Structural insights into SraP-mediated Staphylococcus aureus adhesion to host cells

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.

Crystal structure of the legume lectin-like domain of an ERGIC-53-like protein from Entamoeba histolytica

ERGIC-53-like proteins are type I membrane proteins that belong to the class of intracellular cargo receptors and are known to be indispensable for the intracellular transport of glycoproteins. They are implicated in transporting glycoproteins between the endoplasmic reticulum and the Golgi body. The crystal structure of the legume lectin-like domain of an ERGIC-53-like protein from Entamoeba histolytica has been determined at 2.4 Å resolution. Although the overall structure of the domain resembles those of its mammalian and yeast orthologs (ERGIC-53 and Emp46, respectively), there are significant changes in the carbohydrate-binding site. A sequence-based search revealed the presence of several homologs of ERGIC-53 in different species of Entamoeba. This is the first report of the structural characterization of a member of this class of proteins from a protozoan and serves to further knowledge and understanding regarding the species-specific differences.

ABO blood group antigens may be associated with increased susceptibility to schistosomiasis: a systematic review and meta-analysis

Schistosomiasis or bilharzia is a widespread parasitic disease caused by blood flukes of the genus Schistosoma. Some factors have been investigated previously regarding their effect on the pathophysiological mechanism of human schistosomiasis, but the possible influence of the ABO blood group on the severity of Schistosoma infection has been the most promising. Hence, we performed a systematic review and meta-analysis to further investigate the association of the ABO blood group with schistosomiasis susceptibility. Selected publications were retrieved from PubMed up to 21 August 2018, for related studies written in English. Number of cases (with schistosomiasis) and controls (without schistosomiasis) were extracted across all ABO blood types. Odds ratios (OR) and 95% confidence intervals (CI) were computed, pooled and interpreted. Subgroup analysis by the species of Schistosoma infecting the population and the participants' ethnicity was also performed. The overall analysis revealed heterogeneity in the outcomes, which warranted the identification of the cause using the Galbraith plot. Post-outlier outcomes of the pooled ORs show that individuals who are not blood type O are more susceptible (OR: 1.40; 95% CI: 1.17-1.67; PA < 0.001) to schistosomiasis than those who are blood type O (OR: 0.71; 95% CI: 0.60-0.85; PA < 0.001). Subgroup analysis yielded the same observations regardless of the species of schistosome and the ethnicity of the participants. Results of this meta-analysis suggest that individuals who are blood type B and A are more susceptible to schistosomiasis than those who are blood type O. However, more studies are needed to confirm our claims.

Endo-fucoidan hydrolases from glycoside hydrolase family 107 (GH107) display structural and mechanistic similarities to α-l-fucosidases from GH29

Fucoidans are chemically complex and highly heterogeneous sulfated marine fucans from brown macro algae. Possessing a variety of physicochemical and biological activities, fucoidans are used as gelling and thickening agents in the food industry and have anticoagulant, antiviral, antitumor, antibacterial, and immune activities. Although fucoidan-depolymerizing enzymes have been identified, the molecular basis of their activity on these chemically complex polysaccharides remains largely uninvestigated. In this study, we focused on three glycoside hydrolase family 107 (GH107) enzymes: MfFcnA and two newly identified members, P5AFcnA and P19DFcnA, from a bacterial species of the genus Psychromonas Using carbohydrate-PAGE, we show that P5AFcnA and P19DFcnA are active on fucoidans that differ from those depolymerized by MfFcnA, revealing differential substrate specificity within the GH107 family. Using a combination of X-ray crystallography and NMR analyses, we further show that GH107 family enzymes share features of their structures and catalytic mechanisms with GH29 α-l-fucosidases. However, we found that GH107 enzymes have the distinction of utilizing a histidine side chain as the proposed acid/base catalyst in its retaining mechanism. Further interpretation of the structural data indicated that the active-site architectures within this family are highly variable, likely reflecting the specificity of GH107 enzymes for different fucoidan substructures. Together, these findings begin to illuminate the molecular details underpinning the biological processing of fucoidans.

SssP1, a Streptococcus suis Fimbria-Like Protein Transported by the SecY2/A2 System, Contributes to Bacterial Virulence

Streptococcus suis is an important Gram-positive pathogen in the swine industry and is an emerging zoonotic pathogen for humans. In our previous work, we found a virulent S. suis strain, CZ130302, belonging to a novel serotype, Chz, to be associated with acute meningitis in piglets. However, its underlying mechanisms of pathogenesis remain poorly understood. In this study, we sequenced and analyzed the complete genomes of three Chz serotype strains, including strain CZ130302 and two avirulent strains, HN136 and AH681. By genome comparison, we found two putative genomic islands (GIs) uniquely encoded in strain CZ130302 and designated them 50K GI and 58K GI. In mouse infection model, the deletion of 50K and 58K GIs caused 270-fold and 3-fold attenuation of virulence, respectively. Notably, we identified a complete SecY2/A2 system, coupled with its secretory protein SssP1 encoded in the 50K GI, which contributed to the pathogenicity of strain CZ130302. Immunogold electron microscopy and immunofluorescence analyses indicated that SssP1 could form fimbria-like structures that extend outward from the bacterial cell surface. The sssP1 mutation also attenuated bacterial adherence in human laryngeal epithelial (HEp-2) cells and human brain microvessel endothelial cells (HBMECs) compared with the wild type. Furthermore, we showed that two analogous Ig-like subdomains of SssP1 have sialic acid binding capacities. In conclusion, our results revealed that the 50K GI and the inside SecY2/A2 system gene cluster are related to the virulence of strain CZ130302, and we clarified a new S. suis pathogenesis mechanism mediated by the secretion protein SssP1.IMPORTANCE Streptococcus suis is an important zoonotic pathogen. Here, we managed to identify key factors to clarify the virulence of S. suis strain CZ130302 from a novel serotype, Chz. Notably, it was shown that a fimbria-like structure was significantly connected to the pathogenicity of the CZ130302 strain by comparative genomics analysis and animal infection assays. The mechanisms of how the CZ130302 strain constructs these fimbria-like structures in the cell surface by genes encoding and production transport were subsequently elucidated. Biosynthesis of the fimbria-like structure was achieved by the production of SssP1 glycoproteins, and its construction was dependent on the SecA2/Y2 secretion system. This study identified a visible fimbria-like protein, SssP1, participating in adhesion to host cells and contributing to the virulence in S. suis These findings will promote a better understanding of the pathogenesis of S. suis.

