Enterococcus faecalis capsular polysaccharide serotypes C and D and their contributions to host innate immune evasion

Changes in gut microbiota predict neutropenia after induction treatment in childhood acute lymphoblastic leukemia

ABSTRACT

Delayed neutrophil recovery during acute lymphoblastic leukemia (ALL) treatment increases the risk of infection and causes delay in chemotherapy. Emerging evidence implicates gut microbiota in neutrophil reconstitution after chemotherapy. We explored the interplay between the gut microbiota and neutrophil dynamics, including neutrophil chemoattractants, in 51 children with newly diagnosed ALL. Daily absolute neutrophil count (ANC), weekly plasma chemokines (CXCL1 and CXCL8), granulocyte colony-stimulating factor (G-CSF), and fecal samples were monitored until day 29 during ALL induction treatment. Fecal sequencing using 16S ribosomal RNA revealed an overall significant reduction in bacterial diversity and Enterococcus overgrowth throughout the induction treatment. Prolonged neutropenia (ANC <0.5 × 109 cells per L at day 36) and elevated chemokine levels were associated with a decreased abundance of genera from the Ruminococcaceae and Lachnospiraceae families, decreased Veillonella genus, and Enterococcus overgrowth from diagnosis and throughout induction treatment. G-CSF was upregulated in response to neutropenia but was unrelated to microbiota changes. Overall, this study revealed that a diminished abundance of specific intestinal commensals and Enterococcus overgrowth is associated with delayed neutrophil reconstitution and increased chemokine signaling, indicating that disruption of the microbiota may contribute to prolonged neutropenia. These findings lay the groundwork for future investigations into the mechanisms underlying these associations and their clinical implications for developing gut-sparring strategies to minimize the impact of gut dysbiosis on immune recovery.

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.

Enterococcal-host interactions in the gastrointestinal tract and beyond

The gastrointestinal tract (GIT) is typically considered the natural niche of enterococci. However, these bacteria also inhabit extraintestinal tissues, where they can disrupt organ physiology and cause life-threatening infections. Here, we discuss how enterococci, primarily Enterococcus faecalis, interact with the intestine and other host anatomical locations such as the oral cavity, heart, liver, kidney, and vaginal tract. The metabolic flexibility of these bacteria allows them to quickly adapt to new environments, promoting their persistence in diverse tissues. In transitioning from commensals to pathogens, enterococci must overcome harsh conditions such as nutrient competition, exposure to antimicrobials, and immune pressure. Therefore, enterococci have evolved multiple mechanisms to adhere, colonize, persist, and endure these challenges in the host. This review provides a comprehensive overview of how enterococci interact with diverse host cells and tissues across multiple organ systems, highlighting the key molecular pathways that mediate enterococcal adaptation, persistence, and pathogenic behavior.

Determinants of bacterial survival and proliferation in blood

Bloodstream infection is a major public health concern associated with high mortality and high healthcare costs worldwide. Bacteremia can trigger fatal sepsis whose prevention, diagnosis, and management have been recognized as a global health priority by the World Health Organization. Additionally, infection control is increasingly threatened by antimicrobial resistance, which is the focus of global action plans in the framework of a One Health response. In-depth knowledge of the infection process is needed to develop efficient preventive and therapeutic measures. The pathogenesis of bloodstream infection is a dynamic process resulting from the invasion of the vascular system by bacteria, which finely regulate their metabolic pathways and virulence factors to overcome the blood immune defenses and proliferate. In this review, we highlight our current understanding of determinants of bacterial survival and proliferation in the bloodstream and discuss their interactions with the molecular and cellular components of blood.

The population-level impact of Enterococcus faecalis genetics on intestinal colonization and extraintestinal infection

IMPORTANCE

Enterococcus faecalis causes life-threatening invasive hospital- and community-associated infections that are usually associated with multidrug resistance globally. Although E. faecalis infections cause opportunistic infections typically associated with antibiotic use, immunocompromised immune status, and other factors, they also possess an arsenal of virulence factors crucial for their pathogenicity. Despite this, the relative contribution of these virulence factors and other genetic changes to the pathogenicity of E. faecalis strains remain poorly understood. Here, we investigated whether specific genomic changes in the genome of E. faecalis isolates influence its pathogenicity-infection of hospitalized and nonhospitalized individuals and the propensity to cause extraintestinal infection and intestinal colonization. Our findings indicate that E. faecalis genetics partially influence the infection of hospitalized and nonhospitalized individuals and the propensity to cause extraintestinal infection, possibly due to gut-to-bloodstream translocation, highlighting the potential substantial role of host and environmental factors, including gut microbiota, on the opportunistic pathogenic lifestyle of this bacterium.

Evaluation of Enterococcal Probiotic Usage and Review of Potential Health Benefits, Safety, and Risk of Antibiotic-Resistant Strain Emergence

Enterococci are often used in probiotics but can also cause nosocomial infections. As such, enterococcal consumption may have beneficial health effects, but a thorough evaluation of virulence absence and risk of antibiotic resistance spread is needed at the strain level. This article reviewed ten online health product shopping websites in the US. On these websites, 23 probiotic products using enterococci were found across 12 companies. In addition, this article reviewed studies that demonstrated the probiotic potential of enterococcal consumption (e.g., gastrointestinal and respiratory disease, hyperlipidemia alleviation, as well as infection prevention). To investigate the safety aspects of enterococci, the present work examined studies evaluating virulence factors and antibiotic resistance. Furthermore, this article assessed research that explored these virulent factors, specifically in probiotics containing enterococci, as well as the potential transfer mechanism of their antibiotic resistance. Based on reviewed data, enterococcal probiotic consumption has been proven beneficial for conditions or symptoms of multiple diseases without any apparent adverse effects. However, due to the plasmid- or transposon-mediated gene transfer ability of enterococci, surveillance monitoring and further studies regarding enterococcal consumption are warranted. Future studies that identify enterococcal strains safe to use in probiotics without virulence factors and antibiotic resistance are imperative for evidence-based decisions by health organizations and government agencies.

