Secondary cell wall polymers of Enterococcus faecalis are critical for resistance to complement activation via mannose-binding lectin

Enterococcus faecalis: an overlooked cell invader

SUMMARYEnterococcus faecalis and Enterococcus faecium are human pathobionts that exhibit a dual lifestyle as commensal and pathogenic bacteria. The pathogenic lifestyle is associated with specific conditions involving host susceptibility and intestinal overgrowth or the use of a medical device. Although the virulence of E. faecium appears to benefit from its antimicrobial resistance, E. faecalis is recognized for its higher pathogenic potential. E. faecalis has long been considered a predominantly extracellular pathogen; it adheres to and is taken up by a wide range of mammalian cells, albeit with less efficiency than classical intracellular enteropathogens. Carbohydrate structures, rather than proteinaceous moieties, are likely to be primarily involved in the adhesion of E. faecalis to epithelial cells. Consistently, few adhesins have been implicated in the adhesion of E. faecalis to epithelial cells. On the host side, very little is known about cognate receptors, except for the role of glycosaminoglycans during macrophage infection. Several lines of evidence indicate that E. faecalis internalization may involve a zipper-like mechanism as well as a macropinocytosis pathway. Conversely, E. faecalis can use several strategies to prevent engulfment in phagocytes. However, the bacterial and host mechanisms underlying cell infection by E. faecalis are still in their infancy. The most recent striking finding is the existence of an intracellular lifestyle where E. faecalis can replicate within a variety of host cells. In this review, we summarize and discuss the current knowledge of E. faecalis-host cell interactions and argue on the need for further mechanistic studies to prevent or reduce infections.

Pangenome-spanning epistasis and coselection analysis via de Bruijn graphs

Studies of bacterial adaptation and evolution are hampered by the difficulty of measuring traits such as virulence, drug resistance, and transmissibility in large populations. In contrast, it is now feasible to obtain high-quality complete assemblies of many bacterial genomes thanks to scalable high-accuracy long-read sequencing technologies. To exploit this opportunity, we introduce a phenotype- and alignment-free method for discovering coselected and epistatically interacting genomic variation from genome assemblies covering both core and accessory parts of genomes. Our approach uses a compact colored de Bruijn graph to approximate the intragenome distances between pairs of loci for a collection of bacterial genomes to account for the impacts of linkage disequilibrium (LD). We demonstrate the versatility of our approach to efficiently identify associations between loci linked with drug resistance and adaptation to the hospital niche in the major human bacterial pathogens Streptococcus pneumoniae and Enterococcus faecalis.

Exploring the role of E. faecalis enterococcal polysaccharide antigen (EPA) and lipoproteins in evasion of phagocytosis

Enterococcus faecalis is an opportunistic pathogen frequently causing nosocomial infections. The virulence of this organism is underpinned by its capacity to evade phagocytosis, allowing dissemination in the host. Immune evasion requires a surface polysaccharide produced by all enterococci, known as the enterococcal polysaccharide antigen (EPA). EPA consists of a cell wall-anchored rhamnose backbone substituted by strain-specific polysaccharides called 'decorations', essential for the biological activity of this polymer. However, the structural determinants required for innate immune evasion remain unknown, partly due to a lack of suitable validated assays. Here, we describe a quantitative, in vitro assay to investigate how EPA decorations alter phagocytosis. Using the E. faecalis model strain OG1RF, we demonstrate that a mutant with a deletion of the locus encoding EPA decorations can be used as a platform strain to express heterologous decorations, thereby providing an experimental system to investigate the inhibition of phagocytosis by strain-specific decorations. We show that the aggregation of cells lacking decorations is increasing phagocytosis and that this process does not involve the recognition of lipoproteins by macrophages. Collectively, our work provides novel insights into innate immune evasion by enterococci and paves the way for further studies to explore the structure/function relationship of EPA decorations.

Identification of a capsular polysaccharide from Enterococcus faecium U0317 using a targeted approach to discover immunogenic carbohydrates for vaccine development

Enterococcus faecium, a gram-positive opportunistic pathogen, has become a major concern for nosocomial infections due to its resistance to several antibiotics, including vancomycin. Finding novel alternatives for treatment prevention, such as vaccines, is therefore crucial. In this study, we used various techniques to discover a novel capsular polysaccharide. Firstly, we identified an encapsulated E. faecium strain by evaluating the opsonophagocytic activity of fifteen strains with antibodies targeting the well-known lipoteichoic acid antigen. This activity was attributed to an unknown polysaccharide. We then prepared a crude cell wall glycopolymer and fractionated it, guided by immunodot-blot analysis. The most immunoreactive fractions were used for opsonophagocytic inhibition assays. The fraction containing the inhibitory polysaccharide underwent structural characterization using NMR and chemical analyses. The elucidated structure presents a branched repeating unit, with the linear part being: →)-β-d-Gal-(1 → 4)-β-d-Glc-(1 → 4)-β-d-Gal-(1 → 4)-β-d-GlcNAc-(1→, further decorated with a terminal α-d-Glc and a d-phosphoglycerol moiety, attached to O-2 and O-3 of the 4-linked Gal unit, respectively. This polysaccharide was conjugated to BSA and the synthetic glycoprotein used to immunize mice. The resulting sera exhibited good opsonic activity, suggesting its potential as a vaccine antigen. In conclusion, our effector-function-based approach successfully identified an immunogenic capsular polysaccharide with promising applications in immunotherapy.

The pathogenicity of vancomycin-resistant Enterococcus faecalis to colon cancer cells

BACKGROUND

The aim of this study was to investigate the pathogenicity of vancomycin-resistant Enterococcus faecalis (VREs) to human colon cells in vitro.

METHODS

Three E. faecalis isolates (2 VREs and E. faecalis ATCC 29212) were cocultured with NCM460, HT-29 and HCT116 cells. Changes in cell morphology and bacterial adhesion were assessed at different time points. Interleukin-8 (IL-8) and vascular endothelial growth factor A (VEGFA) expression were measured via RT-qPCR and enzyme-linked immunosorbent assay (ELISA), respectively. Cell migration and human umbilical vein endothelial cells (HUVECs) tube formation assays were used for angiogenesis studies. The activity of PI3K/AKT/mTOR signaling pathway was measured by Western blotting.

