Inducible expression of Enterococcus faecalis aggregation substance surface protein facilitates bacterial internalization by cultured enterocytes

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.

Prevalence and virulence factors of haemolytic Enterococcus faecalis isolated from root filled teeth associated with periradicular lesions: A laboratory investigation in Thailand

AIM

Previous endodontic research has provided limited understanding of the prevalence and roles of haemolytic and non-haemolytic Enterococcus faecalis strains in root filled teeth. This study aimed to determine the prevalence of these strains in root filled teeth with periradicular lesions and investigate their associated virulence factors.

METHODOLOGY

A total of 36 root canal samples were collected from 36 subjects. The prevalence of E. faecalis was determined using culture and PCR methods. Antibiotic susceptibility of haemolytic and non-haemolytic E. faecalis strains was assessed using the broth dilution assay. The cytokine stimulation in periodontal ligament (PDL) cells and neutrophil migration were evaluated using real-time PCR and migration assay, respectively. Cell invasion ability of the strains was assessed using a cell culture model. Additionally, the virulence gene expression of the haemolytic and non-haemolytic strains was investigated using real-time PCR. The Mann-Whitney U and Spearman's ρ tests were used to examine the significant difference between the two strains and to analyse the correlation between phenotype and gene expression, respectively.

RESULTS

Enterococcus faecalis was detected in 33.3% and 88.9% of samples by culture and real-time PCR, respectively. Haemolytic strains were found in 36.4% of subjects. Non-haemolytic strains exhibited susceptibility to erythromycin and varying susceptibility to tetracycline, while all haemolytic strains were resistant to both antibiotics. Haemolytic strains significantly upregulated the expression of IL-8, OPG and RANKL in PDL cells (p < .05). Notably, the fold increases in these genes were higher: IL-8 (556.1 ± 82.9 vs. 249.6 ± 81.8), OPG (2.2 ± 0.5 vs. 1.3 ± 0.2) and RANKL (1.8 ± 0.3 vs. 1.2 ± 0.1). Furthermore, haemolytic strains had a greater effect on neutrophil migration (68.7 ± 15.2% vs. 46.9 ± 11.4%) and demonstrated a higher level of internalization into oral keratinocyte cells (68.6 ± 0.4% vs. 33.8 ± 0.5%) (p < .05). They also showed enhanced expression of virulence genes associated with haemolysin, surface proteins, collagen-binding and aggregation substances. Gelatinase activity was only detectable in non-haemolytic strains.

CONCLUSIONS

This study revealed that haemolytic strains E. faecalis possessed enhanced abilities in host invasion and a higher abundance of virulence factors, suggesting their potential contribution to more severe disease manifestations.

Both biofilm cytotoxicity and monocytes' adhesion may be used as estimators of enterococcal virulence

Our study aimed to identify markers of enterococci's virulence potential by evaluating the properties of strains of different sites of isolation. Enterococcal strains were isolated as commensals from faeces and as invasive strains from the urine and blood of patients from the University Clinical Centre, Gdańsk, Poland. Changes in monocytes' susceptibility to the cytotoxic activity of isolates of different origins and their adherence to biofilm were evaluated using a flow cytometer. The bacterial protein profile was estimated by matrix assisted laser desorption ionization-time of flight mass spectrometer. The cytotoxicity of biofilm and monocytes' adherence to it were the most accurate factors in predicting the prevalence of the strain in the specific niche. Additionally, a bacterial protein with mass-to-charge ratio (m/z) 5000 was found to be responsible for the increased bacterial cytotoxicity, while monocytes' decreased adherence to biofilm was linked with the presence of proteins either with m/z 3330 or 2435. The results illustrate that monocytes' reaction when exposed to the bacterial biofilm can be used as an estimator of pathogens' virulence potential. The observed differences in monocytes' response are explainable by the bacterial proteins' profile. Additionally, the results indicate that the features of both bacteria and monocytes impact the outcome of the infection.

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.

Let Me Upgrade You: Impact of Mobile Genetic Elements on Enterococcal Adaptation and Evolution

Enterococci are Gram-positive bacteria that have evolved to thrive as both commensals and pathogens, largely due to their accumulation of mobile genetic elements via horizontal gene transfer (HGT). Common agents of HGT include plasmids, transposable elements, and temperate bacteriophages. These vehicles of HGT have facilitated the evolution of the enterococci, specifically Enterococcus faecalis and Enterococcus faecium, into multidrug-resistant hospital-acquired pathogens. On the other hand, commensal strains of Enterococcus harbor CRISPR-Cas systems that prevent the acquisition of foreign DNA, restricting the accumulation of mobile genetic elements. In this review, we discuss enterococcal mobile genetic elements by highlighting their contributions to bacterial fitness, examine the impact of CRISPR-Cas on their acquisition, and identify key areas of research that can improve our understanding of enterococcal evolution and ecology.

Artificial Sweeteners Negatively Regulate Pathogenic Characteristics of Two Model Gut Bacteria, E. coli and E. faecalis

Artificial sweeteners (AS) are synthetic sugar substitutes that are commonly consumed in the diet. Recent studies have indicated considerable health risks which links the consumption of AS with metabolic derangements and gut microbiota perturbations. Despite these studies, there is still limited data on how AS impacts the commensal microbiota to cause pathogenicity. The present study sought to investigate the role of commonly consumed AS on gut bacterial pathogenicity and gut epithelium-microbiota interactions, using models of microbiota (Escherichia coli NCTC10418 and Enterococcus faecalis ATCC19433) and the intestinal epithelium (Caco-2 cells). Model gut bacteria were exposed to different concentrations of the AS saccharin, sucralose, and aspartame, and their pathogenicity and changes in interactions with Caco-2 cells were measured using in vitro studies. Findings show that sweeteners differentially increase the ability of bacteria to form a biofilm. Co-culture with human intestinal epithelial cells shows an increase in the ability of model gut bacteria to adhere to, invade and kill the host epithelium. The pan-sweet taste inhibitor, zinc sulphate, effectively blocked these negative impacts. Since AS consumption in the diet continues to increase, understanding how this food additive affects gut microbiota and how these damaging effects can be ameliorated is vital.

Polymer Adhesin Domains in Gram-Positive Cell Surface Proteins

Surface proteins in Gram-positive bacteria are often involved in biofilm formation, host-cell interactions, and surface attachment. Here we review a protein module found in surface proteins that are often encoded on various mobile genetic elements like conjugative plasmids. This module binds to different types of polymers like DNA, lipoteichoic acid and glucans, and is here termed polymer adhesin domain. We analyze all proteins that contain a polymer adhesin domain and classify the proteins into distinct classes based on phylogenetic and protein domain analysis. Protein function and ligand binding show class specificity, information that will be useful in determining the function of the large number of so far uncharacterized proteins containing a polymer adhesin domain.

