The Streptococcus gordonii platelet binding protein GspB undergoes glycosylation independently of export

Safety assessment of five candidate probiotic lactobacilli using comparative genome analysis

Micro-organisms belonging to the Lactobacillus genus complex are often used for oral consumption and are generally considered safe but can exhibit pathogenicity in rare and specific cases. Therefore, screening and understanding genetic factors that may contribute to pathogenicity can yield valuable insights regarding probiotic safety. Limosilactobacillus mucosae LM1, Lactiplantibacillus plantarum SK151, Lactiplantibacillus plantarum BS25, Limosilactobacillus fermentum SK152 and Lactobacillus johnsonii PF01 are current probiotics of interest; however, their safety profiles have not been explored. The genome sequences of LM1, SK151, SK152 and PF01 were downloaded from the NCBI GenBank, while that of L. plantarum BS25 was newly sequenced. These genomes were then annotated using the Rapid Annotation using Subsystem Technology tool kit pipeline. Subsequently, a command line blast was performed against the Virulence Factor Database (VFDB) and the Comprehensive Antibiotic Resistance Database (CARD) to identify potential virulence factors and antibiotic resistance (AR) genes. Furthermore, ResFinder was used to detect acquired AR genes. The query against the VFDB identified genes that have a role in bacterial survivability, platelet aggregation, surface adhesion, biofilm formation and immunoregulation; and no acquired AR genes were detected using CARD and ResFinder. The study shows that the query strains exhibit genes identical to those present in pathogenic bacteria with the genes matched primarily having roles related to survival and surface adherence. Our results contribute to the overall strategies that can be employed in pre-clinical safety assessments of potential probiotics. Gene mining using whole-genome data, coupled with experimental validation, can be implemented in future probiotic safety assessment strategies.

The glycopatterns of Pseudomonas aeruginosa as a potential biomarker for its carbapenem resistance

IMPORTANCE

Bacterial surface glycans are an attractive therapeutic target in response to antibiotics; however, current knowledge of the corresponding mechanisms is rather limited. Antimicrobial susceptibility testing, genome sequencing, and MALDI-TOF MS, commonly used in recent years to analyze bacterial resistance, are unable to rapidly and efficiently establish associations between glycans and resistance. The discovery of new antimicrobial strategies still requires the introduction of promising analytical methods. In this study, we applied lectin microarray technology and a machine-learning model to screen for important glycan structures associated with carbapenem-resistant P. aeruginosa. This work highlights that specific glycopatterns can be important biomarkers associated with bacterial antibiotic resistance, which promises to provide a rapid entry point for exploring new resistance mechanisms in pathogens.

Opportunities and Challenges of Bacterial Glycosylation for the Development of Novel Antibacterial Strategies

Glycosylation is a ubiquitous process that is universally conserved in nature. The various products of glycosylation, such as polysaccharides, glycoproteins, and glycolipids, perform a myriad of intra- and extracellular functions. The multitude of roles performed by these molecules is reflected in the significant diversity of glycan structures and linkages found in eukaryotes and prokaryotes. Importantly, glycosylation is highly relevant for the virulence of many bacterial pathogens. Various surface-associated glycoconjugates have been identified in bacteria that promote infectious behavior and survival in the host through motility, adhesion, molecular mimicry, and immune system manipulation. Interestingly, bacterial glycosylation systems that produce these virulence factors frequently feature rare monosaccharides and unusual glycosylation mechanisms. Owing to their marked difference from human glycosylation, bacterial glycosylation systems constitute promising antibacterial targets. With the rise of antibiotic resistance and depletion of the antibiotic pipeline, novel drug targets are urgently needed. Bacteria-specific glycosylation systems are especially promising for antivirulence therapies that do not eliminate a bacterial population, but rather alleviate its pathogenesis. In this review, we describe a selection of unique glycosylation systems in bacterial pathogens and their role in bacterial homeostasis and infection, with a focus on virulence factors. In addition, recent advances to inhibit the enzymes involved in these glycosylation systems and target the bacterial glycan structures directly will be highlighted. Together, this review provides an overview of the current status and promise for the future of using bacterial glycosylation to develop novel antibacterial strategies.

O-acetylation controls the glycosylation of bacterial serine-rich repeat glycoproteins

The serine-rich repeat (SRR) glycoproteins of gram-positive bacteria are a family of adhesins that bind to a wide range of host ligands, and expression of SRR glycoproteins is linked with enhanced bacterial virulence. The biogenesis of these surface glycoproteins involves their intracellular glycosylation and export via the accessory Sec system. Although all accessory Sec components are required for SRR glycoprotein export, Asp2 of Streptococcus gordonii also functions as an O-acetyltransferase that modifies GlcNAc residues on the SRR adhesin gordonii surface protein B (GspB). Because these GlcNAc residues can also be modified by the glycosyltransferases Nss and Gly, it has been unclear whether the post-translational modification of GspB is coordinated. We now report that acetylation modulates the glycosylation of exported GspB. Loss of O-acetylation due to aps2 mutagenesis led to the export of GspB glycoforms with increased glucosylation of the GlcNAc moieties. Linkage analysis of the GspB glycan revealed that both O-acetylation and glucosylation occurred at the same C6 position on GlcNAc residues and that O-acetylation prevented Glc deposition. Whereas streptococci expressing nonacetylated GspB with increased glucosylation were significantly reduced in their ability to bind human platelets in vitro, deletion of the glycosyltransferases nss and gly in the asp2 mutant restored platelet binding to WT levels. These findings demonstrate that GlcNAc O-acetylation controls GspB glycosylation, such that binding via this adhesin is optimized. Moreover, because O-acetylation has comparable effects on the glycosylation of other SRR adhesins, acetylation may represent a conserved regulatory mechanism for the post-translational modification of the SRR glycoprotein family.

Enhanced biofilm formation of Streptococcus gordonii with lipoprotein deficiency

Streptococcus gordonii is a commensal Gram-positive bacterium that acts as an opportunistic pathogen that can cause apical periodontitis, endocarditis, and pneumonia. Biofilm formation of bacteria is important for the initiation and progression of such diseases. Although lipoproteins play key roles in physiological functions, the role of lipoproteins of S. gordonii in its biofilm formation has not been clearly understood. In this study, we investigated the role of lipoproteins of S. gordonii in the bacterial biofilm formation using its lipoprotein-deficient strain (Δlgt). The S. gordonii Δlgt exhibited increased biofilm formation on the human dentin slices or on the polystyrene surfaces compared to the wild-type strain, while its growth rate did not differ from that of the wild-type. In addition, the S. gordonii Δlgt strain exhibited the enhanced LuxS mRNA expression and AI-2 production, which is known to be a positive regulator of biofilm formation, compared to the wild-type. Concordantly, the augmented biofilm formation of S. gordonii Δlgt was attenuated by an AI-2 inhibitor, D-ribose. In addition, lipoproteins from purified S. gordonii inhibited the biofilm formation of S. gordonii wild-type and Δlgt. Taken together, these results suggest that lipoprotein-deficient S. gordonii form biofilms more effectively than the wild-type strain, which might be related to the AI-2 quorum-sensing system.

The Two Distinct Types of SecA2-Dependent Export Systems

In addition to SecA of the general Sec system, many Gram-positive bacteria, including mycobacteria, express SecA2, a second, transport-associated ATPase. SecA2s can be subdivided into two mechanistically distinct types: (i) SecA2s that are part of the accessory Sec (aSec) system, a specialized transporter mediating the export of a family of serine-rich repeat (SRR) glycoproteins that function as adhesins, and (ii) SecA2s that are part of multisubstrate systems, in which SecA2 interacts with components of the general Sec system, specifically the SecYEG channel, to export multiple types of substrates. Found mainly in streptococci and staphylococci, the aSec system also contains SecY2 and novel accessory Sec proteins (Asps) that are required for optimal export. Asp2 also acetylates glucosamine residues on the SRR domains of the substrate during transport. Targeting of the SRR substrate to SecA2 and the aSec translocon is mediated by a specialized signal peptide. Multisubstrate SecA2 systems are present in mycobacteria, corynebacteria, listeriae, clostridia, and some bacillus species. Although most substrates for this SecA2 have canonical signal peptides that are required for export, targeting to SecA2 appears to depend on structural features of the mature protein. The feature of the mature domains of these proteins that renders them dependent on SecA2 for export may be their potential to fold in the cytoplasm. The discovery of aSec and multisubstrate SecA2 systems expands our appreciation of the diversity of bacterial export pathways. Here we present our current understanding of the mechanisms of each of these SecA2 systems.