RESULTS

The growth and adhesion of E. faecalis at a multiplicity of infection (MOI) of 1:1 were greater than those at a MOI of 100:1(p < 0.05). Compared to E. faecalis ATCC 29212, VREs showed less invasive effect on NCM460 and HT-29 cells. E. faecalis promoted angiogenesis by secreting IL-8 and VEGFA in colon cells, and the cells infected with VREs produced more than those infected with the standard strain (p < 0.05). Additionally, the PI3K/AKT/mTOR signaling pathway was activated in E. faecalis infected cells, with VREs demonstrating a greater activation compared to E. faecalis ATCC 29212 (p < 0.05).

CONCLUSION

VREs contribute to the occurrence and development of CRC by promoting angiogenesis and activating the PI3K/AKT/mTOR signaling pathway.

Structural variations and roles of rhamnose-rich cell wall polysaccharides in Gram-positive bacteria

Rhamnose-rich cell wall polysaccharides (Rha-CWPSs) have emerged as crucial cell wall components of numerous Gram-positive, ovoid-shaped bacteria—including streptococci, enterococci, and lactococci—of which many are of clinical or biotechnological importance. Rha-CWPS are composed of a conserved polyrhamnose backbone with side-chain substituents of variable size and structure. Because these substituents contain phosphate groups, Rha-CWPS can also be classified as polyanionic glycopolymers, similar to wall teichoic acids, of which they appear to be functional homologs. Recent advances have highlighted the critical role of these side-chain substituents in bacterial cell growth and division, as well as in specific interactions between bacteria and infecting bacteriophages or eukaryotic hosts. Here, we review the current state of knowledge on the structure and biosynthesis of Rha-CWPS in several ovoid-shaped bacterial species. We emphasize the role played by multicomponent transmembrane glycosylation systems in the addition of side-chain substituents of various sizes as extracytoplasmic modifications of the polyrhamnose backbone. We provide an overview of the contribution of Rha-CWPS to cell wall architecture and biogenesis and discuss current hypotheses regarding their importance in the cell division process. Finally, we sum up the critical roles that Rha-CWPS can play as bacteriophage receptors or in escaping host defenses, roles that are mediated mainly through their side-chain substituents. From an applied perspective, increased knowledge of Rha-CWPS can lead to advancements in strategies for preventing phage infection of lactococci and streptococci in food fermentation and for combating pathogenic streptococci and enterococci.

Enterococcus Virulence and Resistant Traits Associated with Its Permanence in the Hospital Environment

Enterococcus are opportunistic pathogens that have been gaining importance in the clinical setting, especially in terms of hospital-acquired infections. This problem has mainly been associated with the fact that these bacteria are able to present intrinsic and extrinsic resistance to different classes of antibiotics, with a great deal of importance being attributed to vancomycin-resistant enterococci. However, other aspects, such as the expression of different virulence factors including biofilm-forming ability, and its capacity of trading genetic information, makes this bacterial genus more capable of surviving harsh environmental conditions. All these characteristics, associated with some reports of decreased susceptibility to some biocides, all described in this literary review, allow enterococci to present a longer survival ability in the hospital environment, consequently giving them more opportunities to disseminate in these settings and be responsible for difficult-to-treat infections.

Carbohydrate Metabolism Affects Macrophage-Mediated Killing of Enterococcus faecalis

Enterococcus faecalis, an opportunistic pathogen that causes severe community-acquired and nosocomial infections, has been reported to resist phagocyte-mediated killing, which enables its long-term survival in the host. Metabolism, especially carbohydrate metabolism, plays a key role in the battle between pathogens and hosts. However, the function of carbohydrate metabolism in the long-term survival of E. faecalis in phagocytes has rarely been reported. In this study, we utilized transposon insertion sequencing (TIS) to investigate the function of carbohydrate metabolism during the survival of E. faecalis in RAW264.7 cells. The TIS results showed that the fitness of carbohydrate metabolism-related mutants, especially those associated with fructose and mannose metabolism, were significantly enhanced, suggesting that the attenuation of carbohydrate metabolism promotes the survival of E. faecalis in macrophages. The results of our investigation indicated that macrophages responded to carbohydrate metabolism of E. faecalis and polarized to M1 macrophages to increase nitric oxide (NO) production, leading to the enhancement of macrophage-mediated killing to E. faecalis. Meanwhile, E. faecalis automatically decreased carbohydrate metabolism to escape from the immune clearance of macrophages during intracellular survival. The shift of primary carbon resources for macrophages affected the ability to clear intracellular E. faecalis. In summary, the results of the present study demonstrated that carbohydrate metabolism affects the macrophage-mediated killing of E. faecalis.

IMPORTANCEE. faecalis has become a major pathogen leading to a variety of infections around the world. The metabolic interaction between E. faecalis and its host is important during infection but is rarely investigated. We used transposon insertion sequencing coupled with transcriptome sequencing to explore the metabolic interaction between E. faecalis and macrophages and uncovered that the shift of carbohydrate metabolism dramatically affected the inflammatory response of macrophages. In addition, E. faecalis attenuated carbohydrate metabolism to avoid the activation of the immune response of macrophages. This study provides new insights for the reason why E. faecalis is capable of long-term survival in macrophages and may facilitate the development of novel strategies to treat infectious diseases.

Concanavalin A differentiates gram-positive bacteria through hierarchized nanostructured transducer

Biosensors are pre-prepared diagnostic devices composed of at least one biological probe. These devices are envisaged for the practical identification of specific targets of microbiological interest. In recent years, the use of narrow-specific probes such as lectins has been proven to distinguish bacteria and glycoproteins based on their superficial glycomic pattern. For instance, Concanavalin A is a carbohydrate-binding lectin indicated as a narrow-specific biological probe for Gram-negative bacteria. As a drawback, Gram-positive bacteria are frequently overlooked from lectin-based biosensing studies because their identification results in low resolution and overlapped signals. In this work, the authors explore the effect that platform nanostructuration has over the electrochemical response of ConA-based platforms constructed for bacterial detection; one is formed of chitosan-capped magnetic nanoparticles, and another is composed of gold nanoparticle-decorated magnetic nanoparticles. The biosensing platforms were characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) as a function of bacterial concentration. Our results show that probe-target interaction causes variations in the electrical responses of nanostructured transducers. Moreover, the association of gold nanoparticles to magnetic nanoparticles resulted in an electrical enhancement capable of overcoming low resolution and overlapping Gram-positive identification. Both platforms attained a limit of detection of 10 ° CFU mL^-1, which is useful for water analyses and sanitation concerns, where low CFU mL^-1 are always expected. Although both platforms were able to detect Gram-negative bacteria, Gram-positives were only correctly differentiated by the gold nanoparticle-decorated magnetic nanoparticles, thus demonstrating the positive influence of hierarchically nanostructured platforms.

