Plasminogen- and fibronectin-binding protein B is involved in the adherence of Streptococcus pneumoniae to human epithelial cells

Respiratory tract barrier dysfunction in viral-bacterial co-infection cases

A preceding viral infection of the respiratory tract predisposes the host to secondary bacterial pneumonia, known as a major cause of morbidity and mortality. However, the underlying mechanism of the viral-bacterial synergy that leads to disease progression has remained elusive, thus hampering the production of effective prophylactic and therapeutic intervention options. In addition to viral-induced airway epithelial damage, which allows dissemination of bacteria to the lower respiratory tract and increases their invasiveness, dysfunction of immune defense following a viral infection has been implicated as a factor for enhanced susceptibility to secondary bacterial infections. Given the proximity of the oral cavity to the respiratory tract, where viruses enter and replicate, it is also well-established that oral health status can significantly influence the initiation, progression, and pathology of respiratory viral infections. This review was conducted to focus on the dysfunction of the respiratory barrier, which plays a crucial role in providing physical and secretory barriers as well as immune defense in the context of viral-bacterial synergy. Greater understanding of barrier response to viral-bacterial co-infections, will ultimately lead to development of effective, broad-spectrum therapeutic approaches for prevention of enhanced susceptibility to these pathogens.

The serine-rich repeat glycoprotein Srr2 mediates Streptococcus agalactiae interaction with host fibronectin

BACKGROUND

Group B Streptococcus (GBS) is a commensal of healthy adults and an important pathogen in newborns, the elderly and immunocompromised individuals. GBS displays several virulence factors that promote colonisation and host infection, including the ST-17 strain-specific adhesin Srr2, previously characterised for its binding to fibrinogen. Another common target for bacterial adhesins and for host colonization is fibronectin, a multi-domain glycoprotein found ubiquitously in body fluids, in the extracellular matrix and on the surface of cells.

RESULTS

In this study, fibronectin was identified as a novel ligand for the Srr2 adhesin of GBS. A derivative of the ST-17 strain BM110 overexpressing the srr2 gene showed an increased ability to bind fibrinogen and fibronectin, compared to the isogenic wild-type strain. Conversely, the deletion of srr2 impaired bacterial adhesion to both ligands. ELISA assays and surface plasmon resonance studies using the recombinant binding region (BR) form of Srr2 confirmed a direct interaction with fibronectin with an estimated Kd of 92 nM. Srr2-BR variants defective in fibrinogen binding also exhibited no interaction with fibronectin, suggesting that Srr2 binds this ligand through the dock-lock-latch mechanism, previously described for fibrinogen binding. The fibronectin site responsible for recombinant Srr2-BR binding was identified and localised in the central cell-binding domain of the protein. Finally, in the presence of fibronectin, the ability of a Δsrr2 mutant to adhere to human cervico-vaginal epithelial cells was significantly lower than that of the wild-type strain.

CONCLUSION

By combining genetic and biochemical approaches, we demonstrate a new role for Srr2, namely interacting with fibronectin. We characterised the molecular mechanism of this interaction and demonstrated that it plays a role in promoting the adhesion of GBS to human cervico-vaginal epithelial cells, further substantiating the role of Srr2 as a factor responsible for the hypervirulence of GBS ST-17 strains. The discovery of the previously undescribed interaction between Srr2 and fibronectin establishes this adhesin as a key factor for GBS colonisation of host tissues.

The Pneumococcal Protein SufC Binds to Host Plasminogen and Promotes Its Conversion into Plasmin

Streptococcus pneumoniae causes otitis media, sinusitis, and serious diseases such as pneumonia and bacteremia. However, the in vivo dynamics of S. pneumoniae infections and disease severity are not fully understood. In this study, we investigated pneumococcal proteins detected in the bronchoalveolar lavage fluid of an S. pneumoniae-infected mouse, which were assumed to be expressed during infection. Analysis of three proteins with unknown infection-related functions revealed that recombinant Fe-S cluster assembly ATP-binding protein (SufC) binds to the host plasminogen and promotes its conversion into plasmin. SufC was detected in the bacterial cell-surface protein fraction, but it had no extracellular secretory signal. This study suggests that S. pneumoniae releases SufC extracellularly through LytA-dependent autolysis, binding to the bacterial cell surface and host plasminogen and promoting its conversion into plasmin. The recruitment of plasmin by S. pneumoniae is considered useful for bacterial survival and spread, and SufC is suggested to facilitate this process.

C-Terminal Lysine Residue of Pneumococcal Triosephosphate Isomerase Contributes to Its Binding to Host Plasminogen

The main causative agent of pneumonia, Streptococcus pneumoniae, is also responsible for invasive diseases. S. pneumoniae recruits human plasminogen for the invasion and colonization of host tissues. We previously discovered that S. pneumoniae triosephosphate isomerase (TpiA), an enzyme involved in intracellular metabolism that is essential for survival, is released extracellularly to bind human plasminogen and facilitate its activation. Epsilon-aminocaproic acid, a lysine analogue, inhibits this binding, suggesting that the lysine residues in TpiA are involved in plasminogen binding. In this study, we generated site-directed mutant recombinants in which the lysine residue in TpiA was replaced with alanine and analyzed their binding activities to human plasminogen. Results from blot analysis, enzyme-linked immunosorbent assay, and surface plasmon resonance assay revealed that the lysine residue at the C-terminus of TpiA is primarily involved in binding to human plasminogen. Furthermore, we found that TpiA binding to plasminogen through its C-terminal lysine residue was required for the promotion of plasmin activation by activating factors.

Pneumococcal proteins ClpC and UvrC as novel host plasminogen binding factors

Two plasminogen binding proteins were identified from a mouse infected with Streptococcus pneumoniae. The pneumococcal proteins were annotated as ATP-dependent Clp protease ATP-binding subunit (ClpC) and excinuclease ABC subunit C (UvrC) using the isobaric tags for relative and absolute quantification (iTRAQ) method. Recombinants of both proteins showed significant binding to plasminogen and were found to promote plasminogen activation by tissue-type plasminogen activator. In addition, ClpC and UvrC were LytA-dependently released into the culture supernatant and bound to the bacterial surface. These results suggest that S. pneumoniae releases ClpC and UvrC by autolysis and recruits them to the bacterial surface, where they bind to plasminogen and promote its activation, contributing to extracellular matrix degradation and tissue invasion.