Risks associated with enterococci as probiotics

Probiotics are naturally occurring microorganisms that confer health benefits by altering host commensal microbiota, modulating immunity, enhancing intestinal barrier function, or altering pain perception. Enterococci are human and animal intestinal commensals that are used as probiotics and in food production. These microorganisms, however, express many virulence traits including cytolysin, proteases, aggregation substance, capsular polysaccharide, enterococcal surface protein, biofilm formation, extracellular superoxide, intestinal translocation, and resistance to innate immunity that can lead to serious hospital-acquired infections. In addition, enterococci are facile in acquiring antibiotic resistance genes to many clinically important antibiotics encoded on a wide variety of conjugative plasmids, transposons, and bacteriophages. The pathogenicity and disease burden caused by enterococci render them poor choices as probiotics. No large, randomized, placebo-controlled clinical trials have demonstrated the safety and efficacy of any enterococcal probiotic. As a result, no enterococcal probiotic has been approved by the United States Food and Drug Administration for the treatment, cure, or amelioration of human disease. In 2007, the European Food Safety Authority concluded that enterococci do not meet the standard for "Qualified Presumption of Safety". Enterococcal strains used or proposed for use as probiotics should be carefully screened for efficacy and safety.

Bacterial Cell-Cell Communication in the Host via RRNPP Peptide-Binding Regulators

Human microbiomes are composed of complex and dense bacterial consortia. In these environments, bacteria are able to react quickly to change by coordinating their gene expression at the population level via small signaling molecules. In Gram-positive bacteria, cell–cell communication is mostly mediated by peptides that are released into the extracellular environment. Cell–cell communication based on these peptides is especially widespread in the group Firmicutes, in which they regulate a wide array of biological processes, including functions related to host–microbe interactions. Among the different agents of communication, the RRNPP family of cytoplasmic transcriptional regulators, together with their cognate re-internalized signaling peptides, represents a group of emerging importance. RRNPP members that have been studied so far are found mainly in species of bacilli, streptococci, and enterococci. These bacteria are characterized as both human commensal and pathogenic, and share different niches in the human body with other microorganisms. The goal of this mini-review is to present the current state of research on the biological relevance of RRNPP mechanisms in the context of the host, highlighting their specific roles in commensalism or virulence.

Orthopedic implant infections: Incompetence of Staphylococcus epidermidis, Staphylococcus lugdunensis, and Enterococcus faecalis to invade osteoblasts

Septic failure is still the major complication of prosthetic implants. Entering host cells, bacteria hide from host immune defenses, shelter from extracellular antibiotics, and cause chronic infection. Staphylococcus aureus, the leading etiologic agent of orthopedic implant infections, is able to enter bone cells and induce osteoblast apoptosis, osteoclast recruitment, and highly destructive osteomyelitis. Staphylococcus epidermidis, Staphylococcus lugdunensis, and Enterococcus faecalis are opportunistic pathogens causative of implant-related infections. This study investigated the ability to internalize into osteoblastic MG63 cells of 22 S. epidermidis, 9 S. lugdunensis, and 21 E. faecalis clinical isolates from orthopedic implant infections. Isolates were categorized in clusters by ribotyping. Internalization assay was carried out by means of a microtiter plate-based method. S. epidermidis, S. lugdunensis, and E. faecalis strains turned out incompetent to enter osteoblasts, exhibiting negligible internalization into MG63 cells, nearly three orders of magnitude lower than that of S. aureus. Osteoblast invasion does not appear as a pathogenetic mechanism utilized by S. epidermidis, S. lugdunensis, or E. faecalis for infecting orthopedic implants. Moreover, it can be inferred that intracellularly active antimicrobials should not be necessary against implant infections caused by the three bacterial species. Finally, implications with the uptake of biomaterial microparticles by nonphagocytic cells are enlightened. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 788-801, 2016.

Friend turned foe: evolution of enterococcal virulence and antibiotic resistance

The enterococci are an ancient genus that evolved along with the tree of life. These intrinsically rugged bacteria are highly adapted members of the intestinal consortia of a range of hosts that spans the animal kingdom. Enterococci are also leading opportunistic hospital pathogens, causing infections that are often resistant to treatment with most antibiotics. Despite the importance of enterococci as hospital pathogens, the vast majority live outside of humans, and nearly all of their evolutionary history took place before the appearance of modern humans. Because hospital infections represent evolutionary end points, traits that exacerbate human infection are unlikely to have evolved for that purpose. However, clusters of traits have converged in specific lineages that are well adapted to colonize the antibiotic-perturbed gastrointestinal tracts of patients and that thrive in the hospital environment. Here we discuss these traits in an evolutionary context, as well as how comparative genomics is providing new insights into the evolution of the enterococci.

Identification of a proton-chloride antiporter (EriC) by Himar1 transposon mutagenesis in Lactobacillus reuteri and its role in histamine production

The gut microbiome may modulate intestinal immunity by luminal conversion of dietary amino acids to biologically active signals. The model probiotic organism Lactobacillus reuteri ATCC PTA 6475 is indigenous to the human microbiome, and converts the amino acid L-histidine to the biogenic amine, histamine. Histamine suppresses tumor necrosis factor (TNF) production by human myeloid cells and is a product of L-histidine decarboxylation, which is a proton-facilitated reaction. A transposon mutagenesis strategy was developed based on a single-plasmid nisin-inducible Himar1 transposase/transposon delivery system for L. reuteri. A highly conserved proton-chloride antiporter gene (eriC), a gene widely present in the gut microbiome was discovered by Himar1 transposon (Tn)-mutagenesis presented in this study. Genetic inactivation of eriC by transposon insertion and genetic recombineering resulted in reduced ability of L. reuteri to inhibit TNF production by activated human myeloid cells, diminished histamine production by the bacteria and downregulated expression of histidine decarboxylase cluster genes compared to those of WT 6475. EriC belongs to a large family of ion transporters that includes chloride channels and proton-chloride antiporters and may facilitate the availability of protons for the decarboxylation reaction, resulting in histamine production by L. reuteri. This report leverages the tools of bacterial genetics for probiotic gene discovery. The findings highlight the widely conserved nature of ion transporters in bacteria and how ion transporters are coupled with amino acid decarboxylation and contribute to microbiome-mediated immunomodulation.

Enterococcus faecalis internalization in human umbilical vein endothelial cells (HUVEC)

Initial Enterococcus faecalis-endothelial cell molecular interactions which lead to enterococci associating in the host endothelial tissue, colonizing it and proliferating there can be assessed using in vitro models. Cultured human umbilical vein endothelial cells (HUVEC) have been used to study other Gram-positive bacteria-cell interactions; however, few studies have been aimed at establishing the relationship of E. faecalis with endothelial cells. The aggregation substance (AS) family of adhesins represents an E. faecalis virulence factor which has been implicated in endocarditis severity and bacterial persistence. The Asc10 protein (a member of this family) promotes bacterium-bacterium aggregation and bacterium-host cell binding. Evaluating Asc10 role in bacterial internalization by cultured enterocytes has shown that this adhesin facilitates E. faecalis endocytosis by HT-29 cells. A few eukaryotic cell structural components, such as cytoskeletal proteins, have been involved in E. faecalis entry into cell-lines; it is thus relevant to determine whether Asc10, as well as microtubules and actin microfilaments, play a role in E. faecalis internalization by cultured endothelial cells. The role of Asc10 and cytoskeleton proteins in E. faecalis ability to enter HUVEC was assessed in the present study, as well as cell apoptosis induction by enterococcal internalization by HUVEC; the data indicated increased cell apoptosis and that cytoskeleton components were partially involved in E. faecalis entry to endothelial cells, thereby suggesting that E. faecalis Asc10 protein would not be a critical factor for bacterial entry to cultured HUVEC.

