally, we have shown that antibodies in human serum do not contribute to this phenotype. These results demonstrate that deglycosylation of a human serum glycoconjugate(s) by the combined effects of NanA, BgaA, and StrH, is important for resistance to complement deposition and subsequent phagocytic killing of S. pneumoniae.

The surface-anchored NanA protein promotes pneumococcal brain endothelial cell invasion

In humans, Streptococcus pneumoniae (SPN) is the leading cause of bacterial meningitis, a disease with high attributable mortality and frequent permanent neurological sequelae. The molecular mechanisms underlying the central nervous system tropism of SPN are incompletely understood, but include a primary interaction of the pathogen with the blood–brain barrier (BBB) endothelium. All SPN strains possess a gene encoding the surface-anchored sialidase (neuraminidase) NanA, which cleaves sialic acid on host cells and proteins. Here, we use an isogenic SPN NanA-deficient mutant and heterologous expression of the protein to show that NanA is both necessary and sufficient to promote SPN adherence to and invasion of human brain microvascular endothelial cells (hBMECs). NanA-mediated hBMEC invasion depends only partially on sialidase activity, whereas the N-terminal lectinlike domain of the protein plays a critical role. NanA promotes SPN–BBB interaction in a murine infection model, identifying the protein as proximal mediator of CNS entry by the pathogen.

The NanA neuraminidase of Streptococcus pneumoniae is involved in biofilm formation

Streptococcus pneumoniae remains a major cause of bacteremia, pneumonia, and otitis media despite vaccines and effective antibiotics. The neuraminidase of S. pneumoniae, which catalyzes the release of terminal sialic acid residues from glycoconjugates, is involved in host colonization in animal models of infection and may provide a novel target for preventing pneumococcal infection. We demonstrate that the S. pneumoniae neuraminidase (NanA) cleaves sialic acid and show that it is involved in biofilm formation, suggesting an additional role in pathogenesis, and that it shares this property with the neuraminidase of Pseudomonas aeruginosa even though we show that the two enzymes are phylogenetically divergent. Using an in vitro model of biofilm formation incorporating human airway epithelial cells, we demonstrate that small-molecule inhibitors of NanA block biofilm formation and may provide a novel target for preventative therapy. This work highlights the role played by the neuraminidase in pathogenesis and represents an important step in drug development for prevention of colonization of the respiratory tract by this important pathogen.

Influenza virus infection decreases tracheal mucociliary velocity and clearance of Streptococcus pneumoniae

Influenza virus infections increase susceptibility to secondary bacterial infections, such as pneumococcal pneumonia, resulting in increased morbidity and mortality. Influenza-induced tissue damage is hypothesized to increase susceptibility to Streptococcus pneumoniae infection by increasing adherence to the respiratory epithelium. Using a mouse model of influenza infection followed by S. pneumoniae infection, we found that an influenza infection does not increase the number of pneumococci initially present within the trachea, but does inhibit pneumococcal clearance by 2 hours after infection. To determine whether influenza damage increases pneumococcal adherence, we developed a novel murine tracheal explant system to determine influenza-induced tissue damage and subsequent pneumococcal adherence. Murine tracheas were kept viable ex vivo as shown by microscopic examination of ciliary beating and cellular morphology using continuous media flow for up to 8 days. Tracheas were infected with influenza virus for 0.5-5 days ex vivo, and influenza-induced tissue damage and the early stages of repair to the epithelium were assessed histologically. A prior influenza infection did not increase pneumococcal adherence, even when the basement membrane was maximally denuded or during the repopulation of the basement membrane with undifferentiated epithelial cells. We measured mucociliary clearance in vivo and found it was decreased in influenza-infected mice. Together, our results indicate that exposure of the tracheal basement membrane contributes minimally to pneumococcal adherence. Instead, an influenza infection results in decreased tracheal mucociliary velocity and initial clearance of pneumococci, leading to an increased pneumococcal burden as early as 2 hours after pneumococcal infection.

Advances in microbial biofilm prevention on indwelling medical devices with emphasis on usage of acoustic energy

Microbial biofilms are a major impediment to the use of indwelling medical devices, generating device-related infections with high morbidity and mortality. Major efforts directed towards preventing and eradicating the biofilm problem face difficulties because biofilms protect themselves very effectively by producing a polysaccharide coating, reducing biofilm sensitivity to antimicrobial agents. Techniques applied to combating biofilms have been primarily chemical. These have met with partial and limited success rates, leading to current trends of eradicating biofilms through physico-mechanical strategies. Here we review the different approaches that have been developed to control biofilm formation and removal, focusing on the utilization of acoustic energy to achieve these objectives.