Serine-rich repeat proteins from gut microbes

Serine-rich repeat proteins (SRRPs) have emerged as an important group of cell surface adhesins found in a growing number of Gram-positive bacteria. Studies focused on SRRPs from streptococci and staphylococci demonstrated that these proteins are O-glycosylated on serine or threonine residues and exported via an accessory secretion (aSec) system. In pathogens, these adhesins contribute to disease pathogenesis and represent therapeutic targets. Recently, the non-canonical aSec system has been identified in the genomes of gut microbes and characterization of their associated SRRPs is beginning to unfold, showing their role in mediating attachment and biofilm formation. Here we provide an update of the occurrence, structure, and function of SRRPs across bacteria, with emphasis on the molecular and biochemical properties of SRRPs from gut symbionts, particularly Lactobacilli. These emerging studies underscore the range of ligands recognized by these adhesins and the importance of SRRP glycosylation in the interaction of gut microbes with the host.

Serine-rich repeat protein adhesins from Lactobacillus reuteri display strain specific glycosylation profiles

Lactobacillus reuteri is a gut symbiont inhabiting the gastrointestinal tract of numerous vertebrates. The surface-exposed serine-rich repeat protein (SRRP) is a major adhesin in Gram-positive bacteria. Using lectin and sugar nucleotide profiling of wild-type or L. reuteri isogenic mutants, MALDI-ToF-MS, LC-MS and GC-MS analyses of SRRPs, we showed that L. reuteri strains 100-23C (from rodent) and ATCC 53608 (from pig) can perform protein O-glycosylation and modify SRRP100-23 and SRRP53608 with Hex-Glc-GlcNAc and di-GlcNAc moieties, respectively. Furthermore, in vivo glycoengineering in E. coli led to glycosylation of SRRP53608 variants with α-GlcNAc and GlcNAcβ(1→6)GlcNAcα moieties. The glycosyltransferases involved in the modification of these adhesins were identified within the SecA2/Y2 accessory secretion system and their sugar nucleotide preference determined by saturation transfer difference NMR spectroscopy and differential scanning fluorimetry. Together, these findings provide novel insights into the cellular O-protein glycosylation pathways of gut commensal bacteria and potential routes for glycoengineering applications.

Lipoproteins in Streptococcus gordonii are critical in the infection and inflammatory responses

Gram-positive bacteria such as Streptococcus gordonii causing life-threatening infective endocarditis are mainly recognized by Toll-like receptor 2 (TLR2). Lipoteichoic acid (LTA) and lipoproteins are representative TLR2 ligands that play important roles in bacterial infection and in host inflammatory responses. In the present study, we generated an LTA-deficient mutant (ΔltaS) and a lipoprotein-deficient mutant (Δlgt) and investigated the contributions of LTA and lipoproteins to bacterial morphology and their effect on induction of proinflammatory cytokines in THP-1 and mouse bone-marrow derived macrophages (BMDMs). Deletion of ltaS and lgt was confirmed by PCR analysis of genomic DNA from each mutant. The mutants with absence of LTA or lipoproteins were examined by SDS-PAGE followed by Western blotting with anti-LTA antibodies and silver staining, respectively. Interestingly, scanning and transmission electron microscopies showed no difference in the bacterial cell morphology or size between the wild-type and the mutants even though substantial changes in the cell size and/or morphology have been reported in other Gram-positive bacteria such as Staphylococcus aureus, Listeria monocytogenes, and Bacillus subtilis. However, S. gordonii wild-type and ΔltaS potently induced the expression of proinflammatory cytokines including TNF-α, IL-8, and IL-1β at the mRNA and protein levels, while Δlgt did not have these effects. Furthermore, lipoproteins purified from S. gordonii also induced the expression of the aforementioned cytokines more potently than the purified LTA. Neither LTA nor lipoprotein induced TNF-α, KC (IL-8 counterpart in mouse), and IL-1β in TLR2-deficient BMDMs. S. gordonii Δlgt was less virulent than the wild-type or ΔltaS in a mouse intraperitoneal infection model. Collectively, these results suggest that S. gordonii lipoproteins, but not LTA, are mainly responsible for the infection and inflammatory responses.

Role of Neuraminidase-Producing Bacteria in Exposing Cryptic Carbohydrate Receptors for Streptococcus gordonii Adherence

Streptococcus gordonii is an early colonizer of the oral cavity. Although a variety of S. gordonii adherence mechanisms have been described, current dogma is that the major receptor for S. gordonii is sialic acid. However, as many bacterial species in the oral cavity produce neuraminidase that can cleave terminal sialic acid, it is unclear whether S. gordonii relies on sialic acid for adherence to oral surfaces or if this species has developed alternative binding strategies. Previous studies have examined adherence to immobilized glycoconjugates and identified binding to additional glycans, but no prior studies have defined the contribution of these different glycan structures in adherence to oral epithelial cells. We determined that the majority of S. gordonii strains tested did not rely on sialic acid for efficient adherence. In fact, adherence of some strains was significantly increased following neuraminidase treatment. Further investigation of representative strains that do not rely on sialic acid for adherence revealed binding not only to sialic acid via the serine-rich repeat protein GspB but also to β-1,4-linked galactose. Adherence to this carbohydrate occurs via an unknown adhesin distinct from those utilized by Streptococcus oralis and Streptococcus pneumoniae Demonstrating the potential biological relevance of binding to this cryptic receptor, we established that S. oralis increases S. gordonii adherence in a neuraminidase-dependent manner. These data suggest that S. gordonii has evolved to simultaneously utilize both terminal and cryptic receptors in response to the production of neuraminidase by other species in the oral environment.

Gram-Positive Uropathogens, Polymicrobial Urinary Tract Infection, and the Emerging Microbiota of the Urinary Tract

Gram-positive bacteria are a common cause of urinary-tract infection (UTI), particularly among individuals who are elderly, pregnant, or who have other risk factors for UTI. Here we review the epidemiology, virulence mechanisms, and host response to the most frequently isolated Gram-positive uropathogens: Staphylococcus saprophyticus, Enterococcus faecalis, and Streptococcus agalactiae. We also review several emerging, rare, misclassified, and otherwise underreported Gram-positive pathogens of the urinary tract including Aerococcus, Corynebacterium, Actinobaculum, and Gardnerella. The literature strongly suggests that urologic diseases involving Gram-positive bacteria may be easily overlooked due to limited culture-based assays typically utilized for urine in hospital microbiology laboratories. Some UTIs are polymicrobial in nature, often involving one or more Gram-positive bacteria. We herein review the risk factors and recent evidence for mechanisms of bacterial synergy in experimental models of polymicrobial UTI. Recent experimental data has demonstrated that, despite being cleared quickly from the bladder, some Gram-positive bacteria can impact pathogenic outcomes of co-infecting organisms. When taken together, the available evidence argues that Gram-positive bacteria are important uropathogens in their own right, but that some can be easily overlooked because they are missed by routine diagnostic methods. Finally, a growing body of evidence demonstrates that a surprising variety of fastidious Gram-positive bacteria may either reside in or be regularly exposed to the urinary tract and further suggests that their presence is widespread among women, as well as men. Experimental studies in this area are needed; however, there is a growing appreciation that the composition of bacteria found in the bladder could be a potentially important determinant in urologic disease, including susceptibility to UTI.

Transcriptional organization of pneumococcal psrP-secY2A2 and impact of GtfA and GtfB deletion on PsrP-associated virulence properties

Pneumococcal serine-rich repeat protein (PsrP) is a glycoprotein that mediates Streptococcus pneumoniae attachment to lung cells and promotes biofilm formation. Herein, we investigated the transcriptional organization of psrP-secY2A2, the 37-kbp pathogenicity island encoding PsrP and its accessory genes. PCR amplification of cDNA and RNA-seq analysis found psrP-secY2A2 to be minimally composed of three operons: psrP-glyA, glyB, and glyC-asp5. Transcription of all three operons was greatest during biofilm growth and immunoblot analyses confirmed increased PsrP production by biofilm pneumococci. Using gas chromatography-mass spectrometry we identified monomeric N-acetylglucosamine as the primary glycoconjugate present on a recombinant intracellular version of PsrP, i.e. PsrP1-734. This finding was validated by immunoblot using lectins with known carbohydrate specificities. We subsequently deleted gtfA and gtfB, the GTFs thought to be responsible for addition of O-linked N-acetylglucosamine, and tested for PsrP and its associated virulence properties. These deletions negatively affected our ability to detect PsrP1-734 in bacterial whole cell lysates. Moreover, S. pneumoniae mutants lacking these genes pheno-copied the psrP mutant and were attenuated for: biofilm formation, adhesion to lung epithelial cells, and pneumonia in mice. Our studies identify the transcriptional organization of psrP-secY2A2 and show the indispensable role of GtfA and GtfB on PsrP-mediated pneumococcal virulence.