Cloning and expression of a novel lactococcal aggregation factor from Lactococcus lactis subsp. lactis BGKP1

BACKGROUND

Aggregation may play a main role in the adhesion of bacteria to the gastrointestinal epithelium and their colonization ability, as well as in probiotic effects through co-aggregation with intestinal pathogens and their subsequent removal. The aggregation phenomenon in lactococci is directly associated with the sex factor and lactose plasmid co-integration event or duplication of the cell wall spanning (CWS) domain of PrtP proteinase.

RESULTS

Lactococcus lactis subsp. lactis BGKP1 was isolated from artisanal semi-hard homemade cheese and selected due to its strong auto-aggregation phenotype. Subsequently, non-aggregating derivative (Agg-) of BGKP1, designated as BGKP1-20, was isolated, too. Comparative analysis of cell surface proteins of BGKP1 and derivative BGKP1-20 revealed a protein of approximately 200 kDa only in the parental strain BGKP1. The gene involved in aggregation (aggL) was mapped on plasmid pKP1 (16.2 kb), cloned and expressed in homologous and heterologous lactococci and enterococci. This novel lactococcal aggregation protein was shown to be sufficient for cell aggregation in all tested hosts. In addition to the aggL gene, six more ORFs involved in replication (repB and repX), restriction and modification (hsdS), transposition (tnp) and possible interaction with mucin (mbpL) were also located on plasmid pKP1.

CONCLUSION

AggL is a new protein belonging to the collagen-binding superfamily of proteins and is sufficient for cell aggregation in lactococci.

Enterococci as probiotics and their implications in food safety

Enterococci belong to the lactic acid bacteria (LAB) and they are of importance in foods due to their involvement in food spoilage and fermentations, as well as their utilisation as probiotics in humans and slaughter animals. However, they are also important nosocomial pathogens that cause bacteraemia, endocarditis and other infections. Some strains are resistant to many antibiotics and possess virulence factors such as adhesins, invasins, pili and haemolysin. The role of enterococci in disease has raised questions on their safety for use in foods or as probiotics. Studies on the incidence of virulence traits among enterococcal strains isolated from food showed that some can harbour virulence traits, but it is also thought that virulence is not the result of the presence of specific virulence determinants alone, but is rather a more intricate process. Specific genetic lineages of hospital-adapted strains have emerged, such as E. faecium clonal complex (CC) 17 and E. faecalis CC2, CC9, CC28 and CC40, which are high risk enterococcal clonal complexes. These are characterised by the presence of antibiotic resistance determinants and/or virulence factors, often located on pathogenicity islands or plasmids. Mobile genetic elements thus are considered to play a major role in the establishment of problematic lineages. Although enterococci occur in high numbers in certain types of fermented cheeses and sausages, they are not deliberately added as starter cultures. Some E. faecium and E. faecalis strains are used as probiotics and are ingested in high numbers, generally in the form of pharmaceutical preparations. Such probiotics are administered to treat diarrhoea, antibiotic-associated diarrhoea or irritable bowel syndrome, to lower cholesterol levels or to improve host immunity. In animals, enterococcal probiotics are mainly used to treat or prevent diarrhoea, for immune stimulation or to improve growth. From a food microbiological point of view, the safety of the bacteria used as probiotics must be assured, and data on the major strains in use so far indicate that they are safe. The advantage of use of probiotics in slaughter animals, from a food microbiological point of view, lies in the reduction of zoonotic pathogens in the gastrointestinal tract of animals which prevents the transmission of these pathogens via food. The use of enterococcal probiotics should, in view of the development of problematic lineages and the potential for gene transfer in the gastrointestinal tract of both humans and animals, be carefully monitored, and the advantages of using these and new strains should be considered in a well contemplated risk/benefit analysis.

Pathogenesis and immunity in enterococcal infections

Enterococcus faecalis and Enterococcus faecium have emerged as multi-resistant nosocomial pathogens in immunocompromised and critically ill patients. Multi-resistant strains have acquired virulence genes resulting in hospital-adapted clones. The following review summarizes several proteins and carbohydrate- or glycoconjugates that have been identified as putative virulence factors involved in the pathogenesis of enterococcal infections and may be used as targets for alternative therapies. Several studies describing the host immune response against enterococci are also summarized.

LPxTG surface proteins of enterococci

Enterococci have become an important cause of nosocomial infections since the late 1980s. Several surface proteins have been implicated in contributing to infections caused by Enterococcus faecalis and Enterococcus faecium. Understanding the in vivo function of enterococcal surface proteins, particularly their role in directing interactions with the host during infection, is essential to explain the success of enterococci as nosocomial pathogens. Here we review current knowledge of enterococcal LPxTG surface proteins, including aggregation substance, enterococcal surface protein, three collagen-binding microbial surface components that recognize adhesive matrix molecules (Ace, Acm, Scm) and pili (Ebp, PilA and PilB), their interactions with host molecules and their role in pathogenicity and biofilm development.

The winter ulcer bacterium Moritella viscosa demonstrates adhesion and cytotoxicity in a fish cell model

Moritella viscosa is considered the main aetiological agent of 'winter ulcer' disease in farmed salmonid fish. To further understand the pathogenesis of this disease, M. viscosa interaction with fish cells was studied using a Chinook salmon embryo cell line (CHSE-214). As winter ulcer appears exclusively at temperatures below 7-8 degrees C, we attempted to identify if this connection is explained by temperature regulated bacterial virulence. Therefore, infection studies were performed at a temperature range from 4 to 15 degrees C. At all temperatures, M. viscosa caused CHSE cells to retract and round up, lose their attachment abilities and finally disintegrate. The bacterium adhered to CHSE cells and caused changes to the cytoskeleton, however, it did not invade the cells. Increased adherence was demonstrated at 4 degrees C compared to adherence at higher temperatures. Extracellular proteins exerted rapid pore formation and lysis of CHSE cells at a temperature range from 4 to 22 degrees C. Furthermore, only small differences were found comparing extracellular proteomes of M. viscosa from 4 and 15 degrees C. We propose that the pathogenic mechanisms exerted by M. viscosa on CHSE cells are disruption of the cytoskeleton which affects cell rigidity and structure, followed by pore formation and lysis caused by secreted products from the bacterium. These processes can also occur at temperatures above those experienced from winter ulcer outbreaks. However, the adhesion mechanisms appear to be temperature regulated and may contribute to temperature dependent disease outbreaks.