Comparative Genome Analysis of Enterococcus cecorum Reveals Intercontinental Spread of a Lineage of Clinical Poultry Isolates

Enterococcus cecorum is an emerging pathogen responsible for osteomyelitis, spondylitis, and femoral head necrosis causing animal suffering and mortality and requiring antimicrobial use in poultry. Paradoxically, E. cecorum is a common inhabitant of the intestinal microbiota of adult chickens. Despite evidence suggesting the existence of clones with pathogenic potential, the genetic and phenotypic relatedness of disease-associated isolates remains little investigated. Here, we sequenced and analyzed the genomes and characterized the phenotypes of more than 100 isolates, the majority of which were collected over the last 10 years from 16 French broiler farms. Comparative genomics, genome-wide association studies, and the measured susceptibility to serum, biofilm-forming capacity, and adhesion to chicken type II collagen were used to identify features associated with clinical isolates. We found that none of the tested phenotypes could discriminate the origin of the isolates or the phylogenetic group. Instead, we found that most clinical isolates are grouped phylogenetically, and our analyses selected six genes that discriminate 94% of isolates associated with disease from those that are not. Analysis of the resistome and the mobilome revealed that multidrug-resistant clones of E. cecorum cluster into a few clades and that integrative conjugative elements and genomic islands are the main carriers of antimicrobial resistance. This comprehensive genomic analysis shows that disease-associated clones of E. cecorum belong mainly to one phylogenetic clade. IMPORTANCE Enterococcus cecorum is an important pathogen of poultry worldwide. It causes a number of locomotor disorders and septicemia, particularly in fast-growing broilers. Animal suffering, antimicrobial use, and associated economic losses require a better understanding of disease-associated E. cecorum isolates. To address this need, we performed whole-genome sequencing and analysis of a large collection of isolates responsible for outbreaks in France. By providing the first data set on the genetic diversity and resistome of E. cecorum strains circulating in France, we pinpoint an epidemic lineage that is probably also circulating elsewhere that should be targeted preferentially by preventive strategies in order to reduce the burden of E. cecorum-related diseases.

Risk factors of enterococcal bacteriuria in cats: A retrospective study

Objective

To determine if factors associated with urothelial damage and inflammation, including urinary catheterization, urinary obstruction, and urolithiasis are associated with the presence of enterococcal bacteriuria in cats.

Animals

Thirty-one cats with Enterococcus spp. bacteriuria and 31 cats with Escherichia coli bacteriuria.

Procedure

A retrospective case-control study with cases and controls identified by records search for Enterococcus spp. (case) and E. coli (control) bacteriuria from August 1, 2014 to July 31, 2019. Cases and controls were balanced with respect to average age. Binary logistic regression was used to estimate and test whether the odds of having Enterococcus spp. bacteriuria (instead of E. coli) were associated with the presence of any characteristic.

Results

Urinary catheterization, urinary obstruction, and urolithiasis were not observed more often in Enterococcus cases versus E. coli controls (19% versus 25%, P = 0.543; 19% versus 32%, P = 0.244; and 16% versus 16%, P = 1, respectively). Signs of lower urinary tract disease were significantly less common in Enterococcus cases than in E. coli controls (OR: 0.30; 95% CI: 0.10 to 0.83, P = 0.02). Hematuria was significantly less common in cases than controls (P = 0.048).

Conclusion

No association was identified between urinary catheterization, urolithiasis, or any other comorbidities (hyperthyroidism, chronic kidney disease) and enterococcal bacteriuria in cats.

Clinical relevance

Unlike in humans and dogs, urothelial damage and inflammation caused by factors such as urinary catheterization and urolithiasis may not be the mechanism for enterococcal bacteriuria in cats.

Isolation, Identification, and Fermentation Medium Optimization of a Caproic Acid‑Producing Enterococcus casseliflavus Strain from Pit Mud of Chinese Strong Flavor Baijiu Ecosystem

Caproic acid is the precursor material of ethyl hexanoate, a representative flavor substance in strong flavor baijiu (SFB). Increasing the content of caproic acid in SFB helps to improve its quality. In the present study, caproic acid-producing bacteria from the pit mud of an SFB ecosystem were isolated, purified, and characterized. Strain BF-1 with the highest caproic acid yield (0.88 g/l) was selected. The morphological and molecular identification analysis showed that strain BF-1 was Enterococcus casseliflavus. The genome of E. casseliflavus BF-1 was sequenced and was found to be 2,968,377 bp in length with 3,270 open reading frames (ORFs). The caproic acid biosynthesis pathway in E. casseliflavus BF-1 was predicted based on the KAAS annotation. The virulence factors in the genome of strain BF-1 were annotated, which showed that E. casseliflavus BF-1 is safe at the genetic level. After adding essential nutrients based on the KAAS annotation, the optimum medium conditions for acid production by strain BF-1 were obtained by performing orthogonal experiments. The caproic acid yield of strain BF-1 reached 3.03 g/l, which was 3.44-fold higher than the initial yield. The optimized fer- mentation of caproic acid production by BF-1 was reported for the first time. The strain could be further used to regulate the ecosystem in baijiu production to improve its quality.

Lactococcus garvieae FUA009, a Novel Intestinal Bacterium Capable of Producing the Bioactive Metabolite Urolithin A from Ellagic Acid

Dietary polyphenol ellagic acid has anti-cancer and anti-inflammatory activities, and these biological activities require the conversion of ellagic acid to urolithins by intestinal microbes. However, few gut microbes are capable of metabolizing ellagic acid to produce urolithins, limiting the beneficial effects of ellagic acid on health. Here, we describe an intestinal bacterium Lactococcus garvieae FUA009 isolated from the feces of a healthy volunteer. It was demonstrated via HPLC and UPLC-MS analysis that the end product of ellagic acid metabolism of FUA009 was urolithin A. In addition, we also examined the whole genome sequence of FUA009 and then assessed the safety and probiotic properties of FUA009 based on a complete genome and phenotype analysis. We indicated that FUA009 was safe, which was confirmed by FUA009 being sensitive to multiple antibiotics, having no hemolytic activity, and being free of aggressive putative virulence factors. Moreover, 19 stress-responsive protein genes and 8 adhesion-related genes were predicted in the FUA009 genome. Furthermore, we demonstrated that FUA009 was tolerant to acid and bile salt by determining the cell viability in a stress environment. In summary, Lactococcus garvieae FUA009, as a novel UA-producing bacterium, not only contributes to the study of the metabolic pathway of ellagic acid but is also expected to be a novel probiotic candidate.

Barriers to genetic manipulation of Enterococci: Current Approaches and Future Directions

Enterococcus faecalis and Enterococcus faecium are Gram-positive commensal gut bacteria that can also cause fatal infections. To study clinically relevant multi-drug resistant E. faecalis and E. faecium strains, methods are needed to overcome physical (thick cell wall) and enzymatic barriers that limit the transfer of foreign DNA and thus prevent facile genetic manipulation. Enzymatic barriers to DNA uptake identified in E. faecalis and E. faecium include type I, II and IV restriction modification systems and CRISPR-Cas. This review examines E. faecalis and E. faecium DNA defence systems and the methods with potential to overcome these barriers. DNA defence system bypass will allow the application of innovative genetic techniques to expedite molecular-level understanding of these important, but somewhat neglected, pathogens.