The Plasmin-Sensitive Protein Pls in Methicillin-Resistant Staphylococcus aureus (MRSA) Is a Glycoprotein

Most bacterial glycoproteins identified to date are virulence factors of pathogenic bacteria, i.e. adhesins and invasins. However, the impact of protein glycosylation on the major human pathogen Staphylococcus aureus remains incompletely understood. To study protein glycosylation in staphylococci, we analyzed lysostaphin lysates of methicillin-resistant Staphylococcus aureus (MRSA) strains by SDS-PAGE and subsequent periodic acid-Schiff’s staining. We detected four (>300, ∼250, ∼165, and ∼120 kDa) and two (>300 and ∼175 kDa) glycosylated surface proteins with strain COL and strain 1061, respectively. The ∼250, ∼165, and ∼175 kDa proteins were identified as plasmin-sensitive protein (Pls) by mass spectrometry. Previously, Pls has been demonstrated to be a virulence factor in a mouse septic arthritis model. The pls gene is encoded by the staphylococcal cassette chromosome (SCC)mec type I in MRSA that also encodes the methicillin resistance-conferring mecA and further genes. In a search for glycosyltransferases, we identified two open reading frames encoded downstream of pls on the SCCmec element, which we termed gtfC and gtfD. Expression and deletion analysis revealed that both gtfC and gtfD mediate glycosylation of Pls. Additionally, the recently reported glycosyltransferases SdgA and SdgB are involved in Pls glycosylation. Glycosylation occurs at serine residues in the Pls SD-repeat region and modifying carbohydrates are N-acetylhexosaminyl residues. Functional characterization revealed that Pls can confer increased biofilm formation, which seems to involve two distinct mechanisms. The first mechanism depends on glycosylation of the SD-repeat region by GtfC/GtfD and probably also involves eDNA, while the second seems to be independent of glycosylation as well as eDNA and may involve the centrally located G5 domains. Other previously known Pls properties are not related to the sugar modifications. In conclusion, Pls is a glycoprotein and Pls glycosyl residues can stimulate biofilm formation. Thus, sugar modifications may represent promising new targets for novel therapeutic or prophylactic measures against life-threatening S. aureus infections.

Staphylococcus aureus is a serious pathogen that causes life-threatening infections due to its ability to attach to surfaces, form biofilms, and persist inside the host. One of previously identified virulence factors in S. aureus pathogenesis is the plasmin-sensitive surface protein Pls. We here identified Pls as a posttranslationally modified glycoprotein and characterized the domain within Pls that becomes glycosylated as well as the modifying sugars. Moreover, we found that the glycosyltransferases GtfC and GtfD carry out the glycosylation reactions. In a search for a role for the modifying sugars, we found that Pls can stimulate biofilm formation apparently via two distinct mechanisms, one being dependent on glycosylation by GtfC and GtfD the other being independent of glycosylation as well as eDNA. Moreover, we found that none of the already known Pls functions is mediated by the sugar moieties. Thus, we conclude that GtfC/GtfD-glycosylated Pls may contribute to MRSA pathogenicity via stimulation of biofilm formation and may serve as future target to combat or prevent infections with this serious pathogen.

Diverse evolutionary patterns of pneumococcal antigens identified by pangenome-wide immunological screening

Characterizing the immune response to pneumococcal proteins is critical in understanding this bacterium's epidemiology and vaccinology. Probing a custom-designed proteome microarray with sera from 35 healthy US adults revealed a continuous distribution of IgG affinities for 2,190 potential antigens from the species-wide pangenome. Reproducibly elevated IgG binding was elicited by 208 "antibody binding targets" (ABTs), which included 109 variants of the diverse pneumococcal surface proteins A and C (PspA and PspC) and zinc metalloprotease A and B (ZmpA and ZmpB) proteins. Functional analysis found ABTs were enriched in motifs for secretion and cell surface association, with extensive representation of cell wall synthesis machinery, adhesins, transporter solute-binding proteins, and degradative enzymes. ABTs were associated with stronger evidence for evolving under positive selection, although this varied between functional categories, as did rates of diversification through recombination. Particularly rapid variation was observed at some immunogenic accessory loci, including a phage protein and a phase-variable glycosyltransferase ubiquitous among the diverse set of genomic islands encoding the serine-rich PsrP glycoprotein. Nevertheless, many antigens were conserved in the core genome, and strains' antigenic profiles were generally stable. No strong evidence was found for any epistasis between antigens driving population dynamics, or redundancy between functionally similar accessory ABTs, or age stratification of antigen profiles. These results highlight the paradox of why substantial variation is observed in only a subset of epitopes. This result may indicate only some interactions between immunoglobulins and ABTs clear pneumococcal colonization or that acquired immunity to pneumococci is an accumulation of individually weak responses to ABTs evolving under different levels of functional constraint.

Streptococcus gordonii induces nitric oxide production through its lipoproteins stimulating Toll-like receptor 2 in murine macrophages

Streptococcus gordonii, a Gram-positive commensal in the oral cavity, is an opportunistic pathogen that can cause endodontic and systemic infections resulting in infective endocarditis. Lipoteichoic acid (LTA) and lipoprotein are major virulence factors of Gram-positive bacteria that are preferentially recognized by Toll-like receptor 2 (TLR2) on immune cells. In the present study, we investigated the effect of S. gordonii LTA and lipoprotein on the production of the representative inflammatory mediator nitric oxide (NO) by the mouse macrophages. Heat-killed S. gordonii wild-type and an LTA-deficient mutant (ΔltaS) but not a lipoprotein-deficient mutant (Δlgt) induced NO production in mouse primary macrophages and the cell line, RAW 264.7. S. gordonii wild-type and ΔltaS also induced the expression of inducible NO synthase (iNOS) at the mRNA and protein levels. In contrast, the Δlgt mutant showed little effect under the same condition. Furthermore, S. gordonii wild-type and ΔltaS induced NF-κB activation, STAT1 phosphorylation, and IFN-β expression, which are important for the induction of iNOS gene expression, with little activation by Δlgt. S. gordonii wild-type and ΔltaS showed an increased adherence and internalization to RAW 264.7 cells compared to Δlgt. In addition, S. gordonii wild-type and ΔltaS, but not Δlgt, substantially increased TLR2 activation while none of these induced NO production in TLR2-deficient macrophages. Triton X-114-extracted lipoproteins from S. gordonii were sufficient to induce NO production. Collectively, we suggest that lipoprotein is an essential cell wall component of S. gordonii to induce NO production in macrophages through TLR2 triggering NF-κB and STAT1 activation.

A glimpse into the modulation of post-translational modifications of human-colonizing bacteria

Protein post-translational modifications (PTMs) are a key bacterial feature that holds the capability to modulate protein function and responses to environmental cues. Until recently, their role in the regulation of prokaryotic systems has been largely neglected. However, the latest developments in mass spectrometry-based proteomics have allowed an unparalleled identification and quantification of proteins and peptides that undergo PTMs in bacteria, including in species which directly or indirectly affect human health. Herein, we address this issue by carrying out the largest and most comprehensive global pooling and comparison of PTM peptides and proteins from bacterial species performed to date. Data was collected from 91 studies relating to PTM bacterial peptides or proteins identified by mass spectrometry-based methods. The present analysis revealed that there was a considerable overlap between PTMs across species, especially between acetylation and other PTMs, particularly succinylation. Phylogenetically closer species may present more overlapping phosphoproteomes, but environmental triggers also contribute to this proximity. PTMs among bacteria were found to be extremely versatile and diverse, meaning that the same protein may undergo a wide variety of different modifications across several species, but it could also suffer different modifications within the same species.

Emerging facets of prokaryotic glycosylation

Glycosylation of proteins is one of the most prevalent post-translational modifications occurring in nature, with a wide repertoire of biological implications. Pathways for the main types of this modification, the N- and O-glycosylation, can be found in all three domains of life-the Eukarya, Bacteria and Archaea-thereby following common principles, which are valid also for lipopolysaccharides, lipooligosaccharides and glycopolymers. Thus, studies on any glycoconjugate can unravel novel facets of the still incompletely understood fundamentals of protein N- and O-glycosylation. While it is estimated that more than two-thirds of all eukaryotic proteins would be glycosylated, no such estimate is available for prokaryotic glycoproteins, whose understanding is lagging behind, mainly due to the enormous variability of their glycan structures and variations in the underlying glycosylation processes. Combining glycan structural information with bioinformatic, genetic, biochemical and enzymatic data has opened up an avenue for in-depth analyses of glycosylation processes as a basis for glycoengineering endeavours. Here, the common themes of glycosylation are conceptualised for the major classes of prokaryotic (i.e. bacterial and archaeal) glycoconjugates, with a special focus on glycosylated cell-surface proteins. We describe the current knowledge of biosynthesis and importance of these glycoconjugates in selected pathogenic and beneficial microbes.