Pathogenesis of implant infections by enterococci

Enterococci are commensals of human and animal intestinal tract that have emerged in the last decades as a major cause of nosocomial infections of bloodstream, urinary tract and in infected surgical sites. Enterococcus faecalis is responsible for ca. 80% of all enterococcal infections while Enterococcus faecium accounts for most of the others; among the most relevant risk factors for development of enterococcal infections is the presence of implanted devices. The pathogenesis of such infections is poorly understood, but several virulence factors have been proposed. Among them, the ability to form biofilm has recently been shown to be one of the most prominent features of this microorganism, allowing colonization of inert and biological surfaces, while protecting against antimicrobial substances, and mediating adhesion and invasion of host cells and survival within professional phagocytes. Biofilm formation has been shown to be particularly important in the development of prosthetic valve enterococcal endocarditis and stent occlusion. Enterococci are also able to express other surface factors that may support colonization of both inert and biological surfaces, and that may be involved in the invasion of, and survival within, the host cell.

Heparan sulfate proteoglycans mediate Staphylococcus aureus interactions with intestinal epithelium

Staphylococcus aureus can be internalized by non-professional phagocytes, and may colonize the intestine in normal and antibiotic-treated individuals. Intestinal colonization may depend on the interactions of S. aureus with the intestinal epithelium. The best described mechanism of S. aureus binding to eukaryotic cells involves S. aureus fibronectin binding proteins (FnBPs), using fibronectin as a bridging molecule to beta1 integrins on the eukaryotic cell surface. Because S. aureus can be internalized by enterocytes, and because S. aureus is known to bind heparan sulfate (HS), we hypothesized that heparan sulfate proteoglycans (HSPGs) widely expressed on epithelia may mediate S. aureus interactions with intestinal epithelial cells. Internalization of S. aureus RN6390 by cultured intestinal epithelial cells was inhibited in a dose-dependent fashion by the HS mimic heparin, and by HS itself. Internalization of S. aureus DU5883, which lacks expression of staphylococcal FnBPs, was also inhibited by heparin. S. aureus adherence to ARH-77 cells, transfected to express the HSPG syndecan-1, was greatly increased when compared to adherence to plasmid control ARH-77 cells which have little detergent extractable HS. In addition, compared to wild-type HS-expressing Chinese hamster ovary (CHO) cells, internalization of S. aureus was decreased using mutant CHO cells with decreased HS expression. These findings are consistent with a model wherein S. aureus internalization by intestinal epithelial cells (and perhaps other epithelia) is mediated by S. aureus binding to the HS moiety of cell-surface HSPGs, and this interaction appears independent of fibronectin binding.

ABILITY OF THE HEPARAN SULFATE PROTEOGLYCAN SYNDECAN-1 TO PARTICIPATE IN BACTERIAL TRANSLOCATION ACROSS THE INTESTINAL EPITHELIAL BARRIER

Although hundreds of microbial species reside in the human intestinal tract, comparatively few (e.g., Escherichia coli and other enterobacteria, Enterococcus faecalis, etc.) are typically associated with systemic infection in postsurgical, shock, and trauma patients. Syndecan-1 is the predominant cell surface heparan sulfate proteoglycan expressed on epithelia, and there is substantial evidence that heparan sulfate participates in interactions of a variety of frankly pathogenic microbes with mammalian cells. To investigate the role of syndecan-1 in interactions of enteric flora with intestinal epithelium, bacteria that might use the enterocyte as a portal of entry for systemic infection (including E. faecalis, E. coli, and other enterobacteria, and several species of staphylococci and streptococci) were studied for their abilities to interact with syndecan-1. Streptococcus bovis, S. agalactiae, S. pyogenes, Staphylococcus aureus, and S. epidermidis showed increased adherence to ARH-77 cells transfected to express syndecan-1. Heparin, a heparan sulfate analog, inhibited internalization of S. bovis, S. agalactiae, S. pyogenes, and S. aureus by HT-29 enterocytes (prominent syndecan-1 expression), but not Caco-2 enterocytes (relatively low syndecan-1 expression). Data from experiments with Chinese hamster ovary cells with altered glycosaminoglycan expression indicated that heparan sulfate and chondroitin sulfate (glycosaminoglycans on the syndecan-1 ectodomain) participated in bacterial interactions with mammalian cells. Thus, although E. faecalis, E. coli, and other gram-negative enterobacteria did not appear to interact with syndecan-1, this heparan sulfate proteoglycan may mediate enterocyte interactions with some staphylococci and streptococci that are known to cause systemic infections in specific populations of high-risk, immunosuppressed, postsurgical, and trauma patients.

Enterococcal surface protein Esp does not facilitate intestinal colonization or translocation of Enterococcus faecalis in clindamycin-treated mice

Enterococcal surface protein (Esp) is a cell wall-associated protein of Enterococcus faecalis that has been identified as a potential virulence factor. We used a mouse model to examine whether Esp facilitates intestinal colonization or translocation of E. faecalis to mesenteric lymph nodes. After clindamycin treatment, similar levels of high-density colonization were established after orogastric inoculation of an E. faecalis isolate containing the esp gene within a large pathogenicity island and an isogenic mutant created by allelic replacement of the esp gene with a chloramphenicol resistance cassette (P=0.7); translocation to mesenteric lymph nodes was detected in 3 of 12 (25%) mice in both groups. Isogenic mutants of FA2-2 (a plasmid-free derivative of E. faecalis strain JH2) with or without the esp gene failed to establish colonization of clindamycin-treated mice. These results suggest that Esp does not facilitate intestinal colonization or translocation of E. faecalis.

Characterization of the pheromone response of the Enterococcus faecalis conjugative plasmid pCF10: complete sequence and comparative analysis of the transcriptional and phenotypic responses of pCF10-containing cells to pheromone induction

The sex pheromone plasmids in Enterococcus faecalis are one of the most efficient conjugative plasmid transfer systems known in bacteria. Plasmid transfer rates can reach or exceed 10(-1) transconjugants per donor in vivo and under laboratory conditions. We report the completion of the DNA sequence of plasmid pCF10 and the analysis of the transcription profile of plasmid genes, relative to conjugative transfer ability following pheromone induction. These experiments employed a mini-microarray containing all 57 open reading frames of pCF10 and a set of selected chromosomal genes. A clear peak of transcription activity was observed 30 to 60 min after pheromone addition, with transcription subsiding 2 h after pheromone induction. The transcript activity correlated with the ability of donor cells to transfer pCF10 to recipient cells. Remarkably, aggregation substance (Asc10, encoded by the prgB gene) was present on the cell surface for a long period of time after pheromone-induced transcription of prgB and plasmid transfer ability had ceased. This observation could have relevance for the virulence of E. faecalis.

