Heterologous inducible expression of Enterococcus faecalis pCF10 aggregation substance asc10 in Lactococcus lactis and Streptococcus gordonii contributes to cell hydrophobicity and adhesion to fibrin

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.

High protection of mice against Brucella abortus by oral immunization with recombinant probiotic Lactobacillus casei vector vaccine, expressing the outer membrane protein OMP19 of Brucella species

Brucellosis is a zoonotic disease threatening the public health and hindering the trade of animals and their products, which has a negative impact on the economic development of a country. Vaccination is the most effective way to control brucellosis. The recombinant vector vaccines are promising candidates for immunization in humans and animals. In this study, the gene encoding OMP19 antigen was primarily amplified and cloned into an expression vector called pT1NX, and then transformed to L. casei cell via electroporation technique. The expression was confirmed using specific antibody against the recombinant protein via immunological screening tests such as western blot and immunofluorescence assay. Finally, recombinant L. casei was orally fed to mice and the results were further recorded, indicating that the mice group which received OMP19 through L. casei based vaccine represented a very good general and mucosal immune responses protective against challenges with virulent B. abortus 544 strain compared with negative control recipient groups. Therefore, the vaccine produced in this research plan can be a very good candidate for protection against brucellosis.

Coassociation between Group B Streptococcus and Candida albicans Promotes Interactions with Vaginal Epithelium

Group B Streptococcus (GBS) is a leading cause of neonatal sepsis, pneumonia, and meningitis worldwide. In the majority of cases, GBS is transmitted vertically from mother to neonate, making maternal vaginal colonization a key risk factor for neonatal disease. The fungus Candida albicans is an opportunistic pathogen of the female genitourinary tract and the causative agent of vaginal thrush. Carriage of C. albicans has been shown to be an independent risk factor for vaginal colonization by GBS. However, the nature of interactions between these two microbes is poorly understood. This study provides evidence of a reciprocal, synergistic interplay between GBS and C. albicans that may serve to promote their cocolonization of the vaginal mucosa. GBS strains NEM316 (serotype III) and 515 (serotype Ia) are shown to physically interact with C. albicans, with the bacteria exhibiting tropism for candidal hyphal filaments. This interaction enhances association levels of both microbes with the vaginal epithelial cell line VK2/E6E7. The ability of GBS to coassociate with C. albicans is dependent upon expression of the hypha-specific adhesin Als3. In turn, expression of GBS antigen I/II family adhesins (Bsp polypeptides) facilitates this coassociation and confers upon surrogate Lactococcus lactis the capacity to exhibit enhanced interactions with C. albicans on vaginal epithelium. As genitourinary tract colonization is an essential first step in the pathogenesis of GBS and C. albicans, the coassociation mechanism reported here may have important implications for the risk of disease involving both of these pathogens.

The accessory Sec system (SecY2A2) in Streptococcus pneumoniae is involved in export of pneumolysin toxin, adhesion and biofilm formation

In Streptococcus pneumoniae TIGR4, genes encoding a SecY2A2 accessory Sec system are present within a locus encoding a serine-rich repeat surface protein PsrP. Mutant strains deleted in secA2 or psrP were deficient in biofilm formation, while the ΔsecA2 mutant was reduced in binding to airway epithelial cells. Cell wall protein (CWP) fractions from the ΔsecA2 mutant, but not from the ΔpsrP mutant, were reduced in haemolytic (pneumolysin) activity. Contact-dependent pneumolysin (Ply) activity of wild type TIGR4 cells was ten-fold greater than that of ΔsecA2 mutant cells suggesting that Ply was not active at the ΔsecA2 cell surface. Ply protein was found to be present in the CWP fraction from the ΔsecA2 mutant, but showed aberrant electrophoretic migration indicative of protein modification. Proteomic analyses led to the discovery that the ΔsecA2 mutant CWP fraction was deficient in two glycosidases as well as other enzymes involved in carbohydrate metabolism. Taken collectively the results suggest that positioning of Ply into the cell wall compartment in active form, together with glycosyl hydrolases and adhesins, requires a functional accessory Sec system.

