Effects of the Enterococcus faecalis hypR gene encoding a new transcriptional regulator on oxidative stress response and intracellular survival within macrophages

Insights into ecology, pathogenesis, and biofilm formation of Enterococcus faecalis from functional genomics

SUMMARYEnterococcus faecalis is a significant resident of the gastrointestinal tract of most animals, including humans. Although generally non-pathogenic in healthy hosts, this microbe is adept at the exploitation of compromises in host immune functions, resulting in life-threatening opportunistic infections whose treatments are complicated by a high degree of intrinsic and acquired resistance to antimicrobial chemotherapy. Historically, progress in enterococcal research was limited by a lack of experimental models that replicate natural infection pathways and the relevance of in vitro studies to the natural biology of the organism. In this review, we summarize the history of enterococcal research during the 20th and early 21st centuries and describe more recent genetic and genomic tools and screens developed to address challenges in the field. We also describe how the results of recent studies reveal the importance of previously uncharacterized enterococcal genes, and we provide examples of interesting determinants that have emerged as important contributors to enterococcal biology. These factors may also serve as targets for future vaccines and chemotherapeutic agents to combat life-threatening hospital infections.

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.

LysR-type transcriptional regulators: state of the art

The LysR-type transcriptional regulators (LTTRs) are DNA-binding proteins present in bacteria, archaea, and in algae. Knowledge about their distribution, abundance, evolution, structural organization, transcriptional regulation, fundamental roles in free life, pathogenesis, and bacteria-plant interaction has been generated. This review focuses on these aspects and provides a current picture of LTTR biology.

CCR2-dependent CX3CR1+ colonic macrophages promote Enterococcus faecalis dissemination

Enterococci are common commensal bacteria that colonize the gastrointestinal tracts of most mammals, including humans. Importantly, these bacteria are one of the leading causes of nosocomial infections. This study examined the role of colonic macrophages in facilitating Enterococcus faecalis infections in mice. We determined that depletion of colonic phagocytes resulted in the reduction of E. faecalis dissemination to the gut-draining mesenteric lymph nodes. Furthermore, we established that trafficking of monocyte-derived CX3CR1-expressing macrophages contributed to E. faecalis dissemination in a manner that was not reliant on CCR7, the conventional receptor involved in lymphatic migration. Finally, we showed that E. faecalis mutants with impaired intracellular survival exhibited reduced dissemination, suggesting that E. faecalis can exploit host immune cell migration to disseminate systemically and cause disease. Our findings indicate that modulation of macrophage trafficking in the context of antibiotic therapy could serve as a novel approach for preventing or treating opportunistic infections by disseminating enteric pathobionts like E. faecalis.

Enterococcus spp.: Is It a Bad Choice for a Good Use-A Conundrum to Solve?

Since antiquity, the ubiquitous lactic acid bacteria (LAB) Enterococci, which are just as predominant in both human and animal intestinal commensal flora, have been used (and still are) as probiotics in food and feed production. Their qualities encounter several hurdles, particularly in terms of the array of virulence determinants, reflecting a notorious reputation that nearly prevents their use as probiotics. Additionally, representatives of the Enterococcus spp. genus showed intrinsic resistance to several antimicrobial agents, and flexibility to acquire resistance determinants encoded on a broad array of conjugative plasmids, transposons, and bacteriophages. The presence of such pathogenic aspects among some species represents a critical barrier compromising their use as probiotics in food. Thus, the genus neither has Generally Recognized as Safe (GRAS) status nor has it been included in the Qualified Presumption of Safety (QPS) list implying drastic legislation towards these microorganisms. To date, the knowledge of the virulence factors and the genetic structure of foodborne enterococcal strains is rather limited. Although enterococcal infections originating from food have never been reported, the consumption of food carrying virulence enterococci seems to be a risky path of transfer, and hence, it renders them poor choices as probiotics. Auspiciously, enterococcal virulence factors seem to be strain specific suggesting that clinical isolates carry much more determinants that food isolates. The latter remain widely susceptible to clinically relevant antibiotics and subsequently, have a lower potential for pathogenicity. In terms of the ideal enterococcal candidate, selected strains deemed for use in foods should not possess any virulence genes and should be susceptible to clinically relevant antibiotics. Overall, implementation of an appropriate risk/benefit analysis, in addition to the case-by-case assessment, the establishment of a strain's innocuity, and consideration for relevant guidelines, legislation, and regulatory aspects surrounding functional food development seem to be the crucial elements for industries, health-staff and consumers to accept enterococci, like other LAB, as important candidates for useful and beneficial applications in food industry and food biotechnology. The present review aims at shedding light on the world of hurdles and limitations that hampers the Enterococcus spp. genus and its representatives from being used or proposed for use as probiotics. The future of enterococci use as probiotics and legislation in this field are also discussed.

Characterisation of the manganese superoxide dismutase of Enterococcus faecium

The manganese superoxide dismutase (SodA) of E. faecium strain AUS0004 has been characterised. It is most closely related to Enterococcus hirae, Enterococcus durans, Enterococcus villorium, and Enterococcus mundtii with 100%, 91,55%, 90,85%, and 90,58% homology, respectively, but more distant from SodA of E. faecalis (81.68%). A sodA deletion mutant has been constructed. Compared to the parental strain, the ΔsodA mutant was affected in aerobic growth and more sensitive to hydrogen peroxide (H₂O₂), cumene hydroperoxide (CuOOH), and the superoxide anion (O₂^•-) generator menadione. The E. faecium strain AUS0004 is part of those bacteria accumulating H₂O₂ to high concentrations (around 5 mM) starting from late exponential growth phase. Accumulation of the peroxide was around 25% less in the mutant suggesting that this part of H₂O₂ is due to the dismutation of O₂^•- by SodA. The sodA gene of E. faecium AUS0004 was induced by oxygen, peroxides and menadione but the corresponding regulator remains hitherto unknown. Finally, we showed that SodA activity is important for virulence in the Galleria mellonella model.

Proteomic analysis of an Enterococcus faecalis mutant generated against the exposure to silver nanoparticles

INTRODUCTION

Nanoparticles (NPs) have been widely studied as an alternative to antibiotic use due to their antimicrobial properties at lower concentrations. Enterococcus faecalis is a facultative Gram-positive microorganism inhabiting the gastrointestinal tract of humans and animals. It can also be present in other environments such as the oral cavity, water, sewage, soil and food.

AIMS

We evaluated whether E. faecalis could develop resistance to silver NPs (AgNPs) after exposure to sublethal concentrations of the NPs.