Streptococcal Siglec-like adhesins recognize different subsets of human plasma glycoproteins: implications for infective endocarditis

Streptococcus gordonii and Streptococcus sanguinis are typically found among the normal oral microbiota but can also cause infective endocarditis. These organisms express cell surface serine-rich repeat adhesins containing "Siglec-like" binding regions (SLBRs) that mediate attachment to α2-3-linked sialic acids on human glycoproteins. Two known receptors for the Siglec-like adhesins are the salivary mucin MG2/MUC7 and platelet GPIbα, and the interaction of streptococci with these targets may contribute to oral colonization and endocarditis, respectively. The SLBRs display a surprising diversity of preferences for defined glycans, ranging from highly selective to broader specificity. In this report, we characterize the glycoproteins in human plasma recognized by four SLBRs that prefer different α2-3 sialoglycan structures. We found that the SLBRs recognize a surprisingly small subset of plasma proteins that are extensively O-glycosylated. The preferred plasma protein ligands for a sialyl-T antigen-selective SLBR are proteoglycan 4 (lubricin) and inter-alpha-trypsin inhibitor heavy chain H4. Conversely, the preferred ligand for a 3'sialyllactosamine-selective SLBR is glycocalicin (the extracellular portion of platelet GPIbα). All four SLBRs recognize C1 inhibitor but detect distinctly different glycoforms of this key regulator of the complement and kallikrein protease cascades. The four plasma ligands have potential roles in thrombosis and inflammation, and each has been cited as a biomarker for one or more vascular or other diseases. The combined results suggest that the interaction of Siglec-like adhesins with different subsets of plasma glycoproteins could have a significant impact on the propensity of streptococci to establish endocardial infections.

Discovery of a new Pro-Pro endopeptidase, PPEP-2, provides mechanistic insights into the differences in substrate specificity within the PPEP family

Pro-Pro endopeptidases (PPEPs) belong to a recently discovered family of proteases capable of hydrolyzing a Pro-Pro bond. The first member from the bacterial pathogen Clostridium difficile (PPEP-1) cleaves two C. difficile cell-surface proteins involved in adhesion, one of which is encoded by the gene adjacent to the ppep-1 gene. However, related PPEPs may exist in other bacteria and may shed light on substrate specificity in this enzyme family. Here, we report on the homolog of PPEP-1 in Paenibacillus alvei, which we denoted PPEP-2. We found that PPEP-2 is a secreted metalloprotease, which likewise cleaved a cell-surface protein encoded by an adjacent gene. However, the cleavage motif of PPEP-2, PLP↓PVP, is distinct from that of PPEP-1 (VNP↓PVP). As a result, an optimal substrate peptide for PPEP-2 was not cleaved by PPEP-1 and vice versa. To gain insight into the specificity mechanism of PPEP-2, we determined its crystal structure at 1.75 Å resolution and further confirmed the structure in solution using small-angle X-ray scattering (SAXS). We show that a four-amino-acid loop, which is distinct in PPEP-1 and -2 (GGST in PPEP-1 and SERV in PPEP-2), plays a crucial role in substrate specificity. A PPEP-2 variant, in which the four loop residues had been swapped for those from PPEP-1, displayed a shift in substrate specificity toward PPEP-1 substrates. Our results provide detailed insights into the PPEP-2 structure and the structural determinants of substrate specificity in this new family of PPEP proteases.

Expression of Neu5Acα2,3Gal and Neu5Acα2,6Gal on the nasal mucosa of patients with chronic rhinosinusitis and its possible effect on bacterial biofilm formation

OBJECTIVE

Adherence of pathogen to nasal mucosa and colonization is the first step of bacterial biofilm(BBF) formation in patients with chronic rhinosinusitis (CRS).Terminal sialic acids presenting on cell surface are potential targets for bacterial binding, thus may partly contribute to the pathogenesis of CRS. However, little has been published in this respect, the purpose of our study aimed to investigate the expression of sialic acids on the nasal mucosa in CRS patients and its possible effect on BBF formation.

METHODS

Sinus mucosa were harvested from CRS patients undergoing endoscopic surgery. The positive of BBF formation were detected by scanning electronic microscopy (SEM) and the expression of Neu5Acα2,3Gal(α2,3-linked sialic acid) and Neu5Acα2,6Gal(α2,6-linked sialic acid) on nasal mucosa were determined by fluorescent-immunohistochemical staining (F-IHC) with MAL-II and SNA respectively. A semi-quantitative scoring system was used to assess their different expression between CRS group and the control, as well as BBF positive and negative group.