The Streptococcus gordonii Adhesin CshA Protein Binds Host Fibronectin via a Catch-Clamp Mechanism

Adherence of bacteria to biotic or abiotic surfaces is a prerequisite for host colonization and represents an important step in microbial pathogenicity. This attachment is facilitated by bacterial adhesins at the cell surface. Because of their size and often elaborate multidomain architectures, these polypeptides represent challenging targets for detailed structural and functional characterization. The multifunctional fibrillar adhesin CshA, which mediates binding to both host molecules and other microorganisms, is an important determinant of colonization by Streptococcus gordonii, an oral commensal and opportunistic pathogen of animals and humans. CshA binds the high-molecular-weight glycoprotein fibronectin (Fn) via an N-terminal non-repetitive region, and this protein-protein interaction has been proposed to promote S. gordonii colonization at multiple sites within the host. However, the molecular details of how these two proteins interact have yet to be established. Here we present a structural description of the Fn binding N-terminal region of CshA, derived from a combination of X-ray crystallography, small angle X-ray scattering, and complementary biophysical methods. In vitro binding studies support a previously unreported two-state "catch-clamp" mechanism of Fn binding by CshA, in which the disordered N-terminal domain of CshA acts to "catch" Fn, via formation of a rapidly assembled but also readily dissociable pre-complex, enabling its neighboring ligand binding domain to tightly clamp the two polypeptides together. This study presents a new paradigm for target binding by a bacterial adhesin, the identification of which will inform future efforts toward the development of anti-adhesive agents that target S. gordonii and related streptococci.

Concerted functions of Streptococcus gordonii surface proteins PadA and Hsa mediate activation of human platelets and interactions with extracellular matrix

A range of Streptococcus bacteria are able to interact with blood platelets to form a thrombus (clot). Streptococcus gordonii is ubiquitous within the human oral cavity and amongst the common pathogens isolated from subjects with infective endocarditis. Two cell surface proteins, Hsa and Platelet adherence protein A (PadA), in S. gordonii mediate adherence and activation of platelets. In this study, we demonstrate that PadA binds activated platelets and that an NGR (Asparagine-Glycine-Arginine) motif within a 657 amino acid residue N-terminal fragment of PadA is responsible for this, together with two other integrin-like recognition motifs RGT and AGD. PadA also acts in concert with Hsa to mediate binding of S. gordonii to cellular fibronectin and vitronectin, and to promote formation of biofilms. Evidence is presented that PadA and Hsa are each reliant on the other's active presentation on the bacterial cell surface, suggesting cooperativity in functions impacting both colonization and pathogenesis.

A Critical Evaluation of Bifidobacterial Adhesion to the Host Tissue

Bifidobacteria are common inhabitants of the human gastrointestinal tract that, despite a long history of research, have not shown any pathogenic potential whatsoever. By contrast, some bifidobacteria are associated with a number of health-related benefits for the host. The reported beneficial effects of bifidobacteria include competitive exclusion of pathogens, alleviation of symptoms of irritable bowel syndrome and inflammatory bowel disease, and modulation of intestinal and systemic immune responses. Based on these effects, bifidobacteria are widely used as probiotics by pharmaceutical and dairy industries. In order to exert a beneficial effect bifidobacteria have to, at least transiently, colonize the host in a sufficient population size. Besides other criteria such as resistance to manufacturing processes and intestinal transit, potential probiotic bacteria are tested for adhesion to the host structures including intestinal epithelial cells, mucus, and extracellular matrix components. In the present review article, we summarize the current knowledge on bifidobacterial structures that mediate adhesion to host tissue and compare these to similar structures of pathogenic bacteria. This reveals that most of the adhesive structures and mechanisms involved in adhesion of bifidobacteria to host tissue are similar or even identical to those employed by pathogens to cause disease. It is thus reasonable to assume that these structures and mechanisms are equally important for commensal or probiotic bacteria and play a similar role in the beneficial effects exerted by bifidobacteria.