Characterization of the Streptococcus pneumoniae BgaC protein as a novel surface beta-galactosidase with specific hydrolysis activity for the Galbeta1-3GlcNAc moiety of oligosaccharides

Streptococcus pneumoniae is a causative agent of high morbidity and mortality. Although sugar moieties have been recognized as ligands for initial contact with the host, only a few exoglycosidases have been reported to occur in S. pneumoniae. In this study, a putative beta-galactosidase, encoded by the bgaC gene of S. pneumoniae, was characterized for its enzymatic activity and virulence. The recombinant BgaC protein, expressed and purified from Escherichia coli, was found to have a highly regiospecific and sugar-specific hydrolysis activity for the Galbeta1-3-GlcNAc moiety of oligosaccharides. Interestingly, the BgaC hydrolysis activity was localized at the cell surface of S. pneumoniae, indicating that BgaC is expressed as a surface protein although it does not have a typical signal sequence or membrane anchorage motif. The surface localization of BgaC was further supported by immunofluorescence microscopy analysis using an antibody raised against BgaC and by a reassociation assay with fluorescein isothiocyanate-labeled BgaC. Although the bgaC deletion mutation did not significantly attenuate the virulence of S. pneumoniae in vivo, the bgaC mutant strain showed relatively low numbers of viable cells compared to the wild type after 24 h of infection in vivo, whereas the mutant showed higher colonization levels at 6 and 24 h postinfection in vivo. Our data strongly indicate for the first time that S. pneumoniae bgaC encodes a surface beta-galactosidase with high substrate specificity that is significantly associated with the infection activity of pneumococci.

Structural and functional studies of Streptococcus pneumoniae neuraminidase B: An intramolecular trans-sialidase

The human pathogen Streptococcus pneumoniae expresses neuraminidase proteins that cleave sialic acids from complex carbohydrates. The pneumococcus genome encodes up to three neuraminidase proteins that have been shown to be important virulence factors. Here, we report the first structure of a neuraminidase from S. pneumoniae: the crystal structure of NanB in complex with its reaction product 2,7-anhydro-Neu5Ac. Our structural data, together with biochemical analysis, establish NanB as an intramolecular trans-sialidase with strict specificity towards alpha2-3 linked sialic acid substrates. In addition, we show that NanB differs in its substrate specificity from the other pneumococcal neuraminidase NanA.

PclA, a pneumococcal collagen-like protein with selected strain distribution, contributes to adherence and invasion of host cells

Analysis of Streptococcus pneumoniae sequenced genomes revealed a region present only in selected strains consisting of two ORFs: a putative cell wall anchored protein and a putative transcriptional regulator. The cell wall anchored protein contains large regions of collagen-like repeats, the number of which varies between strains. We have therefore named this protein PclA for pneumococcal collagen-like protein A. The second gene, spr1404, encodes a putative transcriptional regulator. We examined the strain distribution of these two genes among a collection of clinical isolates from invasive pneumococcal disease and found them to be present in 39% of the strains examined. Strains were either positive for both genes or lacked both, with the two genes always present together in the same location of the genome. RT-PCR analysis revealed that pclA is transcribed in vitro, even in the absence of spr1404. Single deletion mutants lacking either gene were not attenuated in a mouse model of invasive pneumonia. However, the pclA mutant was defective in adherence and invasion of host cells in vitro.

The Ashwell receptor mitigates the lethal coagulopathy of sepsis

The Ashwell receptor, the major lectin of hepatocytes, rapidly clears from blood circulation glycoproteins bearing glycan ligands that include galactose and N-acetylgalactosamine. This asialoglycoprotein receptor activity remains a key factor in the development and administration of glycoprotein pharmaceuticals, yet a biological purpose of the Ashwell receptor has remained elusive. We have identified endogenous ligands of the Ashwell receptor as glycoproteins and regulatory components in blood coagulation and thrombosis that include von Willebrand factor (vWF) and platelets. The Ashwell receptor normally modulates vWF homeostasis and is responsible for thrombocytopenia during systemic Streptococcus pneumoniae infection by eliminating platelets desialylated by the bacterium's neuraminidase. Hemostatic adaptation by the Ashwell receptor moderates the onset and severity of disseminated intravascular coagulation during sepsis and improves the probability of host survival.