METHODS AND RESULTS

Proteomic analyses revealed that different pathways were activated during the acquired resistance under sublethal concentrations, and selected genes were validated by qPCR.

CONCLUSIONS

The results of this study showed that E. faecalis is capable of generating resistance to AgNPs.

SIGNIFICANCE AND IMPACT OF THE STUDY

To avoid the generation of resistance against AgNPs, future use of these NPs should be combined with other NPs prepared with different metals to prevent the dissemination of resistant strains.

Single-Nucleotide RNA Maps for the Two Major Nosocomial Pathogens Enterococcus faecalis and Enterococcus faecium

Enterococcus faecalis and faecium are two major representative clinical strains of the Enterococcus genus and are sadly notorious to be part of the top agents responsible for nosocomial infections. Despite their critical implication in worldwide public healthcare, essential and available resources such as deep transcriptome annotations remain poor, which also limits our understanding of post-transcriptional control small regulatory RNA (sRNA) functions in these bacteria. Here, using the dRNA-seq technique in combination with ANNOgesic analysis, we successfully mapped and annotated transcription start sites (TSS) of both E. faecalis V583 and E. faecium AUS0004 at single nucleotide resolution. Analyzing bacteria in late exponential phase, we capture ~40% (E. faecalis) and 43% (E. faecium) of the annotated protein-coding genes, determine 5′ and 3′ UTR (untranslated region) length, and detect instances of leaderless mRNAs. The transcriptome maps revealed sRNA candidates in both bacteria, some found in previous studies and new ones. Expression of candidate sRNAs is being confirmed under biologically relevant environmental conditions. This comprehensive global TSS mapping atlas provides a valuable resource for RNA biology and gene expression analysis in the Enterococci. It can be accessed online at www.helmholtz-hiri.de/en/datasets/enterococcus through an instance of the genomic viewer JBrowse.

Parallel Genomics Uncover Novel Enterococcal-Bacteriophage Interactions

We lack fundamental understanding of how phage infection influences bacterial gene expression and, consequently, how bacterial responses to phage infection affect the assembly of polymicrobial communities. Using parallel genomic approaches, we have discovered novel transcriptional regulators and metabolic genes that influence phage infection. The integration of whole-genome transcriptomic profiling during phage infection has revealed the differential regulation of genes important for group behaviors and polymicrobial interactions. Our work suggests that therapeutic phages could more broadly influence bacterial community composition outside their intended host targets.

Bacteriophages (phages) have been proposed as alternative therapeutics for the treatment of multidrug-resistant bacterial infections. However, there are major gaps in our understanding of the molecular events in bacterial cells that control how bacteria respond to phage predation. Using the model organism Enterococcus faecalis, we used two distinct genomic approaches, namely, transposon library screening and RNA sequencing, to investigate the interaction of E. faecalis with a virulent phage. We discovered that a transcription factor encoding a LytR family response regulator controls the expression of enterococcal polysaccharide antigen (epa) genes that are involved in phage infection and bacterial fitness. In addition, we discovered that DNA mismatch repair mutants rapidly evolve phage adsorption deficiencies, underpinning a molecular basis for epa mutation during phage infection. Transcriptomic profiling of phage-infected E. faecalis revealed broad transcriptional changes influencing viral replication and progeny burst size. We also demonstrate that phage infection alters the expression of bacterial genes associated with intra- and interbacterial interactions, including genes involved in quorum sensing and polymicrobial competition. Together, our results suggest that phage predation has the potential to influence complex microbial behavior and may dictate how bacteria respond to external environmental stimuli. These responses could have collateral effects (positive or negative) on microbial communities, such as the host microbiota, during phage therapy.

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.

Adaptation to Adversity: the Intermingling of Stress Tolerance and Pathogenesis in Enterococci

Enterococcus is a diverse and rugged genus colonizing the gastrointestinal tract of humans and numerous hosts across the animal kingdom. Enterococci are also a leading cause of multidrug-resistant hospital-acquired infections. In each of these settings, enterococci must contend with changing biophysical landscapes and innate immune responses in order to successfully colonize and transit between hosts. Therefore, it appears that the intrinsic durability that evolved to make enterococci optimally competitive in the host gastrointestinal tract also ideally positioned them to persist in hospitals, despite disinfection protocols, and acquire new antibiotic resistances from other microbes. Here, we discuss the molecular mechanisms and regulation employed by enterococci to tolerate diverse stressors and highlight the role of stress tolerance in the biology of this medically relevant genus.

Enterococcal Genetics

The study of the genetics of enterococci has focused heavily on mobile genetic elements present in these organisms, the complex regulatory circuits used to control their mobility, and the antibiotic resistance genes they frequently carry. Recently, more focus has been placed on the regulation of genes involved in the virulence of the opportunistic pathogenic species Enterococcus faecalis and Enterococcus faecium. Little information is available concerning fundamental aspects of DNA replication, partition, and division; this article begins with a brief overview of what little is known about these issues, primarily by comparison with better-studied model organisms. A variety of transcriptional and posttranscriptional mechanisms of regulation of gene expression are then discussed, including a section on the genetics and regulation of vancomycin resistance in enterococci. The article then provides extensive coverage of the pheromone-responsive conjugation plasmids, including sections on regulation of the pheromone response, the conjugative apparatus, and replication and stable inheritance. The article then focuses on conjugative transposons, now referred to as integrated, conjugative elements, or ICEs, and concludes with several smaller sections covering emerging areas of interest concerning the enterococcal mobilome, including nonpheromone plasmids of particular interest, toxin-antitoxin systems, pathogenicity islands, bacteriophages, and genome defense.

Transcriptome Changes of Escherichia coli, Enterococcus faecalis, and Escherichia coli O157:H7 Laboratory Strains in Response to Photo-Degraded DOM

In this study, we investigated gene expression changes in three bacterial strains (Escherichia coli C3000, Escherichia coli O157:H7 B6914, and Enterococcus faecalis ATCC 29212), commonly used as indicators of water quality and as control strains in clinical, food, and water microbiology laboratories. Bacterial transcriptome responses from pure cultures were monitored in microcosms containing water amended with manure-derived dissolved organic matter (DOM), previously exposed to simulated sunlight for 12 h. We used RNA sequencing (RNA-seq) and quantitative real-time reverse transcriptase (qRT-PCR) to compare differentially expressed temporal transcripts between bacteria incubated in microcosms containing sunlight irradiated and non-irradiated DOM, for up to 24 h. In addition, we used whole genome sequencing simultaneously with RNA-seq to identify single nucleotide variants (SNV) acquired in bacterial populations during incubation. These results indicate that E. coli and E. faecalis have different mechanisms for removal of reactive oxygen species (ROS) produced from irradiated DOM. They are also able to produce micromolar concentrations of H₂O₂ from non-irradiated DOM, that should be detrimental to other bacteria present in the environment. Notably, this study provides an assessment of the role of two conjugative plasmids carried by the E. faecalis and highlights the differences in the overall survival dynamics of environmentally-relevant bacteria in the presence of naturally-produced ROS.