Cell wall polysaccharides of Gram positive ovococcoid bacteria and their role as bacteriophage receptors

Gram-positive bacterial cell walls are characterised by the presence of a thick peptidoglycan layer which provides protection from extracellular stresses, maintains cell integrity and determines cell morphology, while it also serves as a foundation to anchor a number of crucial polymeric structures. For ovococcal species, including streptococci, enterococci and lactococci, such structures are represented by rhamnose-containing cell wall polysaccharides, which at least in some instances appear to serve as a functional replacement for wall teichoic acids. The biochemical composition of several streptococcal, lactococcal and enterococcal rhamnose-containing cell wall polysaccharides have been elucidated, while associated functional genomic analyses have facilitated the proposition of models for individual biosynthetic pathways. Here, we review the genomic loci which encode the enzymatic machinery to produce rhamnose-containing, cell wall-associated polysaccharide (Rha cwps) structures of the afore-mentioned ovococcal bacteria with particular emphasis on gene content, biochemical structure and common biosynthetic steps. Furthermore, we discuss the role played by these saccharidic polymers as receptors for bacteriophages and the important role phages play in driving Rha cwps diversification and evolution.

Inhibition of the Classical Pathway of Complement Activation Impairs Bacterial Clearance during Enterococcus faecalis Infection

Enterococcus faecalis infections are considered a major public health concern worldwide. The complement system has a crucial role in the protection against different microbial pathogens, including E. faecalis Complement can be activated through three different pathways, including the classical, lectin, and alternative pathways. There is limited information on the role of the classical pathway (CP) in protection against infections caused by E. faecalis In the present study, we generated Fab fragments that successfully block the CP in mouse via inhibition of a key enzyme, C1s-A. Our results showed that anti-C1s-A Fab fragments block CP-mediated C3b and C4b deposition in vitro We further showed that administration of anti-C1s-A Fab fragments significantly impairs the CP functional activity in vivo Moreover, treatment of mice infected with E. faecalis using anti-C1s-A Fab fragments significantly impairs bacterial clearance as determined from the viable bacterial counts recovered from blood, kidneys, spleens, livers, and lungs of infected mice. Overall, this study highlights the essential role of the CP in host defense against E. faecalis.

Intrinsic resistance of Enterococcus faecalis strains to ΦEf11 phage endolysin is associated with the presence of ΦEf11 prophage

The use of bacteriophage-encoded murein hydrolases (endolysins) is being actively explored as a means of controlling multidrug-resistant pathogens. Previously, we isolated and characterized one such enzyme, the phage ΦEf11 ORF28 lysin, which demonstrated profound antimicrobial activity against many strains of Enterococcus faecalis. Although the lysin is eminently active against many vancomycin-resistant enterococal (VRE) strains, and displays lower minimum inhibitory concentrations than vancomycin against vancomycin-sensitive strains, there is a subset of E. faecalis strains that is not affected by the lysin. Currently, there is no explanation for the disparate sensitivity to ORF28 lysin among E. faecalis strains. In the present investigation, we show that the intrinsic insensitivity of the insusceptible strains to the lysin is associated with the presence of a ΦEf11 prophage. Of the strains harboring phage ΦEf11 genes (N = 28), 68% were insensitive to the lysin, whereas 91% of the strains (N = 75) lacking detectable ΦEf11 genes demonstrated lysin sensitivity. Furthermore, curing a lysin-resistant, lysogenic E. faecalis strain resulted in a lysin-sensitive derivative, whereas lysogenizing a wild-type non-lysogenic strain converted it from lysin sensitivity to lysin resistance. Our results suggest that lysin resistance comes about through lysogenic conversion of non-lysogenic, lysin-sensitive strains.

Advances and Prospects in Vaccine Development against Enterococci

Enterococci are the second most common Gram-positive pathogen responsible for nosocomial infections. Due to the limited number of new antibiotics that reach the medical practice and the resistance of enterococci to the current antibiotic options, passive and active immunotherapies have emerged as a potential prevention and/or treatment strategy against this opportunistic pathogen. In this review, we explore the pathogenicity of these bacteria and their interaction with the host immune response. We provide an overview of the capsular polysaccharides and surface-associated proteins that have been described as potential antigens in anti-enterococcal vaccine formulations. In addition, we describe the current status in vaccine development against enterococci and address the importance and the current advances toward the development of well-defined vaccines with broad coverage against enterococci.

A comprehensive review of bacterial osteomyelitis with emphasis on Staphylococcus aureus

Osteomyelitis, a significant infection of bone tissue, gives rise to two main groups of infection: acute and chronic. These groups are further categorized in terms of the duration of infection. Usually, children and adults are more susceptible to acute and chronic infections, respectively. The aforementioned groups of osteomyelitis share almost 80% of the corresponding bacterial pathogens. Among all bacteria, Staphylococcus aureus (S. aureus) is a significant pathogen and is associated with a high range of osteomyelitis symptoms. S. aureus has many strategies for interacting with host cells including Small Colony Variant (SCV), biofilm formation, and toxin secretion. In addition, it induces an inflammatory response and causes host cell death by apoptosis and necrosis. However, any possible step to take in this respect is dependent on the conditions and host responses. In the absence of any immune responses and antibiotics, bacteria actively duplicate themselves; however, in the presence of phagocytic cell and harassing conditions, they turn into a SCV, remaining sustainable for a long time. SCV is characterized by notable advantages such as (a) intracellular life that mediates a dam against immune cells and (b) low ATP production that mediates resistance against antibiotics.