RESULTS

Expression of Neu5Acα2,3Gal and Neu5Acα2,6Gal were both detected in the epithelium and submucosal glands of all 40 CRS patients and 23 controls, they were significantly up-regulated in CRS group(p < 0.05). Among 24 CRS patients, typical BBF formation were identified in 13 cases while the other 11 were regarded as negative, Between the subgroup of BBF(+) and BBF(-), both of Neu5Acα2,3Gal and Neu5Acα2,6Gal had a trend of increasing in BBF(+) group, however, the increased expression of Neu5Acα2,3Gal was statistical significance (4.77 ± 0.90 versus 3.45 ± 1.40; p = 0.0282), whereas the difference of Neu5Acα2,6Gal was insignificant(4.15 ± 1.27 versus 3.55 ± 1.59; p = 0.4281).

CONCLUSION

Expression of MAL-II binding (most probable Neu5Acα2,3Gal) and SNA binding (Neu5Acα2,6Gal) were up-regulated in inflamed nasal mucosa, and the increased expression of them may contribute to bacterial biofilm formation which deserved a further investigation.

Current therapies in treatment and prevention of fracture wound biofilms: why a multifaceted approach is essential for resolving persistent infections

Traumatic orthopedic injuries, particularly extremity wounds, are a significant cause of morbidity. Despite prophylactic antibiotic treatment and surgical intervention, persistent infectious complications can and do occur. Persistent bacterial infections are often caused by biofilms, communities of antibiotic tolerant bacteria encased within a matrix. The structural and metabolic differences in this mode of growth make treatment difficult. Herein, we describe both established and novel, experimental treatments targeted at various stages of wound healing that are specifically aimed at reducing and eliminating biofilm bacteria. Importantly, the highly tolerant nature of these bacterial communities suggests that most singular approaches could be circumvented and a multifaceted, combinatorial approach will be the most effective strategy for treating these complicated infections.

Structural basis for the role of serine-rich repeat proteins from Lactobacillus reuteri in gut microbe-host interactions

Gut bacteria play a key role in health and disease, but the molecular mechanisms underpinning their interaction with the host remain elusive. The serine-rich repeat proteins (SRRPs) are a family of adhesins identified in many Gram-positive pathogenic bacteria. We previously showed that beneficial bacterial species found in the gut also express SRRPs and that SRRP was required for the ability of Lactobacillus reuteri strain to colonize mice. Here, our structural and biochemical data reveal that L. reuteri SRRP adopts a β-solenoid fold not observed in other structurally characterized SRRPs and functions as an adhesin via a pH-dependent mechanism, providing structural insights into the role of these adhesins in biofilm formation of gut symbionts.

Lactobacillus reuteri, a Gram-positive bacterial species inhabiting the gastrointestinal tract of vertebrates, displays remarkable host adaptation. Previous mutational analyses of rodent strain L. reuteri 100-23C identified a gene encoding a predicted surface-exposed serine-rich repeat protein (SRRP_100-23) that was vital for L. reuteri biofilm formation in mice. SRRPs have emerged as an important group of surface proteins on many pathogens, but no structural information is available in commensal bacteria. Here we report the 2.00-Å and 1.92-Å crystal structures of the binding regions (BRs) of SRRP_100-23 and SRRP₅₃₆₀₈ from L. reuteri ATCC 53608, revealing a unique β-solenoid fold in this important adhesin family. SRRP₅₃₆₀₈-BR bound to host epithelial cells and DNA at neutral pH and recognized polygalacturonic acid (PGA), rhamnogalacturonan I, or chondroitin sulfate A at acidic pH. Mutagenesis confirmed the role of the BR putative binding site in the interaction of SRRP₅₃₆₀₈-BR with PGA. Long molecular dynamics simulations showed that SRRP₅₃₆₀₈-BR undergoes a pH-dependent conformational change. Together, these findings provide mechanistic insights into the role of SRRPs in host–microbe interactions and open avenues of research into the use of biofilm-forming probiotics against clinically important pathogens.

Staphylococcal Adhesion and Host Cell Invasion: Fibronectin-Binding and Other Mechanisms

Opportunistic bacteria from the genus Staphylococcus can cause life-threatening infections such as pneumonia, endocarditis, bone and joint infections, and sepsis. This pathogenicity is closely related to their capacity to bind directly to the extracellular matrix or to host cells. Adhesion is indeed the first step in the formation of biofilm or the invasion of host cells, which protect the bacteria from the host immune system and facilitate chronic infection. Adhesion relies on the expression of a repertoire of surface proteins called adhesins, notably microbial surface components recognizing adhesive matrix molecules. In this short review, we discuss the main pathway (FnBP-Fn-α5β1 integrin), as well as alternatives, through which Staphylococcus aureus adheres to and then invades non-professional phagocytic cells. We then examine the corresponding mechanisms for coagulase negative staphylococci. There is currently a little understanding of the molecular mechanisms that lead to internalization. Filling this gap in the literature would therefore be an important step toward limiting the duration of staphylococci infections in clinical practice.