Composition and Activity of the Non-canonical Gram-positive SecY2 Complex

The accessory Sec system in Streptococcus gordonii DL1 is a specialized export system that transports a large serine-rich repeat protein, Hsa, to the bacterial surface. The system is composed of core proteins SecA2 and SecY2 and accessory Sec proteins Asp1–Asp5. Similar to canonical SecYEG, SecY2 forms a channel for translocation of the Hsa adhesin across the cytoplasmic membrane. Accessory Sec proteins Asp4 and Asp5 have been suggested to work alongside SecY2 to form the translocon, similar to the associated SecY, SecE, and SecG of the canonical system (SecYEG). To test this theory, S. gordonii secY2, asp4, and asp5 were co-expressed in Escherichia coli. The resultant complex was subsequently purified, and its composition was confirmed by mass spectrometry to be SecY2-Asp4-Asp5. Like SecYEG, the non-canonical complex activates the ATPase activity of the SecA motor (SecA2). This study also shows that Asp4 and Asp5 are necessary for optimal adhesion of S. gordonii to glycoproteins gp340 and fibronectin, known Hsa binding partners, as well as for early stage biofilm formation. This work opens new avenues for understanding the structure and function of the accessory Sec system.

Melittin, a honeybee venom‑derived antimicrobial peptide, may target methicillin‑resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is difficult to treat using available antibiotic agents. Honeybee venom has been widely used as an oriental treatment for several inflammatory diseases and bacterial infections. The venom contains predominantly biologically active compounds, however, the therapeutic effects of such materials when used to treat MRSA infections have not been investigated extensively. The present study evaluated bee venom and its principal active component, melittin, in terms of their antibacterial activities and in vivo protection against MRSA infections. In vitro, bee venom and melittin exhibited comparable levels of antibacterial activity, which was more marked against MRSA strains, compared with other Gram-positive bacteria. When MRSA-infected mice were treated with bee venom or melittin, only the latter animals were successfully rescued from MRSA- induced bacteraemia or exhibited recovery from MRSA-infected skin wounds. Together, the data of the present study demonstrated for the first time, to the best of our knowledge, that melittin may be used as a promising antimicrobial agent to enhance the healing of MRSA-induced wounds.

Group B streptococcal infection and activation of human astrocytes

Background

Streptococcus agalactiae (Group B Streptococcus, GBS) is the leading cause of life-threatening meningitis in human newborns in industrialized countries. Meningitis results from neonatal infection that occurs when GBS leaves the bloodstream (bacteremia), crosses the blood-brain barrier (BBB), and enters the central nervous system (CNS), where the bacteria contact the meninges. Although GBS is known to invade the BBB, subsequent interaction with astrocytes that physically associate with brain endothelium has not been well studied.

Methodology/Principal Findings

We hypothesize that human astrocytes play a unique role in GBS infection and contribute to the development of meningitis. To address this, we used a well- characterized human fetal astrocyte cell line, SVG-A, and examined GBS infection in vitro. We observed that all GBS strains of representative clinically dominant serotypes (Ia, Ib, III, and V) were able to adhere to and invade astrocytes. Cellular invasion was dependent on host actin cytoskeleton rearrangements, and was specific to GBS as Streptococcus gordonii failed to enter astrocytes. Analysis of isogenic mutant GBS strains deficient in various cell surface organelles showed that anchored LTA, serine-rich repeat protein (Srr1) and fibronectin binding (SfbA) proteins all contribute to host cell internalization. Wild-type GBS also displayed an ability to persist and survive within an intracellular compartment for at least 12 h following invasion. Moreover, GBS infection resulted in increased astrocyte transcription of interleukin (IL)-1β, IL-6 and VEGF.

Conclusions/Significance

This study has further characterized the interaction of GBS with human astrocytes, and has identified the importance of specific virulence factors in these interactions. Understanding the role of astrocytes during GBS infection will provide important information regarding BBB disruption and the development of neonatal meningitis.

The sweet tooth of bacteria: common themes in bacterial glycoconjugates

Humans have been increasingly recognized as being superorganisms, living in close contact with a microbiota on all their mucosal surfaces. However, most studies on the human microbiota have focused on gaining comprehensive insights into the composition of the microbiota under different health conditions (e.g., enterotypes), while there is also a need for detailed knowledge of the different molecules that mediate interactions with the host. Glycoconjugates are an interesting class of molecules for detailed studies, as they form a strain-specific barcode on the surface of bacteria, mediating specific interactions with the host. Strikingly, most glycoconjugates are synthesized by similar biosynthesis mechanisms. Bacteria can produce their major glycoconjugates by using a sequential or an en bloc mechanism, with both mechanistic options coexisting in many species for different macromolecules. In this review, these common themes are conceptualized and illustrated for all major classes of known bacterial glycoconjugates, with a special focus on the rather recently emergent field of glycosylated proteins. We describe the biosynthesis and importance of glycoconjugates in both pathogenic and beneficial bacteria and in both Gram-positive and -negative organisms. The focus lies on microorganisms important for human physiology. In addition, the potential for a better knowledge of bacterial glycoconjugates in the emerging field of glycoengineering and other perspectives is discussed.

Modification of Streptococcus mutans Cnm by PgfS contributes to adhesion, endothelial cell invasion, and virulence

Expression of the surface protein Cnm has been directly implicated in the ability of certain strains of Streptococcus mutans to bind to collagen and to invade human coronary artery endothelial cells (HCAEC) and in the killing of Galleria mellonella. Sequencing analysis of Cnm(+) strains revealed that cnm is located between the core genes SMU.2067 and SMU.2069. Reverse transcription-PCR (RT-PCR) analysis showed that cnm is cotranscribed with SMU.2067, encoding a putative glycosyltransferase referred to here as PgfS (protein glycosyltransferase of streptococci). Notably, Cnm contains a threonine-rich domain predicted to undergo O-linked glycosylation. The previously shown abnormal migration pattern of Cnm, the presence of the threonine-rich domain, and the molecular linkage of cnm with pgfS lead us to hypothesize that PgfS modifies Cnm. A ΔpgfS strain showed defects in several traits associated with Cnm expression, including collagen binding, HCAEC invasion, and killing of G. mellonella. Western blot analysis revealed that Cnm from the ΔpgfS mutant migrated at a lower molecular weight than that from the parent strain. In addition, Cnm produced by ΔpgfS was highly susceptible to proteinase K degradation, in contrast to the high-molecular-weight Cnm version found in the parent strain. Lectin-binding analyses confirmed the glycosylated nature of Cnm and strongly suggested the presence of N-acetylglucosamine residues attached to Cnm. Based on these findings, the phenotypes observed in ΔpgfS are most likely associated with defects in Cnm glycosylation that affects protein function, stability, or both. In conclusion, this study demonstrates that Cnm is a glycoprotein and that posttranslational modification mediated by PgfS contributes to the virulence-associated phenotypes linked to Cnm.

Stimulation of platelet apoptosis by peptidoglycan from Staphylococcus aureus 113

Peptidoglycan (PGN), a component of bacterial cell wall and belonging to "Microbe-Associated Molecular Patterns" (MAMP) triggers host reactions contributing to the pathophysiology of infectious disease. Host cell responses to PGN exposure include apoptosis. Bacterial infections may result in activation of blood platelets and thrombocytopenia. The present study explored, whether HPLC-purified fractions of PGNs from Staphylococcus aureus 113 triggers apoptosis of platelets. To this end platelets were exposed to PGN fractions and annexin-V binding determined to depict cell membrane scrambling, DiOC6 fluorescence to estimate depolarization of mitochondrial potential, Fluo-3AM staining for intracellular Ca(2+) activity ([Ca(2+)](i)) and immunofluorescence to quantify protein abundance of active caspase-3. As a result, a 30 min exposure to monomeric fraction (mPGN) (≥50 ng/ml) was followed by annexin-V binding, paralleled by increase of [Ca(2+)](i), mitochondrial depolarization, caspase-3 activation and integrin α(IIb)β(3) upregulation. The annexin-V binding was significantly blunted by anti-TLR-2 antibodies, in absence of extracellular Ca(2+), and by pancaspase inhibitor zVAD-FMK (1 μM). In conclusion, PGN triggers apoptosis of platelets in activation-dependent manner, characterized by mitochondrial depolarization, caspase-3 activation and cell membrane scrambling.