Evolution of vancomycin-resistant Enterococcus faecium during colonization and infection in immunocompromised pediatric patients

Immunocompromised patients are at increased risk for multidrug-resistant infections, due to broad-spectrum antibiotic exposure and a host environment with limited innate defenses. This study explored how vancomycin-resistant Enterococcus faecium (VREfm), a pathogen endemic to many hospitals, underwent genomic and phenotypic changes during intestinal colonization and bloodstream infection of immunocompromised pediatric patients. We identified a mutation conferring bacterial growth in alternative sugars that arose de novo in two different patients and was also present in five other patients. We also characterized mutations in surface polysaccharide production associated with better adherence to surfaces and resistance to the innate immune factor lysozyme. These findings suggest that targeting carbohydrate availability and bacterial adherence may be worthwhile strategies to limit VREfm proliferation in immunocompromised hosts.

Patients with hematological malignancies or undergoing hematopoietic stem cell transplantation are vulnerable to colonization and infection with multidrug-resistant organisms, including vancomycin-resistant Enterococcus faecium (VREfm). Over a 10-y period, we collected and sequenced the genomes of 110 VREfm isolates from gastrointestinal and blood cultures of 24 pediatric patients undergoing chemotherapy or hematopoietic stem cell transplantation for hematological malignancy at St. Jude Children’s Research Hospital. We used patient-specific reference genomes to identify variants that arose over time in subsequent gastrointestinal and blood isolates from each patient and analyzed these variants for insight into how VREfm adapted during colonization and bloodstream infection within each patient. Variants were enriched in genes involved in carbohydrate metabolism, and phenotypic analysis identified associated differences in carbohydrate utilization among isolates. In particular, a Y585C mutation in the sorbitol operon transcriptional regulator gutR was associated with increased bacterial growth in the presence of sorbitol. We also found differences in biofilm-formation capability between isolates and observed that increased biofilm formation correlated with mutations in the putative E. faecium capsular polysaccharide (cps) biosynthetic locus, with different mutations arising independently in distinct genetic backgrounds. Isolates with cps mutations showed improved survival following exposure to lysozyme, suggesting a possible reason for the selection of capsule-lacking bacteria. Finally, we observed mutations conferring increased tolerance of linezolid and daptomycin in patients who were treated with these antibiotics. Overall, this study documents known and previously undescribed ways that VREfm evolve during intestinal colonization and subsequent bloodstream infection in immunocompromised pediatric patients.

Complete Structure of the Enterococcal Polysaccharide Antigen (EPA) of Vancomycin-Resistant Enterococcus faecalis V583 Reveals that EPA Decorations Are Teichoic Acids Covalently Linked to a Rhamnopolysaccharide Backbone

Enterococci are opportunistic pathogens responsible for hospital- and community-acquired infections. All enterococci produce a surface polysaccharide called EPA (enterococcal polysaccharide antigen) required for biofilm formation, antibiotic resistance, and pathogenesis. Despite the critical role of EPA in cell growth and division and as a major virulence factor, no information is available on its structure. Here, we report the complete structure of the EPA polymer produced by the model strain E. faecalis V583. We describe the structure of the EPA backbone, made of a rhamnan hexasaccharide substituted by Glc and GlcNAc residues, and show that teichoic acids are covalently bound to this rhamnan chain, forming the so-called “EPA decorations” essential for host colonization and pathogenesis. This report represents a key step in efforts to identify the structural properties of EPA that are essential for its biological activity and to identify novel targets to develop preventive and therapeutic approaches against enterococci.

All enterococci produce a complex polysaccharide called the enterococcal polysaccharide antigen (EPA). This polymer is required for normal cell growth and division and for resistance to cephalosporins and plays a critical role in host-pathogen interaction. The EPA contributes to host colonization and is essential for virulence, conferring resistance to phagocytosis during the infection. Recent studies revealed that the “decorations” of the EPA polymer, encoded by genetic loci that are variable between isolates, underpin the biological activity of this surface polysaccharide. In this work, we investigated the structure of the EPA polymer produced by the high-risk enterococcal clonal complex Enterococcus faecalis V583. We analyzed purified EPA from the wild-type strain and a mutant lacking decorations and elucidated the structure of the EPA backbone and decorations. We showed that the rhamnan backbone of EPA is composed of a hexasaccharide repeat unit of C2- and C3-linked rhamnan chains, partially substituted in the C3 position by α-glucose (α-Glc) and in the C2 position by β-N-acetylglucosamine (β-GlcNAc). The so-called “EPA decorations” consist of phosphopolysaccharide chains corresponding to teichoic acids covalently bound to the rhamnan backbone. The elucidation of the complete EPA structure allowed us to propose a biosynthetic pathway, a first essential step toward the design of antimicrobials targeting the synthesis of this virulence factor.

Exopolysaccharide-mediated surface penetration as new virulence trait in Enterococcus faecalis

Enterococcus faecalis is a commensal bacterium that normally inhabits the gastrointestinal tract of humans. This non-motile microorganism can also cause lethal infections in other organs by penetrating and breaching the intestinal barrier. However, the precise molecular mechanisms enabling E. faecalis movement and translocation across epithelial barriers remain incompletely characterized. We recently reported that E. faecalis utilizes the RpiA-GlnA-EpaX metabolic axis to generate β-1,6-linked poly-N-acetylglucosamine (polyGlcNAc)-containing exopolymers that are necessary for its optimal migration into semisolid surfaces and efficient translocation through human epithelial cell monolayers. These findings provide new evidence indicating that non-motile bacterial pathogens can exploit carbohydrate metabolism to penetrate surfaces. Hence, targeting this process might represent a new strategy to more effectively control systemic infections by E. faecalis.

Enterococcus faecalis Escapes Complement-Mediated Killing via Recruitment of Complement Factor H

BACKGROUND

Enterococcus faecalis is considered to be the most important species of enterococci responsible for blood stream infections in critically ill patients. In blood, the complement system is activated via the classical pathway (CP), the lectin pathway (LP), or the alternative pathway (AP), and it plays a critical role in opsonophagocytosis of bacteria including E faecalis.

METHODS

In a mouse model of enterococcus peritonitis, BALB-C mice were challenged with a high dose of E faecalis 12 hours after intraperitoneal administration of anti-Factor H (FH) antibodies or isotype control. Four hours later, control mice developed higher bacterial burden in blood and organs compared with mice treated with anti-FH antibodies.