Within-host evolution of a gut pathobiont facilitates liver translocation

Gut commensal bacteria with the ability to translocate across the intestinal barrier can drive the development of diverse immune-mediated diseases^1-4. However, the key factors that dictate bacterial translocation remain unclear. Recent studies have revealed that gut microbiota strains can adapt and evolve throughout the lifetime of the host^5-9, raising the possibility that changes in individual commensal bacteria themselves over time may affect their propensity to elicit inflammatory disease. Here we show that within-host evolution of the model gut pathobiont Enterococcus gallinarum facilitates bacterial translocation and initiation of inflammation. Using a combination of in vivo experimental evolution and comparative genomics, we found that E. gallinarum diverges into independent lineages adapted to colonize either luminal or mucosal niches in the gut. Compared with ancestral and luminal E. gallinarum, mucosally adapted strains evade detection and clearance by the immune system, exhibit increased translocation to and survival within the mesenteric lymph nodes and liver, and induce increased intestinal and hepatic inflammation. Mechanistically, these changes in bacterial behaviour are associated with non-synonymous mutations or insertion-deletions in defined regulatory genes in E. gallinarum, altered microbial gene expression programs and remodelled cell wall structures. Lactobacillus reuteri also exhibited broadly similar patterns of divergent evolution and enhanced immune evasion in a monocolonization-based model of within-host evolution. Overall, these studies define within-host evolution as a critical regulator of commensal pathogenicity that provides a unique source of stochasticity in the development and progression of microbiota-driven disease.

Four decades of experience of prosthetic valve endocarditis reflect a high variety of diverse pathogens

Prosthetic valve endocarditis (PVE) remains a serious condition with a high mortality rate. Precise identification of the PVE-associated pathogen/s and their virulence is essential for successful therapy, and patient survival. The commonly described PVE-associated pathogens are staphylococci, streptococci and enterococci, with Staphylococcus aureus being the most frequently diagnosed species. Furthermore, multi-drug resistance pathogens are increasing in prevalence, and continue to pose new challenges mandating a personalized approach. Blood cultures in combination with echocardiography are the most common methods to diagnose PVE, often being the only indication, it exists. In many cases, the diagnostic strategy recommended in the clinical guidelines does not identify the precise microbial agent and to frequently, false negative blood cultures are reported. Despite the fact that blood culture findings are not always a good indicator of the actual PVE agent in the valve tissue, only a minority of re-operated prostheses are subjected to microbiological diagnostic evaluation. In this review, we focus on the diversity and the complete spectrum of PVE-associated bacterial, fungal and viral pathogens in blood, and prosthetic heart valve, their possible virulence potential, and their challenges in making a microbial diagnosis. We are curious to understand if the unacceptable high mortality of PVE is associated with the high number of negative microbial findings in connection with a possible PVE. Herein, we discuss the possibilities and limits of the diagnostic methods conventionally used and make recommendations for enhanced pathogen identification. We also show possible virulence factors of the most common PVE-associated pathogens and their clinical effects. Based on blood culture, molecular biological diagnostics, and specific valve examination, better derivations for the antibiotic therapy as well as possible preventive intervention can be established in the future.

Activity of CcpA-Regulated GH18 Family Glycosyl Hydrolases That Contributes to Nutrient Acquisition and Fitness in Enterococcus faecalis

The ability of Enterococcus faecalis to colonize host anatomical sites is dependent on its adaptive response to host conditions. Three glycosyl hydrolase gene clusters, each belonging to glycosyl hydrolase family 18 (GH18) (ef0114, ef0361, and ef2863), in E. faecalis were previously found to be upregulated under glucose-limiting conditions. The GH18 catalytic domain is present in proteins that are classified as either chitinases or β-1,4 endo-β-N-acetylglucosaminidases (ENGases) based on their β-1,4 endo-N-acetyl-β-d-glucosaminidase activity, and ENGase activity is commonly associated with cleaving N-linked glycoprotein, an abundant glycan structure on host epithelial surfaces. Here, we show that all three hydrolases are negatively regulated by the transcriptional regulator carbon catabolite protein A (CcpA). Additionally, we demonstrate that a constitutively active CcpA variant represses the expression of CcpA-regulated genes irrespective of glucose availability. Previous studies showed that the GH18 catalytic domains of EndoE (EF0114) and EfEndo18A (EF2863) were capable of deglycosylating RNase B, a model high-mannose-type glycoprotein. However, it remained uncertain which glycosidase is primarily responsible for the deglycosylation of high-mannose-type glycoproteins. In this study, we show by mutation analysis as well as a dose-dependent analysis of recombinant protein expression that EfEndo18A is primarily responsible for deglycosylating high-mannose glycoproteins and that the glycans removed by EfEndo18A support growth under nutrient-limiting conditions in vitro. In contrast, IgG is representative of a complex-type glycoprotein, and we demonstrate that the GH18 domain of EndoE is primarily responsible for the removal of this glycan decoration. Finally, our data highlight the combined contribution of glycosidases to the virulence of E. faecalis in vivo.

Metabolism of Poly-β1,4-N-Acetylglucosamine Substrates and Importation of N-Acetylglucosamine and Glucosamine by Enterococcus faecalis

The ability of Enterococcus faecalis to use a variety of carbon sources enables colonization at various anatomic sites within a mammalian host. N-Acetylglucosamine (GlcNAc) is one of the most abundant natural sugars and provides bacteria with a source of carbon and nitrogen when metabolized. N-Acetylglucosamine is also a component of bacterial peptidoglycan, further highlighting the significance of N-acetylglucosamine utilization. In this study, we show that CcpA-regulated enzymes are required for growth on the poly-β1,4-linked GlcNAc substrate, chitopentaose (β1,4-linked GlcNAc₅). We also show that EF0114 (EndoE) is required for growth on chitobiose (β1,4-linked GlcNAc₂) and that the GH20 domain of EndoE is required for the conversion of GlcNAc₂ to N-acetylglucosamine. GlcNAc is transported into the cell via two separate phosphotransferase system (PTS) complexes, either the PTS IICBA encoded by ef1516 (nagE) or the Mpt glucose/mannose permease complex (MptBACD). The Mpt PTS is also the primary glucosamine transporter. In order for N-acetylglucosamine to be utilized as a carbon source, phosphorylated N-acetylglucosamine (GlcNAc-6-P) must be deacetylated, and here, we show that this activity is mediated by EF1317 (an N-acetylglucosamine-6-phosphate deacetylase; NagA homolog), as a deletion of ef1317 is unable to grow on GlcNAc as the carbon source. Deamination of glucosamine to fructose-6-phosphate is required for entry into glycolysis, and we show that growth on glucosamine is dependent on EF0466 (a glucosamine-6-phosphate deaminase; NagB homolog). Collectively, our data highlight the chitinolytic machinery required for breaking down exogenous chitinous substrates, as well as the uptake and cytosolic enzymes needed for metabolizing N-acetylglucosamine. IMPORTANCE Enterococcus faecalis causes life-threatening health care-associated infections in part due to its intrinsic and acquired antibiotic resistance, its ability to form biofilms, and its nutrient versatility. Alternative nutrient acquisition systems are key factors that contribute to enterococcal colonization at biologically unique host anatomic sites. Although E. faecalis can metabolize an array of carbon sources, little is known of how this bacterium acquires these secondary nutrient sources in mammalian hosts. Our research identifies the glycosidase machinery required for degrading exogenous chitinous substrates into N-acetylglucosamine monomers for transport and metabolism of one of the most abundant naturally occurring sugars, N-acetylglucosamine. Disrupting the function of this N-acetylglucosamine acquisition pathway may lead to new treatments against multidrug-resistant enterococcal infections.