Stress Physiology of Lactic Acid Bacteria

Lactic acid bacteria (LAB) are important starter, commensal, or pathogenic microorganisms. The stress physiology of LAB has been studied in depth for over 2 decades, fueled mostly by the technological implications of LAB robustness in the food industry. Survival of probiotic LAB in the host and the potential relatedness of LAB virulence to their stress resilience have intensified interest in the field. Thus, a wealth of information concerning stress responses exists today for strains as diverse as starter (e.g., Lactococcus lactis), probiotic (e.g., several Lactobacillus spp.), and pathogenic (e.g., Enterococcus and Streptococcus spp.) LAB. Here we present the state of the art for LAB stress behavior. We describe the multitude of stresses that LAB are confronted with, and we present the experimental context used to study the stress responses of LAB, focusing on adaptation, habituation, and cross-protection as well as on self-induced multistress resistance in stationary phase, biofilms, and dormancy. We also consider stress responses at the population and single-cell levels. Subsequently, we concentrate on the stress defense mechanisms that have been reported to date, grouping them according to their direct participation in preserving cell energy, defending macromolecules, and protecting the cell envelope. Stress-induced responses of probiotic LAB and commensal/pathogenic LAB are highlighted separately due to the complexity of the peculiar multistress conditions to which these bacteria are subjected in their hosts. Induction of prophages under environmental stresses is then discussed. Finally, we present systems-based strategies to characterize the "stressome" of LAB and to engineer new food-related and probiotic LAB with improved stress tolerance.

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.

Oxidative stress enhances cephalosporin resistance of Enterococcus faecalis through activation of a two-component signaling system

Enterococcus faecalis is a low-GC Gram-positive bacterium, a normal resident of the gastrointestinal (GI) tract, and an important hospital-acquired pathogen. An important risk factor for hospital-acquired enterococcal infections is prior therapy with broad-spectrum cephalosporins, antibiotics that impair cell wall biosynthesis by inhibiting peptidoglycan cross-linking. Enterococci are intrinsically resistant to cephalosporins; however, environmental factors that modulate cephalosporin resistance have not been described. While searching for the genetic determinants of cephalosporin resistance in E. faecalis, we unexpectedly discovered that oxidative stress, whether from external sources or derived from endogenous metabolism, drives enhanced intrinsic resistance to cephalosporins. A particular source of oxidative stress, H2O2, activates signaling through the CroR-CroS two-component signaling system, a known determinant of cephalosporin resistance in E. faecalis. We find that CroR-CroS is required for adaptation to H2O2 stress and that H2O2 potentiates the activities of cephalosporins against E. faecalis when the CroR-CroS signaling system is nonfunctional. Rather than directly detecting H2O2, our data suggest that the CroR-CroS system responds to cell envelope damage caused by H2O2 exposure in order to promote cell envelope repair and enhanced cephalosporin resistance.

Enterococcus faecalis zinc-responsive proteins mediate bacterial defence against zinc overload, lysozyme and oxidative stress

Two Enterococcus faecalis genes encoding the P-type ATPase EF1400 and the putative SapB protein EF0759 were previously shown to be strongly upregulated in the presence of high concentrations of zinc. In the present work, we showed that a Zn(2+)-responsive DNA-binding motif (zim) is present in the promoter regions of these genes. Both proteins were further studied with respect to their involvement in zinc homeostasis and invasion of the host. EF0759 contributed to intramacrophage survival by an as-yet unknown mechanism(s). EF1400, here renamed ZntAEf, is an ATPase with specificity for zinc and plays a role in dealing with several host defences, i.e. zinc overload, oxidative stress and lysozyme; it provides E. faecalis cells with the ability to survive inside macrophages. As these three host defence mechanisms are important at several sites in the host, i.e. inside macrophages and in saliva, this work suggested that ZntAEf constitutes a crucial E. faecalis defence mechanism that is likely to contribute to the ability of this bacterium to endure life inside its host.

Involvement of Enterococcus faecalis small RNAs in stress response and virulence

Candidate small RNAs (sRNAs) have recently been identified in Enterococcus faecalis, a Gram-positive opportunistic pathogen, and six of these candidate sRNAs with unknown functions were selected for a functional study. Deletion mutants and complemented strains were constructed, and their virulence was tested. We were unable to obtain the ef0869-0870 mutant, likely due to an essential role, and the ef0820-0821 sRNA seemed not to be involved in virulence. In contrast, the mutant lacking ef0408-0409 sRNA, homologous to the RNAII component of the toxin-antitoxin system, appeared more virulent and more able to colonize mouse organs. The three other mutants showed reduced virulence. In addition, we checked the responses of these mutant strains to several stresses encountered in the gastrointestinal tract or during the infection process. In parallel, the activities of the sRNA promoters were measured using transcriptional fusion constructions. To attempt to identify the regulons of these candidate sRNAs, proteomics profiles of the mutant strains were compared with that of the wild type. This showed that the selected sRNAs controlled the expression of proteins involved in diverse cellular processes and the stress response. The combined data highlight the roles of certain candidate sRNAs in the adaptation of E. faecalis to environmental changes and in the complex transition process from a commensal to a pathogen.

Global metabolic response of Enterococcus faecalis to oxygen

Oxygen and oxidative stress have become relevant components in clarifying the mechanism that weakens bacterial cells in parallel to the mode of action of bactericidal antibiotics. Given the importance of oxidative stress in the overall defense mechanism of bacteria and their apparent role in the antimicrobial mode of action, it is important to understand how bacteria respond to this stress at a metabolic level. The aim of this study was to determine the impact of oxygen on the metabolism of the facultative anaerobe Enterococcus faecalis using continuous culture, metabolomics, and (13)C enrichment of metabolic intermediates. When E. faecalis was rapidly transitioned from anaerobic to aerobic growth, cellular metabolism was directed toward intracellular glutathione production and glycolysis was upregulated 2-fold, which increased the supply of critical metabolite precursors (e.g., glycine and glutamate) for sulfur metabolism and glutathione biosynthesis as well as reducing power for cellular respiration in the presence of hemin. The ultimate metabolic response of E. faecalis to an aerobic environment was the upregulation of fatty acid metabolism and benzoate degradation, which was linked to important changes in the bacterial membrane composition as evidenced by changes in membrane fatty acid composition and the reduction of membrane-associated demethylmenaquinone. These key metabolic pathways associated with the response of E. faecalis to oxygen may represent potential new targets to increase the susceptibility of this bacterium to bactericidal drugs.