Structural and Functional Analysis of Cell Wall-anchored Polypeptide Adhesin BspA in Streptococcus agalactiae

Streptococcus agalactiae (group B Streptococcus, GBS) is the predominant cause of early-onset infectious disease in neonates and is responsible for life-threatening infections in elderly and immunocompromised individuals. Clinical manifestations of GBS infection include sepsis, pneumonia, and meningitis. Here, we describe BspA, a deviant antigen I/II family polypeptide that confers adhesive properties linked to pathogenesis in GBS. Heterologous expression of BspA on the surface of the non-adherent bacterium Lactococcus lactis confers adherence to scavenger receptor gp340, human vaginal epithelium, and to the fungus Candida albicans Complementary crystallographic and biophysical characterization of BspA reveal a novel β-sandwich adhesion domain and unique asparagine-dependent super-helical stalk. Collectively, these findings establish a new bacterial adhesin structure that has in effect been hijacked by a pathogenic Streptococcus species to provide competitive advantage in human mucosal infections.

Enterococcal Sex Pheromones: Evolutionary Pathways to Complex, Two-Signal Systems

Gram-positive bacteria carry out intercellular communication using secreted peptides. Important examples of this type of communication are the enterococcal sex pheromone systems, in which the transfer of conjugative plasmids is controlled by intercellular signaling among populations of donors and recipients. This review focuses on the pheromone response system of the conjugative plasmid pCF10. The peptide pheromones regulating pCF10 transfer act by modulating the ability of the PrgX transcription factor to repress the transcription of an operon encoding conjugation functions. Many Gram-positive bacteria regulate important processes, including the production of virulence factors, biofilm formation, sporulation, and genetic exchange using peptide-mediated signaling systems. The key master regulators of these systems comprise the RRNPP (RggRap/NprR/PlcR/PrgX) family of intracellular peptide receptors; these regulators show conserved structures. While many RRNPP systems include a core module of two linked genes encoding the regulatory protein and its cognate signaling peptide, the enterococcal sex pheromone plasmids have evolved to a complex system that also recognizes a second host-encoded signaling peptide. Additional regulatory genes not found in most RRNPP systems also modulate signal production and signal import in the enterococcal pheromone plasmids. This review summarizes several structural studies that cumulatively demonstrate that the ability of three pCF10 regulatory proteins to recognize the same 7-amino-acid pheromone peptide arose by convergent evolution of unrelated proteins from different families. We also focus on the selective pressures and structure/function constraints that have driven the evolution of pCF10 from a simple, single-peptide system resembling current RRNPPs in other bacteria to the current complex inducible plasmid transfer system.

Antigen I/II encoded by integrative and conjugative elements of Streptococcus agalactiae and role in biofilm formation

Streptococcus agalactiae (i.e. Group B streptococcus, GBS) is a major human and animal pathogen. Genes encoding putative surface proteins and in particular an antigen I/II have been identified on Integrative and Conjugative Elements (ICEs) found in GBS. Antigens I/II are multimodal adhesins promoting colonization of the oral cavity by streptococci such as Streptococcus gordonii and Streptococcus mutans. The prevalence and diversity of antigens I/II in GBS were studied by a bioinformatic analysis. It revealed that antigens I/II, which are acquired by horizontal transfer via ICEs, exhibit diversity and are widespread in GBS, in particular in the serotype Ia/ST23 invasive strains. This study aimed at characterizing the impact on GBS biology of proteins encoded by a previously characterized ICE of S. agalactiae (ICE_515_tRNA(Lys)). The production and surface exposition of the antigen I/II encoded by this ICE was examined using RT-PCR and immunoblotting experiments. Surface proteins of ICE_515_tRNA(Lys) were found to contribute to GBS biofilm formation and to fibrinogen binding. Contribution of antigen I/II encoded by SAL_2056 to biofilm formation was also demonstrated. These results highlight the potential for ICEs to spread microbial adhesins between species.