Streptococcus pneumoniae binds collagens and C1q via the SSURE repeats of the PfbB adhesin

The binding of Streptococcus pneumoniae to collagen is likely an important step in the pathogenesis of pneumococcal infections, but little is known of the underlying molecular mechanisms. Streptococcal surface repeats (SSURE) are highly conserved protein domains present in cell wall adhesins from different Streptococcus species. We find here that SSURE repeats of the pneumococcal adhesin plasminogen and fibronectin binding protein B (PfbB) bind to various types of collagen. Moreover, deletion of the pfbB gene resulted in a significant impairment of the ability of encapsulated or unencapsulated pneumococci to bind collagen. Notably, a PfbB SSURE domain is also bound to the complement component C1q that bears a collagen‐like domain and promotes adherence of pneumococci to host cells by acting as a bridge between bacteria and epithelial cells. Accordingly, deletion of PfbB or pre‐treatment with anti‐SSURE antibodies markedly decreased pneumococcal binding to C1q as well as C1q‐dependent adherence to epithelial and endothelial cells. Further data indicated that C1q promotes pneumococcal adherence by binding to integrin α₂β₁. In conclusion, our results indicate that the SSURE domains of the PfbB protein promote interactions of pneumococci with various types of collagen and with C1q. These repeats may be useful targets in strategies to control S. pneumoniae infections.

Triosephosphate isomerase of Streptococcus pneumoniae is released extracellularly by autolysis and binds to host plasminogen to promote its activation

Recruitment of plasminogen is an important infection strategy of the human pathogen Streptococcus pneumoniae to invade host tissues. In Streptococcus aureus, triosephosphate isomerase (TPI) has been reported to bind plasminogen. In this study, the TPI of S. pneumoniae (TpiA) was identified through proteomic analysis of bronchoalveolar lavage fluid from a murine pneumococcal pneumonia model. The binding kinetics of recombinant pneumococcal TpiA with plasminogen were characterized using surface plasmon resonance (SPR, Biacore), ligand blot analyses, and enzyme‐linked immunosorbent assay. Enhanced plasminogen activation and subsequent degradation by plasmin were also shown. Release of TpiA into the culture medium was observed to be dependent on autolysin. These findings suggest that S. pneumoniae releases TpiA via autolysis, which then binds to plasminogen and promotes its activation, thereby contributing to tissue invasion via degradation of the extracellular matrix.

Lysine Residues in the MK-Rich Region Are Not Required for Binding of the PbsP Protein From Group B Streptococci to Plasminogen

Binding to plasminogen (Plg) enables bacteria to associate with and invade host tissues. The cell wall protein PbsP significantly contributes to the ability of group B streptococci, a frequent cause of invasive infection, to bind Plg. Here we sought to identify the molecular regions involved in the interactions between Plg and PbsP. The K4 Kringle domain of the Plg molecule was required for binding of Plg to whole PbsP and to a PbsP fragment encompassing a region rich in methionine and lysine (MK-rich domain). These interactions were inhibited by free L-lysine, indicating the involvement of lysine binding sites in the Plg molecule. However, mutation to alanine of all lysine residues in the MK-rich domain did not decrease its ability to bind Plg. Collectively, our data identify a novel bacterial sequence that can interact with lysine binding sites in the Plg molecule. Notably, such binding did not require the presence of lysine or other positively charged amino acids in the bacterial receptor. These data may be useful for developing alternative therapeutic strategies aimed at blocking interactions between group B streptococci and Plg.

Invasion and trafficking of hypervirulent group B streptococci in polarized enterocytes

Streptococcus agalactiae (group B streptococcus or GBS) is a commensal bacterium that can frequently behave as a pathogen, particularly in the neonatal period and in the elderly. The gut is a primary site of GBS colonization and a potential port of entry during neonatal infections caused by hypervirulent clonal complex 17 (CC17) strains. Here we studied the interactions between the prototypical CC17 BM110 strain and polarized enterocytes using the Caco-2 cell line. GBS could adhere to and invade these cells through their apical or basolateral surfaces. Basolateral invasion was considerably more efficient than apical invasion and predominated under conditions resulting in weakening of cell-to-cell junctions. Bacterial internalization occurred by a mechanism involving caveolae- and lipid raft-dependent endocytosis and actin re-organization, but not clathrin-dependent endocytosis. In the first steps of Caco-2 invasion, GBS colocalized with the early endocytic marker EEA-1, to later reside in acidic vacuoles. Taken together, these data suggest that CC17 GBS selectively adheres to the lateral surface of enterocytes from which it enters through caveolar lipid rafts using a classical, actin-dependent endocytic pathway. These data may be useful to develop alternative preventive strategies aimed at blocking GBS invasion of the intestinal barrier.

Investigation of potential anti-pneumococcal effects of l-sulforaphane and metabolites: Insights from synchrotron-FTIR microspectroscopy and molecular docking studies

Streptococcus pneumoniae infection can lead to pneumococcal disease, a major cause of mortality in children under the age of five years. In low- and middle-income country settings where pneumococcal disease burden is high, vaccine use is low and widespread antibiotic use has led to increased rates of multi-drug resistant pneumococci. l-sulforaphane (LSF), derived from broccoli and other cruciferous vegetables, has established anti-inflammatory, antioxidant, and anti-microbial properties. Hence, we sought to investigate the potential role of LSF against pneumococcal infection. Using a combination of in vitro and computational methods, the results showed that LSF and relevant metabolites had a potential to reduce pneumococcal adherence through modulation of host receptors, regulation of inflammation, or through direct modification of bacterial factors. Treatment with LSF and metabolites reduced pneumococcal adherence to respiratory epithelial cells. Synchrotron-Fourier transform infrared microspectroscopy (S-FTIR) revealed biochemical changes in protein and lipid profiles of lung epithelial cells following treatment with LSF or metabolites. Molecular docking studies of 116 pneumococcal and 89 host factors revealed a potent effect for the metabolite LSF-glutathione (GSH). A comprehensive list of factors involved in interactions between S. pneumoniae and host cells was compiled to construct a bacterium and host interaction network. Network analysis revealed plasminogen, fibronectin, and RrgA as key factors involved in pneumococcal-host interactions. Therefore, we propose that these constitute critical targets for direct inhibition by LSF and/or metabolites, which may disrupt pneumococcal-host adherence. Overall, our findings further enhance understanding of the potential role of LSF to modulate pneumococcal-host dynamics.

Fibronectin and Its Role in Human Infective Diseases

Fibronectin is a multidomain glycoprotein ubiquitously detected in extracellular fluids and matrices of a variety of animal and human tissues where it functions as a key link between matrices and cells. Fibronectin has also emerged as the target for a large number of microorganisms, particularly bacteria. There are clear indications that the binding of microorganism’ receptors to fibronectin promotes attachment to and infection of host cells. Each bacterium may use different receptors which recognize specific fibronectin domains, mostly the N-terminal domain and the central cell-binding domain. In many cases, fibronectin receptors have actions over and above that of simple adhesion: In fact, adhesion is often the prerequisite for invasion and internalization of microorganisms in the cells of colonized tissues. This review updates the current understanding of fibronectin receptors of several microorganisms with emphasis on their biochemical and structural properties and the role they can play in the onset and progression of host infection diseases. Furthermore, we describe the antigenic profile and discuss the possibility of designing adhesion inhibitors based on the structure of the fibronectin-binding site in the receptor or the receptor-binding site in fibronectin.