The equine antimicrobial peptide eCATH1 is effective against the facultative intracellular pathogen Rhodococcus equi in mice

Rhodococcus equi, the causal agent of rhodococcosis, is a major pathogen of foals and is also responsible for severe infections in immunocompromised humans. Of great concern, strains resistant to currently used antibiotics have emerged. As the number of drugs that are efficient in vivo is limited because of the intracellular localization of the bacterium inside macrophages, new active but cell-permeant drugs will be needed in the near future. In the present study, we evaluated, by in vitro and ex vivo experiments, the ability of the alpha-helical equine antimicrobial peptide eCATH1 to kill intracellular bacterial cells. Moreover, the therapeutic potential of the peptide was assessed in experimental rhodococcosis induced in mice, while the in vivo toxicity was evaluated by behavioral and histopathological analysis. The study revealed that eCATH1 significantly reduced the number of bacteria inside macrophages. Furthermore, the bactericidal potential of the peptide was maintained in vivo at doses that appeared to have no visible deleterious effects for the mice even after 7 days of treatment. Indeed, daily subcutaneous injections of 1 mg/kg body weight of eCATH1 led to a significant reduction of the bacterial load in organs comparable to that obtained after treatment with 10 mg/kg body weight of rifampin. Interestingly, the combination of the peptide with rifampin showed a synergistic interaction in both ex vivo and in vivo experiments. These results emphasize the therapeutic potential that eCATH1 represents in the treatment of rhodococcosis.

EfaR is a major regulator of Enterococcus faecalis manganese transporters and influences processes involved in host colonization and infection

Metal ions, in particular manganese, are important modulators of bacterial pathogenicity. However, little is known about the role of manganese-dependent proteins in the nosocomial pathogen Enterococcus faecalis, a major cause of bacterial endocarditis. The present study demonstrates that the DtxR/MntR family metalloregulator EfaR of E. faecalis controls the expression of several of its regulon members in a manganese-dependent way. We also show that efaR inactivation impairs the ability of E. faecalis to form biofilms, to survive inside macrophages, and to tolerate oxidative stress. Our results reveal that EfaR is an important modulator of E. faecalis virulence and link manganese homeostasis to enterococcal pathogenicity.

Enterococcus faecalis overcomes foreign body-mediated inflammation to establish urinary tract infections

Urinary catheterization elicits major histological and immunological changes that render the bladder susceptible to microbial invasion, colonization, and dissemination. However, it is not understood how catheters induce these changes, how these changes act to promote infection, or whether they may have any protective benefit. In the present study, we examined how catheter-associated inflammation impacts infection by Enterococcus faecalis, a leading cause of catheter-associated urinary tract infection (CAUTI), a source of significant societal and clinical challenges. Using a recently optimized murine model of foreign body-associated UTI, we found that the implanted catheter itself was the primary inducer of inflammation. In the absence of the silicone tubing implant, E. faecalis induced only minimal inflammation and was rapidly cleared from the bladder. The catheter-induced inflammation was only minimally altered by subsequent enterococcal infection and was not suppressed by inhibitors of the neurogenic pathway and only partially by dexamethasone. Despite the robust inflammatory response induced by urinary implantation, E. faecalis produced biofilm and high bladder titers in these animals. Induction of inflammation in the absence of an implanted catheter failed to promote infection, suggesting that the presence of the catheter itself is essential for E. faecalis persistence in the bladder. Immunosuppression prior to urinary catheterization enhanced E. faecalis colonization, suggesting that implant-mediated inflammation contributes to the control of enterococcal infection. Thus, this study underscores the need for novel strategies against CAUTIs that seek to reduce the deleterious effects of implant-mediated inflammation on bladder homeostasis while maintaining an active immune response that effectively limits bacterial invaders.

The Spx regulator modulates stress responses and virulence in Enterococcus faecalis

The ability to cope with endogenous or host-generated reactive oxygen species is considered a key virulence attribute of the opportunistic pathogen Enterococcus faecalis, a leading cause of hospital-acquired infections. In this study, we used in silico and mutational analyses to identify and characterize the role of the Spx global regulator in oxidative stress tolerance and virulence in E. faecalis. While the Δspx strain grew as well as the wild-type strain under anaerobic conditions, the mutant strain exhibited impaired growth under aerobic conditions and was highly sensitive to oxidative stress agents. The spx mutant strain was also sensitive to a variety of other stressful conditions, including antibiotic stress and killing by the mouse-derived macrophage cell line J774. Using a murine model of foreign body-associated peritonitis, we demonstrated that the ability of the Δspx strain to colonize the peritoneum and disseminate in the bloodstream was significantly reduced compared to that of the parent strain. Transcriptional analysis revealed that a large number of known oxidative stress genes are under positive control by Spx. Collectively, our results show that Spx is a major stress gene regulator and is implicated in the pathophysiology of E. faecalis. The relationship of Spx to other oxidative stress regulators is also discussed.

Large-scale screening of a targeted Enterococcus faecalis mutant library identifies envelope fitness factors

Spread of antibiotic resistance among bacteria responsible for nosocomial and community-acquired infections urges for novel therapeutic or prophylactic targets and for innovative pathogen-specific antibacterial compounds. Major challenges are posed by opportunistic pathogens belonging to the low GC% Gram-positive bacteria. Among those, Enterococcus faecalis is a leading cause of hospital-acquired infections associated with life-threatening issues and increased hospital costs. To better understand the molecular properties of enterococci that may be required for virulence, and that may explain the emergence of these bacteria in nosocomial infections, we performed the first large-scale functional analysis of E. faecalis V583, the first vancomycin-resistant isolate from a human bloodstream infection. E. faecalis V583 is within the high-risk clonal complex 2 group, which comprises mostly isolates derived from hospital infections worldwide. We conducted broad-range screenings of candidate genes likely involved in host adaptation (e.g., colonization and/or virulence). For this purpose, a library was constructed of targeted insertion mutations in 177 genes encoding putative surface or stress-response factors. Individual mutants were subsequently tested for their i) resistance to oxidative stress, ii) antibiotic resistance, iii) resistance to opsonophagocytosis, iv) adherence to the human colon carcinoma Caco-2 epithelial cells and v) virulence in a surrogate insect model. Our results identified a number of factors that are involved in the interaction between enterococci and their host environments. Their predicted functions highlight the importance of cell envelope glycopolymers in E. faecalis host adaptation. This study provides a valuable genetic database for understanding the steps leading E. faecalis to opportunistic virulence.