Selective transport by SecA2: an expanding family of customized motor proteins

The SecA2 proteins are a special class of transport-associated ATPases that are related to the SecA component of the general Sec system, and are found in an increasingly large number of Gram-positive bacterial species. The SecA2 substrates are typically linked to the cell wall, but may be lipid-linked, peptidoglycan-linked, or non-covalently associated S-layer proteins. These substrates can have a significant impact on virulence of pathogenic organisms, but may also aid colonization by commensals. The SecA2 orthologues range from being highly similar to their SecA paralogues, to being distinctly different in apparent structure and function. Two broad classes of SecA2 are evident. One transports multiple substrates, and may interact with the general Sec system, or with an as yet unidentified transmembrane channel. The second type transports a single substrate, and is a component of the accessory Sec system, which includes the SecY paralogue SecY2 along with the accessory Sec proteins Asp1-3. Recent studies indicate that the latter three proteins may have a unique role in coordinating post-translational modification of the substrate with transport by SecA2. Comparative functional and phylogenetic analyses suggest that each SecA2 may be uniquely adapted for a specific type of substrate. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Novel staphylococcal glycosyltransferases SdgA and SdgB mediate immunogenicity and protection of virulence-associated cell wall proteins

Infection of host tissues by Staphylococcus aureus and S. epidermidis requires an unusual family of staphylococcal adhesive proteins that contain long stretches of serine-aspartate dipeptide-repeats (SDR). The prototype member of this family is clumping factor A (ClfA), a key virulence factor that mediates adhesion to host tissues by binding to extracellular matrix proteins such as fibrinogen. However, the biological siginificance of the SDR-domain and its implication for pathogenesis remain poorly understood. Here, we identified two novel bacterial glycosyltransferases, SdgA and SdgB, which modify all SDR-proteins in these two bacterial species. Genetic and biochemical data demonstrated that these two glycosyltransferases directly bind and covalently link N-acetylglucosamine (GlcNAc) moieties to the SDR-domain in a step-wise manner, with SdgB appending the sugar residues proximal to the target Ser-Asp repeats, followed by additional modification by SdgA. GlcNAc-modification of SDR-proteins by SdgB creates an immunodominant epitope for highly opsonic human antibodies, which represent up to 1% of total human IgG. Deletion of these glycosyltransferases renders SDR-proteins vulnerable to proteolysis by human neutrophil-derived cathepsin G. Thus, SdgA and SdgB glycosylate staphylococcal SDR-proteins, which protects them against host proteolytic activity, and yet generates major eptopes for the human anti-staphylococcal antibody response, which may represent an ongoing competition between host and pathogen.

Staphylococcus aureus and S. epidermidis are major bacterial pathogens that can cause life-threatening human diseases. Following entry into the circulation, S.aureus can infect virtually any organ. However, it must first counter antibacterial mechanisms of the innate immune system, including those involving macrophages and neutrophils. Important for staphylococcal adhesion to and successful colonization of host tissues, is a family of bacterial surface proteins containing multiple repeats of serine-aspartate repeats (SDR) adjacent to an adhesive A-domain. The biological functions of the SDR-domain of these SDR proteins remain elusive. We found that the SDR-domain of all staphylococcal SDR proteins is heavily glycosylated. We identified two novel glycosylases, SdgA and SdgB, which are responsible for glycosylation in two steps, and found that this glycosylation protects the adhesive SDR proteins against proteolytic attack by human neutrophil cathepin G. Since pathogen binding to human tissues, including the extracellular matrix protein fibrinogen, depends on SDR proteins, this glycosylation may be important for successful colonization of the human host. We also show that the SdgB-mediated glycosylation creates an immunodominant epitope for highly opsonic antibodies in humans. These antibodies account for a significant proportion of the total anti-staphylococcal IgG response.

Binding of glycoprotein Srr1 of Streptococcus agalactiae to fibrinogen promotes attachment to brain endothelium and the development of meningitis

The serine-rich repeat glycoprotein Srr1 of Streptococcus agalactiae (GBS) is thought to be an important adhesin for the pathogenesis of meningitis. Although expression of Srr1 is associated with increased binding to human brain microvascular endothelial cells (hBMEC), the molecular basis for this interaction is not well defined. We now demonstrate that Srr1 contributes to GBS attachment to hBMEC via the direct interaction of its binding region (BR) with human fibrinogen. When assessed by Far Western blotting, Srr1 was the only protein in GBS extracts that bound fibrinogen. Studies using recombinant Srr1-BR and purified fibrinogen in vitro confirmed a direct protein-protein interaction. Srr1-BR binding was localized to amino acids 283–410 of the fibrinogen Aα chain. Structural predictions indicated that the conformation of Srr1-BR is likely to resemble that of SdrG and other related staphylococcal proteins that bind to fibrinogen through a “dock, lock, and latch” mechanism (DLL). Deletion of the predicted latch domain of Srr1-BR abolished the interaction of the BR with fibrinogen. In addition, a mutant GBS strain lacking the latch domain exhibited reduced binding to hBMEC, and was significantly attenuated in an in vivo model of meningitis. These results indicate that Srr1 can bind fibrinogen directly likely through a DLL mechanism, which has not been described for other streptococcal adhesins. This interaction was important for the pathogenesis of GBS central nervous system invasion and subsequent disease progression.

Streptococcus agalactiae (Group B streptococcus, GBS) is a leading cause of meningitis in newborns and infants. This life-threatening infection of the brain and surrounding tissues continues to result in a high incidence of morbidity and mortality, despite antibiotic therapy. A key factor in disease production is the ability of this organism to invade the central nervous system, via the bloodstream. We now report that a GBS surface protein called Srr1 binds fibrinogen, a major protein in human blood. This interaction enhances the attachment of GBS to brain vascular endothelial cells, and contributes to the development of meningitis. A mutation in Srr1 that specifically disrupted binding to fibrinogen significantly reduced GBS attachment to brain endothelium, and markedly reduced virulence in an in vivo model of GBS disease. These studies have identified a new mechanism by which Srr1 contributes to GBS invasion of the central nervous system and may provide a basis for novel therapies targeting Srr1 binding.

Emerging themes in SecA2-mediated protein export

The conserved general secretion (Sec) pathway carries out most protein export in bacteria and is powered by the essential ATPase SecA. Interestingly, mycobacteria and some Gram-positive bacteria possess two SecA proteins: SecA1 and SecA2. In these species, SecA1 is responsible for exporting most proteins, whereas SecA2 exports only a subset of substrates and is implicated in virulence. However, despite the impressive body of knowledge about the canonical SecA1, less is known concerning SecA2 function. Here, we review our current understanding of the different types of SecA2 systems and outline future directions for their study.

The accessory Sec protein Asp2 modulates GlcNAc deposition onto the serine-rich repeat glycoprotein GspB

The accessory Sec system is a specialized transport system that exports serine-rich repeat (SRR) glycoproteins of Gram-positive bacteria. This system contains two homologues of the general secretory (Sec) pathway (SecA2 and SecY2) and several other essential proteins (Asp1 to Asp5) that share no homology to proteins of known function. In Streptococcus gordonii, Asp2 is required for the transport of the SRR adhesin GspB, but its role in export is unknown. Tertiary structure predictions suggest that the carboxyl terminus of Asp2 resembles the catalytic region of numerous enzymes that function through a Ser-Asp-His catalytic triad. Sequence alignment of all Asp2 homologues identified a highly conserved pentapeptide motif (Gly-X-Ser(362)-X-Gly) typical of most Ser-Asp-His catalytic triads, where Ser forms the reactive residue. Site-directed mutagenesis of residues comprising the predicted catalytic triad of Asp2 of S. gordonii had no effect upon GspB transport but did result in a marked change in the electrophoretic mobility of the protein. Lectin-binding studies and monosaccharide content analysis of this altered glycoform revealed an increase in glucosamine deposition. Random mutagenesis of the Asp2 region containing this catalytic domain also disrupted GspB transport. Collectively, our findings suggest that Asp2 is a bifunctional protein that is essential for both GspB transport and correct glycosylation. The catalytic domain may be responsible for controlling the glycosylation of GspB, while other surrounding regions are functionally required for glycoprotein transport.

A role for glycosylated serine-rich repeat proteins in gram-positive bacterial pathogenesis

Bacterial attachment to host surfaces is a pivotal event in the biological and infectious processes of both commensal and pathogenic bacteria, respectively. Serine-rich repeat proteins (SRRPs) are a family of adhesins in Gram-positive bacteria that mediate attachment to a variety of host and bacterial surfaces. As such, they contribute towards a wide-range of diseases including sub-acute bacterial endocarditis, community-acquired pneumonia, and meningitis. SRRPs are unique in that they are glycosylated, require a non-canonical Sec-translocase for transport, and are largely composed of a domain containing hundreds of alternating serine residues. These serine-rich repeats are thought to extend a unique non-repeat (NR) domain outward away from the bacterial surface to mediate adhesion. So far, NR domains have been determined to bind to sialic acid moieties, keratins, or other NR domains of a similar SRRP. This review summarizes how this important family of bacterial adhesins mediates bacterial attachment to host and bacterial cells, contributes to disease pathogenesis, and might be targeted for pharmacological intervention or used as novel protective vaccine antigens. This review also highlights recent structural findings on the NR domains of these proteins.