Protein secretion in Lactococcus lactis : an efficient way to increase the overall heterologous protein production

Lactococcus lactis, the model lactic acid bacterium (LAB), is a food grade and well-characterized Gram positive bacterium. It is a good candidate for heterologous protein delivery in foodstuff or in the digestive tract. L. lactis can also be used as a protein producer in fermentor. Many heterologous proteins have already been produced in L. lactis but only few reports allow comparing production yields for a given protein either produced intracellularly or secreted in the medium. Here, we review several works evaluating the influence of the localization on the production yields of several heterologous proteins produced in L. lactis. The questions of size limits, conformation, and proteolysis are addressed and discussed with regard to protein yields. These data show that i) secretion is preferable to cytoplasmic production; ii) secretion enhancement (by signal peptide and propeptide optimization) results in increased production yield; iii) protein conformation rather than protein size can impair secretion and thus alter production yields; and iv) fusion of a stable protein can stabilize labile proteins. The role of intracellular proteolysis on heterologous cytoplasmic proteins and precursors is discussed. The new challenges now are the development of food grade systems and the identification and optimization of host factors affecting heterologous protein production not only in L. lactis, but also in other LAB species.

An amino-terminal domain of Enterococcus faecalis aggregation substance is required for aggregation, bacterial internalization by epithelial cells and binding to lipoteichoic acid

Aggregation substance (AS), a plasmid-encoded surface protein of Enterococcus faecalis, plays important roles in virulence and antibiotic resistance transfer. Previous studies have suggested that AS-mediated aggregation of enterococcal cells could involve the binding of this protein to cell wall lipoteichoic acid (LTA). Here, a method to purify an undegraded form of Asc10, the AS of the plasmid pCF10, is described. Using this purified protein, direct binding of Asc10 to purified E. faecalis LTA was demonstrated. Equivalent binding of Asc10 to LTA purified from INY3000, an E. faecalis strain that is incapable of aggregation, was also observed. Surprisingly, mutations in a previously identified aggregation domain from amino acids 473 to 683 that abolished aggregation had no effect on LTA binding. In frame deletion analysis of Asc10 was used to identify a second aggregation domain located in the N-terminus of the protein from amino acids 156 to 358. A purified Asc10 mutant protein lacking this domain showed reduced LTA binding, while a purified N-terminal fragment from amino acids 44-331 had high LTA binding. Like the previously described aggregation domain, the newly identified Asc10((156-358)) aggregation domain was also required for efficient internalization of E. faecalis into HT-29 enterocytes. Thus, Asc10 possess two distinct domains required for aggregation and eukaryotic cell internalization: an N-terminal domain that promotes binding to LTA and a second domain located near the middle of the protein.

Sex pheromone response, clumping, and slime production in enterococcal strains isolated from occluded biliary stents

Bile-resistant bacteria, particularly gram-positive Enterococcus faecalis and Enterococcus faecium, play an important role in biliary stent occlusion, because their sessile mode of growth protects them against host defenses and antimicrobial agents. Twelve E. faecalis and seven E. faecium strains isolated from occluded biliary stents have been investigated for slime production, presence of aggregation substance genes, and ability to adhere to Caco-2 cells. Ten isolates were strong producers of slime, and seven isolates produced clumps when exposed to pheromones of E. faecalis JH2-2 and/or OG1RF. The small E. faecium clumps differed from the large clumps of E. faecalis and were similar to those of E. faecium LS10(pBRG1) carrying a pheromone response plasmid. After induction with pheromones, the adhesion to Caco-2 cells of clumping-positive strains was found to increase from two- to fourfold. Amplicons of the expected size were detected in three clumping-positive and three clumping-negative E. faecalis isolates by using primers (agg) internal to a highly conserved region of the E. faecalis pheromone response plasmids pAD1, pPD1, and pCF10 and primers internal to prgB of the E. faecalis plasmid pCF10. The agg/prgB-positive E. faecalis strains were also positive in Southern hybridization experiments with a prgB-specific probe. No PCR products were obtained with the same primers from four clumping-positive isolates (one E. faecalis and three E. faecium strains), which were also Southern hybridization negative. Our results demonstrate that slime production and pheromone response are both present in isolated enterococci, suggesting that clinical strains with these features might have a selective advantage in colonizing biliary stents.

Enterococcal infections: host response, therapeutic, and prophylactic possibilities

The emergence of resistance against multiple antibiotics and the increasing frequency with which Enterococcus faecalis and Enterococcus faecium are isolated from hospitalized patients underscore the necessity for a better understanding of the virulence mechanisms of this pathogen and the development of alternatives to current antibiotic treatments. The genetic plasticity of enterococci and their ability to rapidly acquire and/or develop resistance against many clinically important antibiotics and to transfer these resistance determinants to other more pathogenic microorganisms makes the search for alternative treatment and preventive options even more important. A capsular polysaccharide antigen has recently been characterized that is the target of opsonic antibodies. A limited number of clinically relevant serotypes exist, and the development of an enterococcal vaccine based on capsular polysaccharides may improve our ability to prevent and treat these infections. Additional enterococcal surface antigens, including ABC transporter proteins and other virulence factors, such as aggregation substance (AS), may also be useful targets for therapeutic antibodies.

Assessment of the role of antibiotics and enterococcal virulence factors in a mouse model of extraintestinal translocation

OBJECTIVE

To study the relative contribution of antibiotics and bacterial virulence factors in the process of translocation of Enterococcus faecalis from the gut to extraintestinal organs.

DESIGN

Prospective controlled animal study.

SETTING

Animal experimental laboratory at a university medical center.

SUBJECTS

Fifty-two female Balb/c mice.

INTERVENTIONS

We developed a mouse model to study the translocation of Enterococcus faecalis from the intestinal tract. Balb/c mice received sterile drinking water or antibiotic combinations to deplete their indigenous intestinal microflora. The animals subsequently were fed genetically engineered enterococci expressing different combinations of the putative enterococcal virulence factors aggregation substance and binding substance. Animals were killed, and their livers, spleens, and mesenteric lymph nodes were aseptically removed and cultured along with fecal samples for enumeration of bacteria.

MEASUREMENTS AND MAIN RESULTS

All animals were colonized with the test strains at 2-6 x 109 colony forming units/g of feces; in the antibiotic-treated animals, feces were free from anaerobes and Enterobacteriaceae. In animals fed the identical bacterial mutant, the colony counts in mesenteric lymph nodes were significantly lower in mice not treated with antibiotics than in those treated with antibiotics (p =.016). Multigroup analysis of variance revealed no significant differences of the translocation frequencies for the different mutant strains; however, the differences were statistically significant for all groups receiving antibiotics vs. the group not receiving antibiotics (p <.05-.01). There was a trend (although not statistically significant) for a higher proportion of positive cultures from either spleen or liver in mice that had enterococci recovered from their mesenteric lymph nodes (28%) relative to those that did not have enterococci isolated from the lymph nodes (12%; rate ratio 2.39, p =.30 by logistic regression analysis).