Group B Streptococcus pili mediate adherence to salivary glycoproteins

Group B Streptococcus (GBS) is a leading cause of neonatal sepsis, pneumonia and meningitis, and is responsible for a rising number of severe invasive infections in adults. For all disease manifestations, colonisation is a critical first step. GBS has frequently been isolated from the oropharynx of neonates and adults. However, little is understood about the mechanisms of GBS colonisation at this site. In this study it is shown that three GBS strains (COH1, NEM316, 515) have capacity to adhere to human salivary pellicle. Heterologous expression of GBS pilus island (PI) genes in Lactococcus lactis to form surface-expressed pili demonstrated that GBS PI-2a and PI-1 pili bound glycoprotein-340 (gp340), a component of salivary pellicle. By contrast, PI-2b pili did not interact with gp340. The variation was attributable to differences in capacities for backbone and ancillary protein subunits of each pilus to bind gp340. Furthermore, while GBS strains were aggregated by fluid-phase gp340, this mechanism was not mediated by pili, which displayed specificity for immobilised gp340. Thus pili may enable GBS to colonise the soft and hard tissues of the oropharynx, while evading an innate mucosal defence, with implications for risk of progression to severe diseases such as meningitis and sepsis.

Enterococcus faecalis pCF10-encoded surface proteins PrgA, PrgB (aggregation substance) and PrgC contribute to plasmid transfer, biofilm formation and virulence

Enterococcus faecalis pCF10 transfers at high frequencies upon pheromone induction of the prgQ transfer operon. This operon codes for three cell wall-anchored proteins - PrgA, PrgB (aggregation substance) and PrgC - and a type IV secretion system through which the plasmid is delivered to recipient cells. Here, we defined the contributions of the Prg surface proteins to plasmid transfer, biofilm formation and virulence using the Caenorhabditis elegans infection model. We report that a combination of PrgB and extracellular DNA (eDNA), but not PrgA or PrgC, was required for extensive cellular aggregation and pCF10 transfer at wild-type frequencies. In addition to PrgB and eDNA, production of PrgA was necessary for extensive binding of enterococci to abiotic surfaces and development of robust biofilms. However, although PrgB is a known virulence factor in mammalian infection models, we determined that PrgA and PrgC, but not PrgB, were required for efficient killing in the worm infection model. We propose that the pheromone-responsive, conjugative plasmids of E. faecalis have retained Prg-like surface functions over evolutionary time for attachment, colonization and robust biofilm development. In natural settings, these biofilms are polymicrobial in composition and constitute optimal environments for signal exchange, mating pair formation and widespread lateral gene transfer.

Peptide pheromone signaling in Streptococcus and Enterococcus

Intercellular chemical signaling in bacteria, commonly referred to as quorum sensing (QS), relies on the production and detection of compounds known as pheromones to elicit coordinated responses among members of a community. Pheromones produced by Gram-positive bacteria are comprised of small peptides. Based on both peptide structure and sensory system architectures, Gram-positive bacterial signaling pathways may be classified into one of four groups with a defining hallmark: cyclical peptides of the Agr type, peptides that contain Gly-Gly processing motifs, sensory systems of the RNPP family, or the recently characterized Rgg-like regulatory family. The recent discovery that Rgg family members respond to peptide pheromones increases substantially the number of species in which QS is likely a key regulatory component. These pathways control a variety of fundamental behaviors including conjugation, natural competence for transformation, biofilm development, and virulence factor regulation. Overlapping QS pathways found in multiple species and pathways that utilize conserved peptide pheromones provide opportunities for interspecies communication. Here we review pheromone signaling identified in the genera Enterococcus and Streptococcus, providing examples of all four types of pathways.

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.