The Streptococcus agalactiae cell wall-anchored protein PbsP mediates adhesion to and invasion of epithelial cells by exploiting the host vitronectin/αv integrin axis

Binding of microbial pathogens to host vitronectin (Vtn) is a common theme in the pathogenesis of invasive infections. In this study, we characterized the role of Vtn in the invasion of mucosal epithelial cells by Streptococcus agalactiae (i.e. group B streptococcus or GBS), a frequent human pathogen. Moreover, we identified PbsP, a previously described plasminogen-binding protein of GBS, as a dual adhesin that can also interact with human Vtn through its streptococcal surface repeat (SSURE) domains. Deletion of the pbsP gene decreases both bacterial adhesion to Vtn-coated inert surfaces and the ability of GBS to interact with epithelial cells. Bacterial adherence to and invasion of epithelial cells were either inhibited or enhanced by cell pretreatment with, respectively, anti-Vtn antibodies or Vtn, confirming the role of Vtn as a GBS ligand on host cells. Finally, antibodies directed against the integrin α_v subunit inhibited Vtn-dependent cell invasion by GBS. Collectively, these results indicate that Vtn acts as a bridge between the SSURE domains of PbsP on the GBS surface and host integrins to promote bacterial invasion of epithelial cells. Therefore, inhibition of interactions between PbsP and extracellular matrix components could represent a viable strategy to prevent colonization and invasive disease by GBS.

The plasminogen binding protein PbsP is required for brain invasion by hypervirulent CC17 Group B streptococci

Streptococcus agalactiae (Group B Streptococcus or GBS) is a frequent cause of serious disease in newborns and adults. Epidemiological evidence indicates a strong association between GBS strains belonging to the hypervirulent CC17 clonal complex and the occurrence of meningitis in neonates. We investigate here the role of PbsP, a cell wall plasminogen binding protein, in colonization of the central nervous system by CC17 GBS. Deletion of pbsP selectively impaired the ability of the CC17 strain BM110 to colonize the mouse brain after intravenous challenge, despite its unchanged capacity to persist at high levels in the blood and to invade the kidneys. Moreover, immunization with a recombinant form of PbsP considerably reduced brain infection and lethality. In vitro, pbsP deletion markedly decreased plasmin-dependent transmigration of BM110 through brain microvascular endothelial cells. Although PbsP was modestly expressed in bacteria grown under standard laboratory conditions, pbsP expression was markedly upregulated during in vivo infection or upon contact with cultured brain endothelial cells. Collectively, our studies indicate that PbsP is a highly conserved Plg binding adhesin, which is functionally important for invasion of the central nervous system by the hypervirulent CC17 GBS. Moreover, this antigen is a promising candidate for inclusion in a universal GBS vaccine.

A Vaginal Tract Signal Detected by the Group B Streptococcus SaeRS System Elicits Transcriptomic Changes and Enhances Murine Colonization

Streptococcus agalactiae (group B streptococcus [GBS]) can colonize the human vaginal tract, leading to both superficial and serious infections in adults and neonates. To study bacterial colonization of the reproductive tract in a mammalian system, we employed a murine vaginal carriage model. Using transcriptome sequencing (RNA-Seq), the transcriptome of GBS growing in vivo during vaginal carriage was determined. Over one-quarter of the genes in GBS were found to be differentially regulated during in vivo colonization compared to laboratory cultures. A two-component system (TCS) homologous to the staphylococcal virulence regulator SaeRS was identified as being upregulated in vivo One of the SaeRS targets, pbsP, a proposed GBS vaccine candidate, is shown to be important for colonization of the vaginal tract. A component of vaginal lavage fluid acts as a signal to turn on pbsP expression via SaeRS. These data demonstrate the ability to quantify RNA expression directly from the murine vaginal tract and identify novel genes involved in vaginal colonization by GBS. They also provide more information about the regulation of an important virulence and colonization factor of GBS, pbsP, by the TCS SaeRS.

PbsP, a cell wall-anchored protein that binds plasminogen to promote hematogenous dissemination of group B Streptococcus

Streptococcus agalactiae (Group B Streptococcus or GBS) is a leading cause of invasive infections in neonates whose virulence is dependent on its ability to interact with cells and host components. We here characterized a surface protein with a critical function in GBS pathophysiology. This adhesin, designated PbsP, possesses two Streptococcal Surface Repeat domains, a methionine and lysine-rich region, and a LPXTG cell wall-anchoring motif. PbsP mediates plasminogen (Plg) binding both in vitro and in vivo and we showed that cell surface-bound Plg can be activated into plasmin by tissue plasminogen activator to increase the bacterial extracellular proteolytic activity. Absence of PbsP results in a decreased bacterial transmigration across brain endothelial cells and impaired virulence in a murine model of infection. PbsP is conserved among the main GBS lineages and is a major plasminogen adhesin in non-CC17 GBS strains. Importantly, immunization of mice with recombinant PbsP confers protective immunity. Our results indicate that GBS have evolved different strategies to recruit Plg which indicates that the ability to acquire cell surface proteolytic activity is essential for the invasiveness of this bacterium.

Discovery of Immunodominant B Cell Epitopes within Surface Pneumococcal Virulence Proteins in Pediatric Patients with Invasive Pneumococcal Disease

The identification of immunodominant B cell epitopes within surface pneumococcal virulence proteins in pediatric patients with invasive pneumococcal disease (IPD) is a valuable approach to define novel vaccine candidates. To this aim, we evaluated sera from children with IPD and age-matched controls against 141 20-mer synthetic peptides covering the entire sequence of major antigenic fragments within pneumococcal virulence proteins; namely, choline-binding protein D (CbpD), pneumococcal histidine triad proteins (PhtD and PhtE), pneumococcal surface protein A (PspA), plasminogen and fibronectin binding protein B (PfbB), and zinc metalloproteinase B (ZmpB). Ten immunodominant B cell epitopes were identified: CbpD-pep4 (amino acids (aa) 291-310), PhtD-pep11 (aa 88-107), PhtD-pep17 (aa 172-191), PhtD-pep19 (aa 200-219), PhtE-pep32 (aa 300-319), PhtE-pep40 (aa 79-98), PfbB-pep76 (aa 180-199), PfbB-pep79 (aa 222-241), PfbB-pep90 (aa 484-503), and ZmpB-pep125 (aa 431-450). All epitopes were highly conserved among different pneumococcal serotypes, and four of them were located within the functional zinc-binding domain of the histidine triad proteins PhtD and PhtE. Peptides CbpD-pep4, PhtD-pep19, and PhtE-pep40 were broadly recognized by IPD patient sera with prevalences of 96.4%, 92.9%, and 71.4%, respectively, whereas control sera exhibited only minor reactivities (<10.7%). Their specificities for IPD were 93.3%, 95%, and 96.7%; their sensitivities were 96.4%, 92.9%, and 71.4% and their positivity likelihood ratios for IPD were 14.5, 18.6, and 21.4, respectively. Furthermore, purified antibodies against CbpD-pep4, PhtD-pep19, and PhtE-pep40 readily bound on the surfaces of different pneumococcal serotypes, as assessed by FACS and immunofluorescence analysis. The identified immunodominant B cell epitopes provide a better understanding of immune response in IPD and are worth evaluation in additional studies as potential vaccine candidates.