CONCLUSIONS

Oral antibiotics can select for extraintestinal translocation of Enterococcus faecalis, and neither aggregation substance nor binding substance seems to be required for this process. The experiments encourage further exploration of host and microbial factors contributing to translocation and may provide a better understanding of the pathogenesis of enterococcal infections in patients in intensive care units.

Pseudomonas aeruginosa expresses a lethal virulence determinant, the PA-I lectin/adhesin, in the intestinal tract of a stressed host: the role of epithelia cell contact and molecules of the Quorum Sensing Signaling System

OBJECTIVE

We have previously demonstrated that P. aeruginosa can have profound effects on the intestinal epithelial barrier via one of its virulence factors, the PA-I lectin/adhesin. The aims of the present study were to further characterize the interaction of P. aeruginosa and the intestinal epithelium using both in vitro and in vivo approaches.

METHODS

In vitro assays examining the effect of bacterial growth phase, epithelial cell contact, and butanoyl homoserine lactone (C4-HSL), a quorum sensing signaling molecule know to affect various extracellular virulence factors in P. aeruginosa, on PA-I expression in P. aeruginosa were performed. In vivo studies were carried out by modeling catabolic stress in mice using a 30% surgical hepatectomy and direct introduction of P. aeruginosa and various virulence components into the cecum. The effect of this model on PA-I expression in P. aeruginosa was determined.

RESULTS

Results demonstrated that PA-I expression in P. aeruginosa is affected by its phase of growth, its contact to the intestinal epithelium, and its exposure to the quorum sensing molecule, C4-HSL. Furthermore, data from the present study suggest that the PA-I lectin/adhesin of P. aeruginosa may be increased in vivo by local factors within the cecum of mice in response to surgical stress.

CONCLUSIONS

These data indicate that multiple factors present in the intestinal microenvironment of a stressed host may induce certain opportunistic pathogens to express key virulence factors leading to a state of lethal gut-derived sepsis.

Gene transfer of vancomycin and tetracycline resistances among Enterococcus faecalis during cheese and sausage fermentations

This study assessed the frequency of transfer of two mobile genetic elements coding for virulence determinants and antibiotic resistance factors, into food associated enterococci during fermentation processes. First, the transfer of the pheromone-inducible pCF10 plasmid, carrying tetracycline resistance and aggregation substance (AS) as virulence factor, between clinical and food strains of Enterococcus faecalis, was investigated in models of cheese and fermented sausage. The experiments demonstrated that even in the absence of selective tetracycline pressure, plasmid pCF10 was transferred from E. faecalis OG1rf cells to food strain E. faecalis BF3098c and that the plasmid subsequently persisted in these environments. Very high frequency of transfer was observed in sausage (10(-3)/recipient) if compared to cheese (10(-6)) and plate mating (10(-4)). Transconjugants were subsequently verified by PCR. The second transmissible element was the plasmid harbouring the vancomycin resistance (VanA phenotype) from E. faecalis A256. The transfer of this antibiotic resistance to a food strain of E. faecalis was studied in vitro and in food models. Although the transfer of vancomycin resistance was achieved in all the environments, the highest conjugation frequencies were observed during the ripening of fermented sausages, reaching 10(-3) transconjugants/recipient cell. PCR confirmed the transfer of the VanA genotype into a food associated Enterococcus strain. This study showed that even in the absence of selective pressure, mobile genetic elements carrying antibiotic resistance and virulence determinants can be transferred at high frequency to food associated enterococci during cheese and sausage fermentation.

The aggregation domain of aggregation substance, not the RGD motifs, is critical for efficient internalization by HT-29 enterocytes

Aggregation substance (AS), a surface protein encoded on the pheromone-inducible plasmids of Enterococcus faecalis, has been shown to increase adherence and internalization into a number of different cell types, presumably through integrin binding mediated by the N-terminal RGD motif of AS. Here, defined mutations constructed in Asc10, the AS encoded by the plasmid pCF10, are analyzed for their ability to promote increased internalization levels into HT-29 enterocytes. The results clearly show that the previously identified Asc10 functional domain, not the RGD motifs, is critical for Asc10-directed internalization of E. faecalis into HT-29 enterocytes. Also, expression of Asc10 in the nonaggregating E. faecalis strain INY3000 is unable to mediate HT-29 internalization. However, Asc10-expressing E. faecalis cells are not internalized as bacterial aggregates, suggesting bacterial aggregation is not a prerequisite for HT-29 internalization. These data show that Asc10 directs internalization of E. faecalis into HT-29 enterocytes through a non-RGD-dependent mechanism.

High-resolution backscatter electron imaging of colloidal gold in LVSEM

High-resolution backscatter electron (BSE) imaging of colloidal gold can be accomplished at low voltage using in-lens or below-the-lens FESEMs equipped with either Autrata-modified yttrium aluminium garnet (YAG) scintillators doped with cerium, or with BSE to secondary electron (SE) conversion plates. The threshold for BSE detection of colloidal gold was 1.8 keV for the YAG detector, and the BSE/SE conversion was sensitive down to 1 keV. Gold particles (6, 12 and 18 nm) have an atomic number of 79 and were clearly distinguished at 500,000x by materials contrast and easily discriminated from cell surfaces coated with platinum with an atomic number of 78. BSE imaging was relatively insensitive to charging, and build up of carbon contamination on the specimen was transparent to the higher energy BSE.

Intracellular survival of Staphylococcus aureus within cultured enterocytes

BACKGROUND

Little is known about the mechanisms involved in bacterial translocation from the intestinal lumen to extraintestinal sites. Because Staphylococcus aureus can colonize the intestinal tract, and because the intestinal tract is a reservoir for antibiotic resistant S. aureus, experiments were designed to clarify the interactions of S. aureus with cultured intestinal epithelial cells, and assays included measurements of bacterial internalization, enterocyte apoptosis, and epithelial barrier function.

METHODS AND RESULTS

Mature, confluent enterocytes were incubated 1 h with S. aureus, and the gentamicin protection assay was used to quantify intracellular bacterial survival at various time intervals up to 120 h later. Enterocyte apoptosis was assessed using Annexin V, and the permeability of confluent enterocyte cultures was measured by transepithelial electrical resistance and by transmigration of Escherichia coli across confluent enterocytes.S. aureus was internalized by cultured enterocytes and remained viable for up to 120 h within both HT-29 and Caco-2 enterocytes. S. aureus intracellular survival was associated with enterocyte apoptosis and with decreased transepithelial electrical resistance across confluent Caco-2 enterocytes. S. aureus intracellular survival over time was also associated with increased E. coli transmigration across confluent Caco-2, but not HT-29, enterocytes.

CONCLUSIONS

S. aureus appeared to survive within cultured enterocytes for prolonged time periods, up to several days. Survival of S. aureus within host eukaryotic cells, such as enterocytes, might facilitate persistence of S. aureus in infected tissue despite appropriate antibiotic therapy.