Application of subproteomics in the characterization of Gram-positive bacteria

Gram-positive bacteria cause a series of diseases in human, animals and plants. There has been increasing interest in efforts to investigate pathogenesis of bacteria using multiple "omic" strategies including proteomics. Proteins in different cell fractions of bacteria may play different vital roles in various physiological processes, such as adhesion, invasion, internalization, sensing, respiration, oxidative stress protection and pathogenicity. Subproteomics specifically focuses on the pre-fractionated cellular proteins and thus may be able to characterize more low-abundance molecules that are usually overlooked by the traditional whole-cell proteomics, providing comprehensive information for further investigations. This review intends to outline the current progress, challenges and future development of subproteomics in the characterization of Gram-positive bacteria. This article is part of a Special Issue entitled: Proteomics: The clinical link.

Streptococcus pneumoniae in biofilms are unable to cause invasive disease due to altered virulence determinant production

It is unclear whether Streptococcus pneumoniae in biofilms are virulent and contribute to development of invasive pneumococcal disease (IPD). Using electron microscopy we confirmed the development of mature pneumococcal biofilms in a continuous-flow-through line model and determined that biofilm formation occurred in discrete stages with mature biofilms composed primarily of dead pneumococci. Challenge of mice with equal colony forming units of biofilm and planktonic pneumococci determined that biofilm bacteria were highly attenuated for invasive disease but not nasopharyngeal colonization. Biofilm pneumococci of numerous serotypes were hyper-adhesive and bound to A549 type II pneumocytes and Detroit 562 pharyngeal epithelial cells at levels 2 to 11-fold greater than planktonic counterparts. Using genomic microarrays we examined the pneumococcal transcriptome and determined that during biofilm formation S. pneumoniae down-regulated genes involved in protein synthesis, energy production, metabolism, capsular polysaccharide (CPS) production, and virulence. We confirmed these changes by measuring CPS by ELISA and immunoblotting for the toxin pneumolysin and the bacterial adhesins phosphorylcholine (ChoP), choline-binding protein A (CbpA), and Pneumococcal serine-rich repeat protein (PsrP). We conclude that biofilm pneumococci were avirulent due to reduced CPS and pneumolysin production along with increased ChoP, which is known to bind C-reactive protein and is opsonizing. Likewise, biofilm pneumococci were hyper-adhesive due to selection for the transparent phase variant, reduced CPS, and enhanced production of PsrP, CbpA, and ChoP. These studies suggest that biofilms do not directly contribute to development of IPD and may instead confer a quiescent mode of growth during colonization.

Trimeric autotransporter adhesins in members of the Burkholderia cepacia complex: a multifunctional family of proteins implicated in virulence

Trimeric autotransporter adhesins (TAAs) are multimeric surface proteins exclusively found in bacteria. They are involved in various biological traits of pathogenic Gram-negative bacteria including adherence, biofilm formation, invasion, survival within eukaryotic cells, serum resistance, and cytotoxicity. TAAs have a modular architecture composed by a conserved membrane-anchored C-terminal domain and a variable number of stalk and head domains. In this study, a bioinformatic approach has been used to analyze the distribution and architecture of TAAs among Burkholderia cepacia complex (Bcc) genomes. Fifteen genomes were probed revealing a total of 74 encoding sequences. Compared with other bacterial species, the Bcc genomes contain a large number of TAAs (two genes to up to eight genes, such as in B. cenocepacia). Phylogenetic analysis showed that the TAAs grouped into at least eight distinct clusters. TAAs with serine-rich repeats are clearly well separated from others, thereby representing a different evolutionary lineage. Comparative gene mapping across Bcc genomes reveals that TAA genes are inserted within conserved synteny blocks. We further focused our analysis on the epidemic strain B. cenocepacia J2315 in which seven TAAs were annotated. Among these, three TAA-encoding genes (BCAM019, BCAM0223, and BCAM0224) are organized into a cluster and are candidates for multifunctional virulence factors. Here we review the current insights into the functional role of BCAM0224 as a model locus.

The major autolysin Acm2 from Lactobacillus plantarum undergoes cytoplasmic O-glycosylation

The major autolysin Acm2 from the probiotic strain Lactobacillus plantarum WCFS1 contains high proportions of alanine, serine, and threonine in its N-terminal so-called AST domain. It has been suggested that this extracellular protein might be glycosylated, but this has not been experimentally verified. We used high-resolution liquid chromatography-tandem mass spectrometry (LC-MS/MS) to study the possible occurrence of glycans on peptides generated from lactobacillary surface proteins by protease treatment. This approach yielded five glycopeptides in various glycoforms, all derived from the AST domain of Acm2. All five glycopeptides contained the hydroxy-amino acids serine and threonine, suggesting that Acm2 is O-glycosylated. By using lectin blotting with succinylated wheat germ agglutinin, and by comparing the wild-type strain with an Acm2-negative derivative (NZ3557), we found that the attached N-acetylhexosamines are most likely N-acetylglucosamines (GlcNAc). NZ3557 was further used as a genetic background to express an Acm2 variant lacking its secretion signal, resulting in intracellular expression of Acm2. We show that this intracellular version of Acm2 is also glycosylated, indicating that the GlcNAc modification is an intracellular process.

Serine-rich repeat proteins and pili promote Streptococcus agalactiae colonization of the vaginal tract

Streptococcus agalactiae (group B streptococcus [GBS]) is a Gram-positive bacterium found in the female rectovaginal tract and is capable of producing severe disease in susceptible hosts, including newborns and pregnant women. The vaginal tract is considered a major reservoir for GBS, and maternal vaginal colonization poses a significant risk to the newborn; however, little is known about the specific bacterial factors that promote GBS colonization and persistence in the female reproductive tract. We have developed in vitro models of GBS interaction with the human female cervicovaginal tract using human vaginal and cervical epithelial cell lines. Analysis of isogenic mutant GBS strains deficient in cell surface organelles such as pili and serine-rich repeat (Srr) proteins shows that these factors contribute to host cell attachment. As Srr proteins are heavily glycosylated, we confirmed that carbohydrate moieties contribute to the effective interaction of Srr-1 with vaginal epithelial cells. Antibody inhibition assays identified keratin 4 as a possible host receptor for Srr-1. Our findings were further substantiated in an in vivo mouse model of GBS vaginal colonization, where mice inoculated with an Srr-1-deficient mutant exhibited decreased GBS vaginal persistence compared to those inoculated with the wild-type (WT) parental strain. Furthermore, competition experiments in mice showed that WT GBS exhibited a significant survival advantage over the ΔpilA or Δsrr-1 mutant in the vaginal tract. Our results suggest that these GBS surface proteins contribute to vaginal colonization and may offer new insights into the mechanisms of vaginal niche establishment.

Characterization of the fibrinogen binding domain of bacteriophage lysin from Streptococcus mitis

The binding of bacteria to human platelets is a likely central mechanism in the pathogenesis of infective endocarditis. Platelet binding by Streptococcus mitis SF100 is mediated in part by a lysin encoded by the lysogenic bacteriophage SM1. In addition to its role in the phage life cycle, lysin mediates the binding of S. mitis to human platelets via its interaction with fibrinogen on the platelet surface. To better define the region of lysin mediating fibrinogen binding, we tested a series of purified lysin truncation variants for their abilities to bind this protein. These studies revealed that the fibrinogen binding domain of lysin is contained within the region spanned by amino acid residues 102 to 198 (lysin(102-198)). This region has no sequence homology to other known fibrinogen binding proteins. Lysin(102-198) bound fibrinogen comparably to full-length lysin and with the same selectivity for the fibrinogen Aα and Bβ chains. Lysin(102-198) also inhibited the binding in vitro of S. mitis to human fibrinogen and platelets. When assessed by platelet aggregometry, the disruption of the lysin gene in SF100 resulted in a significantly longer time to the onset of aggregation of human platelets than that of the parent strain. The preincubation of platelets with purified lysin(102-198) also delayed the onset of aggregation by SF100. These results indicate that the binding of lysin to fibrinogen is mediated by a specific domain of the phage protein and that this interaction is important for both platelet binding and aggregation by S. mitis.

Asp2 and Asp3 interact directly with GspB, the export substrate of the Streptococcus gordonii accessory Sec System

GspB is a serine-rich glycoprotein adhesin of Streptococcus gordonii that is exported to the bacterial surface by the accessory Sec system. This dedicated export pathway is comprised of seven components (SecA2, SecY2, and five accessory Sec proteins [Asp1 to Asp5]). The latter proteins have no known homologs beyond the Asps of other species. Asp1 to Asp3 are absolutely required for export of the substrate GspB, but their roles in this process are unknown. Using copurification analysis and far-Western blotting, we found that Asp2 and Asp3 could individually bind the serine-rich repeat (SRR) domains of GspB. Deletion of both SRR regions of GspB led to a decrease in its export, suggesting that binding of the Asps to the SRR regions is important for GspB transport by the accessory Sec system. The Asps also bound a heterologous substrate for the accessory Sec system containing a slow-folding MalE variant, but they did not bind wild-type MalE. The combined results indicate that the Asps may recognize the export substrate through preferential interactions with its unstructured or unfolded regions. Glycosylation of the SRR domains on GspB prevented Asp binding, suggesting that binding of the Asps to the preprotein occurs prior to its full glycosylation. Together, these findings suggest that Asp2 and Asp3 are likely to function in part as chaperones in the early phase of GspB transport.