Identification of PblB mediating galactose-specific adhesion in a successful Streptococcus pneumoniae clone

The pneumococcal genome is variable and there are minimal data on the influence of the accessory genome on phenotype. Pneumococcal serotype 14 sequence type (ST) 46 had been the most prevalent clone causing pneumonia in children in Taiwan. A microarray was constructed using the genomic DNA of a clinical strain (NTUH-P15) of serotype 14 ST46. Using DNA hybridization, genomic variations in NTUH-P15 were compared to those of 3 control strains. Microarray analysis identified 7 genomic regions that had significant increases in hybridization signals in the NTUH-P15 strain compared to control strains. One of these regions encoded PblB, a phage-encoded virulence factor implicated (in Streptococcus mitis) in infective endocarditis. The isogenic pblB mutant decreased adherence to A549 human lung epithelial cell compared to wild-type NTUH-P15 strain (P = 0.01). Complementation with pblB restored the adherence. PblB is predicted to contain a galactose-binding domain-like region. Preincubation of NTUH-P15 with D-galactose resulted in decreases of adherence to A549 cell in a dose-dependent manner. Challenge of mice with NTUH-P15, isogenic pblB mutant and pblB complementation strains determined that PblB was required for bacterial persistence in the nasopharynx and lung. PblB, as an adhesin mediating the galactose-specific adhesion activity of pneumococci, promote pneumococcal clonal success.

FbsC, a novel fibrinogen-binding protein, promotes Streptococcus agalactiae-host cell interactions

Streptococcus agalactiae (group B Streptococcus or GBS) is a common cause of invasive infections in newborn infants and adults. The ability of GBS to bind human fibrinogen is of crucial importance in promoting colonization and invasion of host barriers. We characterized here a novel fibrinogen-binding protein of GBS, designated FbsC (Gbs0791), which is encoded by the prototype GBS strain NEM316. FbsC, which bears two bacterial immunoglobulin-like tandem repeat domains and a C-terminal cell wall-anchoring motif (LPXTG), was found to be covalently linked to the cell wall by the housekeeping sortase A. Studies using recombinant FbsC indicated that it binds fibrinogen in a dose-dependent and saturable manner, and with moderate affinity. Expression of FbsC was detected in all clinical GBS isolates, except those belonging to the hypervirulent lineage ST17. Deletion of fbsC decreases NEM316 abilities to adhere to and invade human epithelial and endothelial cells, and to form biofilm in vitro. Notably, bacterial adhesion to fibrinogen and fibrinogen binding to bacterial cells were abolished following fbsC deletion in NEM316. Moreover, the virulence of the fbsC deletion mutant and its ability to colonize the brain were impaired in murine models of infection. Finally, immunization with recombinant FbsC significantly protected mice from lethal GBS challenge. In conclusion, FbsC is a novel fibrinogen-binding protein expressed by most GBS isolates that functions as a virulence factor by promoting invasion of epithelial and endothelial barriers. In addition, the protein has significant immunoprotective activity and may be a useful component of an anti-GBS vaccine.

Rapid profiling of the antigen regions recognized by serum antibodies using massively parallel sequencing of antigen-specific libraries

There is a need for techniques capable of identifying the antigenic epitopes targeted by polyclonal antibody responses during deliberate or natural immunization. Although successful, traditional phage library screening is laborious and can map only some of the epitopes. To accelerate and improve epitope identification, we have employed massive sequencing of phage-displayed antigen-specific libraries using the Illumina MiSeq platform. This enabled us to precisely identify the regions of a model antigen, the meningococcal NadA virulence factor, targeted by serum antibodies in vaccinated individuals and to rank hundreds of antigenic fragments according to their immunoreactivity. We found that next generation sequencing can significantly empower the analysis of antigen-specific libraries by allowing simultaneous processing of dozens of library/serum combinations in less than two days, including the time required for antibody-mediated library selection. Moreover, compared with traditional plaque picking, the new technology (named Phage-based Representation OF Immuno-Ligand Epitope Repertoire or PROFILER) provides superior resolution in epitope identification. PROFILER seems ideally suited to streamline and guide rational antigen design, adjuvant selection, and quality control of newly produced vaccines. Furthermore, this method is also susceptible to find important applications in other fields covered by traditional quantitative serology.

Comparison of the transmembrane mucins MUC1 and MUC16 in epithelial barrier function

Membrane-anchored mucins are present in the apical surface glycocalyx of mucosal epithelial cells, each mucosal epithelium having at least two of the mucins. The mucins have been ascribed barrier functions, but direct comparisons of their functions within the same epithelium have not been done. In an epithelial cell line that expresses the membrane-anchored mucins, MUC1 and MUC16, the mucins were independently and stably knocked down using shRNA. Barrier functions tested included dye penetrance, bacterial adherence and invasion, transepithelial resistance, tight junction formation, and apical surface size. Knockdown of MUC16 decreased all barrier functions tested, causing increased dye penetrance and bacterial invasion, decreased transepithelial resistance, surprisingly, disruption of tight junctions, and greater apical surface cell area. Knockdown of MUC1 did not decrease barrier function, in fact, barrier to dye penetrance and bacterial invasion increased significantly. These data suggest that barrier functions of membrane-anchored mucins vary in the context of other membrane mucins, and MUC16 provides a major barrier when present.