Oral immunization of mice with recombinant Lactococcus lactis expressing Cu,Zn superoxide dismutase of Brucella abortus triggers protective immunity

Brucella infections mainly occur through mucosal surfaces. Thus, the development of mucosal administered vaccines could be instrumental for the control of brucellosis. Here, we evaluated the usefulness of recombinant Lactococcus lactis secreting Brucella abortus Cu-Zn superoxide dismutase (SOD) as oral antigen delivery system, when administered alone or in combination with L. lactis expressing IL-12. To this end, mice were vaccinated by oral route with L. lactis NZ9000 transformed with pSEC derivatives encoding for SOD (pSEC:SOD) and IL-12 (pSEC:scIL-12). In animals receiving L. lactis pSEC:SOD alone, anti-SOD-specific IgM antibodies were detected in sera at day 28 post-vaccination, together with an IgG2a dominated IgG response. SOD-specific sIgA was also detected in nasal and bronchoalveolar lavages. In addition, T-cell-proliferative responses upon re-stimulation with either recombinant SOD or crude Brucella protein extracts were observed up to 6 months after the last boost, suggesting the induction of long term memory. Vaccinated animals were also protected against challenge with the virulent B. abortus 2308 strain. Responses were mildly improved when L. lactis pSEC:SOD was co-administered with L. lactis pSEC:scIL-12. These results indicated that vaccines based on lactococci-derived live carriers are promising interventions against B. abortus infections.

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.

Green fluorescent protein-labeled monitoring tool to quantify conjugative plasmid transfer between Gram-positive and Gram-negative bacteria

On the basis of pIP501, a green fluorescent protein (GFP)-tagged monitoring tool was constructed for quantifying plasmid mobilization among Gram-positive bacteria and between Gram-positive Enterococcus faecalis and Gram-negative Escherichia coli. Furthermore, retromobilization of the GFP-tagged monitoring tool was shown from E. faecalis OG1X into the clinical isolate E. faecalis T9.

Staphylococcus aureus keratinocyte invasion is dependent upon multiple high-affinity fibronectin-binding repeats within FnBPA

Staphylococcus aureus is a commensal organism and a frequent cause of skin and soft tissue infections, which can progress to serious invasive disease. This bacterium uses its fibronectin binding proteins (FnBPs) to invade host cells and it has been hypothesised that this provides a protected niche from host antimicrobial defences, allows access to deeper tissues and provides a reservoir for persistent or recurring infections. FnBPs contain multiple tandem fibronectin-binding repeats (FnBRs) which bind fibronectin with varying affinity but it is unclear what selects for this configuration. Since both colonisation and skin infection are dependent upon the interaction of S. aureus with keratinocytes we hypothesised that this might select for FnBP function and thus composition of the FnBR region. Initial experiments revealed that S. aureus attachment to keratinocytes is rapid but does not require FnBRs. By contrast, invasion of keratinocytes was dependent upon the FnBR region and occurred via similar cellular processes to those described for endothelial cells. Despite this, keratinocyte invasion was relatively inefficient and appeared to include a lag phase, most likely due to very weak expression of α₅β₁ integrins. Molecular dissection of the role of the FnBR region revealed that efficient invasion of keratinocytes was dependent on the presence of at least three high-affinity (but not low-affinity) FnBRs. Over-expression of a single high-affinity or three low-affinity repeats promoted invasion but not to the same levels as S. aureus expressing an FnBPA variant containing three high-affinity repeats. In summary, invasion of keratinocytes by S. aureus requires multiple high-affinity FnBRs within FnBPA, and given the importance of the interaction between these cell types and S. aureus for both colonisation and infection, may have provided the selective pressure for the multiple binding repeats within FnBPA.