A new piece of the Shigella Pathogenicity puzzle: spermidine accumulation by silencing of the speG gene [corrected]

The genome of Shigella, a gram negative bacterium which is the causative agent of bacillary dysentery, shares strong homologies with that of its commensal ancestor, Escherichia coli. The acquisition, by lateral gene transfer, of a large plasmid carrying virulence determinants has been a crucial event in the evolution towards the pathogenic lifestyle and has been paralleled by the occurrence of mutations affecting genes, which negatively interfere with the expression of virulence factors. In this context, we have analysed to what extent the presence of the plasmid-encoded virF gene, the major activator of the Shigella regulon for invasive phenotype, has modified the transcriptional profile of E. coli. Combining results from transcriptome assays and comparative genome analyses we show that in E. coli VirF, besides being able to up-regulate several chromosomal genes, which potentially influence bacterial fitness within the host, also activates genes which have been lost by Shigella. We have focused our attention on the speG gene, which encodes spermidine acetyltransferase, an enzyme catalysing the conversion of spermidine into the physiologically inert acetylspermidine, since recent evidence stresses the involvement of polyamines in microbial pathogenesis. Through identification of diverse mutations, which prevent expression of a functional SpeG protein, we show that the speG gene has been silenced by convergent evolution and that its inactivation causes the marked increase of intracellular spermidine in all Shigella spp. This enhances the survival of Shigella under oxidative stress and allows it to better face the adverse conditions it encounters inside macrophage. This is supported by the outcome of infection assays performed in mouse peritoneal macrophages and of a competitive-infection assay on J774 macrophage cell culture. Our observations fully support the pathoadaptive nature of speG inactivation in Shigella and reveal that the accumulation of spermidine is a key determinant in the pathogenicity strategy adopted by this microrganism.

SlyA regulator is involved in bile salts stress response of Enterococcus faecalis

SlyA is a newly transcriptional regulator identified in Enterococcus faecalis that is involved in the virulence, persistence in mouse kidneys and liver, and survival inside peritoneal macrophages. In this study we searched for environmental conditions that affect expression of the corresponding gene. Of the several stress conditions tested, only bile salts (0.08%) significantly induced transcription of slyA. In addition, the growth of ΔslyA mutant strain was significantly impaired in the presence of bile salts. To increase knowledge of SlyA regulon, real-time quantitative PCR was performed and revealed that expression of EF_3005, which encodes a choloylglycine hydrolase, is negatively regulated by SlyA.

Comparative genome-wide analysis of small RNAs of major Gram-positive pathogens: from identification to application

In the recent years, the number of drug- and multi-drug-resistant microbial strains has increased rapidly. Therefore, the need to identify innovative approaches for development of novel anti-infectives and new therapeutic targets is of high priority in global health care. The detection of small RNAs (sRNAs) in bacteria has attracted considerable attention as an emerging class of new gene expression regulators. Several experimental technologies to predict sRNA have been established for the Gram-negative model organism Escherichia coli. In many respects, sRNA screens in this model system have set a blueprint for the global and functional identification of sRNAs for Gram-positive microbes, but the functional role of sRNAs in colonization and pathogenicity for Listeria monocytogenes, Staphylococcus aureus, Streptococcus pyogenes, Enterococcus faecalis and Clostridium difficile is almost completely unknown. Here, we report the current knowledge about the sRNAs of these socioeconomically relevant Gram-positive pathogens, overview the state-of-the-art high-throughput sRNA screening methods and summarize bioinformatics approaches for genome-wide sRNA identification and target prediction. Finally, we discuss the use of modified peptide nucleic acids (PNAs) as a novel tool to inactivate potential sRNA and their applications in rapid and specific detection of pathogenic bacteria.

Comparative genomic analysis of pathogenic and probiotic Enterococcus faecalis isolates, and their transcriptional responses to growth in human urine

Urinary tract infection (UTI) is the most common infection caused by enterococci, and Enterococcus faecalis accounts for the majority of enterococcal infections. Although a number of virulence related traits have been established, no comprehensive genomic or transcriptomic studies have been conducted to investigate how to distinguish pathogenic from non-pathogenic E. faecalis in their ability to cause UTI. In order to identify potential genetic traits or gene regulatory features that distinguish pathogenic from non-pathogenic E. faecalis with respect to UTI, we have performed comparative genomic analysis, and investigated growth capacity and transcriptome profiling in human urine in vitro. Six strains of different origins were cultivated and all grew readily in human urine. The three strains chosen for transcriptional analysis showed an overall similar response with respect to energy and nitrogen metabolism, stress mechanism, cell envelope modifications, and trace metal acquisition. Our results suggest that citrate and aspartate are significant for growth of E. faecalis in human urine, and manganese appear to be a limiting factor. The majority of virulence factors were either not differentially regulated or down-regulated. Notably, a significant up-regulation of genes involved in biofilm formation was observed. Strains from different origins have similar capacity to grow in human urine. The overall similar transcriptional responses between the two pathogenic and the probiotic strain suggest that the pathogenic potential of a certain E. faecalis strain may to a great extent be determined by presence of fitness and virulence factors, rather than the level of expression of such traits.

On the origin of reactive oxygen species and antioxidative mechanisms in Enterococcus faecalis

Enterococci cause serious infections due to a number of virulence factors and wide-spread antibiotic resistance. A molecular mechanism involved in the pathogenesis of enterococcal infections is oxidative stress. Enterococcus faecalis produces a variety of antioxidative enzymes involved in the oxidative stress response, a process that is regulated by several transcriptional regulators. In addition, direct production of free radicals derived from oxygen has been proved and hypothesized, respectively, to contribute to the pathogenesis of colorectal cancer and periodontitis. The understanding of molecular mechanisms behind the production of free radicals and the antioxidative status in E. faecalis might suggest new alternatives for the treatment of enterococcal infections and related diseases.