Characterization of the immunogenicity and pathogenicity of malate dehydrogenase in Brucella abortus

Brucella abortus is a gram-negative, facultative intracellular pathogen that causes brucellosis, a chronic zoonotic disease resulting in abortion in pregnant cattle and undulant fever in humans. Malate dehydrogenase (MDH), a key enzyme in the tricarboxylic acid cycle, plays important metabolic roles in aerobic energy producing pathways and in malate shuttle. In this study, the MDH-encoding gene for malate dehydrogenase mdh of B. abortus S2308 was cloned, sequenced and expressed. Western blot analysis demonstrated that MDH is an immunogenic membrane-associated protein. In addition, recombinant MDH showed sero-reactivity with 30 individual bovine B. abortus-positive sera by enzyme-linked immunosorbent assay, indicates that MDH may be used as a candidate marker for sero-diagnosis of brucellosis. Furthermore, MDH exhibits fibronectin and plasminogen-binding ability in immunoblotting assay. Inhibition assays on HeLa cells demonstrated that rabbit anti-serum against MDH significantly reduced both bacterial adherence and invasion abilities (p < 0.05), suggesting that MDH play a role in B. abortus colonization. Our results indicated that MDH is not only an immunogenic protein, but is also related to bacterial pathogenesis and may act as a new virulent factor, which will benefit for further understanding the MDH's roles in B. abortus metabolism, pathogenesis and immunity.

Interaction of streptococcal plasminogen binding proteins with the host fibrinolytic system

The ability to take advantage of plasminogen and its activated form plasmin is a common mechanism used by commensal as well as pathogenic bacteria in interaction with their respective host. Hence, a huge variety of plasminogen binding proteins and activation mechanisms exist. This review solely focuses on the genus Streptococcus and, in particular, on the so-called non-activating plasminogen binding proteins. Based on structural and functional differences, as well as on their mode of surface linkaging, three groups can be assigned: M-(like) proteins, surface displayed cytoplasmatic proteins with enzymatic activities (“moonlighting proteins”) and other surface proteins. Here, the plasminogen binding sites and the interaction mechanisms are compared. Recent findings on the functional consequences of these interactions on tissue degradation and immune evasion are summarized.

Immunogenic properties of Streptococcus agalactiae FbsA fragments

Several species of Gram-positive bacteria can avidly bind soluble and surface-associated fibrinogen (Fng), a property that is considered important in the pathogenesis of human infections. To gain insights into the mechanism by which group B Streptococcus (GBS), a frequent neonatal pathogen, interacts with Fng, we have screened two phage displayed genomic GBS libraries. All of the Fng-binding phage clones contained inserts encoding fragments of FbsA, a protein displaying multiple repeats. Since the functional role of this protein is only partially understood, representative fragments were recombinantly expressed and analyzed for Fng binding affinity and ability to induce immune protection against GBS infection. Maternal immunization with 6pGST, a fragment containing five repeats, significantly protected mouse pups against lethal GBS challenge and these protective effects could be recapitulated by administration of anti-6pGST serum from adult animals. Notably, a monoclonal antibody that was capable of neutralizing Fng binding by 6pGST, but not a non-neutralizing antibody, could significantly protect pups against lethal GBS challenge. These data suggest that FbsA-Fng interaction promotes GBS pathogenesis and that blocking such interaction is a viable strategy to prevent or treat GBS infections.

Identification of potential new protein vaccine candidates through pan-surfomic analysis of pneumococcal clinical isolates from adults

Purified polysaccharide and conjugate vaccines are widely used for preventing infections in adults and in children against the Gram-positive bacterium Streptococcus pneumoniae, a pathogen responsible for high morbidity and mortality rates, especially in developing countries. However, these polysaccharide-based vaccines have some important limitations, such as being serotype-dependent, being subjected to losing efficacy because of serotype replacement and high manufacturing complexity and cost. It is expected that protein-based vaccines will overcome these issues by conferring a broad coverage independent of serotype and lowering production costs. In this study, we have applied the “shaving” proteomic approach, consisting of the LC/MS/MS analysis of peptides generated by protease treatment of live cells, to a collection of 16 pneumococcal clinical isolates from adults, representing the most prevalent strains circulating in Spain during the last years. The set of unique proteins identified in all the isolates, called “pan-surfome”, consisted of 254 proteins, which included most of the protective protein antigens reported so far. In search of new candidates with vaccine potential, we identified 32 that were present in at least 50% of the clinical isolates analyzed. We selected four of them (Spr0012, Spr0328, Spr0561 and SP670_2141), whose protection capacity has not yet been tested, for assaying immunogenicity in human sera. All of them induced the production of IgM antibodies in infected patients, thus indicating that they could enter the pipeline for vaccine studies. The pan-surfomic approach shows its utility in the discovery of new proteins that can elicit protection against infectious microorganisms.

Analysis of the Streptococcus agalactiae exoproteome

The two-component regulatory system CovRS is the main regulator of virulence gene expression in Group B Streptococcus (GBS), the leading cause of invasive infections in neonates. In this study we analyzed by mass spectrometry the GBS extracellular protein complex (i.e. the exoproteome) of NEM316 wild-type (WT) strain and its isogenic covRS deletion mutant (ΔcovRS). A total of 53 proteins, 49 of which had classical secretion signals, were identified: 12 were released by both strains while 21 and 20 were released exclusively by WT and ΔcovRS strains, respectively. In addition to known surface proteins, we detected here unstudied cell-wall associated proteins and/or orthologs of putative virulence factors present in other pathogenic streptococci. While the functional role of these proteins remains to be elucidated, our data suggest that the analysis of the exoproteome of bacterial pathogens under different gene expression conditions may be a powerful tool for the rapid identification of novel virulence factors and vaccine candidates.

BIOLOGICAL SIGNIFICANCE

We believe that this manuscript will be of interest to Journal of Proteomics readers since the paper describes the identification of several putative virulence factors and vaccine candidates of the group B streptococcus, an important pathogen, using a simple proteomics strategy involving LC-MS analysis of culture supernatants obtained from two strains with divergent gene expression patterns. This technique provided the most comprehensive inventory of extracellular proteins obtained from a single streptococcal species thus far. The approach described has the added benefit of being easily applicable to a large number of different strains, making it ideal for the identification of conserved vaccine candidates.

The interaction between bacterial enolase and plasminogen promotes adherence of Streptococcus pneumoniae to epithelial and endothelial cells

Binding and conversion of the plasma protein plasminogen is an important pathogenesis mechanism of the human pathogen Streptococcus pneumoniae. Once converted into plasmin, the proteolytic activity of this major fibrinolysis component promotes degradation of extracellular matrix and the dissolution of fibrin clots. Here, we present the exploitation of plasminogen-binding as a further pivotal strategy of pneumococci facilitating adherence to eukaryotic cells. Flow cytometric measurements demonstrated the immobilization of plasminogen on host cell surfaces of human alveolar type II pneumocytes (A549), nasopharyngeal epithelium (Detroit 562) and brain-derived endothelial cells (HBMEC). These host-derived cells were employed in cell culture infection analyses followed by confocal microscopy to monitor the plasminogen-mediated adherence. Results of these studies revealed that host cell-bound plasminogen promotes pneumococcal adherence to human epithelial and endothelial cells in dose-dependent manner, whereas pneumococcal internalization was not enhanced. As an opposed effect pneumococcal-bound plasminogen reduced attachment to the epithelial and endothelial cells, and increased the interaction with neutrophil granulocytes. Moreover, the surface-displayed enolase, which serves as major pneumococcal plasminogen receptor, was identified as a key factor for plasminogen-mediated bacterial attachment in infection analyses with S. pneumoniae enolase mutants.