Staphylococcus aureus host cell invasion and virulence in sepsis is facilitated by the multiple repeats within FnBPA

Entry of Staphylococcus aureus into the bloodstream can lead to metastatic abscess formation and infective endocarditis. Crucial to the development of both these conditions is the interaction of S. aureus with endothelial cells. In vivo and in vitro studies have shown that the staphylococcal invasin FnBPA triggers bacterial invasion of endothelial cells via a process that involves fibronectin (Fn) bridging to alpha(5)beta(1) integrins. The Fn-binding region of FnBPA usually contains 11 non-identical repeats (FnBRs) with differing affinities for Fn, which facilitate the binding of multiple Fn molecules and may promote integrin clustering. We thus hypothesized that multiple repeats are necessary to trigger the invasion of endothelial cells by S. aureus. To test this we constructed variants of fnbA containing various combinations of FnBRs. In vitro assays revealed that endothelial cell invasion can be facilitated by a single high-affinity, but not low-affinity FnBR. Studies using a nisin-inducible system that controlled surface expression of FnBPA revealed that variants encoding fewer FnBRs required higher levels of surface expression to mediate invasion. High expression levels of FnBPA bearing a single low affinity FnBR bound Fn but did not invade, suggesting that FnBPA affinity for Fn is crucial for triggering internalization. In addition, multiple FnBRs increased the speed of internalization, as did higher expression levels of FnBPA, without altering the uptake mechanism. The relevance of these findings to pathogenesis was demonstrated using a murine sepsis model, which showed that multiple FnBRs were required for virulence. In conclusion, multiple FnBRs within FnBPA facilitate efficient Fn adhesion, trigger rapid bacterial uptake and are required for pathogenesis.

The terminal A domain of the fibrillar accumulation-associated protein (Aap) of Staphylococcus epidermidis mediates adhesion to human corneocytes

The opportunistic pathogen Staphylococcus epidermidis colonizes indwelling medical devices by biofilm formation but is primarily a skin resident. In many S. epidermidis strains biofilm formation is mediated by a cell wall-anchored protein, the accumulation-associated protein (Aap). Here, we investigate the role of Aap in skin adhesion. Aap is an LPXTG protein with a domain architecture including a terminal A domain and a B-repeat region. S. epidermidis NCTC 11047 expresses Aap as localized, lateral tufts of fibrils on one subpopulation of cells (Fib(+)), whereas a second subpopulation does not express these fibrils of Aap (Fib(-)). Flow cytometry showed that 72% of NCTC 11047 cells expressed Aap and that 28% of cells did not. Aap is involved in the adhesion of Fib(+) cells to squamous epithelial cells from the hand (corneocytes), as the recombinant A-domain protein partially blocked binding to corneocytes. To confirm the role of the Aap A domain in corneocyte attachment, Aap was expressed on the surface of Lactococcus lactis MG1363 as sparsely distributed, peritrichous fibrils. The expression of Aap increased corneocyte adhesion 20-fold compared to L. lactis carrying Aap without an A domain. S. epidermidis isolates from catheters, artificial joints, skin, and the nose also used the A domain of Aap to adhere to corneocytes, emphasizing the role of Aap in skin adhesion. In addition, L. lactis expressing Aap with different numbers of B repeats revealed a positive correlation between the number of B repeats and adhesion to corneocytes, suggesting an additional function for the B region in enhancing A-domain-dependent attachment to skin. Therefore, in addition to its established role in biofilm formation, Aap can also promote adhesion to corneocytes and is likely to be an important adhesin in S. epidermidis skin colonization.

Multiple adhesin proteins on the cell surface of Streptococcus gordonii are involved in adhesion to human fibronectin

Adhesion of bacterial cells to fibronectin (FN) is thought to be a pivotal step in the pathogenesis of invasive infectious diseases. Viridans group streptococci such as Streptococcus gordonii are considered commensal members of the oral microflora, but are important pathogens in infective endocarditis. S. gordonii expresses a battery of cell-surface adhesins that act alone or in concert to bind host receptors. Here, we employed molecular genetic approaches to determine the relative contributions of five known S. gordonii surface proteins to adherence to human FN. Binding levels to FN by isogenic mutants lacking Hsa glycoprotein were reduced by 70 %, while mutants lacking CshA and CshB fibrillar proteins showed approximately 30 % reduced binding. By contrast, disruption of antigen I/II adhesin genes sspA and sspB in a wild-type background did not result in reduced FN binding. Enzymic removal of sialic acids from FN led to reduced S. gordonii DL1 adhesion (>50 %), but did not affect binding by the hsa mutant, indicating that Hsa interacts with sialic acid moieties on FN. Conversely, desialylation of FN did not affect adherence levels of Lactococcus lactis cells expressing SspA or SspB polypeptides. Complementation of the hsa mutant partially restored adhesion to FN. A model is proposed for FN binding by S. gordonii in which Hsa and CshA/CshB are primary adhesins, and SspA or SspB play secondary roles. Understanding the basis of oral streptococcal interactions with FN will provide a foundation for development of new strategies to control infective endocarditis.