Effect of lipopolysaccharide on virulence of intestinal candida albicans

BACKGROUND

Candida albicans is a polymorphic fungus that frequently causes systemic infection in postsurgical and trauma patients. Others have reported that Escherichia coli lipopolysaccharide (LPS) acts as a copathogen to enhance the virulence of parenteral C. albicans. Experiments were designed to clarify the effect of parenteral LPS on systemic candidiasis initiated via the oral route.

MATERIALS AND METHODS

Antibiotic-treated mice were orally inoculated with C. albicans CAF2 (wild-type) or mutant HLC54 (defective in filament formation), and were given 100 microg parenteral LPS 16 h before sacrifice. Separate groups of mice were additionally exposed to intermittent hypoxia prior to LPS. At sacrifice, cecal flora and microbial translocation to the mesenteric lymph nodes were quantified. C. albicans adherence to cultured HT-29 and Caco-2 enterocytes (pretreated with LPS, or calcium-free medium to expose the enterocyte lateral surface, or both) was quantified by enzyme-linked immunoabsorbent assay.

RESULTS

All mice had high numbers of cecal C. albicans, and LPS was associated with an additional increase in cecal concentrations of HLC54 but not CAF2. Translocation of HLC54, but not CAF2, appeared facilitated by hypoxia, but LPS did not facilitate translocation in any treatment group. Exposure of the lateral surface of cultured enterocytes had no effect on C. albicans adherence, although LPS consistently decreased adherence of both C. albicans strains.

CONCLUSIONS

In contrast to experiments where systemic candidiasis was initiated by the parenteral route, parenteral LPS did not act as a copathogen in mice with systemic candidiasis initiated by the oral route, and these results might be related to LPS-induced alterations in C. albicans adherence to host enterocytes.

Incidence of virulence determinants in clinical Enterococcus faecium and Enterococcus faecalis isolates collected in Sardinia (Italy)

Enterococci are widely distributed in the environment; within the human body, they are normal commensals of the oral cavity, gastrointestinal tract and vagina. In recent years, enterococci have become one of the most frequent causes of acquired nosocomial infections worldwide. The molecular mechanism of virulence of these bacteria is still not completely understood. The aims of this work were to characterize phenotypically 47 isolates of Enterococcus faecalis and Enterococcus faecium collected in Sardinia (Italy) by their abilities to adhere to different epithelial cell lines (Vero and Caco-2 cells) and to associate their phenotypes with the presence of known virulence genes detected within their genomes by PCR. The following genes were amplified: AS (aggregation substance), esp (surface protein gene), ace (accessory colonization factor), efaA (E. faecalis endocarditis antigen) and gelE (gelatinase). The virulence genes were detected in E. faecalis isolates only, with the exception of esp, which was found in both species. The phenotypic and genotypic results were also compared with the susceptibility of isolates to various antibiotics.

Extracellular adherence protein from Staphylococcus aureus enhances internalization into eukaryotic cells

In this study we have shown that Eap (extracellular adherence protein) plays a role in the internalization process of Staphylococcus aureus into eukaryotic cells. Eap is a protein that is mostly extracellularly and to a lesser extent is bound to the bacterial surface as a result of rebinding. Eap is able to bind to several plasma proteins, such as fibronectin, fibrinogen, and prothrombin. It has the capacity to form oligomers and is able to agglutinate S. aureus. A mutant strain, Newman mAH12 (eap:: Ery(r)), with a deficient eap gene was used in the present study. We have demonstrated that (i) strain Newman mAH12 could adhere to and become internalized to a higher extent by eukaryotic cells than the isogenic mutant, (ii) strain Newman mAH12 complemented with the eap gene displayed restoration of the internalization level, (iii) externally added Eap enhanced the internalization of laboratory and clinical S. aureus strains as well as of S. carnosus (a coagulase-negative species devoid of proteins important for internalization), and (iv) antibodies against Eap were able to block the internalization process in strain Newman mAH12 and clinical isolates. Eap, with its broad binding capacity and its surface localization, thus seems to contribute to the internalization of S. aureus into eukaryotic cells. We therefore propose a novel internalization pathway for S. aureus in which Eap plays an enhancing role.

Presence of a vanA-carrying pheromone response plasmid (pBRG1) in a clinical isolate of Enterococcus faecium

Sex pheromone plasmids, frequently found in Enterococcus faecalis, have rarely been detected in Enterococcus faecium. pBRG1 is an approximately 50-kb vanA-carrying conjugative plasmid of an E. faecium clinical isolate (LS10) that is transferable to E. faecalis laboratory strains. In cell infection experiments, E. faecium LS10 exhibited remarkably high invasion efficiency and produced cytopathogenic effects in Caco-2 cell monolayers. Growth in the presence of sex pheromones produced by E. faecalis JH2-2 was found to cause self-aggregation of both E. faecium LS10 and E. faecalis JH-RFV(pBRG1) (a transconjugant obtained by transfer of pBRG1 to E. faecalis JH2-2) and to increase the cell adhesion and invasion efficiencies of both E. faecium LS10 and E. faecalis JH-RFV(pBRG1). Sex pheromone cCF10 caused clumping of E. faecalis OG1RF(pBRG1) (a transconjugant obtained by transfer of pBRG1 to E. faecalis OG1RF) at a concentration approximately 100-fold higher than the one required for the control strain E. faecalis OG1RF(pCF10). PCR products of the expected sizes were obtained with primers internal to aggregation substance genes of E. faecalis pheromone response plasmids pAD1, pPD1, and pCF10 and primers internal to ash701 of E. faecium pheromone plasmid pHKK701. These findings suggest that pBRG1 of E. faecium LS10 is a sex pheromone response plasmid.

Virulence factors in food, clinical and reference Enterococci: A common trait in the genus?

The occurrence of several virulence traits (cytolysin, adhesins and hydrolytic enzymes) was investigated in a collection of 164 enterococci, including food and clinical isolates (from human and veterinary origin), as well as type and reference strains from 20 enterococcal species. Up to fifteen different cyl genotypes were found, as well as silent cyl genes. The occurrence of the cyl operon and haemolytic potential seems to be widespread in the genus. A significant association of this virulent trait with clinical isolates was found (p < 0.05). High levels of incidence were also observed for genes encoding surface adhesins (esp, efaA(fs), efaA(fm)), agg and gelE, irrespectively of species allocation and origin of strains. Although gelE behaves as silent in the majority of the strains, gelatinase activity predominates in clinical isolates, whereas lipase and DNase were mainly detected in food isolates pointing to their minor role as virulence determinants. No hyaluronidase activity was detected for all strains. Numerical hierarchic data analysis grouped the strains in three main clusters, two of them including a total of 50 strains with low number of virulence determinants (from 2 to 7) and the other with 114 strains with a high virulence potential (up to 12 determinants). No statistical association was found between virulence clusters and species allocation (p > 0.10), strongly suggesting that virulence determinants are a common trait in the genus Enterococcus. Clinical strains seem to be significantly associated with high virulence potential, whereas food, commensal and environmental strains harbour fewer virulence determinants (p < 0.01). A high level of relative diversity in virulence patterns was observed (Shannon's index varies from 0.95 to 1.0 among clusters), reinforcing the strain-specific nature of the association of virulence factors. Although a low risk seems to be associated with the use of enterococci in long-established artisanal cheeses, screening of virulence traits and their cross-synergies must be performed, particularly for commercial starters, probiotic strains and products to be used by high risk population groups.