Phenotypic, genomic, and transcriptional characterization of Streptococcus pneumoniae interacting with human pharyngeal cells

BACKGROUND

Streptococcus pneumoniae is a leading cause of childhood morbidity and mortality worldwide, despite the availability of effective pneumococcal vaccines. Understanding the molecular interactions between the bacterium and the host will contribute to the control and prevention of pneumococcal disease.

RESULTS

We used a combination of adherence assays, mutagenesis and functional genomics to identify novel factors involved in adherence. By contrasting these processes in two pneumococcal strains, TIGR4 and G54, we showed that adherence and invasion capacities vary markedly by strain. Electron microscopy showed more adherent bacteria in association with membranous pseudopodia in the TIGR4 strain. Operons for cell wall phosphorylcholine incorporation (lic), manganese transport (psa) and phosphate utilization (phn) were up-regulated in both strains on exposure to epithelial cells. Pneumolysin, pili, stress protection genes (adhC-czcD) and genes of the type II fatty acid synthesis pathway were highly expressed in the naturally more invasive strain, TIGR4. Deletion mutagenesis of five gene regions identified as regulated in this study revealed attenuation in adherence. Most strikingly, ∆SP_1922 which was predicted to contain a B-cell epitope and revealed significant attenuation in adherence, appeared to be expressed as a part of an operon that includes the gene encoding the cytoplasmic pore-forming toxin and vaccine candidate, pneumolysin.

CONCLUSION

This work identifies a list of novel potential pneumococcal adherence determinants.

Group B Streptococcus hijacks the host plasminogen system to promote brain endothelial cell invasion

Group B Streptococcus (GBS) is the leading cause of meningitis in neonates. We have previously shown that plasminogen, once recruited to the GBS cell surface and converted into plasmin by host-derived activators, leads to an enhancement of bacterial virulence. Here, we investigated whether plasmin(ogen) bound at the GBS surface contributes to blood-brain barrier penetration and invasion of the central nervous system. For that purpose, GBS strain NEM316 preincubated with or without plasminogen plus tissue type plasminogen activator was analyzed for the capacity to adhere to, invade and transmigrate the human brain microvascular endothelial cell (hBMEC) monolayer, and to penetrate the central nervous system using a neonatal mouse model. At earlier times of infection, plasmin(ogen)-treated GBS exhibited a significant increase in adherence to and invasion of hBMECs. Later, injury of hBMECs were observed with plasmin(ogen)-treated GBS that displayed a plasmin-like activity. The same results were obtained when hBMECs were incubated with whole human plasma and infected with untreated GBS. To confirm that the observed effects were due to the recruitment and activation of plasminogen on GBS surface, the bacteria were first incubated with epsilon-aminocaproic acid (εACA), an inhibitor of plasminogen binding, and thereafter with plasmin(ogen). A significant decrease in the hBMECs injury that was correlated with a decrease of the GBS surface proteolytic activity was observed. Furthermore, plasmin(ogen)-treated GBS infected more efficiently the brain of neonatal mice than the untreated bacteria, indicating that plasmin(ogen) bound to GBS surface may facilitate the traversal of the blood-brain barrier. A higher survival rate was observed in offspring born from εACA-treated mothers, compared to untreated mice, and no brain infection was detected in these neonates. Our findings suggest that capture of the host plasmin(ogen) by the GBS surface promotes the crossing of the blood-brain barrier and contributes to the establishment of meningitis.

The choline-binding protein PspC of Streptococcus pneumoniae interacts with the C-terminal heparin-binding domain of vitronectin

Adherence of Streptococcus pneumoniae is directly mediated by interactions of adhesins with eukaryotic cellular receptors or indirectly by exploiting matrix and serum proteins as molecular bridges. Pneumococci engage vitronectin, the human adhesive glycoprotein and complement inhibitor, to facilitate attachment to epithelial cells of the mucosal cavity, thereby modulating host cell signaling. In this study, we identified PspC as a vitronectin-binding protein interacting with the C-terminal heparin-binding domain of vitronectin. PspC is a multifunctional surface-exposed choline-binding protein displaying various adhesive properties. Vitronectin binding required the R domains in the mature PspC protein, which are also essential for the interaction with the ectodomain of the polymeric immunoglobulin receptor and secretory IgA. Consequently, secretory IgA competitively inhibited binding of vitronectin to purified PspC and to PspC-expressing pneumococci. In contrast, Factor H, which binds to the N-terminal part of mature PspC molecules, did not interfere with the PspC-vitronectin interaction. Using a series of vitronectin peptides, the C-terminal heparin-binding domain was shown to be essential for the interaction of soluble vitronectin with PspC. Binding experiments with immobilized vitronectin suggested a region N-terminal to the identified heparin-binding domain as an additional binding region for PspC, suggesting that soluble, immobilized, as well as cellularly bound vitronectin possesses different conformations. Finally, vitronectin bound to PspC was functionally active and inhibited the deposition of the terminal complement complex. In conclusion, this study identifies and characterizes (on the molecular level) the interaction between the pneumococcal adhesin PspC and the human glycoprotein vitronectin.

The Spr1875 protein confers resistance to the microglia-mediated killing of Streptococcus pneumoniae

By screening a whole-genome λ-display library of Streptococcus pneumoniae, we have previously identified a novel surface protein, named Spr1875, that exhibited immunogenic properties and was closely related to pneumococcal virulence. In the present study, we investigated the role of the Spr1875 antigen in the interaction of S. pneumoniae with microglia, the resident brain macrophages. By using an in vitro infection model, the BV2 microglial cell line was challenged with the S. pneumoniae strain DP1004 and its isogenic spr1875-deleted mutant (Δspr1875). Both strains were phagocytosed by microglia efficiently and to a similar extent; however, the DP1004 strain was more resistant than the Δspr1875 mutant to the intracellular killing, as assessed by antibiotic protection and phagosome maturation assays. Moreover, significant differences between the two strains were also observed in terms of susceptibility to microglia-mediated killing. Taken together, these results indicate that S. pneumoniae-microglial cell interplay is influenced by the presence of Spr1875, suggesting that this protein may play a role in the pathogenesis of pneumococcal meningitis.