Enterococcus faecalis PcfC, a spatially localized substrate receptor for type IV secretion of the pCF10 transfer intermediate

Upon sensing of peptide pheromone, Enterococcus faecalis efficiently transfers plasmid pCF10 through a type IV secretion (T4S) system to recipient cells. The PcfF accessory factor and PcfG relaxase initiate transfer by catalyzing strand-specific nicking at the pCF10 origin of transfer sequence (oriT). Here, we present evidence that PcfF and PcfG spatially coordinate docking of the pCF10 transfer intermediate with PcfC, a membrane-bound putative ATPase related to the coupling proteins of gram-negative T4S machines. PcfC and PcfG fractionated with the membrane and PcfF with the cytoplasm, yet all three proteins formed several punctate foci at the peripheries of pheromone-induced cells as monitored by immunofluorescence microscopy. A PcfC Walker A nucleoside triphosphate (NTP) binding site mutant (K156T) fractionated with the E. faecalis membrane and also formed foci, whereas PcfC deleted of its N-terminal putative transmembrane domain (PcfCDelta N103) distributed uniformly throughout the cytoplasm. Native PcfC and mutant proteins PcfCK156T and PcfCDelta N103 bound pCF10 but not pcfG or Delta oriT mutant plasmids as shown by transfer DNA immunoprecipitation, indicating that PcfC binds only the processed form of pCF10 in vivo. Finally, purified PcfCDelta N103 bound DNA substrates and interacted with purified PcfF and PcfG in vitro. Our findings support a model in which (i) PcfF recruits PcfG to oriT to catalyze T-strand nicking, (ii) PcfF and PcfG spatially position the relaxosome at the cell membrane to stimulate substrate docking with PcfC, and (iii) PcfC initiates substrate transfer through the pCF10 T4S channel by an NTP-dependent mechanism.

Expression of bioactive porcine interferon-gamma by recombinant Lactococcus lactis

Lactococcus lactis is an immunomodulator and candidate live mucosal delivery vehicle for vaccine antigens and for biologically active molecules, including immunoregulatory cytokines such as interferon-gamma (IFN-gamma). To provide a tool for investigating downregulation of allergic predisposition of pigs to experimental food allergy, porcine IFN-gamma was cloned and expressed as a fusion protein with the usp45 secretion signal. Immunoblot analysis with polyclonal anti-pIFN-gamma-antibody demonstrated that the recombinant porcine IFN-gamma (rpIFN-gamma) protein was expressed in the L. lactis transformants as a secreted product. Activity of rpIFN-gamma was confirmed by ability to upregulate class II major histocompatibility complex (MHC) on cells of the porcine monocytic cell line 3D4/31. The L. lactis producing biologically active rpIFN-gamma will be used to investigate its possible ability to modulate the allergic immune response phenotype of pigs.