RESULTS

We demonstrate that complement recognition molecules C1q, CL-11, and murine ficolin-A bind the enterococcus and drive the CP and the LP in human and mouse. We further describe that E faecalis evades the AP by recruitment of FH on its surface. Our results show a strong C3b deposition on E faecalis via both the CP and the LP but not through the AP.

CONCLUSIONS

These findings indicate that E faecalis avoids the complement phagocytosis by the AP via sequestering complement FH from the host blood.

Fitness Restoration of a Genetically Tractable Enterococcus faecalis V583 Derivative To Study Decoration-Related Phenotypes of the Enterococcal Polysaccharide Antigen

E. faecalis strain VE14089 was derived from V583 cured of its plasmids. Although VE14089 had no major DNA rearrangements, it presented significant growth and host adaptation differences from the reference strain V583 of our collection. To construct a strain with better fitness, we sequenced the genome of VE14089, identified single nucleotide polymorphisms (SNPs), and repaired the genes that could account for these changes. Using this reference-derivative strain, we provide a novel genetic system to understand the role of the variable region of epa in the enterococcal lifestyle.

Commensal and generally harmless in healthy individuals, Enterococcus faecalis causes opportunistic infections in immunocompromised patients. Plasmid-cured E. faecalis strain VE14089, derived from sequenced reference strain V583, is widely used for functional studies due to its improved genetic amenability. Although strain VE14089 has no major DNA rearrangements, with the exception of an ∼20-kb integrated region of pTEF1 plasmid, the strain presented significant growth differences from the V583 reference strain of our collection (renamed VE14002). In the present study, genome sequencing of strain VE14089 identified additional point mutations. Excision of the integrated pTEF1 plasmid region and sequential restoration of wild-type alleles showing nonsilent mutations were performed to obtain the VE18379 reference-derivative strain. Recovery of the growth ability of the restored VE18379 strain at a level similar to that seen with the reference strain points to GreA and Spx as bacterial fitness determinants. Virulence potential in Galleria mellonella and intestinal colonization in mouse demonstrated host adaptation of the VE18379 strain equivalent to VE14002 host adaptation. We further demonstrated that deletion of the 16.8-kb variable region of the epa locus recapitulates the key role of Epa decoration in host adaptation, providing a genetic system to study the role of specific epa-variable regions in host adaptation independently of other genetic variations.

IMPORTANCEE. faecalis strain VE14089 was derived from V583 cured of its plasmids. Although VE14089 had no major DNA rearrangements, it presented significant growth and host adaptation differences from the reference strain V583 of our collection. To construct a strain with better fitness, we sequenced the genome of VE14089, identified single nucleotide polymorphisms (SNPs), and repaired the genes that could account for these changes. Using this reference-derivative strain, we provide a novel genetic system to understand the role of the variable region of epa in the enterococcal lifestyle.

Pathogenicity of Enterococci

Enterococci are unusually well adapted for survival and persistence in a variety of adverse environments, including on inanimate surfaces in the hospital environment and at sites of infection. This intrinsic ruggedness undoubtedly played a role in providing opportunities for enterococci to interact with other overtly drug-resistant microbes and acquire additional resistances on mobile elements. The rapid rise of antimicrobial resistance among hospital-adapted enterococci has rendered hospital-acquired infections a leading therapeutic challenge. With about a quarter of a genome of additional DNA conveyed by mobile elements, there are undoubtedly many more properties that have been acquired that help enterococci persist and spread in the hospital setting and cause diseases that have yet to be defined. Much remains to be learned about these ancient and rugged microbes, particularly in the area of pathogenic mechanisms involved with human diseases.

Detection and characterization of bacterial polysaccharides in drug-resistant enterococci

Enterococcus faecium (E. faecium) has emerged as one of today's leading causes of health care-associated infections that is difficult to treat with the available antibiotics. These pathogens produce capsular polysaccharides on the cell surface which play a significant role in adhesion, virulence and evasion. Therefore, we aimed at the identification and characterization of bacterial polysaccharide antigens which are central for the development of vaccine-based prophylactic approaches. The crude cell wall-associated polysaccharides from E. faecium, its mutant and complemented strains were purified and analyzed by a primary antibody raised against lipoteichoic acid (LTA) and diheteroglycan (DHG). The resistant E. faecium strains presumably possess novel capsular polysaccharides that allow them to avoid the evasion from opsonic killing. The E. faecium U0317 strain was very well opsonized by anti-U0317 (~95%), an antibody against the whole bacterial cell. The deletion mutant showed a significantly increased susceptibility to opsonophagocytic killing (90-95%) against the penicillin binding protein (anti-PBP-5). By comparison, in a mouse urinary tract and rat endocarditis infection model, respectively, there were no significant differences in virulence. In this study we explored the biological role of the capsule of E. faecium. Our findings showed that the U0317 strain is not only sensitive to anti-LTA but also to antibodies against other enterococcal surface proteins. Our findings demonstrate that polysaccharides capsule mediated-resistance to opsonophagocytosis. We also found that the capsular polysaccharides do not play an important role in bacterial virulence in urinary tract and infective endocarditis in vivo models.

Bacteriophage Resistance Alters Antibiotic-Mediated Intestinal Expansion of Enterococci

Enterococcus faecalis is a human intestinal pathobiont with intrinsic and acquired resistance to many antibiotics, including vancomycin. Nature provides a diverse and virtually untapped repertoire of bacterial viruses, or bacteriophages (phages), that could be harnessed to combat multidrug-resistant enterococcal infections. Bacterial phage resistance represents a potential barrier to the implementation of phage therapy, emphasizing the importance of investigating the molecular mechanisms underlying the emergence of phage resistance. Using a cohort of 19 environmental lytic phages with tropism against E. faecalis, we found that these phages require the enterococcal polysaccharide antigen (Epa) for productive infection. Epa is a surface-exposed heteroglycan synthesized by enzymes encoded by both conserved and strain-specific genes. We discovered that exposure to phage selective pressure favors mutation in nonconserved epa genes both in culture and in a mouse model of intestinal colonization. Despite gaining phage resistance, epa mutant strains exhibited a loss of resistance to cell wall-targeting antibiotics. Finally, we show that an E. faecalis epa mutant strain is deficient in intestinal colonization, cannot expand its population upon antibiotic-driven intestinal dysbiosis, and fails to be efficiently transmitted to juvenile mice following birth. This study demonstrates that phage therapy could be used in combination with antibiotics to target enterococci within a dysbiotic microbiota. Enterococci that evade phage therapy by developing resistance may be less fit at colonizing the intestine and sensitized to vancomycin, preventing their overgrowth during antibiotic treatment.