Growth of Streptococcus pneumoniae on human glycoconjugates is dependent upon the sequential activity of bacterial exoglycosidases

In the human host, Streptococcus pneumoniae encounters a variety of glycoconjugates, including mucin, host defense molecules, and glycans associated with the epithelial surface. S. pneumoniae is known to encode a number of glycosidases that may modify these glycoconjugates in vivo. Three exoglycosidases, a neuraminidase (NanA), beta-galactosidase (BgaA), and N-acetylglucosaminidase (StrH), have been previously demonstrated to sequentially deglycosylate N-linked glycans on host defense molecules, which coat the pneumococcal surface in vivo. This cleavage is proposed to alter the clearance function of these molecules, allowing pneumococci to persist in the airway. However, we propose that the exoglycosidase-dependent liberation of monosaccharides from these glycoconjugates in close proximity to the pneumococcal surface provides S. pneumoniae with a convenient source of fermentable carbohydrate in vivo. In this study, we demonstrate that S. pneumoniae is able to utilize complex N-linked human glycoconjugates as a sole source of carbon to sustain growth and that efficient growth is dependent upon the sequential deglycosylation of the glycoconjugate substrate by pneumococcal exoglycosidases. In addition to demonstrating a role for NanA, BgaA, and StrH, we have identified a function for the second pneumococcal neuraminidase, NanB, in the deglycosylation of host glycoconjugates and have demonstrated that NanB activity can partially compensate for the loss or dysfunction of NanA. To date, all known functions of pneumococcal neuraminidase have been attributed to NanA. Thus, this study describes the first proposed role for NanB by which it may contribute to S. pneumoniae colonization and pathogenesis.

Adherence of nontypeable Streptococcus pneumoniae to human conjunctival epithelial cells

Conjunctivitis outbreaks have occurred in the US in which nontypeable (NT) Streptococcus pneumoniae (Pnc) strains have been identified as the etiologic agent; however, the pathogenesis of Pnc conjunctivitis has not been extensively evaluated. Here we assessed the adhesive and invasive properties of 13 NT US conjunctivitis outbreak strains (cPnc) using an immortalized human conjunctival epithelial cell (HCjE) line expressing high or low levels of mucin as a surrogate for in vivo ocular surface events. Studies reveal differential binding efficiencies (up to 18-fold) among cPnc strains to HCjE cells and reduced or little adherence efficiency to high mucin-expressing (HME-HCjE). Additionally, in the presence of exogenous mucin there is considerable inhibition (20% to approximately 100%) of bacterial binding to the HCjE cells. Invasion assays suggest that the cPnc are internalized in HCjE, and less in HME-HCjE cells. Microarray analysis of cPnc isolates revealed an up-regulation of Pnc neuraminidases, and treatment of HME-HCjE cells with exogenous neuraminidase resulted in a 2-13-fold enhancement in cPnc binding. The results indicate that mucin acts as a protective barrier in vitro and that neuraminidases, which can degrade mucin, may be contributing factors leading to bacterial adherence, a first step in the pathogenesis of this transmissible infection.

Modulation of adherence, invasion, and tumor necrosis factor alpha secretion during the early stages of infection by Streptococcus pneumoniae ClpL

Heat shock proteins (HSPs) play a pivotal role as chaperones in the folding of native and denatured proteins and can help pathogens penetrate host defenses. However, the underlying mechanism(s) of modulation of virulence by HSPs has not been fully determined. In this study, the role of the chaperone ClpL in the pathogenicity of Streptococcus pneumoniae was assessed. A clpL mutant adhered to and invaded nasopharyngeal or lung cells much more efficiently than the wild type adhered to and invaded these cells in vitro, as well as in vivo, although it produced the same amount of capsular polysaccharide. However, the level of secretion of tumor necrosis factor alpha (TNF-alpha) from macrophages infected with the clpL mutant was significantly lower than the level of secretion elicited by the wild type during the early stages of infection. Interestingly, treatment of the human lung epithelial carcinoma A549 and murine macrophage RAW 264.7 cell lines with cytochalasin D, an inhibitor of actin polymerization, increased adherence of the mutant to the host cells. In contrast, cytochalasin D treatment of RAW 264.7 cells decreased TNF-alpha secretion after infection with either the wild type or the mutant. However, pretreatment of cell lines with the actin polymerization activator jasplakinolide reversed these phenotypes. These findings indicate, for the first time, that the ClpL chaperone represses adherence of S. pneumoniae to host cells and induces secretion of TNF-alpha via a mechanism dependent upon actin polymerization during the initial infection stage.