Real-Time Induction of Macrophage Apoptosis, Pyroptosis, and Necroptosis by Enterococcus faecalis OG1RF and Two Root Canal Isolated Strains

To investigate the effects of two Enterococcus faecalis root canal isolated strains (CA1 and CA2) and of the OG1RF strain on apoptosis, pyroptosis, and necroptosis in macrophages. The virulence factors of E. faecalis CA1 and CA2 pathogenic strains were annotated in the Virulence Factors Database (VFDB). E. faecalis CA1, CA2, and OG1RF strains were used to infect RAW264.7 macrophages (MOI, 100:1). We assessed the viability of intracellular and extracellular bacteria and of macrophages at 2, 6, and 12 h post-infection. We used a live cell imaging analysis system to obtain a dynamic curve of cell death after infection by each of the three E. faecalis strains. At 6 and 12 h post-infection, we quantified the mRNA expression levels of PANoptosis-related genes and proteins by RT-qPCR and western blot, respectively. We identified ultrastructural changes in RAW264.7 cells infected with E. faecalis OG1RF using transmission electron microscopy. We found 145 and 160 virulence factors in the CA1 and CA2 strains, respectively. The extracellular CA1 strains grew faster than the CA2 and OG1RF strains, and the amount of intracellular viable bacteria in the OG1RF group was highest at 6 and 12 h post-infection. The macrophages in the CA1 infection group were the first to reach the maximum PI-positivity in the cell death time point curve. We found the expressions of mRNA expression of caspase-1, GSDMD, caspase-3, MLKL, RIPK3, NLRP3, IL-1β and IL-18 and of proteins cleaved caspase-1, GSDMD, cleaved caspase-3 and pMIKL in the macrophages of the three infection groups to be upregulated (P<0.05). We detected ultrastructural changes of apoptosis, pyroptosis, and necroptosis in macrophages infected with E. faecalis. The three E. faecalis strains induced varying degrees of apoptosis, pyroptosis, and necroptosis that were probably associated with PANoptosis in macrophages. The E. faecalis CA1 strain exhibited faster growth and a higher real-time MOI, and it induced higher expression levels of some PANoptosis-related genes and proteins in the infected macrophages than the other strains tested.

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.

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.

Conjugation of Different Immunogenic Enterococcal Vaccine Target Antigens Leads to Extended Strain Coverage

Enterococci have emerged as important nosocomial pathogens due to their resistance to the most commonly used antibiotics. Alternative treatments or prevention options are aimed at polysaccharides and surface-related proteins that play important roles in pathogenesis. Previously, we have shown that 2 Enterococcus faecium proteins, the secreted antigen A and the peptidyl-prolyl cis-trans isomerase, as well as the Enterococcus faecalis polysaccharide diheteroglycan, are able to induce opsonic and cross-protective antibodies. Here, we evaluate the use of glycoconjugates consisting of these proteins and an enterococcal polysaccharide to develop a vaccine with broader strain coverage. Diheteroglycan was conjugated to these 2 enterococcal proteins. Rabbit sera raised against these glycoconjugates showed Immunoglobulin G titers against the corresponding conjugate, as well as against the respective protein and carbohydrate antigens. Effective opsonophagocytic killing for the 2 sera was observed against different E. faecalis and E. faecium strains. Enzyme-linked immunosorbent assays against whole bacterial cells showed immune recognition of 22 enterococcal strains by the sera. Moreover, the sera conferred protection against E. faecalis and E. faecium strains in a mouse infection model. Our results suggest that these glycoconjugates are promising candidates for vaccine formulations with a broader coverage against these nosocomial pathogens and that the evaluated proteins are potential carrier proteins.

Drivers and regulators of humoral innate immune responses to infection and cancer

The complement cascade consists of cell bound and serum proteins acting together to protect the host from pathogens, remove cancerous cells and effectively links innate and adaptive immune responses. Despite its usefulness in microbial neutralization and clearance of cancerous cells, excessive complement activation causes an immune imbalance and tissue damage in the host. Hence, a series of complement regulatory proteins present at a higher concentration in blood plasma and on cell surfaces tightly regulate the cascade. The complement cascade can be initiated by B-1 B cell production of natural antibodies. Natural antibodies arise spontaneously without any known exogenous antigenic or microbial stimulus and protect against invading pathogens, clear apoptotic cells, provide tissue homeostasis, and modulate adaptive immune functions. Natural IgM antibodies recognize microbial and cancer antigens and serve as an activator of complement mediated lysis. This review will discuss advances in complement activation and regulation in bacterial and viral infections, and cancer. We will also explore the crosstalk of natural antibodies with bacterial populations and cancer.

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.

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.

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.

Enterococcus faecalis Modulates Immune Activation and Slows Healing During Wound Infection

Enterococcus faecalis is one of the most frequently isolated bacterial species in wounds yet little is known about its pathogenic mechanisms in this setting. Here, we used a mouse wound excisional model to characterize the infection dynamics of E faecalis and show that infected wounds result in 2 different states depending on the initial inoculum. Low-dose inocula were associated with short-term, low-titer colonization whereas high-dose inocula were associated with acute bacterial replication and long-term persistence. High-dose infection and persistence were also associated with immune cell infiltration, despite suppression of some inflammatory cytokines and delayed wound healing. During high-dose infection, the multiple peptide resistance factor, which is involved in resisting immune clearance, contributes to E faecalis fitness. These results comprehensively describe a mouse model for investigating E faecalis wound infection determinants, and suggest that both immune modulation and resistance contribute to persistent, nonhealing wounds.

Potential targets for next generation antimicrobial glycoconjugate vaccines

Cell surface carbohydrates have been proven optimal targets for vaccine development. Conjugation of polysaccharides to a carrier protein triggers a T-cell-dependent immune response to the glycan moiety. Licensed glycoconjugate vaccines are produced by chemical conjugation of capsular polysaccharides to prevent meningitis caused by meningococcus, pneumococcus and Haemophilus influenzae type b. However, other classes of carbohydrates (O-antigens, exopolysaccharides, wall/teichoic acids) represent attractive targets for developing vaccines. Recent analysis from WHO/CHO underpins alarming concern toward antibiotic-resistant bacteria, such as the so called ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp.) and additional pathogens such as Clostridium difficile and Group A Streptococcus. Fungal infections are also becoming increasingly invasive for immunocompromised patients or hospitalized individuals. Other emergencies could derive from bacteria which spread during environmental calamities (Vibrio cholerae) or with potential as bioterrorism weapons (Burkholderia pseudomallei and mallei, Francisella tularensis). Vaccination could aid reducing the use of broad-spectrum antibiotics and provide protection by herd immunity also to individuals who are not vaccinated.

This review analyzes structural and functional differences of the polysaccharides exposed on the surface of emerging pathogenic bacteria, combined with medical need and technological feasibility of corresponding glycoconjugate vaccines.