Conservation of Ebp-type pilus genes among Enterococci and demonstration of their role in adherence of Enterococcus faecalis to human platelets

Ebp are endocarditis- and biofilm-associated pili of Enterococcus faecalis that are also important in experimental urinary tract infections (UTIs). Our analyses, using available genomes, found that the ebp locus is unique to enterococci. In E. faecalis, the ebp locus is very highly conserved and only 1/473 E. faecalis isolates tested lacked ebpABC, while only 1.2% had the bee pilus locus. No other pilus-encoding operon was identified in 55 available genomes, indicating that the vast majority of E. faecalis strains (unlike Enterococcus faecium and streptococci) have a single pilus locus. Surface expression studies showed that Ebp pili were produced in vitro by 91/91 brain heart infusion (BHI) plus serum-grown E. faecalis isolates and that strain OG1RF expressed pili at even higher levels in rat endocarditis vegetations. However, Ebp expression was restricted to 30 to 72% of E. faecalis cells, consistent with a bistability mode of expression. We also evaluated E. faecalis interactions with human platelets and found that growth of E. faecalis in BHI plus serum significantly enhanced adherence to human platelets and that sortase deletion mutants (the ΔsrtA, Δbps, and ΔbpsΔsrtA mutants) were markedly defective. Further studies identified that Ebp pili, but not the microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) Ace and Fss2, mediate adherence of E. faecalis to platelets. Taken together, our data show that the immunogenic (in human endocarditis patients) and commonly expressed Ebp pili, which are known to be important for experimental endocarditis, are highly conserved and mediate adherence to platelets, suggesting that Ebp pili may be a reasonable immunotherapeutic target for prevention or possibly treatment of endocarditis caused by this species.

Similarities and differences in the glycosylation mechanisms in prokaryotes and eukaryotes

Recent years have witnessed a rapid growth in the number and diversity of prokaryotic proteins shown to carry N- and/or O-glycans, with protein glycosylation now considered as fundamental to the biology of these organisms as it is in eukaryotic systems. This article overviews the major glycosylation pathways that are known to exist in eukarya, bacteria and archaea. These are (i) oligosaccharyltransferase (OST)-mediated N-glycosylation which is abundant in eukarya and archaea, but is restricted to a limited range of bacteria; (ii) stepwise cytoplasmic N-glycosylation that has so far only been confirmed in the bacterial domain; (iii) OST-mediated O-glycosylation which appears to be characteristic of bacteria; and (iv) stepwise O-glycosylation which is common in eukarya and bacteria. A key aim of the review is to integrate information from the three domains of life in order to highlight commonalities in glycosylation processes. We show how the OST-mediated N- and O-glycosylation pathways share cytoplasmic assembly of lipid-linked oligosaccharides, flipping across the ER/periplasmic/cytoplasmic membranes, and transferring “en bloc” to the protein acceptor. Moreover these hallmarks are mirrored in lipopolysaccharide biosynthesis. Like in eukaryotes, stepwise O-glycosylation occurs on diverse bacterial proteins including flagellins, adhesins, autotransporters and lipoproteins, with O-glycosylation chain extension often coupled with secretory mechanisms.

UafB is a serine-rich repeat adhesin of Staphylococcus saprophyticus that mediates binding to fibronectin, fibrinogen and human uroepithelial cells

Staphylococcus saprophyticus is an important cause of urinary tract infection (UTI), particularly among young women, and is second only to uropathogenic Escherichia coli as the most frequent cause of UTI. The molecular mechanisms of urinary tract colonization by S. saprophyticus remain poorly understood. We have identified a novel 6.84 kb plasmid-located adhesin-encoding gene in S. saprophyticus strain MS1146 which we have termed uro-adherence factor B (uafB). UafB is a glycosylated serine-rich repeat protein that is expressed on the surface of S. saprophyticus MS1146. UafB also functions as a major cell surface hydrophobicity factor. To characterize the role of UafB we generated an isogenic uafB mutant in S. saprophyticus MS1146 by interruption with a group II intron. The uafB mutant had a significantly reduced ability to bind to fibronectin and fibrinogen. Furthermore, we show that a recombinant protein containing the putative binding domain of UafB binds specifically to fibronectin and fibrinogen. UafB was not involved in adhesion in a mouse model of UTI; however, we observed a striking UafB-mediated adhesion phenotype to human uroepithelial cells. We have also identified genes homologous to uafB in other staphylococci which, like uafB, appear to be located on transposable elements. Thus, our data indicate that UafB is a novel adhesin of S. saprophyticus that contributes to cell surface hydrophobicity, mediates adhesion to fibronectin and fibrinogen, and exhibits tropism for human uroepithelial cells.

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.

Asp3 mediates multiple protein-protein interactions within the accessory Sec system of Streptococcus gordonii

Bacterial binding to human platelets is an important step in the pathogenesis of infective endocarditis. Streptococcus gordonii can mediate its platelet attachment through a cell wall glycoprotein termed GspB ('gordonii surface protein B'). GspB export is mediated by a seven-component accessory Sec system, containing two homologues of the general secretory pathway (SecA2 and SecY2) and five accessory Sec proteins (Asps1-5). Here we show that the Asps are required for optimal export of GspB independent of the glycosylation process. Furthermore, yeast two-hybrid screening of the accessory Sec system revealed interactions occurring between Asp3 and the other components of the system. Asp3 was shown to bind SecA2, Asp1, Asp2 and itself. Mutagenesis of Asp3 identified N- and C-terminal regions that are essential for GspB transport, and conserved residues within the C-terminal domain mediated Asp3 binding to other accessory Sec components. The loss of binding by Asp3 also resulted in an impaired ability of S. gordonii to secrete GspB. These studies indicate that Asp3 is a central element mediating multiple interactions among accessory Sec components that are essential for GspB transport to the cell surface.

The pneumococcal serine-rich repeat protein is an intra-species bacterial adhesin that promotes bacterial aggregation in vivo and in biofilms

The Pneumococcal serine-rich repeat protein (PsrP) is a pathogenicity island encoded adhesin that has been positively correlated with the ability of Streptococcus pneumoniae to cause invasive disease. Previous studies have shown that PsrP mediates bacterial attachment to Keratin 10 (K10) on the surface of lung cells through amino acids 273–341 located in the Basic Region (BR) domain. In this study we determined that the BR domain of PsrP also mediates an intra-species interaction that promotes the formation of large bacterial aggregates in the nasopharynx and lungs of infected mice as well as in continuous flow-through models of mature biofilms. Using numerous methods, including complementation of mutants with BR domain deficient constructs, fluorescent microscopy with Cy3-labeled recombinant (r)BR, Far Western blotting of bacterial lysates, co-immunoprecipitation with rBR, and growth of biofilms in the presence of antibodies and competitive peptides, we determined that the BR domain, in particular amino acids 122–166 of PsrP, promoted bacterial aggregation and that antibodies against the BR domain were neutralizing. Using similar methodologies, we also determined that SraP and GspB, the Serine-rich repeat proteins (SRRPs) of Staphylococcus aureus and Streptococcus gordonii, respectively, also promoted bacterial aggregation and that their Non-repeat domains bound to their respective SRRPs. This is the first report to show the presence of biofilm-like structures in the lungs of animals infected with S. pneumoniae and show that SRRPs have dual roles as host and bacterial adhesins. These studies suggest that recombinant Non-repeat domains of SRRPs (i.e. BR for S. pneumoniae) may be useful as vaccine antigens to protect against Gram-positive bacteria that cause infection.

Serine-rich repeat proteins (SRRPs) are a family of surface-expressed proteins found in numerous Gram-positive pathogens, including Staphylococcus aureus, Streptococcus pneumoniae, Group B streptococci, and the oral streptococci that cause infective endocarditis. For all of these bacteria, SRRPs have been demonstrated to play pivotal roles in adhesion to tissues and the development of invasive disease. It is now known that biofilm formation is an important step for bacterial pathogenesis. Bacteria in biofilms have been shown to have differences in metabolism, gene expression, and protein production that contribute to enhanced surface adhesion and the persistence of an infection. Herein we describe a novel role for PsrP, the S. pneumoniae SRRP, as an intra-species bacterial adhesin that promotes bacterial aggregation in the lungs of infected mice during pneumonia. In vitro we show that the Basic Region domain of PsrP promotes self-interactions that result in denser biofilms, greater biofilm biomass, and altered architectures of surface grown cultures; these interactions could be neutralized by antibodies to PsrP that are protective against pneumococcal infection. We also demonstrate that the SRRPs of S. aureus and Streptococcus gordonii also function as intra-species bacterial adhesins. Therefore we conclude that SRRPs have dual roles as host-cell and intra-species bacterial adhesins.