The (p)ppGpp synthetase RelA contributes to stress adaptation and virulence in Enterococcus faecalis V583

Guanosine penta- and tetraphosphate [(p)ppGpp] are two unusual nucleotides implied in the bacterial stringent response. In many pathogenic bacteria, mutants unable to synthesize these molecules lose their virulence. In Gram-positive bacteria such as Enterococcus faecalis, the synthesis and degradation of (p)ppGpp mainly depend on the activity of a bifunctional enzyme, encoded by the relA gene. By analysing DeltarelA and DeltarelQ (which encodes a protein harbouring a ppGpp synthetase activity) deletion mutants, we showed that RelA is by far the main system leading to (p)ppGpp production under our experimental conditions, and during the development of a stringent response induced by mupirocin. We also constructed a mutant (DeltarelAsp) in which a small part of the relA gene (about 0.7 kbp) encoding the carboxy-terminal domain of the RelA protein was deleted. Both relA mutants were more resistant than the wild-type strain to 0.3 % bile salts, 25 % ethanol and acid (pH 2.3) challenges. Interestingly, the DeltarelAsp mutant grew better than the two other strains in the presence of 1 mM H(2)O(2), but did not display increased tolerance when subjected to lethal doses of H(2)O(2) (45 mM). By contrast, the DeltarelA mutant was highly sensitive to 45 mM H(2)O(2) and displayed reduced growth in a medium containing 1 M NaCl. The two mutants also displayed contrasting virulence phenotypes towards larvae of the Greater Wax Moth infection model Galleria mellonella. Indeed, although the DeltarelA mutant did not display any phenotype, the DeltarelAsp mutant was more virulent than the wild-type strain. This virulent phenotype should stem from its increased ability to proliferate under oxidative environments.

ace, Which encodes an adhesin in Enterococcus faecalis, is regulated by Ers and is involved in virulence

Enterococcus faecalis is an opportunistic pathogen that causes numerous infectious diseases in humans and is a major agent of nosocomial infections. In this work, we showed that the recently identified transcriptional regulator Ers (PrfA like), known to be involved in the cellular metabolism and the virulence of E. faecalis, acts as a repressor of ace, which encodes a collagen-binding protein. We characterized the promoter region of ace, and transcriptional analysis by reverse transcription-quantitative PCR and mobility shift protein-DNA binding assays revealed that Ers directly regulates the expression of ace. Transcription of ace appeared to be induced by the presence of bile salts, probably via the deregulation of ers. Moreover, with an ace deletion mutant and the complemented strain and by using an insect (Galleria mellonella) virulence model, as well as in vivo-in vitro murine macrophage models, we demonstrated for the first time that Ace can be considered a virulence factor for E. faecalis. Furthermore, animal experiments revealed that Ace is also involved in urinary tract infection by E. faecalis.

A phenotypic microarray analysis of a Streptococcus mutans liaS mutant

Streptococcus mutans, a biofilm-forming Gram-positive bacterium that resides in the human oral cavity, is considered to be the primary aetiological agent of human dental caries. A cell-envelope stress-sensing histidine kinase, LiaS, is considered to be important for expression of virulence factors such as glucan-binding protein C and mutacin production. In this study, a liaS mutant was subjected to phenotypic microarray (PM) analysis of about 2000 phenotypes, including utilization of various carbon, nitrogen, phosphate and sulfur sources; osmolytes; metabolic inhibitors; and susceptibility to toxic compounds, including several types of antibiotics. Compared to the parental strain UA159, the liaS mutant strain (IBS148) was more tolerant to various inhibitors that target protein synthesis, DNA synthesis and cell-wall biosynthesis. Some of the key findings of the PM analysis were confirmed in independent growth studies and by using antibiotic discs and E-test strips for susceptibility testing.

Evolution of intracellular pathogens

The evolution of intracellular pathogens is considered in the context of ambiguities in basic definitions and the diversity of host-microbe interactions. Intracellular pathogenesis is a subset of a larger world of host-microbe interactions that includes amoeboid predation and endosymbiotic existence. Intracellular pathogens often reveal genome reduction. Despite the uniqueness of each host-microbe interaction, there are only a few general solutions to the problem of intracellular survival, especially in phagocytic cells. Similarities in intracellular pathogenic strategies between phylogenetically distant microbes suggest convergent evolution. For discerning such patterns, it is useful to consider whether the microbe is acquired from another host or directly from the environment. For environmentally acquired microbes, biotic pressures, such as amoeboid predators, may select for the capacity for virulence. Although often viewed as a specialized adaptation, the capacity for intracellular survival may be widespread among microbes, thus questioning whether the intracellular lifestyle warrants a category of special distinctiveness.

An AraC-type transcriptional regulator encoded on the Enterococcus faecalis pathogenicity island contributes to pathogenesis and intracellular macrophage survival

A gene encoding a putative AraC-type transcriptional regulator was identified on the 153-kb pathogenicity island (PAI) found among virulent Enterococcus faecalis strains. In an effort to understand the function of this regulator, designated PerA (for pathogenicity island-encoded regulator), we first examined the expression of the perA gene in the original PAI strain MMH594 and in an unrelated clinical isolate E99 by reverse transcription-PCR. Interestingly, expression analysis revealed no detectable perA transcript in MMH594, whereas a transcript was observed in strain E99. Nucleotide sequence analysis revealed that this altered expression between the two strains was attributable to the differential location of an IS1191 element within the putative promoter region upstream of the perA gene. In order to determine the role of this putative regulator in E. faecalis pathogenesis, a perA-deficient mutant was created in strain E99, and the wild-type and mutant pair were compared for phenotypic differences. In in vitro biofilm assays, the mutant strain showed a significantly higher level of growth medium-specific biofilm formation compared to the wild type. However, in a murine intraperitoneal infection model, the mutant strain was significantly less pathogenic. The mutant was also attenuated for survival within macrophages in vitro. These findings highlight the importance of PerA as a regulator of biofilm formation and survival within macrophages and is likely a regulator controlling determinants important to pathogenesis.

Characterization of the Ers regulon of Enterococcus faecalis

Ers has been qualified as the PrfA-like transcriptional regulator of Enterococcus faecalis. In a previous study we reported that Ers is important for the survival within macrophages of this opportunist pathogenic bacterium. In the present work we have used proteomic and microarray expression profiling of E. faecalis JH2-2 and an ers-deleted mutant (Delta ers mutant) strains to define the Ers regulon. In addition to EF_0082 (encoding a putative facilitator family transporter), already known to be under Ers regulation, three genes or operons displayed a significant decrease (confirmed by reverse transcription quantitative PCR) in expression in the Delta ers mutant. The first locus corresponds to three genes: arcA, arcB, and arcC1 (arcABC). These genes are members of the ADI operon, encoding enzymes of the arginine deiminase system. The second is the EF_1459 gene, which encodes a hypothetical protein and is located within a putative phage genetic element. Lastly, Ef_3319 is annotated as the alpha subunit of the citrate lyase encoded by citF. citF is a member of a putative 12-gene operon involved in citrate catabolism. Moreover, the promoter sequence, similar to the "PrfA box" and found in the promoter regions of ers and EF_0082, has been shown to be included in the DNA segment recognized by Ers. Phenotypic analysis of the Delta ers mutant strain revealed a growth defect when cultured with arginine or citrate as the energy source; this was not seen for the wild type. As expected, similar results were obtained with mutants in which arcA and citF were inactivated. In addition, in the mouse peritonitis model of virulence, the Delta ers mutant appeared significantly less lethal than the JH2-2 wild-type strain. Taken together, these results indicate that the regulator Ers has a pleiotropic effect, especially in the cellular metabolism and virulence of E. faecalis.