Differential expression of virulence-related genes in Enterococcus faecalis in response to biological cues in serum and urine

Enterococci rank among leading causes of nosocomial bacteremia and urinary tract infection and are also a leading cause of community acquired subacute endocarditis. Limited evidence suggests that biological cues in serum and urine may play an important role in modulating enterococcal virulence at sites of infection. To determine the extent to which biological cues affect enterococcal virulence-associated gene expression, we used quantitative real-time PCR to compare mRNA levels in Enterococcus faecalis cultures grown in serum or urine to that achieved in laboratory medium. Both environment- and growth phase-specific variations were observed, demonstrating the occurrence of as-yet-uncharacterized mechanisms for control of gene expression in E. faecalis that may play an important role in vivo.

Effect of tumor necrosis factor alpha, interferon gamma, and interleukin-4 on bacteria-enterocyte interactions

BACKGROUND

Little is known about the mechanisms involved in bacterial translocation from the intestinal lumen to extraintestinal sites. Because the cytokine cascade associated with sepsis, inflammation, and trauma has been shown to affect intestinal epithelial permeability, experiments were designed to clarify the effects of selected cytokines on bacterial adherence to and internalization by cultured HT-29 and Caco-2 enterocytes.

METHODS

Mature, confluent enterocytes were pretreated 48 to 72 h with tumor necrosis factor alpha (TNF-alpha), interferon gamma, (IFN-gamma), or interleukin-4 (IL-4). Adherence of Listeria monocytogenes, Salmonella typhimurium, Proteus mirabilis, and Escherichia coli was measured by enzyme-linked immunosorbent assay and bacterial internalization was quantified by the gentamicin protection assay. Enterocyte permeability was measured by transepithelial electrical resistance and by flux of 40-kDa fluorescent dextran. Bacterial transmigration across confluent enterocytes was measured using enterocytes cultivated on permeable supports.

RESULTS

TNF-alpha, IFN-gamma, and IL-4 had variable effects on bacterial adherence to HT-29 and Caco-2 enterocytes, although the most consistent finding was increased bacterial adherence associated with INF-gamma. However, none of these cytokines had a noticeable effect on bacterial internalization by either Caco-2 or HT-29 enterocytes. In addition, none of these cytokines had a noticeable effect on the permeability of confluent enterocytes as measured by transepithelial electrical resistance or dextran flux. Bacterial transmigration across confluent HT-29 enterocytes was not altered by TNF-alpha, IFN-gamma, or IL-4; however, IL-4 consistently decreased bacterial transmigration across confluent Caco-2 enterocytes.

CONCLUSIONS

IFN-gamma may augment the epithelial adherence of selected species of enteric bacteria, and IL-4 may act as a barrier-sustaining agent to decrease bacterial migration across the intestinal epithelium.

In vivo induction of virulence and antibiotic resistance transfer in Enterococcus faecalis mediated by the sex pheromone-sensing system of pCF10

Enterococcus faecalis has become one of the most notable nosocomial pathogens in the last decade. Aggregation substance (AS) on the sex pheromone plasmids of E. faecalis has been implicated as a virulence factor in several model systems. We investigated the AS-encoding plasmid pCF10 for its ability to increase virulence in a rabbit endocarditis model. Cells containing pCF10 increased the virulence in the model significantly, as assessed by an increase in aortic valve vegetation size. The results confirmed in vivo induction of the normally tightly controlled AS. In addition to the expression of AS when E. faecalis cells were in contact with plasma, plasmid transfer of the tetracycline resistance-carrying plasmid was also activated in vitro and in vivo. In vivo, plasmid transfer reached remarkable frequencies of 8 x 10(-2) to 9 x 10(-2). These values are comparable to the highest frequencies ever observed in vitro. Cells harboring pCF10 had a significant survival advantage over plasmid-free cells indicated by pCF10 present in two-thirds of the recipient population. Plasma induction was dependent on the presence of the plasmid-encoded PrgZ protein, indicating the requirement of the pheromone-sensing system in the induction process. The data suggested that the mechanism of in vivo induction may involve interference of plasma with the normal function of the pheromone peptide and its inhibitor.

Effect of pheromone induction on transfer of the Enterococcus faecalis plasmid pCF10 in intestinal mucus ex vivo

The effect of synthetic sex pheromone on pheromone-inducible conjugation between the isogenic Enterococcus faecalis strains OG1RF and OG1SS was investigated in (i) Todd-Hewitt broth medium and (ii) intestinal mucus isolated from germ-free rats. In broth, the presence of synthetic pheromone cCF10 had no detectable effect on the transfer kinetics observed for the tetracycline resistance encoding plasmid pCF10. In mucus, presence of the same pheromone significantly increased the transfer efficiency observed during the first 2 h of conjugation, while the effect was less pronounced later in the experiment. We suggest that due to differences in diffusion rates and medium-binding of the pheromones, the effect of the synthetic cCF10 was immediately dominated by the effect of pheromones produced by the recipient E. faecalis strain in broth, while this happened later in mucus.

Integrin expression, enterocyte maturation, and bacterial internalization

BACKGROUND

Little is known about the molecular mechanisms involved in the translocation of enteric bacteria. Adhesion molecules mediate interactions between some enteric pathogens and mammalian cells, but no such interactions have been identified for enterocytes and normal enteric bacteria. Using enteric pathogens, adhesion molecule expression has been linked to bacterial internalization and to enterocyte differentiation. Therefore, experiments were designed to study enterocyte integrin expression and differentiation, as well as enterocyte internalization of Salmonella typhimurium, Proteus mirabilis, and Escherichia coli.

MATERIALS AND METHODS

Relative expression of the alpha2, alpha3, and beta1 integrin subunits on Caco-2 and HT-29 enterocytes (mature and immature) was measured by ELISA. Bacteria-enterocyte surface interactions were observed by light and scanning electron microscopy. Bacterial internalization by enterocytes was quantified using the gentamicin protection assay.

RESULTS

Expression of the alpha2, alpha3, and beta1 integrin subunits was consistently increased in immature compared to mature Caco-2 enterocytes; however, compared to mature enterocytes, immature HT-29 enterocytes had similar expression of alpha3 and beta1 but decreased alpha2. Compared to untreated mature enterocytes, bacterial internalization was increased in immature enterocytes as well as mature enterocytes with lateral membranes artifactually exposed. However, there was no difference in bacterial internalization between immature enterocytes and mature enterocytes treated to expose the lateral membrane.

CONCLUSIONS

Bacterial internalization by enterocytes appeared to be due to factors other than integrin expression or enterocyte differentiation. Exposure of the lateral enterocyte membrane may play an important role in facilitating bacterial internalization by enterocytes.