A novel glucosyltransferase is required for glycosylation of a serine-rich adhesin and biofilm formation by Streptococcus parasanguinis

Fap1-like serine-rich glycoproteins are conserved in streptococci, staphylococci, and lactobacilli, and are required for bacterial biofilm formation and pathogenesis. Glycosylation of Fap1 is mediated by a gene cluster flanking the fap1 locus. The key enzymes responsible for the first step of Fap1 glycosylation are glycosyltransferases Gtf1 and Gtf2. They form a functional enzyme complex that catalyzes the transfer of N-acetylglucosamine (GlcNAc) residues to the Fap1 polypeptide. However, until now nothing was known about the subsequent step in Fap1 glycosylation. Here, we show that the second step in Fap1 glycosylation is catalyzed by nucleotide-sugar synthetase-like (Nss) protein. The nss gene located upstream of fap1 is also highly conserved in streptococci and lactobacilli. Nss-deficient mutants failed to catalyze the second step of Fap1 glycosylation in vivo in Streptococcus parasanguinis and in a recombinant Fap1 glycosylation system. Nss catalyzed the direct transfer of the glucosyl residue to the GlcNAc-modified Fap1 substrate in vitro, demonstrating that Nss is a glucosyltransferase. Thus we renamed Nss as glucosyltransferase 3 (Gtf3). A gtf3 mutant exhibited a biofilm defect. Taken together, we conclude that this new glucosyltransferase mediates the second step of Fap1 glycosylation and is required for biofilm formation.

The group B streptococcal serine-rich repeat 1 glycoprotein mediates penetration of the blood-brain barrier

BACKGROUND

Group B Streptococcus (GBS) is the leading cause of bacterial meningitis in newborn infants. Because GBS is able to invade, survive, and cross the blood-brain barrier, we sought to identify surface-expressed virulence factors that contribute to blood-brain barrier penetration and the pathogenesis of meningitis.

METHODS

Targeted deletion and insertional mutants were generated in different GBS clinical isolates. Wild-type and mutant bacteria were analyzed for their capacity to adhere to and invade human brain microvascular endothelial cells (hBMECs) and to penetrate the blood-brain barrier using our model of hematogenous meningitis.

RESULTS

Analysis of a GBS (serotype V) clinical isolate revealed the presence of a surface-anchored serine-rich protein, previously designated serine-rich repeat 1 (Srr-1). GBS Srr-1 is a glycosylated protein with high molecular weight. Deletion of srr1 in NCTC 10/84 resulted in a significant decrease in adherence to and invasion of hBMECs. Additional mutants in other GBS serotypes commonly associated with meningitis showed a similar decrease in hBMEC invasion, compared with parental strains. Finally, in mice, wild-type GBS penetrated the blood-brain barrier and established meningitis more frequently than did the Deltasrr1 mutant strain.

CONCLUSIONS

Our data suggest that GBS Srr glycoproteins play an important role in crossing the blood-brain barrier and in the development of streptococcal meningitis.

The Streptococcus pneumoniae adhesin PsrP binds to Keratin 10 on lung cells

Pneumococcal serine-rich repeat protein (PsrP) is a pathogenicity island-encoded adhesin that mediates attachment to lung cells. It is a member of the serine-rich repeat protein family and the largest bacterial protein known. PsrP production by S. pneumoniae was confirmed by immunoblotting and a truncated version of the protein was determined to be glycosylated. Using isogenic psrP mutants complemented with various PsrP constructs and competitive inhibition assays with recombinant proteins, we determined that PsrP requires an extended SRR2 domain for function and that adhesion is mediated through amino acids 273-341 of its basic region (BR) domain. Affinity chromatography, immunoprecipitation, enzyme-linked immunosorbent assay (ELISA), fluorescent-activated cell sorting (FACS) and immunofluorescent colocalization studies determined that PsrP binds to Keratin 10 (K10) on the surface of lung but not nasopharyngeal epithelial cells. Unglycosylated K10 bound to wild type but not psrP deficient pneumococci; suggesting that unlike other serine-rich repeat proteins, PsrP-mediated adhesion is independent of lectin activity. Finally, mice immunized with recombinant (r)PsrP(BR) had significantly less bacteria in their blood and improved survival versus controls following intranasal challenge. We conclude that the BR domain of PsrP binds to K10 in a lectin-independent manner, that K10 is expressed on lung cells and that vaccination with rPsrP(BR) is protective against pneumococcal disease.

Molecular dissection of the secA2 locus of group B Streptococcus reveals that glycosylation of the Srr1 LPXTG protein is required for full virulence

In streptococci, the secA2 locus includes genes encoding the following: (i) the accessory Sec components (SecA2, SecY2, and at least three accessory secretion proteins), (ii) two essential glycosyltranferases (GTs) (GtfA and GtfB), (iii) a variable number of dispensable additional GTs, and (iv) a secreted serine-rich LPXTG protein which is glycosylated in the cytoplasm and transported to the cell surface by this accessory Sec system. The secA2 locus of Streptococcus agalactiae strain NEM316 is structurally related to those found in other streptococci and encodes the serine-rich surface protein Srr1. We demonstrated that expression of Srr1 but not that of the SecA2 components and the associated GTs is regulated by the standalone transcriptional regulator Rga. Srr1 is synthesized as a glycosylated precursor, secreted by the SecA2 system, and anchored to the cell wall by the housekeeping sortase A. Srr1 was localized preferentially at the old poles. GtfA and/or GtfB, but not the six additional GTs, is essential for the production of Srr1. These GTs are involved in the attachment of GlcNac and sialic acid to Srr1. Full glycosylation of Srr1 is associated with the cell surface display of a protein that is more resistant to proteolytic attack. Srr1 contributes to bacterial adherence to human epithelial cell lines and virulence in a neonatal rat model. The extent of Srr1 glycosylation by GtfC to -H modulates bacterial adherence and virulence.

Characterization of Streptococcus gordonii SecA2 as a paralogue of SecA

The accessory Sec system of Streptococcus gordonii is essential for transport of the glycoprotein GspB to the bacterial cell surface. A key component of this dedicated transport system is SecA2. The SecA2 proteins of streptococci and staphylococci are paralogues of SecA and are presumed to have an analogous role in protein transport, but they may be specifically adapted for the transport of large, serine-rich glycoproteins. We used a combination of genetic and biochemical methods to assess whether the S. gordonii SecA2 functions similarly to SecA. Although mutational analyses demonstrated that conserved amino acids are essential for the function of SecA2, replacing such residues in one of two nucleotide binding folds had only minor effects on SecA2 function. SecA2-mediated transport is highly sensitive to azide, as is SecA-mediated transport. Comparison of the S. gordonii SecA and SecA2 proteins in vitro revealed that SecA2 can hydrolyze ATP at a rate similar to that of SecA and is comparably sensitive to azide but that the biochemical properties of these enzymes are subtly different. That is, SecA2 has a lower solubility in aqueous solutions and requires higher Mg(2+) concentrations for maximal activity. In spite of the high degree of similarity between the S. gordonii paralogues, analysis of SecA-SecA2 chimeras indicates that the domains are not readily interchangeable. This suggests that specific, unique contacts between SecA2 and other components of the accessory Sec system may preclude cross-functioning with the canonical Sec system.

Glycosylation and biogenesis of a family of serine-rich bacterial adhesins

Glycosylation of bacterial proteins is an important process for bacterial physiology and pathophysiology. Both O- and N-linked glycan moieties have been identified in bacterial glycoproteins. The N-linked glycosylation pathways are well established in Gram-negative bacteria. However, the O-linked glycosylation pathways are not well defined due to the complex nature of known O-linked glycoproteins in bacteria. In this review, we examine a new family of serine-rich O-linked glycoproteins which are represented by fimbriae-associated adhesin Fap1 of Streptococcus parasanguinis and human platelet-binding protein GspB of Streptococcus gordonii. This family of glycoproteins is conserved in streptococcal and staphylococcal species. A gene cluster coding for glycosyltransferases and accessory Sec proteins has been implicated in the protein glycosylation. A two-step glycosylation model is proposed. Two glycosyltransferases interact with each other and catalyse the first step of the protein glycosylation in the cytoplasm; the cross-talk between glycosylation-associated proteins and accessory Sec components mediates the second step of the protein glycosylation, an emerging mechanism for bacterial O-linked protein glycosylation. Dissecting the molecular mechanism of this conserved biosynthetic pathway offers opportunities to develop new therapeutic strategies targeting this previously unrecognized pathway, as serine-rich glycoproteins have been shown to play a role in bacterial pathogenesis.