Pneumococcal neuraminidases A and B both have essential roles during infection of the respiratory tract and sepsis

We examined the role of the neuraminidases NanA and NanB in colonization and infection in the upper and lower respiratory tract by Streptococcus pneumoniae, as well as the role of these neuraminidases in the onset and development of septicemia following both intranasal and intravenous infection. We demonstrated for the first time using outbred MF1 mouse models of infection that both NanA and NanB were essential for the successful colonization and infection of the upper and lower respiratory tract, respectively, as well as pneumococcal survival in nonmucosal sites, such as the blood. Our studies have shown that in vivo a neuraminidase A mutant is cleared from the nasopharynx, trachea, and lungs within 12 h postinfection, while a neuraminidase B mutant persists but does not increase in either the nasopharynx, trachea, or lungs. We also demonstrated both neuraminidase mutants were unable to cause sepsis following intranasal infections. When administered intravenously, however, both mutants survived initially but were unable to persist in the blood beyond 48 h postinfection and were progressively cleared. The work presented here demonstrates the importance of pneumococcal neuraminidase A and for the first time neuraminidase B in the development of upper and lower respiratory tract infection and sepsis.

Versatility of pneumococcal surface proteins

Surface-exposed proteins are key players during the infectious process of pathogenic bacteria. The cell surface of the Gram-positive human pathogen Streptococcus pneumoniae is decorated not only by typical Gram-positive surface proteins, but also by a family of proteins that recognizes the phosphorylcholine of the lipoteichoic and teichoic acids, namely the choline-binding proteins, and by non-classical surface proteins that lack a leader peptide and membrane-anchor motif. A comprehensive understanding of how microbial proteins subvert host immunity or host protein functions is a prerequisite for the development of novel therapeutic strategies to combat pneumococcal infections. This article reviews recent progress in the investigation of the versatility and sophistication of the virulence functions of surface-exposed pneumococcal proteins.

Deglycosylation of human glycoconjugates by the sequential activities of exoglycosidases expressed by Streptococcus pneumoniae

Streptococcus pneumoniae produces three surface-associated exoglycosidases; a neuraminidase, NanA, a beta-galactosidase, BgaA, and a beta-N-acetylglucosaminidase, StrH. the proposed functions of NanA, which removes terminal sialic acid, include revealing receptors for adherence, affecting the function of glycosylated host clearance molecules, modifying the surface of other bacteria coinhabiting the same niche, and providing a nutrient source. However, it is unclear whether following desialylation S. pneumoniae can further deglycosylate human targets through the activity of BgaA or StrH. We demonstrate that NanA, BgaA and StrH act sequentially to remove sialic acid, galactose and N-acetylglucosamine and expose mannose on human glycoproteins that bind to the pneumococcus and protect the airway. In addition, both BgaA and NanA were shown to contribute to the adherence of unencapsulated pneumococci, to human epithelial cells. Despite these findings, triple exoglycosidase mutants colonized mice as well as their parental strains, suggesting that any effect of these genes on colonization and disease may be host species-specific. These studies highlight the importance of considering the complete ability of S. pneumoniae to deglycosylate human targets and suggest that in addition to NanA, BgaA and StrH also contribute to pneumococcal colonization and/or pathogenesis.

Neuraminidase decreases in vitro adherence of slime-forming coagulase-negative staphylococci to biosynthetic ovine collagen vascular graft

Vascular prosthetic graft infection is a major complication of vascular surgery that starts with adhesion of the microorganism to the graft. Because slime-forming microorganisms are the major causative agents in graft infection, the goals of investigators in this study were (1) to investigate the bacterial adherence of slime-forming and non-slime-forming coagulase-negative staphylococci (CNS), and (2) to determine the role of neuraminidase (NANase) in bacterial adherence to the biosynthetic ovine collagen graft. Human plasma was instilled and incubated at 37 degrees C in preparation for fibrin deposition of grafts. After 48 hours, incubation grafts were drained and inoculated with slime-forming and non-slime-forming CNS in tryptic soy broth in the presence and in the absence of neuraminidase. After 24 hours of incubation at 36 degrees C, grafts were vortexed and cultured for colony count. Bacterial counts were expressed as total colony-forming units per longitudinal centimeter of the graft. Slime-forming CNS had greater affinity to the collagen graft compared with non-slime-forming CNS (P<.05). Adherence of slime-forming CNS was impaired by NANase treatment (P<.001). NANase treatment of patients with non-slime-forming CNS did not change adherence to the graft (P>.05). Results show that slime plays an important role in the pathogenesis of vascular graft infection. Adherence of slime-forming CNS can be decreased through the administration of NANase. This may have implications for the development of neuraminidase-embedded vascular grafts designed to reduce the occurrence of biomaterial-related infection.