Metabolites Associated With Circulating Interleukin-6 in Older Adults

Background

Circulating levels of the pro-inflammatory cytokine interleukin-6 (IL-6) levels are elevated in older adults, but mechanisms are unclear. In the current study, we used an untargeted metabolomic approach to develop an improved understanding about mechanisms related to circulating IL-6 in older adults.

Methods

Serum IL-6 values were log-transformed to normalize its distribution. Multivariable-adjusted linear regression was used to examine the association between 324 serum metabolites with log IL-6. Backward elimination linear regression was used to develop a metabolite predictor set representative of log IL-6.

Results

Thirty-six metabolites were significantly associated (p < 0.05 and q < 0.30) with log IL-6 in 73 older adults (average age, 78 years). Metabolites related to tryptophan metabolism (kynurenine, 3-indoxyl sulfate, indoleacetate, indolepropionate, C-glycosyltryptophan), infectious burden (C-glycosyltryptophan, N6-carbamoylthreonyladenosine, 1-methylurate, N-formylmethionine, N1-methyladenosine, 3-indoxyl sulfate, bilirubin (E,E), indoleacetate, γ-CEHC, N-acetylneuraminate), aryl hydrocarbon receptor activation and cytochrome P450 (CYP) 1A expression (kynurenine, 3-indoxyl sulfate, indoleacetate, N6-carbamoylthreonyladenosine, bilirubin, 1-methylurate) were positively associated, whereas metabolites related to CYP-mediated ω-oxidation (adipate, 8-hydroxyoctanoate, azelate, sebacate, undecanedioate, γ-CEHC), and peroxisome proliferator activated receptor-alpha (PPAR-α) activation (13 + 9-HODE, bilirubin, 5-oxoproline, cholesterol, glycerate, uridine) were negatively associated with log IL-6. The use of backward elimination regression identified tyrosine, cysteine, uridine, bilirubin, N-formylmethionine, indoleacetate, and 3-indoxyl sulfate to collectively explain 51% of the variance inherent in log IL-6.

Conclusions

These data suggest roles for tryptophan metabolism, infectious burden, activation of host defense, and detoxification through CYP1A-mediated pathways in mechanisms related to elevated inflammation, whereas CYP-mediated ω-oxidation and PPAR-α activation may be related to decreased inflammation in older adults.

The Staphylococcal Biofilm: Adhesins, Regulation, and Host Response

The staphylococci comprise a diverse genus of Gram-positive, nonmotile commensal organisms that inhabit the skin and mucous membranes of humans and other mammals. In general, staphylococci are benign members of the natural flora, but many species have the capacity to be opportunistic pathogens, mainly infecting individuals who have medical device implants or are otherwise immunocompromised. Staphylococcus aureus and Staphylococcus epidermidis are major sources of hospital-acquired infections and are the most common causes of surgical site infections and medical device-associated bloodstream infections. The ability of staphylococci to form biofilms in vivo makes them highly resistant to chemotherapeutics and leads to chronic diseases. These biofilm infections include osteomyelitis, endocarditis, medical device infections, and persistence in the cystic fibrosis lung. Here, we provide a comprehensive analysis of our current understanding of staphylococcal biofilm formation, with an emphasis on adhesins and regulation, while also addressing how staphylococcal biofilms interact with the immune system. On the whole, this review will provide a thorough picture of biofilm formation of the staphylococcus genus and how this mode of growth impacts the host.

Molecular interactions and inhibition of the staphylococcal biofilm-forming protein SdrC

Staphylococcus aureus forms biofilms on indwelling medical devices using a variety of cell-surface proteins. There is growing evidence that specific homophilic interactions between these proteins represent an important mechanism of cell accumulation during biofilm formation, but the underlying molecular mechanisms are still not well-understood. Here we report the direct measurement of homophilic binding forces by the serine-aspartate repeat protein SdrC and their inhibition by a peptide. Using single-cell and single-molecule force measurements, we find that SdrC is engaged in low-affinity homophilic bonds that promote cell-cell adhesion. Low-affinity intercellular adhesion may play a role in favoring biofilm dynamics. We show that SdrC also mediates strong cellular interactions with hydrophobic surfaces, which are likely to be involved in the initial attachment to biomaterials, the first stage of biofilm formation. Furthermore, we demonstrate that a peptide derived from β-neurexin is a powerful competitive inhibitor capable of efficiently blocking surface attachment, homophilic adhesion, and biofilm accumulation. Molecular modeling suggests that this blocking activity may originate from binding of the peptide to a sequence of SdrC involved in homophilic interactions. Our study opens up avenues for understanding the role of homophilic interactions in staphylococcal adhesion, and for the design of new molecules to prevent biofilm formation during infection.

Streptococcus oralis Neuraminidase Modulates Adherence to Multiple Carbohydrates on Platelets

Adherence to host surfaces is often mediated by bacterial binding to surface carbohydrates. Although it is widely appreciated that some bacterial species express glycosidases, previous studies have not considered whether bacteria bind to multiple carbohydrates within host glycans as they are modified by bacterial glycosidases. Streptococcus oralis is a leading cause of subacute infective endocarditis. Binding to platelets is a critical step in disease; however, the mechanisms utilized by S. oralis remain largely undefined. Studies revealed that S. oralis, like Streptococcus gordonii and Streptococcus sanguinis, binds platelets via terminal sialic acid. However, unlike those organisms, S. oralis produces a neuraminidase, NanA, which cleaves terminal sialic acid. Further studies revealed that following NanA-dependent removal of terminal sialic acid, S. oralis bound exposed β-1,4-linked galactose. Adherence to both these carbohydrates required Fap1, the S. oralis member of the serine-rich repeat protein (SRRP) family of adhesins. Mutation of a conserved residue required for sialic acid binding by other SRRPs significantly reduced platelet binding, supporting the hypothesis that Fap1 binds this carbohydrate. The mechanism by which Fap1 contributes to β-1,4-linked galactose binding remains to be defined; however, binding may occur via additional domains of unknown function within the nonrepeat region, one of which shares some similarity with a carbohydrate binding module. This study is the first demonstration that an SRRP is required to bind β-1,4-linked galactose and the first time that one of these adhesins has been shown to be required for binding of multiple glycan receptors.