Decoration of the enterococcal polysaccharide antigen EPA is essential for virulence, cell surface charge and interaction with effectors of the innate immune system

Enterococcus faecalis is an opportunistic pathogen with an intrinsically high resistance to lysozyme, a key effector of the innate immune system. This high level of resistance requires a complex network of transcriptional regulators and several genes (oatA, pgdA, dltA and sigV) acting synergistically to inhibit both the enzymatic and cationic antimicrobial peptide activities of lysozyme. We sought to identify novel genes modulating E. faecalis resistance to lysozyme. Random transposon mutagenesis carried out in the quadruple oatA/pgdA/dltA/sigV mutant led to the identification of several independent insertions clustered on the chromosome. These mutations were located in a locus referred to as the enterococcal polysaccharide antigen (EPA) variable region located downstream of the highly conserved epaA-epaR genes proposed to encode a core synthetic machinery. The epa variable region was previously proposed to be responsible for EPA decorations, but the role of this locus remains largely unknown. Here, we show that EPA decoration contributes to resistance towards charged antimicrobials and underpins virulence in the zebrafish model of infection by conferring resistance to phagocytosis. Collectively, our results indicate that the production of the EPA rhamnopolysaccharide backbone is not sufficient to promote E. faecalis infections and reveal an essential role of the modification of this surface polymer for enterococcal pathogenesis.

Enterococcus faecalis is a commensal bacterium colonizing the gastro-intestinal tract of humans. This organism can cause life-threatening opportunistic infections and represents a reservoir for the transmission of antibiotic resistance genes such as resistance to vancomycin. E. faecalis strains responsible for nosocomial infections are also found in healthy individuals and the virulence factors identified so far are not strictly associated with clinical isolates. The molecular basis underpinning E. faecalis infections therefore remains unclear. In this work, we identify several mutations clustered on the chromosome, which play a role in the resistance of E. faecalis to effectors of the innate immune system such as lysozyme and bile salts. We show that the corresponding genes contribute to the decoration of a conserved polysaccharide called the enterococcal polysaccharide antigen and that this decoration is essential for E. faecalis virulence. This mechanism critical for pathogenesis represents an attractive therapeutic target to control enterococcal infections.

Broad-spectrum capture of clinical pathogens using engineered Fc-mannose-binding lectin enhanced by antibiotic treatment

Background: Fc-mannose-binding lectin (FcMBL), an engineered version of the blood opsonin MBL that contains the carbohydrate recognition domain (CRD) and flexible neck regions of MBL fused to the Fc portion of human IgG1, has been shown to bind various microbes and pathogen-associated molecular patterns (PAMPs). FcMBL has also been used to create an enzyme-linked lectin sorbent assay (ELLecSA) for use as a rapid (<1 h) diagnostic of bloodstream infections. Methods: Here we extended this work by using the ELLecSA to test FcMBL's ability to bind to more than 190 different isolates from over 95 different pathogen species. Results: FcMBL bound to 85% of the isolates and 97 of the 112 (87%) different pathogen species tested, including bacteria, fungi, viral antigens and parasites. FcMBL also bound to PAMPs including, lipopolysaccharide endotoxin (LPS) and lipoteichoic acid (LTA) from Gram-negative and Gram-positive bacteria, as well as lipoarabinomannan (LAM) and phosphatidylinositol mannoside 6 (PIM 6) from Mycobacterium tuberculosis. Conclusions: The efficiency of pathogen detection and variation between binding of different strains of the same species could be improved by treating the bacteria with antibiotics, or mechanical disruption using a bead mill, prior to FcMBL capture to reveal previously concealed binding sites within the bacterial cell wall. As FcMBL can bind to pathogens and PAMPs in urine as well as blood, its broad-binding capability could be leveraged to develop a variety of clinically relevant technologies, including infectious disease diagnostics, therapeutics, and vaccines.

Evolution of virulence in Enterococcus faecium, a hospital-adapted opportunistic pathogen

Enterococci are long-standing members of the human microbiome and they are also widely distributed in nature. However, with the surge of antibiotic-resistance in recent decades, two enterococcal species (Enterococcus faecalis and Enterococcus faecium) have emerged to become significant nosocomial pathogens, acquiring extensive antibiotic resistance. In this review, we summarize what is known about the evolution of virulence in E. faecium, highlighting a specific clone of E. faecium called ST796 that has emerged recently and spread globally.

Cell wall glycopolymers of Firmicutes and their role as nonprotein adhesins

Cell wall glycopolymers (CWGs) of gram-positive bacteria have gained increasing interest with respect to their role in colonization and infection. In most gram-positive pathogens they constitute a large fraction of the cell wall biomass and represent major cell envelope determinants. Depending on their chemical structure they modulate interaction with complement factors and play roles in immune evasion or serve as nonprotein adhesins that mediate, especially under dynamic conditions, attachment to different host cell types. In particular, covalently peptidoglycan-attached CWGs that extend well above the cell wall seem to interact with glyco-receptors on host cell surfaces. For example, in the case of Staphylococcus aureus, the cell wall-attached teichoic acid (WTA) has been identified as a major CWG adhesin. A recent report indicates that a type-F scavenger receptor, termed SR-F1 (SREC-I), is the predominant WTA receptor in the nasal cavity and that WTA-SREC-I interaction plays an important role in S. aureus nasal colonization. Therefore, understanding the role of CWGs in complex processes that mediate colonization and infection will allow novel insights into the mechanisms of host-microbiota interaction.