Streptococcus pneumoniae endopeptidase O (PepO) is a multifunctional plasminogen- and fibronectin-binding protein, facilitating evasion of innate immunity and invasion of host cells

Streptococcus pneumoniae infections remain a major cause of morbidity and mortality worldwide. Therefore a detailed understanding and characterization of the mechanism of host cell colonization and dissemination is critical to gain control over this versatile pathogen. Here we identified a novel 72-kDa pneumococcal protein endopeptidase O (PepO), as a plasminogen- and fibronectin-binding protein. Using a collection of clinical isolates, representing different serotypes, we found PepO to be ubiquitously present both at the gene and protein level. In addition, PepO protein was secreted in a growth phase-dependent manner to the culture supernatants of the pneumococcal isolates. Recombinant PepO bound human plasminogen and fibronectin in a dose-dependent manner and plasminogen did not compete with fibronectin for binding PepO. PepO bound plasminogen via lysine residues and the interaction was influenced by ionic strength. Moreover, upon activation of PepO-bound plasminogen by urokinase-type plasminogen activator, generated plasmin cleaved complement protein C3b thus assisting in complement control. Furthermore, direct binding assays demonstrated the interaction of PepO with epithelial and endothelial cells that in turn blocked pneumococcal adherence. Moreover, a pepO-mutant strain showed impaired adherence to and invasion of host cells compared with their isogenic wild-type strains. Taken together, the results demonstrated that PepO is a ubiquitously expressed plasminogen- and fibronectin-binding protein, which plays role in pneumococcal invasion of host cells and aids in immune evasion.

Bacterial plasminogen receptors: mediators of a multifaceted relationship

Multiple species of bacteria are able to sequester the host zymogen plasminogen to the cell surface. Once localised to the bacterial surface, plasminogen can act as a cofactor in adhesion, or, following activation to plasmin, provide a source of potent proteolytic activity. Numerous bacterial plasminogen receptors have been identified, and the mechanisms by which they interact with plasminogen are diverse. Here we provide an overview of bacterial plasminogen receptors and discuss the diverse role bacterial plasminogen acquisition plays in the relationship between bacteria and the host.

Multiple activities of LigB potentiate virulence of Leptospira interrogans: inhibition of alternative and classical pathways of complement

Microbial pathogens acquire the immediate imperative to avoid or counteract the formidable defense of innate immunity as soon as they overcome the initial physical barriers of the host. Many have adopted the strategy of directly disrupting the complement system through the capture of its components, using proteins on the pathogen's surface. In leptospirosis, pathogenic Leptospira spp. are resistant to complement-mediated killing, in contrast to the highly vulnerable non-pathogenic strains. Pathogenic L. interrogans uses LenA/LfhA and LcpA to respectively sequester and commandeer the function of two regulators, factor H and C4BP, which in turn bind C3b or C4b to interrupt the alternative or classical pathways of complement activation. LigB, another surface-proximal protein originally characterized as an adhesin binding multiple host proteins, has other activities suggesting its importance early in infection, including binding extracellular matrix, plasma, and cutaneous repair proteins and inhibiting hemostasis. In this study, we used a recent model of ectopic expression of LigB in the saprophyte, L. biflexa, to test the hypothesis that LigB also interacts with complement proteins C3b and C4b to promote the virulence of L. interrogans. The surface expression of LigB partially rescued the non-pathogen from killing by 5% normal human serum, showing 1.3- to 48-fold greater survival 4 to 6 d following exposure to complement than cultures of the non-expressing parental strain. Recombinant LigB7'-12 comprising the LigB-specific immunoglobulin repeats binds directly to human complement proteins, C3b and C4b, with respective K(d)s of 43±26 nM and 69±18 nM. Repeats 9 to 11, previously shown to contain the binding domain for fibronectin and fibrinogen, are also important in LigB-complement interactions, which interfere with the alternative and classical pathways measured by complement-mediated hemolysis of erythrocytes. Thus, LigB is an adaptable interface for L. interrogans to efficiently counteract the multiple homeostatic processes of the host.

SP0454, a putative threonine dehydratase, is required for pneumococcal virulence in mice

Increasing pressure in antibiotic resistance and the requirement for the design of new vaccines are the objectives of clarifying the putative virulence factors in pneumococcal infection. In this study, the putative threonine dehydratase sp0454 was inactivated by erythromycin-resistance cassette replacement in Streptococcus pneumoniae CMCC 31203 strain. The sp0454 mutant was tested for cell growth, adherence, colonization, and virulence in a murine model. The Δsp0454 mutant showed decreased ability for colonization and impaired ability to adhere to A549 cells. However, the SP0454 polypeptide or its antiserum did not affect pneumococcal CMCC 31203 adhesion to A549 cells. The sp0454 deletion mutant was less virulent in a murine intranasal infection model. Real-time RT-PCR analysis revealed significant decrease of the pneumococcal surface antigen A expression in the sp0454 mutant. These results suggest that SP0454 contributes to virulence and colonization, which could be explained in part by modulating the expression of other virulence factors, such as psaA in pneumococcal infection.

Protective activity of Streptococcus pneumoniae Spr1875 protein fragments identified using a phage displayed genomic library

There is considerable interest in pneumococcal protein antigens capable of inducing serotype-independent immunoprotection and of improving, thereby, existing vaccines. We report here on the immunogenic properties of a novel surface antigen encoded by ORF spr1875 in the R6 strain genome. An antigenic fragment encoded by spr1875, designated R4, was identified using a Streptococcus pneumoniae phage displayed genomic library after selection with a human convalescent serum. Immunofluorescence analysis with anti-R4 antisera showed that Spr1875 was expressed on the surface of strains belonging to different serotypes. Moreover, the gene was present with little sequence variability in 27 different pneumococcal strains isolated worldwide. A mutant lacking Spr1875 was considerably less virulent than the wild type D39 strain in an intravenous mouse model of infection. Moreover, immunization with the R4 recombinant fragment, but not with the whole Spr1875 protein, induced significant protection against sepsis in mice. Lack of protection after immunization with the whole protein was related to the presence of immunodominant, non-protective epitopes located outside of the R4 fragment. In conclusion, our data indicate that Spr1875 has a role in pneumococcal virulence and is immunogenic. As the R4 fragment conferred immunoprotection from experimental sepsis, selected antigenic fragments of Spr1875 may be useful for the development of a pneumococcal protein-based vaccine.