Peroxiredoxins in bacterial antioxidant defense

Peroxiredoxins constitute an important component of the bacterial defense against toxic peroxides. These enzymes use reactive cysteine thiols to reduce peroxides with electrons ultimately derived from reduced pyridine dinucleotides. Studies examining the regulation and physiological roles of AhpC, Tpx, Ohr and OsmC reveal the multilayered nature of bacterial peroxide defense. AhpC is localized in the cytoplasm and has a wide substrate range that includes H2O2, organic peroxides and peroxynitrite. This enzyme functions in both the control of endogenous peroxides, as well as in the inducible defense response to exogenous peroxides or general stresses. Ohr, OsmC and Tpx are organic peroxide specific. Tpx is localized to the periplasm and can be involved in either constitutive peroxide defense or participate in oxidative stress inducible responses depending on the organism. Ohr is an organic peroxide specific defense system that is under the control of the organic peroxide sensing repressor OhrR. In some organisms Ohr homologs are regulated in response to general stress. Clear evidence indicates that AhpC, Tpx and Ohr are involved in virulence. The role of OsmC is less clear. Regulation of OsmC expression is not oxidative stress inducible, but is controlled by multiple general stress responsive regulators.

Relationships between oxidative stress response and virulence in Enterococcus faecalis

Enterococcus faecalis is a Gram-positive bacterium, member of the lactic acid bacteria group and is also one of the major causative agents of nosocomial infections. Moreover, the oxidative stress is one of the main challenges bacteria have to cope with during their process of infection. This review gives a description of this opportunist pathogen bacterium and an overview of the oxidative stress response and its regulation. We subsequently summarize results that suggest a relationship between this stress response and the virulence in E. faecalis.

Comparative study of the physiological roles of three peroxidases (NADH peroxidase, Alkyl hydroperoxide reductase and Thiol peroxidase) in oxidative stress response, survival inside macrophages and virulence of Enterococcus faecalis

The opportunistic pathogen Enterococcus faecalis is well equipped with peroxidatic activities. It harbours three loci encoding a NADH peroxidase, an alkyl hydroperoxide reductase and a protein (EF2932) belonging to the AhpC/TSA family. We present results demonstrating that ef2932 does encode a thiol peroxidase (Tpx) and show that it is part of the regulon of the hydrogen peroxide regulator HypR. Characterization of unmarked deletion mutants showed that all three peroxidases are important for the defence against externally provided H(2)O(2). Exposure to internal generated H(2)O(2) by aerobic growth on glycerol, lactose, galactose or ribose showed that Npr was absolutely required for aerobic growth on glycerol and optimal growth on the other substrates. Growth on glycerol was also dependent on Ahp. Addition of catalase restored growth of the mutants, and therefore, extracellular H(2)O(2) concentrations have been determined. This showed that the time point of growth arrest of the Deltanpr mutant correlated with the highest H(2)O(2) concentration measured. Analysis of the survival of the different strains inside peritoneal macrophages revealed that Tpx was the most important antioxidant activity for protecting the cells against the hostile phagocyte environment. Finally, the Deltatpx and the triple mutant showed attenuated virulence in a mouse peritonitis model.

Enterococcus faecalis constitutes an unusual bacterial model in lysozyme resistance

Lysozyme is an important and widespread compound of the host constitutive defense system, and it is assumed that Enterococcus faecalis is one of the few bacteria that are almost completely lysozyme resistant. On the basis of the sequence analysis of the whole genome of E. faecalis V583 strain, we identified two genes that are potentially involved in lysozyme resistance, EF_0783 and EF_1843. Protein products of these two genes share significant homology with Staphylococcus aureus peptidoglycan O-acetyltransferase (OatA) and Streptococcus pneumoniae N-acetylglucosamine deacetylase (PgdA), respectively. In order to determine whether EF_0783 and EF_1843 are involved in lysozyme resistance, we constructed their corresponding mutants and a double mutant. The DeltaEF_0783 mutant and DeltaEF_0783 DeltaEF_1843 double mutant were shown to be more sensitive to lysozyme than the parental E. faecalis JH2-2 strain and DeltaEF_1843 mutant were. However, compared to other bacteria, such as Listeria monocytogenes or S. pneumoniae, the tolerance of DeltaEF_0783 and DeltaEF_0783 DeltaEF_1843 mutants towards lysozyme remains very high. Peptidoglycan structure analysis showed that EF_0783 modifies the peptidoglycan by O acetylation of N-acetyl muramic acid, while the EF_1843 deletion has no obvious effect on peptidoglycan structure under the same conditions. Moreover, the EF_0783 and EF_1843 deletions seem to significantly affect the ability of E. faecalis to survive within murine macrophages. In all, while EF_0783 is currently involved in the lysozyme resistance of E. faecalis, peptidoglycan O acetylation and de-N-acetylation are not the main mechanisms conferring high levels of lysozyme resistance to E. faecalis.

Enterococcal leucine-rich repeat-containing protein involved in virulence and host inflammatory response

Enterococcus faecalis is an important nosocomial pathogen associated with high morbidity and mortality for patients who are immunocompromised or who have severe underlying diseases. The E. faecalis genome encodes numerous surface-exposed proteins that may be involved in virulence. This work describes the characterization of the first internalin-like protein in E. faecalis, ElrA, belonging to the recently identified WxL family of surface proteins. ElrA contains an N-terminal signal peptide for export, a leucine-rich repeat domain that may interact with host cells, and a C-terminal WxL domain that interacts with the peptidoglycan. Disruption of the elrA gene significantly attenuates bacterial virulence in a mouse peritonitis model. The elrA deletion mutant also displays a defect in infection of host macrophages and a decreased interleukin-6 response in vivo. Finally, elrA expression is induced in vivo. Altogether, these results demonstrate a role for ElrA in the E. faecalis infectious process in vivo and suggest that this surface protein may contribute to E. faecalis virulence by stimulating the host inflammatory response.