Identification and functional characterization of the putative polysaccharide biosynthesis protein (CapD) of Enterococcus faecium U0317

Most bacterial species produce capsular polysaccharides that contribute to disease pathogenesis through evasion of the host innate immune system and are also involved in inhibiting leukocyte killing. In the present study, we identified a gene in Enterococcus faecium U0317 with homologies to the polysaccharide biosynthesis protein CapD that is made up of 336 amino acids and putatively catalyzes N-linked glycosylation. A capD deletion mutant was constructed and complemented by homologous recombination that was confirmed by PCR and sequencing. The mutant revealed different growth behavior and morphological changes compared to wild-type by scanning electron microscopy, also the capD mutant showed a strong hydrophobicity and that was reversed in the reconstituted mutant. For further characterization and functional analyses, in-vitro cell culture and in-vivo a mouse infection models were used. Antibodies directed against alpha lipotechoic acid (αLTA) and the peptidyl-prolyl cis-trans isomerase (αPpiC), effectively mediated the opsonophagocytic killing in the capD knock-out mutant, while this activity was not observed in the wild-type and reconstituted mutant. By comparison more than 2-fold decrease was seen in mutant colonization and adherence to both T24 and Caco2 cells. However, a significant higher bacterial colonization was observed in capD mutant during bacteremia in the animal model, while virulence in a mouse UTI (urinary tract infection) model, there were no obvious differences. Further studies are needed to elucidate the function of capsular polysaccharide synthesis gene clusters and its involvement in the disease pathogenesis with the aim to develop targeted therapies to treat multidrug-resistant E. faecium infections.

Surface-Associated Lipoproteins Link Enterococcus faecalis Virulence to Colitogenic Activity in IL-10-Deficient Mice Independent of Their Expression Levels

The commensal Enterococcus faecalis is among the most common causes of nosocomial infections. Recent findings regarding increased abundance of enterococci in the intestinal microbiota of patients with inflammatory bowel diseases and induction of colitis in IL-10-deficient (IL-10-/-) mice put a new perspective on the contribution of E. faecalis to chronic intestinal inflammation. Based on the expression of virulence-related genes in the inflammatory milieu of IL-10-/- mice using RNA-sequencing analysis, we characterized the colitogenic role of two bacterial structures that substantially impact on E. faecalis virulence by different mechanisms: the enterococcal polysaccharide antigen and cell surface-associated lipoproteins. Germ-free wild type and IL-10-/- mice were monoassociated with E. faecalis wild type OG1RF or the respective isogenic mutants for 16 weeks. Intestinal tissue and mesenteric lymph nodes (MLN) were collected to characterize tissue pathology, loss of intestinal barrier function, bacterial adhesion to intestinal epithelium and immune cell activation. Bone marrow-derived dendritic cells (BMDC) were stimulated with bacterial lysates and E. faecalis virulence was additionally investigated in three invertebrate models. Colitogenic activity of wild type E. faecalis (OG1RF score: 7.2±1.2) in monoassociated IL-10-/- mice was partially impaired in E. faecalis lacking enterococcal polysaccharide antigen (ΔepaB score: 4.7±2.3; p<0.05) and was almost completely abrogated in E. faecalis deficient for lipoproteins (Δlgt score: 2.3±2.3; p<0.0001). Consistently both E. faecalis mutants showed significantly impaired virulence in Galleria mellonella and Caenorhabditis elegans. Loss of E-cadherin in the epithelium was shown for all bacterial strains in inflamed IL-10-/- but not wild type mice. Inactivation of epaB in E. faecalis reduced microcolony and biofilm formation in vitro, altered bacterial adhesion to intestinal epithelium of germ-free Manduca sexta larvae and impaired penetration into the colonic mucus layer of IL-10-/- mice. Lipoprotein-deficient E. faecalis exhibited an impaired TLR2-mediated activation of BMDCs in vitro despite their ability to fully reactivate MLN cells as well as MLN-derived colitogenic T cells ex vivo. E. faecalis virulence factors accounting for bacterial adhesion to mucosal surfaces as well as intestinal barrier disruption partially contribute to colitogenic activity of E. faecalis. Beyond their well-known role in infections, cell surface-associated lipoproteins are essential structures for colitogenic activity of E. faecalis by mediating innate immune cell activation.

Enterococcus faecalis is a commensal of the human intestinal core microbiota harboring several putative virulence factors, which highlight its role as opportunistic pathogen. This dualistic character is supported by recent evidence linking Enterococcus spp. to the pathogenesis of inflammatory bowel diseases (IBD). Although several studies suggest a crucial role for opportunistic pathogens in IBD pathogenesis targeting genetically susceptible individuals, the dynamic relationship between disease-relevant host compartments and specific bacterial structures able to trigger intestinal inflammation remain unclear. Here, we report that cell surface-associated lipoproteins and the enterococcal polysaccharide antigen, which are relevant for E. faecalis virulence in invertebrate infection models, but whose expression is minimally affected by the intestinal inflammatory milieu, exhibit colitogenic activity in a mouse model susceptible for chronic colitis. Bacterial lipoproteins trigger innate immune cell activation and are a critical prerequisite for E. faecalis-induced colitis. The enterococcal polysaccharide antigen mediates bacterial mucus penetration and adhesion to mucosal surfaces, promotes the formation of biofilm and contributes to E. faecalis colitogenic activity. Using E. faecalis as a model organism, we demonstrate that colitogenic activity of opportunistic pathogens can be assigned to specific bacterial structures, a finding that may help to identify the most essential steps in IBD-related microbe-host interactions.

Enterococcus faecalis lipoteichoic acid suppresses Aggregatibacter actinomycetemcomitans lipopolysaccharide-induced IL-8 expression in human periodontal ligament cells

Periodontitis is caused by multi-bacterial infection and Aggregatibacter actinomycetemcomitans and Enterococcus faecalis are closely associated with inflammatory periodontal diseases. Although lipopolysaccharide (LPS) of A. actinomycetemcomitans (Aa.LPS) and lipoteichoic acid of E. faecalis (Ef.LTA) are considered to be major virulence factors evoking inflammatory responses, their combinatorial effect on the induction of chemokines has not been investigated. In this study, we investigated the interaction between Aa.LPS and Ef.LTA on IL-8 expression in human periodontal ligament (PDL) cells. Aa.LPS, but not Ef.LTA, substantially induced IL-8 expression at the protein and mRNA levels. Interestingly, Ef.LTA suppressed Aa.LPS-induced IL-8 expression without affecting the binding of Aa.LPS to Toll-like receptor (TLR) 4. Ef.LTA reduced Aa.LPS-induced phosphorylation of mitogen-activated protein kinases, including ERK, JNK and p38 kinase. Furthermore, Ef.LTA inhibited the Aa.LPS-induced transcriptional activities of the activating protein 1, CCAAT/enhancer-binding protein and nuclear factor-kappa B transcription factors, all of which are known to regulate IL-8 gene expression. Ef.LTA augmented the expression of IL-1 receptor-associated kinase-M (IRAK-M), a negative regulator of TLR intracellular signaling pathways, in the presence of Aa.LPS at both the mRNA and protein levels. Small interfering RNA silencing IRAK-M reversed the attenuation of Aa.LPS-induced IL-8 expression by Ef.LTA. Collectively, these results suggest that Ef.LTA down-regulates Aa.LPS-induced IL-8 expression in human PDL cells through up-regulation of the negative regulator IRAK-M.