Immunization with recombinant Streptococcus pneumoniae neuraminidase NanA protects chinchillas against nasopharyngeal colonization

Immunization with recombinant S. pneumoniae neuraminidase NanA (rNanA) resulted in a significant reduction in pneumococcal colonization in the chinchilla model. The bacteria were eliminated from the nasopharynx 1 week earlier than that from the control cohort. Our data suggest that rNanA affords protection against pneumococcal nasopharyngeal colonization.

Arcanobacterium pyogenes: molecular pathogenesis of an animal opportunist

Arcanobacterium pyogenes is a commensal and an opportunistic pathogen of economically important livestock, causing diseases as diverse as mastitis, liver abscessation and pneumonia. This organism possesses a number of virulence factors that contribute to its pathogenic potential. A. pyogenes expresses a cholesterol-dependent cytolysin, pyolysin, which is a haemolysin and is cytolytic for immune cells, including macrophages. Expression of pyolysin is required for virulence and this molecule is the most promising vaccine candidate identified to date. A. pyogenes also possesses a number of adherence mechanisms, including two neuraminidases, the action of which are required for full adhesion to epithelial cells, and several extracellular matrix-binding proteins, including a collagen-binding protein, which may be required for adhesion to collagen-rich tissue. A. pyogenes also expresses fimbriae, which are similar to the type 2 fimbriae of Actinomyces naeslundii, and forms biofilms. However, the role of these factors in the pathogenesis of A. pyogenes infections remains to be elucidated. A. pyogenes also invades and survives within epithelial cells and can survive within J774A.1 macrophages for up to 72 h, suggesting an important role for A. pyogenes interaction with host cells during pathogenesis. The two component regulatory system, PloSR, up-regulates pyolysin expression and biofilm formation but down-regulates expression of proteases, suggesting that it may act as a global regulator of A. pyogenes virulence. A. pyogenes is a versatile pathogen, with an arsenal of virulence determinants. However, most aspects of the pathogenesis of infection caused by this important opportunistic pathogen remain poorly characterized.

Stopping Bacterial Adhesion: A Novel Approach to Treating Infections

Adhesion and colonization are prerequisites for the establishment of bacterial pathogenesis. The prevention of adhesion is an attractive target for the development of new therapies in the prevention of infection. Bacteria have developed a multiplicity of adhesion mechanisms commonly targeting surface carbohydrate structures, but our ability to rationally design effective antiadhesives is critically affected by the limitations of our knowledge of the human 'glycome' and of the bacterial function in relation to it. The potential for the future development of carbohydrate-based antiadhesives has been demonstrated by a significant number of in vitro and in vivo studies. Such therapies will be particularly relevant for infections of mucosal surfaces where topical application or delivery is possible.

Localization of UDP-GlcNAc 2-epimerase/ManAc kinase (GNE) in the Golgi complex and the nucleus of mammalian cells

The bifunctional enzyme UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE) is essential for early embryonic development and catalyzes the rate limiting step in sialic acid biosynthesis. Although epimerase and kinase activities have been attributed to GNE, little is known about the regulation, differential expression, and subcellular localization of GNE in vivo. Mutations in GNE cause a rare inherited muscle disorder in humans called hereditary inclusion body myopathy (HIBM). However, the role of GNE in HIBM pathogenesis has not been defined yet. Here, we show that the GNE protein is expressed in various mammalian cells and tissues with highest levels found in cancer cells and liver. In human skeletal muscle, GNE protein is developmentally regulated: high levels are found in immature myoblasts but low levels in mature skeletal muscle. The GNE protein colocalizes with resident proteins of the Golgi compartment in a variety of human cells including muscle. Drug-induced disruption of the Golgi and subsequent recovery reveals co-distribution of GNE along with Golgi-targeted proteins. This subcellular localization of GNE is in good agreement with its established role as the key enzyme of sialic acid biosynthesis, since the sialylation of glycoconjugates takes place in the Golgi complex. Surprisingly, GNE is also detected in the nucleus. Upon nocodazole treatment, GNE redistributes to the cytoplasm suggesting that GNE may act as a nucleocytoplasmic shuttling protein. A regulatory role for GNE shifting between the nuclear and the Golgi compartment is proposed. Further insight into GNE regulation may promote the understanding of HIBM pathogenesis.