The remarkably multifunctional fibronectin binding proteins of Staphylococcus aureus

Staphylococcus aureus expresses two distinct but closely related multifunctional cell wall-anchored (CWA) proteins that bind to the host glycoprotein fibronectin. The fibronectin binding proteins FnBPA and FnBPB comprise two distinct domains. The C-terminal domain comprises a tandem array of repeats that bind to the N-terminal type I modules of fibronectin by the tandem β-zipper mechanism. This causes allosteric activation of a cryptic integrin binding domain, allowing fibronectin to act as a bridge between bacterial cells and the α5β1 integrin on host cells, triggering bacterial uptake by endocytosis. Variants of FnBPA with polymorphisms in fibronectin binding repeats (FnBRs) that increase affinity for the ligand are associated with strains that infect cardiac devices and cause endocarditis, suggesting that binding affinity is particularly important in intravascular infections. The N-terminal A domains of FnBPA and FnBPB have diverged into seven antigenically distinct isoforms. Each binds fibrinogen by the 'dock, lock and latch' mechanism characteristic of clumping factor A. However, FnBPs can also bind to elastin, which is probably important in adhesion to connective tissue in vivo. In addition, they can capture plasminogen from plasma, which can be activated to plasmin by host and bacterial plasminogen activators. The bacterial cells become armed with a host protease which destroys opsonins, contributing to immune evasion and promotes spreading during skin infection. Finally, some methicillin-resistant S. aureus (MRSA) strains form biofilm that depends on the elaboration of FnBPs rather than polysaccharide. The A domains of the FnBPs can interact homophilically, allowing cells to bind together as the biofilm accumulates.

The lectin-like protein 1 in Lactobacillus rhamnosus GR-1 mediates tissue-specific adherence to vaginal epithelium and inhibits urogenital pathogens

The probiotic Lactobacillus rhamnosus GR-1 has been documented to survive implantation onto the vaginal epithelium and interfere with urogenital pathogens. However, the molecular mechanisms involved are largely unknown. Here, we report for the first time the construction of dedicated knock-out mutants in L. rhamnosus GR-1 to enable the study of gene functions. In a search for genes responsible for the adherence capacity of L. rhamnosus GR-1, a genomic region encoding a protein with homology to lectin-like proteins was identified. Phenotypic analyses of the knock-out mutant of L. rhamnosus GR-1 revealed a two-fold decreased adhesion to the vaginal and ectocervical epithelial cell lines compared to wild-type. In contrast, the adhesion to gastro-intestinal epithelial (Caco2) and endocervical cell lines (Hela and End1/E6E7) was not drastically affected by the mutation, suggesting that the LGR-1_Llp1 lectins mediates tissue tropism. The purified LGR-1_Llp1 protein also inhibited biofilm formation and adhesion of uropathogenic Escherichia coli. For the first time, an important role for a novel lectin-like protein in the adhesion capacity and host cell-specific interaction of a vaginal probiotic Lactobacillus strain has been discovered, with an additional role in pathogen inhibition.

High mannose-specific lectin Msl mediates key interactions of the vaginal Lactobacillus plantarum isolate CMPG5300

To characterize the interaction potential of the human vaginal isolate Lactobacillus plantarum CMPG5300, its genome was mined for genes encoding lectin-like proteins. cmpg5300.05_29 was identified as the gene encoding a putative mannose-binding lectin. Phenotypic analysis of a gene knock-out mutant of cmpg5300.05_29 showed that expression of this gene is important for auto-aggregation, adhesion to the vaginal epithelial cells, biofilm formation and binding to mannosylated glycans. Purification of the predicted lectin domain of Cmpg5300.05_29 and characterization of its sugar binding capacity confirmed the specificity of the lectin for high- mannose glycans. Therefore, we renamed Cmpg5300.05_29 as a mannose-specific lectin (Msl). The purified lectin domain of Msl could efficiently bind to HIV-1 glycoprotein gp120 and Candida albicans, and showed an inhibitory activity against biofilm formation of uropathogenic Escherichia coli, Staphylococcus aureus and Salmonella Typhimurium. Thus, using a combination of molecular lectin characterization and functional assays, we could show that lectin-sugar interactions play a key role in host and pathogen interactions of a prototype isolate of the vaginal Lactobacillus microbiota.

Lectin-Like Molecules of Lactobacillus rhamnosus GG Inhibit Pathogenic Escherichia coli and Salmonella Biofilm Formation

Objectives

Increased antibiotic resistance has catalyzed the research on new antibacterial molecules and alternative strategies, such as the application of beneficial bacteria. Since lectin molecules have unique sugar-recognizing capacities, and pathogens are often decorated with sugars that affect their survival and infectivity, we explored whether lectins from the probiotic strain Lactobacillus rhamnosus GG have antipathogenic properties.