Enterococcus faecalis AHG0090 is a Genetically Tractable Bacterium and Produces a Secreted Peptidic Bioactive that Suppresses Nuclear Factor Kappa B Activation in Human Gut Epithelial Cells

Enterococcus faecalis is an early coloniser of the human infant gut and contributes to the development of intestinal immunity. To better understand the functional capacity of E. faecalis, we constructed a broad host range RP4 mobilizable vector, pEHR513112, that confers chloramphenicol resistance and used a metaparental mating approach to isolate E. faecalis AHG0090 from a fecal sample collected from a healthy human infant. We demonstrated that E. faecalis AHG0090 is genetically tractable and could be manipulated using traditional molecular microbiology approaches. E. faecalis AHG0090 was comparable to the gold-standard anti-inflammatory bacterium Faecalibacterium prausnitzii A2-165 in its ability to suppress cytokine-mediated nuclear factor kappa B (NF-κB) activation in human gut-derived LS174T goblet cell like and Caco-2 enterocyte-like cell lines. E. faecalis AHG0090 and F. prausnitzii A2-165 produced secreted low molecular weight NF-κB suppressive peptidic bioactives. Both bioactives were sensitive to heat and proteinase K treatments although the E. faecalis AHG0090 bioactive was more resilient to both forms of treatment. As expected, E. faecalis AHG0090 suppressed IL-1β-induced NF-κB-p65 subunit nuclear translocation and expression of the NF-κB regulated genes IL-6, IL-8 and CXCL-10. Finally, we determined that E. faecalis AHG0090 is distantly related to other commensal strains and likely encodes niche factors that support effective colonization of the infant gut.

A Subset of Polysaccharide Capsules in the Human Symbiont Bacteroides thetaiotaomicron Promote Increased Competitive Fitness in the Mouse Gut

Capsular polysaccharides (CPSs) play multiple roles in protecting bacteria from host and environmental factors, and many commensal bacteria can produce multiple capsule types. To better understand the roles of different CPSs in competitive intestinal colonization, we individually expressed the eight different capsules of the human gut symbiont Bacteroides thetaiotaomicron. Certain CPSs were most advantageous in vivo, and increased anti-CPS immunoglobulin A correlated with increased fitness of a strain expressing one particular capsule, CPS5, suggesting that it promotes avoidance of adaptive immunity. A strain with the ability to switch between multiple capsules was more competitive than those expressing any single capsule except CPS5. After antibiotic perturbation, only the wild-type, capsule-switching strain remained in the gut, shifting to prominent expression of CPS5 only in mice with intact adaptive immunity. These data suggest that different capsules equip mutualistic gut bacteria with the ability to thrive in various niches, including those influenced by immune responses and antibiotic perturbations.

Proinflammatory MG-63 cells response infection with Enterococcus faecalis cps2 evaluated by the expression of TLR-2, IL-1β, and iNOS mRNA

OBJECTIVE

We have previously demonstrated that unencapsulated Enterococcus faecalis cps2 inhibits biofilm formation of Candida albicans, a fungus commonly found with E. faecalis in periapical lesion. In this study, we compared encapsulated and unencapsulated E. faecalis cps2 strains relationship with osteoblastic (MG-63) cells, whereas E. faecalis ATCC 29212 were used as a reference strain.

RESULTS

The binding capacity of E. faecalis to MG-63 cells as shown by each tested strain was comparable, but the unencapsulated strain was less invasive compared to the encapsulated and the reference strains. Moreover, quantitative real time-PCR (qPCR) results showed that infecting unencapsulated E. faecalis cps2 is a stronger stimulator for toll like receptor 2 (TLR2) and interleukin-1β (IL-1β) mRNAs, but not for inducible nitric oxide synthase (iNOS) mRNA in osteoblastic cells. In conclusion, the performance of unencapsulated E. faecalis cps2 when the bacterium interacts with osteoblastic cells is quite different from that of encapsulated E. faecalis cps2 and reference strains. It appears that the unencapsulated strain might contribute to the persistence of the periapical inflammatory response, depending on down-regulation of iNOS mRNA expression.

Genomic insights into the pathogenicity and environmental adaptability of Enterococcus hirae R17 isolated from pork offered for retail sale

Genetic information about Enterococcus hirae is limited, a feature that has compromised our understanding of these clinically challenging bacteria. In this study, comparative analysis was performed of E. hirae R17, a daptomycin‐resistant strain isolated from pork purchased from a retail market in Beijing, China, and three other enterococcal genomes (Enterococcus faecium DO, Enterococcus faecalis V583, and E. hirae ATCC^™9790). Some 1,412 genes were identified that represented the core genome together with an additional 139 genes that were specific to E. hirae R17. The functions of these R17 strain‐specific coding sequences relate to the COGs categories of carbohydrate transport and metabolism and transcription, a finding that suggests the carbohydrate utilization capacity of E. hirae R17 may be more extensive when compared with the other three bacterial species (spp.). Analysis of genomic islands and virulence genes highlighted the potential that horizontal gene transfer played as a contributor of variations in pathogenicity in this isolate. Drug‐resistance gene prediction and antibiotic susceptibility testing indicated E. hirae R17 was resistant to several antimicrobial compounds, including bacitracin, ciprofloxacin, daptomycin, erythromycin, and tetracycline, thereby limiting chemotherapeutic treatment options. Further, tolerance to biocides and metals may confer a phenotype that facilitates the survival and adaptation of this isolate against food preservatives, disinfectants, and antibacterial coatings. The genomic plasticity, mediated by IS elements, transposases, and tandem repeats, identified in the E. hirae R17 genome may support adaptation to new environmental niches, such as those that are found in hospitalized patients. A predicted transmissible plasmid, pRZ1, was found to carry several antimicrobial determinants, along with some predicted pathogenic genes. These data supported the previously determined phenotype confirming that the foodborne E. hirae R17 is a multidrug‐resistant pathogenic bacterium with evident genome plasticity and environmental adaptability.

Novel vitamin B12-producing Enterococcus spp. and preliminary in vitro evaluation of probiotic potentials

Vitamin B₁₂ is an essential nutrient required for crucial metabolic processes in humans. Vitamin B₁₂-producing lactic acid bacteria (LAB) have been attracting increased attentions currently because of the generally recognized as safe (GRAS) status. Most of recent studies focused on Lactobacillus, and little is known about B₁₂-producing Enterococcus. In the present study, five Enterococcus strains isolated from infant feces were identified as vitamin B₁₂ producers. Among them, Enterococcus faecium LZ86 had the highest B₁₂ production (499.8 ± 83.7 μg/L), and the B₁₂ compound from LZ86 was identified as the biological active adenosylcobalamin, using reversed phase high-performance liquid (RP-HPLC) chromatogram. We examined basic probiotic and safety properties of E. faecium LZ86 and found that it was able to survive harsh environmental conditions (hot temperature, cold temperature, ethanol and osmotic stresses), tolerate gastric acid (pH 2.0, 3 h) and bile salts (0.3%), and adhere to Caco-2 cells. We also showed that E. faecium LZ86 is devoid of transferable antibiotic resistance and potential virulence factors. Together, here we report a B₁₂-producing E. faecium strain LZ86 firstly, which has desirable probiotic properties and may serve as a good candidate for vitamin B₁₂ fortification in food industry.