New insights into the Enterococcus faecalis CroRS two-component system obtained using a differential-display random arbitrarily primed PCR approach

Using a modified random arbitrarily primed PCR approach, the operon encoding the Enterococcus faecalis JH2-2 CroRS two-component regulatory system was shown to be repressed during stationary phase, and a CroRS-regulated operon (glnQHMP) was identified. Gel retardation assays showed that the CroR regulator binds specifically to the glnQHMP promoter.

Implication of (Mn)superoxide dismutase of Enterococcus faecalis in oxidative stress responses and survival inside macrophages

The gene encoding the manganese-containing superoxide dismutase (MnSOD) of Enterococcus faecalis was characterized. It is transcribed monocistronically from an upstream promoter identified by rapid amplification of cDNA ends (RACE)-PCR. A sodA mutant was constructed and characterized. Growth of the mutant strain was not significantly different from that of its wild-type counterpart in standing and aerated cultures. However, the mutant was more sensitive towards menadione and hydroperoxide stresses. The response to H(2)O(2) stress was analysed in more detail, and the mode of killing of this oxidant was different under anaerobic and aerobic conditions. Cultures grown and challenged under anaerobic conditions were highly sensitive to treatment with 35 mM H(2)O(2). They were largely protected by the iron chelator deferoxamine, which suggested that killing was mainly due to an enhanced Fenton reaction. In contrast, neither strain was protected by the iron chelators deferoxamine and diethylenetriaminepentaacteic acid when grown and challenged under aerobic conditions, which suggested that inactivation of the cells by H(2)O(2) was due to another killing mode. The sodA mutant was more sensitive under these conditions, showing that MnSOD is also important for protecting the cells from damage under aerobic conditions. Finally, the MnSOD of Ent. faecalis may be considered to be a virulence factor, since survival of the corresponding mutant strain was highly affected inside mouse peritoneal macrophages.

Comparison of OG1RF and an isogenic fsrB deletion mutant by transcriptional analysis: the Fsr system of Enterococcus faecalis is more than the activator of gelatinase and serine protease

The FsrABC system of Enterococcus faecalis controls the expression of gelatinase and a serine protease via a quorum-sensing mechanism, and recent studies suggest that the Fsr system may also regulate other genes important for virulence. To investigate the possibility that Fsr influences the expression of additional genes, we used transcriptional profiling, with microarrays based on the E. faecalis strain V583 sequence, to compare the E. faecalis strain OG1RF with its isogenic mutant, TX5266, an fsrB deletion mutant. We found that the presence of an intact fsrB influences expression of numerous genes throughout the growth phases tested, namely, late log to early stationary phase. In addition, the Fsr regulon is independent of the activity of the proteases, GelE and SprE, whose expression was confirmed to be activated at all three time points tested. While expression of some genes (i.e., ef1097 and ef0750 to -757, encoding hypothetical proteins) was activated in late log phase in OG1RF versus the fsrB deletion mutant, expression of ef1617 to -1634 (eut-pdu orthologues) was highly repressed by the presence of an intact Fsr at entry into stationary phase. This is the first time that Fsr has been characterized as a negative regulator. The newly recognized Fsr-regulated targets include other factors, besides gelatinase, described as important for biofilms (BopD), and genes predicted to encode the surface proteins EF0750 to -0757 and EF1097, along with proteins implicated in several metabolic pathways, indicating that the FsrABC system may be an important regulator in strain OG1RF, with both positive and negative effects.

The response regulator CroR modulates expression of the secreted stress-induced SalB protein in Enterococcus faecalis

The Enterococcus faecalis two-component signal transduction system CroRS, also referred as the RR-HK05 pair, is required for intrinsic beta-lactam resistance (Y. R. Comenge, R. Quintiliani, Jr., L. Li, L. Dubost, J. P. Brouard, J. E. Hugonnet, and M. Arthur, J. Bacteriol. 185:7184-7192, 2003) and is also suspected to be involved in the expression of salB (previously referred to as sagA), a gene important for resistance to environmental stress and cell morphology (Y. Le Breton, G. Boël, A. Benachour, H. Prévost, Y. Auffray, and A. Rincé, Environ. Microbiol. 5:329-337, 2003). In this report, we provide genetic and biochemical evidence that salB encodes a secreted protein that is expressed from a monocistronic stress-inducible operon. Consistent with CroR being a direct transcriptional activator of the salB expression, CroR was found to bind to the salB promoter region in electrophoretic mobility shift assays. Interestingly, we provide evidence that SalB does not play a role in the intrinsic beta-lactam resistance associated with CroRS. We also show that the CroRS system is able to regulate its own expression. The sequence of the CroRS binding site in the salB and croR promoter regions was determined using DNase I footprinting assays.

Characterization of Ers, a PrfA-like regulator ofEnterococcus faecalis

We have identified a transcriptional regulator, named Ers (for enterococcal regulator of survival), of Enterococcus faecalis, an important opportunistic bacterium commonly recovered from hospitalized patients. Ers is a member of the Crp/Fnr family and is 69% similar to Srv, a PrfA-like regulator of Streptococcus pyogenes implicated in virulence, and is the E. faecalis protein most closely related to PrfA, a positive regulator of virulence genes in Listeria monocytogenes. In an in vivo-in vitro macrophage infection model, the survival of an ers mutant was highly significantly decreased compared with that of the parental strain JH2-2. This mutant was more than 10-fold more sensitive to oxidative challenge by hydrogen peroxide. In order to identify genes whose expression was under Ers control, the RNA levels of 31 likely candidates were measured by real-time quantitative PCR. The results indicate that ers may be autoregulated and that the locus ef0082 appears to be positively regulated by Ers. Nevertheless, mutation of ef0082 did not result in any detectable changes in the survival of the bacterium within murine macrophages.

Contribution of a PerR-like regulator to the oxidative-stress response and virulence of Enterococcus faecalis

PerR is one of the most important transcriptional regulators involved in the oxidative-stress response in Bacillus subtilis. Here, the homologous gene in Enterococcus faecalis, ranked among the leading causes of nosocomial infection, was characterized and analysed. Phenotype analysis showed that the perR mutant was significantly more resistant to H2O2 challenge (P<0·05). Expression of eight genes with potential roles in the oxidative-stress response was determined in the wild-type and perR-mutant strains by real-time quantitative PCR. Surprisingly, low quantitative differences in the transcriptional activity of these genes in the mutant versus wild-type were observed. Likewise, this locus was not involved in survival within murine macrophages, but in the mouse peritonitis model, the perR mutant appeared less lethal than the JH2-2 wild-type strain. The combined results show that PerR affects E. faecalis virulence and that its implication in the transcriptional regulation in this bacterium deviates from the B. subtilis model.