Methods

The genome sequence of L. rhamnosus GG was screened for the presence of lectin-like proteins. Two genes, LGG_RS02780 and LGG_RS02750, encoding for polypeptides with an N-terminal conserved L-type lectin domain were detected and designated Llp1 (lectin-like protein 1) and Llp2. The capacity of Llp1 and Llp2 to inhibit biofilm formation of various pathogens was investigated. Sugar specificity was determined by Sepharose beads assays and glycan array screening.

Results

The isolated lectin domains of Llp1 and Llp2 possess pronounced inhibitory activity against biofilm formation by various pathogens, including clinical Salmonella species and uropathogenic E. coli, with Llp2 being more active than Llp1. In addition, sugar binding assays with Llp1 and Llp2 indicate specificity for complex glycans. Both proteins are also involved in the adhesion capacity of L. rhamnosus GG to gastrointestinal and vaginal epithelial cells.

Conclusions

Lectins isolated from or expressed by beneficial lactobacilli could be considered promising bio-active ingredients for improved prophylaxis of urogenital and gastrointestinal infections.

Staphylococcal Bap Proteins Build Amyloid Scaffold Biofilm Matrices in Response to Environmental Signals

Biofilms are communities of bacteria that grow encased in an extracellular matrix that often contains proteins. The spatial organization and the molecular interactions between matrix scaffold proteins remain in most cases largely unknown. Here, we report that Bap protein of Staphylococcus aureus self-assembles into functional amyloid aggregates to build the biofilm matrix in response to environmental conditions. Specifically, Bap is processed and fragments containing at least the N-terminus of the protein become aggregation-prone and self-assemble into amyloid-like structures under acidic pHs and low concentrations of calcium. The molten globule-like state of Bap fragments is stabilized upon binding of the cation, hindering its self-assembly into amyloid fibers. These findings define a dual function for Bap, first as a sensor and then as a scaffold protein to promote biofilm development under specific environmental conditions. Since the pH-driven multicellular behavior mediated by Bap occurs in coagulase-negative staphylococci and many other bacteria exploit Bap-like proteins to build a biofilm matrix, the mechanism of amyloid-like aggregation described here may be widespread among pathogenic bacteria.

Major components of the biofilm matrix scaffold are proteins that assemble to create a unified structure that maintain bacteria attached to each other and to surfaces. We provide evidence that a surface protein present in several staphylococcal species forms functional amyloid aggregates to build the biofilm matrix in response to specific environmental conditions. Under low Ca²⁺ concentrations and acidic pH, Bap is processed and forms insoluble aggregates with amyloidogenic properties. When the Ca²⁺ concentration increases, metal-coordinated Bap adopts a structurally more stable conformation and as a consequence, the N-terminal region is unable to assemble into amyloid aggregates. The control of Bap cleavage and assembly helps to regulate biofilm matrix development as a function of environmental changes.

The Staphylococcus aureus Global Regulator MgrA Modulates Clumping and Virulence by Controlling Surface Protein Expression

Staphylococcus aureus is a human commensal and opportunistic pathogen that causes devastating infections in a wide range of locations within the body. One of the defining characteristics of S. aureus is its ability to form clumps in the presence of soluble fibrinogen, which likely has a protective benefit and facilitates adhesion to host tissue. We have previously shown that the ArlRS two-component regulatory system controls clumping, in part by repressing production of the large surface protein Ebh. In this work we show that ArlRS does not directly regulate Ebh, but instead ArlRS activates expression of the global regulator MgrA. Strains lacking mgrA fail to clump in the presence of fibrinogen, and clumping can be restored to an arlRS mutant by overexpressing either arlRS or mgrA, indicating that ArlRS and MgrA constitute a regulatory pathway. We used RNA-seq to show that MgrA represses ebh, as well as seven cell wall-associated proteins (SraP, Spa, FnbB, SasG, SasC, FmtB, and SdrD). EMSA analysis showed that MgrA directly represses expression of ebh and sraP. Clumping can be restored to an mgrA mutant by deleting the genes for Ebh, SraP and SasG, suggesting that increased expression of these proteins blocks clumping by steric hindrance. We show that mgrA mutants are less virulent in a rabbit model of endocarditis, and virulence can be partially restored by deleting the genes for the surface proteins ebh, sraP, and sasG. While mgrA mutants are unable to clump, they are known to have enhanced biofilm capacity. We demonstrate that this increase in biofilm formation is partially due to up-regulation of SasG, a surface protein known to promote intercellular interactions. These results confirm that ArlRS and MgrA constitute a regulatory cascade, and that they control expression of a number of genes important for virulence, including those for eight large surface proteins.

Staphylococcus causes a wide range of diseases, ranging from skin infections to deadly invasive condition like endocarditis, septicemia, osteomyelitis, and pneumonia. In this work we examine the ArlRS two-component regulatory system, which controls interactions with the host plasma protein fibrinogen. S. aureus normally forms large aggregates called clumps in the presence of fibrinogen, but the arlRS mutant is unable to clump. We demonstrate that ArlRS activates expression of the DNA-binding protein MgrA, and that mgrA is also required for clumping. Transcriptional analysis of an mgrA mutant shows that MgrA regulates expression of eight surface proteins. Expression of these surface proteins affects clumping, possibly by physically interfering with fibrinogen binding. Strains lacking mgrA are less virulent in an endocarditis model, and virulence can be partially restored by deleting genes for three of these surface proteins. An mgrA mutant is also known to have enhanced biofilm formation, and we show that this is partially due to increased production of one of these surface proteins. These results demonstrate that ArlRS and MgrA constitute a regulatory cascade in S. aureus that is crucial for pathogenesis and may be a good candidate to target for drug development.