Distribution of antimicrobial resistance determinants, virulence-associated factors and clustered regularly interspaced palindromic repeats loci in isolates of Enterococcus faecalis from various settings and genetic lineages

Enterococcus faecalis represents an important factor of hospital-associated infections (HAIs). The knowledge on its evolution from a commensal to an opportunistic pathogen is still limited; thus, we performed a study to characterise distribution of factors that may contribute to this adaptation. Using a collection obtained from various settings (hospitalised patients, community carriers, animals, fresh food, sewage, water), we investigated differences in antimicrobial susceptibility, distribution of antimicrobial resistance genes, virulence-associated determinants and phenotypes, and CRISPR loci in the context of the clonal relatedness of isolates. Bayesian Analysis of Population Structure revealed the presence of three major groups; two subgroups comprised almost exclusively HAI isolates, belonging to previously proposed enterococcal high-risk clonal complexes (HiRECCs) 6 and 28. Isolates of these two subgroups were significantly enriched in antimicrobial resistance genes, presumably produced a polysaccharide capsule and often carried the aggregation substance asa1; distribution of other virulence-associated genes, such as esp and cyl, formation of a biofilm and gelatinase production were more variable. Moreover, both subgroups showed a low prevalence of CRISPR-Cas 1 and 3 and presence of small CRISPR2 variants. Our study confirms the importance of HiRECCs in the population of E. faecalis and their confinement to the hospital settings.

Inhibition of Candida albicans biofilm development by unencapsulated Enterococcus faecalis cps2

Background/purpose

In the oral environment, Candida albicans interacts with many bacteria, including Enterococcus faecalis. We investigated the susceptibility of C. albicans biofilm development to the presence of unencapsulated E. faecalis cps2 in comparison with reference strains (E. faecalis ATCC 29212) or their respective spent medium (collected at 6 hours).

Material and methods

Crystal violet stain was used to measure the total biofilm mass, whereas quantitative real-time polymerase chain reaction was used to analyze the change in expression of the mRNA of hypha morphology (ALS1 and ALS3) and biofilm maturation (EFB1).

Results

At the intermediate stage, C. albicans resisted the presence of each E. faecalis strain tested and their spent medium. However, at the maturation stage, the unencapsulated strain was stronger in reducing C. albicans biofilms than the reference strain (P < 0.05). At this maturation stage, the transcription levels of each gene tested decreased in the presence of either E. faecalis strains or their respective spent medium. The unencapsulated strain was more pronounced in reducing ALS1/ALS3 expression, whereas the respective spent medium had a similar capability to restrict the expression of EFB1.

Conclusion

This study showed, the unencapsulated strain is more effective in inhibiting C. albicans biofilm development compared with the reference strains. In contrast, the secreted molecules produced by each strain tested are necessary in controlling the growths of C. albicans biofilm.

Antimicrobial activity and the presence of virulence factors and bacteriocin structural genes in Enterococcus faecium CM33 isolated from ewe colostrum

Screening of lactic acid bacteria (LAB) isolated from ewe colostrum led to the identification and isolation of Enterococcus faecium CM33 with interesting features like high survival rates under acidic or bile salts condition, high tolerance for the simulated gastrointestinal condition, and high adhesive potential to Caco-2 cells. According the inhibition of pathogen adhesion test results, this strain can reduce more than 50% adhesion capacity of Escherichia coli, Shigella flexneri, Klebsiella pneumoniae, Listeria monocytogenes, and Staphylococcus aureus to Caco-2 cells. Based on the antibiotic sensitivity test findings, E. faecium CM33 was susceptible to gentamycin, vancomycin, erythromycin, ampicillin, penicillin, tetracycline, and rifampicin, but resistant to chloramphenicol, clindamycin, and kanamycin. Upon assessment of the virulence determinants for E. faecium CM33, this strain was negative for all tested virulence genes. Furthermore, the genome of this strain was evaluated for the incidence of the known enterocin genes by specific PCR amplification and discovered the genes encoding enterocins A, 31, X, and Q. Based on this study findings, the strain E. faecium CM33 can be considered as a valuable nutraceutical and can be introduced as a new potential probiotic.

Overexpression of Enterococcus faecalis elr operon protects from phagocytosis

Background

Mechanisms underlying the transition from commensalism to virulence in Enterococcus faecalis are not fully understood. We previously identified the enterococcal leucine-rich protein A (ElrA) as a virulence factor of E. faecalis. The elrA gene is part of an operon that comprises four other ORFs encoding putative surface proteins of unknown function.

Results

In this work, we compared the susceptibility to phagocytosis of three E. faecalis strains, including a wild-type (WT), a ΔelrA strain, and a strain overexpressing the whole elr operon in order to understand the role of this operon in E. faecalis virulence. While both WT and ΔelrA strains were efficiently phagocytized by RAW 264.7 mouse macrophages, the elr operon-overexpressing strain showed a decreased capability to be internalized by the phagocytic cells. Consistently, the strain overexpressing elr operon was less adherent to macrophages than the WT strain, suggesting that overexpression of the elr operon could confer E. faecalis with additional anti-adhesion properties. In addition, increased virulence of the elr operon-overexpressing strain was shown in a mouse peritonitis model.

Conclusions

Altogether, our results indicate that overexpression of the elr operon facilitates the E. faecalis escape from host immune defenses.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0448-y) contains supplementary material, which is available to authorized users.

The fsr Quorum-Sensing System and Cognate Gelatinase Orchestrate the Expression and Processing of Proprotein EF_1097 into the Mature Antimicrobial Peptide Enterocin O16

A novel antimicrobial peptide designated enterocin O16 was purified from Enterococcus faecalis. Mass spectrometry showed a monoisotopic mass of 7,231 Da, and N-terminal Edman degradation identified a 29-amino-acid sequence corresponding to residues 90 to 119 of the EF_1097 protein. Bioinformatic analysis showed that enterocin O16 is composed of the 68 most C-terminal residues of the EF_1097 protein. Introduction of an in-frame isogenic deletion in the ef1097 gene abolished the production of enterocin O16. Enterocin O16 has a narrow inhibitory spectrum, as it inhibits mostly lactobacilli. Apparently, E. faecalis is intrinsically resistant to the antimicrobial peptide, as no immunity connected to the production of enterocin O16 could be identified. ef1097 has previously been identified as one of three loci regulated by the fsr quorum-sensing system. The introduction of a nonsense mutation into fsrB consistently impaired enterocin O16 production, but externally added gelatinase biosynthesis-activating pheromone restored the antimicrobial activity. Functional genetic analysis showed that the EF_1097 proprotein is processed extracellularly into enterocin O16 by the metalloprotease GelE. Thus, it is evident that the fsr quorum-sensing system constitutes the regulatory unit that controls the expression of the EF_1097 precursor protein and the protease GelE and that the latter is required for the formation of enterocin O16. On the basis of these results, this study identified antibacterial antagonism as a novel aspect related to the function of fsr and provides a rationale for why ef1097 is part of the fsr regulon.