Wall teichoic acid structure governs horizontal gene transfer between major bacterial pathogens

Mobile genetic elements (MGEs) encoding virulence and resistance genes are widespread in bacterial pathogens, but it has remained unclear how they occasionally jump to new host species. Staphylococcus aureus clones exchange MGEs such as S. aureus pathogenicity islands (SaPIs) with high frequency via helper phages. Here we report that the S. aureus ST395 lineage is refractory to horizontal gene transfer (HGT) with typical S. aureus but exchanges SaPIs with other species and genera including Staphylococcus epidermidis and Listeria monocytogenes. ST395 produces an unusual wall teichoic acid (WTA) resembling that of its HGT partner species. Notably, distantly related bacterial species and genera undergo efficient HGT with typical S. aureus upon ectopic expression of S. aureus WTA. Combined with genomic analyses, these results indicate that a ‘glycocode’ of WTA structures and WTA-binding helper phages permits HGT even across long phylogenetic distances thereby shaping the evolution of Gram-positive pathogens.

Horizontal gene transfer of mobile genetic elements contributes to bacterial evolution and emergence of new pathogens. Here the authors demonstrate that the highly diverse structure of wall teichoic acid polymers governs horizontal gene transfer among Gram-positive pathogens, even across long phylogenetic distances.

The cell wall architecture of Enterococcus faecium: from resistance to pathogenesis

The cell wall of Gram-positive bacteria functions as a surface organelle that continuously interacts with its environment through a plethora of cell wall-associated molecules. Enterococcus faecium is a normal inhabitant of the GI tract of mammals, but has recently become an important etiological agent of hospital-acquired infections in debilitated patients. Insights into the assembly and function of enterococcal cell wall components and their interactions with the host during colonization and infection are essential to explain the worldwide emergence of E. faecium as an important multiantibiotic-resistant nosocomial pathogen. Understanding the biochemistry of cell wall biogenesis and principles of antibiotic resistance at the molecular level may open up new frontiers in research on enterococci, particularly for the development of novel antimicrobial strategies. In this article, we outline the current knowledge on the most important antimicrobial resistance mechanisms that involve peptidoglycan synthesis and the role of cell wall constituents, including lipoteichoic acid, wall teichoic acid, capsular polysaccharides, LPxTG cell wall-anchored surface proteins, WxL-type surface proteins and pili, in the pathogenesis of E. faecium.

Pathways and roles of wall teichoic acid glycosylation in Staphylococcus aureus

The thick peptidoglycan layers of Gram-positive bacteria are connected to polyanionic glycopolymers called wall teichoic acids (WTA). Pathogens such as Staphylococcus aureus, Listeria monocytogenes, or Enterococcus faecalis produce WTA with diverse, usually strain-specific structure. Extensive studies on S. aureus WTA mutants revealed important functions of WTA in cell division, growth, morphogenesis, resistance to antimicrobials, and interaction with host or phages. While most of the S. aureus WTA-biosynthetic genes have been identified it remained unclear for long how and why S. aureus glycosylates WTA with α- or β-linked N-acetylglucosamine (GlcNAc). Only recently the discovery of two WTA glycosyltransferases, TarM and TarS, yielded fundamental insights into the roles of S. aureus WTA glycosylation. Mutants lacking WTA GlcNAc are resistant towards most of the S. aureus phages and, surprisingly, TarS-mediated WTA β-O-GlcNAc modification is essential for β-lactam resistance in methicillin-resistant S. aureus. Notably, S. aureus WTA GlcNAc residues are major antigens and activate the complement system contributing to opsonophagocytosis. WTA glycosylation with a variety of sugars and corresponding glycosyltransferases were also identified in other Gram-positive bacteria, which paves the way for detailed investigations on the diverse roles of WTA modification with sugar residues.

Molecular genetics of familial hematuric diseases

The familial hematuric diseases are a genetically heterogeneous group of monogenic conditions, caused by mutations in one of several genes. The major genes involved are the following: (i) the collagen IV genes COL4A3/A4/A5 that are expressed in the glomerular basement membranes (GBM) and are responsible for the most frequent forms of microscopic hematuria, namely Alport syndrome (X-linked or autosomal recessive) and thin basement membrane nephropathy (TBMN). (ii) The FN1 gene, expressed in the glomerulus and responsible for a rare form of glomerulopathy with fibronectin deposits (GFND). (iii) CFHR5 gene, a recently recognized regulator of the complement alternative pathway and mutated in a recently revisited form of inherited C3 glomerulonephritis (C3GN), characterized by isolated C3 deposits in the absence of immune complexes. A hallmark feature of all conditions is the age-dependent penetrance and a broad phenotypic heterogeneity in the sense that subsets of patients progress to added proteinuria or proteinuria and chronic renal failure that may or may not lead to end-stage kidney disease (ESKD) anywhere between the second and seventh decade of life. In addition to other excellent laboratory tools that assist the clinician in reaching the correct diagnosis, the molecular analysis emerges as the gold standard in establishing the diagnosis in many cases of doubt due to equivocal findings that complicate the differential diagnosis. Recent work led to the description of candidate genetic modifiers which confer a variable risk for progressing to chronic renal failure when co-inherited on the background of a primary glomerulopathy. Finally, more families are still waiting to be studied and more genes to be mapped and cloned that are responsible for other forms of heritable hematuric diseases. The study of such genes and their protein products will likely shed more light on the structure and function of the glomerular filtration barrier and other important glomerular components.