Novel aspects of sialoglycan recognition by the Siglec-like domains of streptococcal SRR glycoproteins

Serine-rich repeat glycoproteins are adhesins expressed by commensal and pathogenic Gram-positive bacteria. A subset of these adhesins, expressed by oral streptococci, binds sialylated glycans decorating human salivary mucin MG2/MUC7, and platelet glycoprotein GPIb. Specific sialoglycan targets were previously identified for the ligand-binding regions (BRs) of GspB and Hsa, two serine-rich repeat glycoproteins expressed by Streptococcus gordonii While GspB selectively binds sialyl-T antigen, Hsa displays broader specificity. Here we examine the binding properties of four additional BRs from Streptococcus sanguinis or Streptococcus mitis and characterize the molecular determinants of ligand selectivity and affinity. Each BR has two domains that are essential for sialoglycan binding by GspB. One domain is structurally similar to the glycan-binding module of mammalian Siglecs (sialic acid-binding immunoglobulin-like lectins), including an arginine residue that is critical for glycan recognition, and that resides within a novel, conserved YTRY motif. Despite low sequence similarity to GspB, one of the BRs selectively binds sialyl-T antigen. Although the other three BRs are highly similar to Hsa, each displayed a unique ligand repertoire, including differential recognition of sialyl Lewis antigens and sulfated glycans. These differences in glycan selectivity were closely associated with differential binding to salivary and platelet glycoproteins. Specificity of sialoglycan adherence is likely an evolving trait that may influence the propensity of streptococci expressing Siglec-like adhesins to cause infective endocarditis.

Clinical applications of bacterial glycoproteins

There is an ongoing race between bacterial evolution and medical advances. Pathogens have the advantages of short generation times and horizontal gene transfer that enable rapid adaptation to new host environments and therapeutics that currently outpaces clinical research. Antibiotic resistance, the growing impact of nosocomial infections, cancer-causing bacteria, the risk of zoonosis, and the possibility of biowarfare all emphasize the increasingly urgent need for medical research focussed on bacterial pathogens. Bacterial glycoproteins are promising targets for alternative therapeutic intervention since they are often surface exposed, involved in host-pathogen interactions, required for virulence, and contain distinctive glycan structures. The potential exists to exploit these unique structures to improve clinical prevention, diagnosis, and treatment strategies. Translation of the potential in this field to actual clinical impact is an exciting prospect for fighting infectious diseases.

Monoclonal Antibody Targeting Staphylococcus aureus Surface Protein A (SasA) Protect Against Staphylococcus aureus Sepsis and Peritonitis in Mice

Epidemic methicillin-resistant Staphylococcus aureus (MRSA) imposes an increasing impact on public health. Due to multi-antibiotics resistance in MRSA strains, there is an urgent need to develop novel therapeutics such as effective monoclonal antibodies (mAbs) against MRSA infections. Staphylococcus aureus surface protein A (SasA), a large surface-located protein (~240 kDa), is one of MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) and a potential target for immunotherapeutic approaches against S. aureus infections. In the present study, we analyzed the sequence of SasA with bioinformatics tools and generated a protective monoclonal antibody (2H7) targeting the conserved domain of SasA. 2H7 was shown to recognize wild-type S. aureus and promote opsonophagocytic killing of S. aureus. In both sepsis and peritoneal infection models, prophylactic administration of 2H7 improved the survival of BALB/c mice challenged by S. aureus strain USA300 and ST239 (prevalent MRSA clones in North America and Asian countries, respectively) and enhanced bacterial clearance in kidneys. Additionally, 2H7 prophylaxis prevented the formation of intraperitoneal abscess in a murine model of peritoneal infection and therapeutic administration of 2H7 showed protective efficacy in a murine sepsis model. Our results presented here provide supporting evidences that an anti-SasA mAb might be a potential component in an antibody-based immunotherapeutic treatment of MRSA infections.

Structural Basis for Sialoglycan Binding by the Streptococcus sanguinis SrpA Adhesin

Streptococcus sanguinisis a leading cause of infective endocarditis, a life-threatening infection of the cardiovascular system. An important interaction in the pathogenesis of infective endocarditis is attachment of the organisms to host platelets.S. sanguinisexpresses a serine-rich repeat adhesin, SrpA, similar in sequence to platelet-binding adhesins associated with increased virulence in this disease. In this study, we determined the first crystal structure of the putative binding region of SrpA (SrpABR) both unliganded and in complex with a synthetic disaccharide ligand at 1.8 and 2.0 Å resolution, respectively. We identified a conserved Thr-Arg motif that orients the sialic acid moiety and is required for binding to platelet monolayers. Furthermore, we propose that sequence insertions in closely related family members contribute to the modulation of structural and functional properties, including the quaternary structure, the tertiary structure, and the ligand-binding site.