IMPORTANCE

The fsr quorum-sensing system modulates important physiological functions in E. faecalis via the activity of GelE. The present study presents a new facet of fsr signaling. The system controls the expression of three primary target operons (fsrABCD, gelE-sprE, and ef1097-ef1097b). We demonstrate that the concerted expression of these operons constitutes the elements necessary for the production of a bacteriocin-type peptide and that antimicrobial antagonism is an intrinsic function of fsr. The bacteriocin enterocin O16 consists of the 68 most C-terminal residues of the EF_1097 secreted proprotein. The GelE protease processes the EF_1097 proprotein into enterocin O16. In this manner, fsr signaling enables E. faecalis populations to express antimicrobial activity in a cell density-dependent manner.

Envelope Structures of Gram-Positive Bacteria

Gram-positive organisms, including the pathogens Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecalis, have dynamic cell envelopes that mediate interactions with the environment and serve as the first line of defense against toxic molecules. Major components of the cell envelope include peptidoglycan (PG), which is a well-established target for antibiotics, teichoic acids (TAs), capsular polysaccharides (CPS), surface proteins, and phospholipids. These components can undergo modification to promote pathogenesis, decrease susceptibility to antibiotics and host immune defenses, and enhance survival in hostile environments. This chapter will cover the structure, biosynthesis, and important functions of major cell envelope components in gram-positive bacteria. Possible targets for new antimicrobials will be noted.

Virulence Plasmids of Nonsporulating Gram-Positive Pathogens

Gram-positive bacteria are leading causes of many types of human infection, including pneumonia, skin and nasopharyngeal infections, as well as urinary tract and surgical wound infections among hospitalized patients. These infections have become particularly problematic because many of the species causing them have become highly resistant to antibiotics. The role of mobile genetic elements, such as plasmids, in the dissemination of antibiotic resistance among Gram-positive bacteria has been well studied; less well understood is the role of mobile elements in the evolution and spread of virulence traits among these pathogens. While these organisms are leading agents of infection, they are also prominent members of the human commensal ecology. It appears that these bacteria are able to take advantage of the intimate association between host and commensal, via virulence traits that exacerbate infection and cause disease. However, evolution into an obligate pathogen has not occurred, presumably because it would lead to rejection of pathogenic organisms from the host ecology. Instead, in organisms that exist as both commensal and pathogen, selection has favored the development of mechanisms for variability. As a result, many virulence traits are localized on mobile genetic elements, such as virulence plasmids and pathogenicity islands. Virulence traits may occur within a minority of isolates of a given species, but these minority populations have nonetheless emerged as a leading problem in infectious disease. This chapter reviews virulence plasmids in nonsporulating Gram-positive bacteria, and examines their contribution to disease pathogenesis.

MRP8/14 induces autophagy to eliminate intracellular Mycobacterium bovis BCG

OBJECTIVE

To explore the role of myeloid-related protein 8/14 in mycobacterial infection.

METHODS

The mRNA and protein expression levels of MRP8 or MRP14 were measured by real-time PCR and flow cytometry, respectively. Role of MRP8/14 was tested by overexpression or RNA interference assays. Flow cytometry and colony forming unit were used to test the phagocytosis and the survival of intracellular Mycobacterium bovis BCG (BCG), respectively. Autophagy mediated by MRP8/14 was detected by Western blot and immunofluorescence. The colocalization of BCG phagosomes with autophagosomes or lysosomes was by detected by confocal microscopy. ROS production was detected by flow cytometry.

RESULTS

MRP8/14 expressions were up-regulated in human monocytic THP1 cells and primary macrophages after mycobacterial challenge. Silencing of MRP8/14 suppressed bacterial killing, but had no influence on the phagocytosis of BCG. Importantly, silencing MRP8/14 decreased autophagy and BCG phagosome maturation in THP1-derived macrophages, thereby increasing the BCG survival. Additionally, we demonstrated that MRP8/14 promoted autophagy in a ROS-dependent manner.

CONCLUSIONS

The present study revealed a novel role of MRP8/14 in the autophagy-mediated elimination of intracellular BCG by promoting ROS generation, which may provide a promising therapeutic target for tuberculosis and other intracellular bacterial infectious diseases.

Depolymerase improves gentamicin efficacy during Klebsiella pneumoniae induced murine infection

Background

Presence of capsule enhances the virulence of bacteria that cause pneumonia, meningitis, cystic fibrosis, dental caries, periodontitis. Capsule is an important virulence factor for Klebsiella pneumoniae and infections due to this pathogen have been associated with high mortality rates. In the present study, use of an Aeromonas punctata derived capsule depolymerase against K. pneumoniae, to reinstate the efficacy of gentamicin during pneumonia and septicemia was investigated.

Methods

Depolymerase was administered in mice intraperitoneally (50 μg) alone as well in combination with gentamicin (1.5 mg/kg), 24 h post infection during acute lung infection and 6 h later during septicemia. Bacterial load, neutrophil infiltration and cytokine levels were estimated. The immunogenicity of protein was also studied.

Results

In comparison to groups treated with gentamicin alone, combination treatment with depolymerase and gentamicin significantly reduced (P < 0.01) bacterial titer in the lungs, liver, kidney, spleen and blood of experimental animals. Highly significant reduction in neutrophil infiltration and levels of pro-inflammatory and anti-inflammatory cytokines was also observed. This indicated an efficient capsule removal by the enzyme, that improved gentamicin efficacy in vivo. Although the enzyme was found to be immunogenic, but no significant reduction in treatment efficacy was observed in the preimmunized as well as naïve mice. In addition, as confirmed through flow cytometry, the hyperimmune sera raised against the enzyme did not neutralize its activity.

Conclusion

The results confirm that administration of enzyme ‘depolymerase’ along with gentamicin not only checked the virulence of K. pneumoniae in vivo but it also increased its susceptibility to gentamicin at a lower concentration. Such a strategy would help to avoid exposure to higher concentration of gentamicin. Moreover, since this decapsulating protein does not possess a lytic activity therefore there would be no chances of development of bacterial resistance against it. Therefore, it should be studied further for its successful inclusion in our prophylactic/therapeutic regimes.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2334-14-456) contains supplementary material, which is available to authorized users.