Phase variable desialylation of host proteins that bind to Streptococcus pneumoniae in vivo and protect the airway

Most clinical isolates of Streptococcus pneumoniae consist of heterogeneous populations of at least two colony phenotypes, opaque and transparent, selected for in the bloodstream and nasopharynx, respectively. Microarray analysis revealed 24 orfs that demonstrated differences in expression greater than twofold between variants of independent strains. Twenty-one of these showed increased expression in the transparent variants, including 11 predicted to be involved in sugar metabolism. A single genomic region contains seven of these loci including the gene that encodes the neuraminidase, NanA. In contrast to previous studies, there was no contribution of NanA to adherence of S. pneumoniae to epithelial cells or colonization in an animal model. However, we observed NanA-dependent desialylation of human airway components that bind to the organism and may mediate bacterial clearance. Targets of desialylation included human lactoferrin, secretory component, and IgA2 that were shown to be present on the surface of the pneumococcus in vivo during pneumococcal pneumonia. The efficiency of desialylation was increased in the transparent variants and enhanced for host proteins binding to the surface of S. pneumoniae. Because deglycosylation affects the function of many host proteins, NanA may contribute to a protease-independent mechanism to modify bound targets and facilitate enhanced survival of the bacterium.

Immunization with native or recombinant Streptococcus pneumoniae neuraminidase affords protection in the chinchilla otitis media model

Streptococcus pneumoniae neuraminidase has been implicated as a virulence factor in the pathogenesis of pneumococcal otitis media. In this study, native neuraminidase was partially purified from cultures of S. pneumoniae by serial chromatography with DEAE-Sepharose and Sephacryl S-200. Recombinant neuraminidase, a 3,038-bp fragment of the neuraminidase A (nanA) gene, was cloned into the pET-28b vector and then expressed at high levels in Escherichia coli. Chinchillas were immunized subcutaneously with either the gel-purified native or recombinant neuraminidase, and all responded with elevated titers of antineuraminidase antibody in serum. Immunization with neuraminidase resulted in a significant reduction in nasopharyngeal colonization as well as in the incidence of otitis media with effusion. These data demonstrate for the first time that neuraminidase affords protection against S. pneumoniae nasopharyngeal colonization and experimental otitis media.

Lec3 Chinese Hamster Ovary Mutants Lack UDP-N-acetylglucosamine 2-Epimerase Activity Because of Mutations in the Epimerase Domain of the Gne Gene

Lec3 Chinese hamster ovary (CHO) cell glycosylation mutants have a defect in sialic acid biosynthesis that is shown here to be reflected most sensitively in reduced polysialic acid (PSA) on neural cell adhesion molecules. To identify the genetic origin of the phenotype, genes encoding different factors required for sialic acid biosynthesis were transfected into Lec3 cells. Only a Gne cDNA encoding UDP-GlcNAc 2-epimerase:ManNAc kinase rescued PSA synthesis. In an in vitro UDP-GlcNAc 2-epimerase assay, Lec3 cells had no detectable UDP-GlcNAc 2-epimerase activity, and Lec3 cells grown in serum-free medium were essentially devoid of sialic acid on glycoproteins. The Lec3 phenotype was rescued by exogenously added N-acetylmannosamine or mannosamine but not by the same concentrations of N-acetylglucosamine, glucosamine, glucose, or mannose. Sequencing of CHO Gne cDNAs identified a nonsense (E35stop) and a missense (G135E) mutation, respectively, in two independent Lec3 mutants. The G135E Lec3 mutant transfected with a rat Gne cDNA had restored in vitro UDP-GlcNAc 2-epimerase activity and cell surface PSA expression. Both Lec3 mutants were similarly rescued with a CHO Gne cDNA and with CHO Gne encoding the known kinase-deficient D413K mutation. However, cDNAs encoding the known epimerase-deficient mutation H132A or the new Lec3 G135E Gne mutation did not rescue the Lec3 phenotype. The G135E Gne missense mutation is a novel mechanism for inactivating UDP-GlcNAc 2-epimerase activity. Lec3 mutants with no UDP-GlcNAc 2-epimerase activity represent sensitive hosts for characterizing disease-causing mutations in the human GNE gene that give rise to sialuria, hereditary inclusion body myopathy, and Nonaka myopathy.