Insights into bacterial protein glycosylation in human microbiota

The study of human microbiota is an emerging research topic. The past efforts have mainly centered on studying the composition and genomic landscape of bacterial species within the targeted communities. The interaction between bacteria and hosts is the pivotal event in the initiation and progression of infectious diseases. There is a great need to identify and characterize the molecules that mediate the bacteria-host interaction. Bacterial surface exposed proteins play an important role in the bacteria- host interaction. Numerous surface proteins are glycosylated, and the glycosylation is crucial for their function in mediating the bacterial interaction with hosts. Here we present an overview of surface glycoproteins from bacteria that inhabit three major mucosal environments across human body: oral, gut and skin. We describe the important enzymes involved in the process of protein glycosylation, and discuss how the process impacts the bacteria-host interaction. Emerging molecular details underlying glycosylation of bacterial surface proteins may lead to new opportunities for designing anti-infective small molecules, and developing novel vaccines in order to treat or prevent bacterial infection.

Involvement of the flagellar assembly pathway in Vibrio alginolyticus adhesion under environmental stresses

Adhesion is an important virulence factor of Vibrio alginolyticus. This factor may be affected by environmental conditions; however, its molecular mechanism remains unclear. In our previous research, adhesion deficient strains were obtained by culturing V. alginolyticus under stresses including Cu, Pb, Hg, and low pH. With RNA-seq and bioinformatics analysis, we found that all of these stress treatments significantly affected the flagellar assembly pathway, which may play an important role in V. alginolyticus adhesion. Therefore, we hypothesized that the environmental stresses of the flagellar assembly pathway may be one way in which environmental conditions affect adhesion. To verify our hypothesis, a bioinformatics analysis, QPCR, RNAi, in vitro adhesion assay and motility assay were performed. Our results indicated that (1) the flagellar assembly pathway was sensitive to environmental stresses, (2) the flagellar assembly pathway played an important role in V. alginolyticus adhesion, and (3) motility is not the only way in which the flagellar assembly pathway affects adhesion.

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.

Blood Groups in Infection and Host Susceptibility

Blood group antigens represent polymorphic traits inherited among individuals and populations. At present, there are 34 recognized human blood groups and hundreds of individual blood group antigens and alleles. Differences in blood group antigen expression can increase or decrease host susceptibility to many infections. Blood groups can play a direct role in infection by serving as receptors and/or coreceptors for microorganisms, parasites, and viruses. In addition, many blood group antigens facilitate intracellular uptake, signal transduction, or adhesion through the organization of membrane microdomains. Several blood groups can modify the innate immune response to infection. Several distinct phenotypes associated with increased host resistance to malaria are overrepresented in populations living in areas where malaria is endemic, as a result of evolutionary pressures. Microorganisms can also stimulate antibodies against blood group antigens, including ABO, T, and Kell. Finally, there is a symbiotic relationship between blood group expression and maturation of the gastrointestinal microbiome.

Cell Wall-Anchored Surface Proteins of Staphylococcus aureus: Many Proteins, Multiple Functions

Staphylococcus aureus persistently colonizes about 20 % of the population and is intermittently associated with the remainder. The organism can cause superficial skin infections and life-threatening invasive diseases. The surface of the bacterial cell displays a variety of proteins that are covalently anchored to peptidoglycan. They perform many functions including adhesion to host cells and tissues, invasion of non-phagocytic cells, and evasion of innate immune responses. The proteins have been categorized into distinct classes based on structural and functional analysis. Many surface proteins are multifunctional. Cell wall-anchored proteins perform essential functions supporting survival and proliferation during the commensal state and during invasive infections. The ability of cell wall-anchored proteins to bind to desquamated epithelial cells is important during colonization, and the binding to fibrinogen is of particular significance in pathogenesis.

Protein-based biofilm matrices in Staphylococci

Staphylococcus aureus and Staphylococcus epidermidis are the most important etiological agents of biofilm associated-infections on indwelling medical devices. Biofilm infections may also develop independently of indwelling devices, e.g., in native valve endocarditis, bone tissue, and open wounds. After attachment to tissue or indwelling medical devices that have been conditioned with host plasma proteins, staphylococcal biofilms grow, and produce a specific environment which provides the conditions for cell–cell interaction and formation of multicellular communities. Bacteria living in biofilms express a variety of macromolecules, including exopolysaccharides, proteins, extracellular eDNA, and other polymers. The S. aureus surface protein C and G (SasC and SasG), clumping factor B (ClfB), serine aspartate repeat protein (SdrC), the biofilm-associated protein (Bap), and the fibronectin/fibrinogen-binding proteins (FnBPA and FnBPB) are individually implicated in biofilm matrix formation. In S. epidermidis, a protein named accumulation-associated protein (Aap) contributes to both the primary attachment phase and the establishment of intercellular connections by forming fibrils on the cell surface. In S. epidermidis, proteinaceous biofilm formation can also be mediated by the extracellular matrix binding protein (Embp) and S. epidermidis surface protein C (SesC). Additionally, multifunctional proteins such as extracellular adherence protein (Eap) and extracellular matrix protein binding protein (Emp) of S. aureus and the iron-regulated surface determinant protein C (IsdC) of S. lugdunensis can promote biofilm formation in iron-depleted conditions. This multitude of proteins intervene at different stages of biofilm formation with certain proteins contributing to biofilm accumulation and others mediating primary attachment to surfaces. This review examines the contribution of proteins to biofilm formation in Staphylococci. The potential to develop vaccines to prevent protein-dependent biofilm formation during staphylococcal infection is discussed.