The majority of a collection of U.S. endocarditis Enterococcus faecalis isolates obtained from 1974 to 2004 lack capsular genes and belong to diverse, non-hospital-associated lineages

Eighty-one endocarditis-derived Enterococcus faecalis isolates that were collected from individual patients in the United States between 1974 and 2004 were sequence typed and analyzed for the presence of various genes, including some previously associated with virulence. Overall, using our previously described trilocus sequence typing (TLST), 44 different sequence types (STs) were found within this collection; 26 isolates were singletons (a unique TLST sequence type [ST(T)]), some ST(T)s contained multiple isolates (up to 6 isolates), and 16% of the isolates (13 isolates) could be grouped by additional sequence typing into clonal cluster 21 (CC21). Of note, only four isolates (7%) of the 56 whose multilocus sequence types were determined were found to belong to one of the previously described hospital-associated clonal clusters CC2 and CC9, and only 15% and 37% of all isolates had high-level resistance to gentamicin and streptomycin, respectively, including 10% that were resistant to both. We also found that 64% of the isolates lacked the genes for production of capsule polysaccharide, which has been proposed to enhance the pathogenic potential of the hospital-associated clonal clusters. In summary, while our collection is not a random sample of cases of E. faecalis endocarditis, these results indicate that nonencapsulated strains belonging to non-hospital-associated lineages were predominant among endocarditis E. faecalis isolates recovered during this time period.

Boosted rat natural xenoantibodies cross-react with Enterococcus faecalis by targeting melibiose and L-rhamnose

Natural antibodies include a subset described as xenoantibodies considered to be directed at microorganisms and also cross-react with antigens of unrelated species. In this study, we generated T-cell-independent (TI) and T-cell-dependent (TD) xenoantibodies in Lewis rats with hamster and pig blood injections. TI anti-hamster and anti-pig IgM and IgG xenoantibodies cross-reacted with Enterococcus faecalis but not with Escherichia coli isolated from the blood of Lewis rats after cecal ligation and puncture (CLP). TI anti-pig IgM xenoantibodies also showed some reactivity with two human blood isolates of E. faecalis. In contrast, TD xenoantibodies did not show any reactivity with rat or human bacteria. TI and TD anti-hamster and anti-pig IgM and IgG xenoantibodies showed cross-reactivity with lymphocytes and endothelial cells from species distinct to that used for immunization. Glycan array analysis and inhibition assays identified antibodies against melibiose and L-rhamnose as mediators of anti-hamster and anti-porcine xenoantibody cross-reactivity with E. faecalis. A rise in TI anti-hamster and anti-pig xenoantibodies was accompanied by decreased survival of Lewis rats in a low-severity sepsis model of CLP. Therefore, TI xenoantibodies in the rat include anti-carbohydrate antibodies reactive to bacteria of endogenous flora. Enhancement of these antibodies may result in more severe infectious diseases caused by these microorganisms.

Bacterial interference with canonical NFκB signalling

The human body is constantly challenged by a variety of commensal and pathogenic micro-organisms that trigger the immune system. Central in the first line of defence is the pattern-recognition receptor (PRR)-induced stimulation of the NFκB pathway, leading to NFκB activation. The subsequent production of pro-inflammatory cytokines and/or antimicrobial peptides results in recruitment of professional phagocytes and bacterial clearance. To overcome this, bacteria have developed mechanisms for targeted interference in every single step in the PRR–NFκB pathway to dampen host inflammatory responses. This review aims to briefly overview the PRR–NFκB pathway in relation to the immune response and give examples of the diverse bacterial evasion mechanisms including changes in the bacterial surface, decoy production and injection of effector molecules. Targeted regulation of inflammatory responses is needed and bacterial molecules developed for immune evasion could provide future anti-inflammatory agents.

Drosophila host model reveals new enterococcus faecalis quorum-sensing associated virulence factors

Enterococcus faecalis V583 is a vancomycin-resistant clinical isolate which belongs to the hospital-adapted clade, CC2. This strain harbours several factors that have been associated with virulence, including the fsr quorum-sensing regulatory system that is known to control the expression of GelE and SprE proteases. To discriminate between genes directly regulated by Fsr, and those indirectly regulated as the result of protease expression or activity, we compared gene expression in isogenic mutants of V583 variously defective in either Fsr quorum sensing or protease expression. Quorum sensing was artificially induced by addition of the quorum signal, GBAP, exogenously in a controlled manner. The Fsr regulon was found to be restricted to five genes, gelE, sprE, ef1097, ef1351 and ef1352. Twelve additional genes were found to be dependent on the presence of GBAP-induced proteases. Induction of GelE and SprE by GBAP via Fsr resulted in accumulation of mRNA encoding lrgAB, and this induction was found to be lytRS dependent. Drosophila infection was used to discern varying levels of toxicity stemming from mutations in the fsr quorum regulatory system and the genes that it regulates, highlighting the contribution of LrgAB and bacteriocin EF1097 to infection toxicity. A contribution of SprE to infection toxicity was also detected. This work brought to light new players in E. faecalis success as a pathogen and paves the way for future studies on host tolerance mechanisms to infections caused by this important nosocomial pathogen.

Structure, function, and biology of the Enterococcus faecalis cytolysin

Enterococcus faecalis is a Gram-positive commensal member of the gut microbiota of a wide range of organisms. With the advent of antibiotic therapy, it has emerged as a multidrug resistant, hospital-acquired pathogen. Highly virulent strains of E. faecalis express a pore-forming exotoxin, called cytolysin, which lyses both bacterial and eukaryotic cells in response to quorum signals. Originally described in the 1930s, the cytolysin is a member of a large class of lanthionine-containing bacteriocins produced by Gram-positive bacteria. While the cytolysin shares some core features with other lantibiotics, it possesses unique characteristics as well. The current understanding of cytolysin biosynthesis, structure/function relationships, and contribution to the biology of E. faecalis are reviewed, and opportunities for using emerging technologies to advance this understanding are discussed.

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

The complement system is part of our first line of defense against invading pathogens. The strategies used by Enterococcus faecalis to evade recognition by human complement are incompletely understood. In this study, we identified an insertional mutant of the wall teichoic acid (WTA) synthesis gene tagB in E. faecalis V583 that exhibited an increased susceptibility to complement-mediated killing by neutrophils. Further analysis revealed that increased killing of the mutant was due to a higher rate of phagocytosis by neutrophils, which correlated with higher C3b deposition on the bacterial surface. Our studies indicated that complement activation via the lectin pathway was much stronger on the tagB mutant compared with wild type. In concordance, we found an increased binding of the key lectin pathway components mannose-binding lectin and mannose-binding lectin-associated serine protease-2 (MASP-2) on the mutant. To understand the mechanism of lectin pathway inhibition by E. faecalis, we purified and characterized cell wall carbohydrates of E. faecalis wild type and V583ΔtagB. NMR analysis revealed that the mutant strain lacked two WTAs with a repeating unit of →6)[α-l-Rhap-(1→3)]β-D-GalpNAc-(1→5)-Rbo-1-P and →6) β-D-Glcp-(1→3) [α-D-Glcp-(1→4)]-β-D-GalpNAc-(1→5)-Rbo-1-P→, respectively (Rbo, ribitol). In addition, compositional changes in the enterococcal rhamnopolysaccharide were noticed. Our study indicates that in E. faecalis, modification of peptidoglycan by secondary cell wall polymers is critical to evade recognition by the complement system.