Stick to your gums: mechanisms of oral microbial adherence

Studies on the adherence properties of oral bacteria have been a major focus in microbiology research for several decades. The ability of bacteria to adhere to the variety of surfaces present in the oral cavity, and to become integrated within the resident microbial communities, confers growth and survival properties. Molecular analyses have revealed several families of Gram-positive bacterial surface proteins, including serine-rich repeat, antigen I/II, and pilus families, that mediate adherence to a variety of salivary and oral bacterial receptors. In Gram-negative bacteria, pili, auto-transporters, and extracellular matrix-binding proteins provide components for host tissue recognition and building of complex microbial communities. Future studies will reveal in greater detail the binding pockets for these adhesin families and their receptors. This information will be crucial for the development of new inhibitors or vaccines that target the functional regions of bacterial proteins that are involved in colonization and pathogenesis.

Expression and potential biological role of α(1,2)fucosylated glycotopes on amniotic and seminal fibronectins

The present paper describes concisely the expression and role of α(1,2)-linked fucose on some glycoconjugates as well as the detection, distribution and potential role of that glycotope on human soluble plasma and cellular fibronectins in addition to the expression on both normal and pathological amniotic fluid and seminal plasma fibronectins. The determination of α(1,2)fucosylated glycans is considered with respect to its usefulness as a potential clinically applicable biomarker in obstetrics to monitor pregnancy and in andrology to evaluate the ejaculate of infertile men and in vitro fertilization.

Cellular interactions by LPxTG-anchored pneumococcal adhesins and their streptococcal homologues

In this review we focus on three important families of LPxTG-anchored adhesins in the human pathogen Streptococcus pneumoniae, but also their homologues in related streptococci. We discuss the contribution of these streptococcal adhesins to host tropism, pathogenesis and their interactions with different host cell types. The first surface structures discussed are the heteropolymeric pili that have been found in important streptococcal pathogens such as S. pneumoniae, S. pyogenes, S. agalactiae and E. faecalis/faecium. Major and minor pilus subunit proteins are covalently joined and finally attached to the cell wall through the action of specific sortases. The role of pili and individual pilin subunits in adhesion and pathogenesis and their structure and assembly in different streptococcal species are being covered. Furthermore, we address recent findings regarding a family of large glycosylated serine-rich repeat (SRR) proteins that act as fibrillar adhesins for which homologues have been found in several streptococcal species including pneumococci. In the pneumococcal genome both pili and its giant SRR protein are encoded by accessory genes present in particular clonal lineages for which epidemiological information is available. Finally, we briefly discuss the role played by the pneumococcal neuraminidase NanA in adhesion and pathogenesis.

Assessment of Streptococcus pneumoniae capsule in conjunctivitis and keratitis in vivo neuraminidase activity increases in nonencapsulated pneumococci following conjunctival infection

PURPOSE

The pneumococcal capsule is required for pathogenesis in systemic infections, yet reports show most conjunctivitis outbreaks are caused by nonencapsulated pneumococci, while keratitis infections are caused by encapsulated strains. This study aims to determine the effect of capsule in pneumococcal keratitis and conjunctivitis in rabbit models of infection.

METHODS

A capsule-deficient isogenic mutant was created using homologous transformation. Parent and mutant strains were injected within the upper bulbar conjunctiva (conjunctivitis) or into the corneal stroma (keratitis) of New Zealand white rabbits. Clinical examinations were performed 24 and 48 hr post-infection at which time corneas or conjunctivae were removed, homogenized, and plated to determine the recovered bacterial load. Whole eyes were removed for histological examination. The neuraminidase activity was determined following in vitro and in vivo growth.

RESULTS

There were no significant differences in clinical scores between the eyes infected with the parent or mutant for either infection, nor was there a difference in the amount of bacteria recovered from the cornea. In the conjunctivae, however, the mutant strain was cleared by the host faster than the parent strain. Histological examination showed slightly more infiltrating polymorphonuclear leukocytes (PMN) and macrophages in the conjunctivae infected with the parent strain. The neuraminidase activity of both strains was not significantly different when the strains were grown in vitro. However, the neuraminidase activity of the parent was significantly less than that of the mutant at 3 and 12 hr post conjunctival infection.

CONCLUSIONS

Although more outbreaks of pneumococcal conjunctivitis are tied to nonencapsulated S. pneumoniae strains, this study showed that an encapsulated strain was capable of establishing conjunctivitis in a rabbit injection model and survive attack by the host immune system longer than its nonencapsulated isogenic mutant. Nonetheless, the nonencapsulated pneumococci had an increased neuraminidase activity level in vivo when compared to the parent strain.

The two variants of the Streptococcus pneumoniae pilus 1 RrgA adhesin retain the same function and elicit cross-protection in vivo

Thirty percent of Streptococcus pneumoniae isolates contain pilus islet 1, coding for a pilus composed of the backbone subunit RrgB and two ancillary proteins, RrgA and RrgC. RrgA is the major determinant of in vitro adhesion associated with pilus 1, is protective in vivo in mouse models, and exists in two variants (clades I and II). Mapping of the sequence variability onto the RrgA structure predicted from X-ray data showed that the diversity was restricted to the "head" of the protein, which contains the putative binding domains, whereas the elongated "stalk" was mostly conserved. To investigate whether this variability could influence the adhesive capacity of RrgA and to map the regions important for binding, two full-length protein variants and three recombinant RrgA portions were tested for adhesion to lung epithelial cells and to purified extracellular matrix (ECM) components. The two RrgA variants displayed similar binding abilities, whereas none of the recombinant fragments adhered at levels comparable to those of the full-length protein, suggesting that proper folding and structural arrangement are crucial to retain protein functionality. Furthermore, the two RrgA variants were shown to be cross-reactive in vitro and cross-protective in vivo in a murine model of passive immunization. Taken together, these data indicate that the region implicated in adhesion and the functional epitopes responsible for the protective ability of RrgA may be conserved and that the considerable level of variation found within the "head" domain of RrgA may have been generated by immunologic pressure without impairing the functional integrity of the pilus.

Pneumococcal microbial surface components recognizing adhesive matrix molecules targeting of the extracellular matrix

The attachment of bacteria to host cells and tissues, and their subsequent invasion and dissemination are key processes during pathogenesis. In this issue of Molecular Microbiology, Jensch and co-workers provide further molecular insight into these events during infection with the Gram positive bacterium Streptococcus pneumoniae. Their characterization of pneumococcal adherence and virulence factor B (PavB), a bacterial surface protein with orthologues in other streptococci, show that it binds to the extracellar matrix components fibronection and plasminogen by virtue of repetitive sequences-designated streptococcal surface repeats. In mice, a pavB mutant showed reduced nasopharyngeal colonization and was attenuated in a lung infection model. As discussed here in the context of the pneumococcus, the study of PavB highlights the central role during microbal pathogenesis of targetting the extracellular matrix by so-called microbial surface components recognizing adhesive matrix molecules (MSCRAMMs).