Characterization of monolaurin resistance in Enterococcus faecalis

There is increasing concern regarding the presence of vancomycin-resistant enterococci in domestically farmed animals, which may act as reservoirs and vehicles of transmission for drug-resistant enterococci to humans, resulting in serious infections. In order to assess the potential for the use of monolaurin as a food preservative, it is important to understand both its target and potential mechanisms of resistance. A Tn917 mutant library of Enterococcus faecalis AR01/DGVS was screened for resistance (MIC, >100 microg/ml) to monolaurin. Three mutants were identified as resistant to monolaurin and were designated DGRM2, DGRM5, and DGRM12. The gene interrupted in all three mutants was identified as traB, which encodes an E. faecalis pheromone shutdown protein and whose complementation in trans restored monolaurin sensitivity in all three mutants. DGRM2 was selected for further characterization. E. faecalis DGRM2 showed increased resistance to gentamicin and chloramphenicol (inhibitors of protein synthesis), while no difference in the MIC was observed with the cell wall-active antibiotics penicillin and vancomycin. E. faecalis AR01/DGVS and DGRM2 were shown to have similar rates (30% cell lysis after 4 h) of cell autolytic activity when activated by monolaurin. Differences in cell surface hydrophobicity were observed between the wild type and the mutant, with the cell surface of the parent strain being significantly more hydrophobic. Analysis of the cell wall structure of DGRM2 by transmission electron microscopy revealed an increase in the apparent cell wall thickness and contraction of its cytoplasm. Taken together, these results suggest that the increased resistance of DGRM2 was due to a change in cell surface hydrophobicity, consequently limiting the diffusion of monolaurin to a potential target in the cytoplasmic membrane and/or cytoplasm of E. faecalis.

Improvement of a nisin-inducible expression vector for use in lactic acid bacteria

The plasmid pMSP3535 is a popular vector for nisin-inducible expression of heterologous genes in lactic acid bacteria. However, the maximum protein expression level achievable with pMSP3535 is relatively low. In an effort to increase expression we modified pMSP3535 to create a high expression variant termed pMSP3535H2. Modifications included removal of a small NisA peptide fragment from the P nisA promoter and addition of a bidirectional transcription terminator. In addition the plasmid copy number was increased by replacing the pMSP3535 copy number control region with that of a high copy variant of the same replicon. As a result of these modifications, expression of two target proteins, the green fluorescent protein and the Escherichia coli antigen intimin, increased 5.0- and 7.5-fold, respectively. The increased range of inducible expression achieved with pMSP3535H2 will facilitate molecular studies in a range of lactic acid bacteria.

Expression of virulence-related genes by Enterococcus faecalis in response to different environments

Enterococci are ubiquitous organisms used to both improve the flavor and texture of fermented foods, and provide protective mechanisms as either a probiotic or antimicrobial additive. However, two species, E. faecalis and E. faecium, are also associated with 10% of nosocomial infections of the bloodstream, wounds, urinary tract and heart. While the genes involved in the pathogenicity of these organisms are slowly identified along with the mechanisms behind their regulation, the environmental signals involved in the conversion to pathogenicity remain unclear. The distribution of virulence genes was determined in 13 E. faecalis isolates from medical, food and animal sources. Regardless of their source of isolation, all isolates harbored between eight and thirteen virulence genes. Relative differences in expression of the virulence associated genes clpP, clpX, gls24, agg, efaA, gelE, and cylBL(L) were examined in E. faecalis TMW 2.63 and TMW 2.622 exposed to different environments (LB, BHI, respective supernatants, pig fecal extract, LB+6.5% NaCl, LB+pH5, LB+6.5% NaCl+pH5, and sausage medium) using RT-PCR and Lightcycler technology. Significant differences in expression were influenced by growth phase, environment, and isolate, which suggests that these three factors be taken into consideration during the selection of enterococci for use in foods or as probiotics rather than their source of isolation or set of virulence genes.

Cryptic plasmid pSKU146 from the wall-less plant pathogen Spiroplasma kunkelii encodes an adhesin and components of a type IV translocation-related conjugation system

A cryptic plasmid of the wall-less plant pathogenic mollicute, Spiroplasma kunkelii CR2-3X, was cloned and its sequence analyzed. The 14,615 bp plasmid, designated pSKU146, has a nucleotide content of 28 mol% G + C, and contains 18 potential protein-coding regions (open reading frames, ORFs), of which six encode proteins that exhibit similarity to virulence-associated proteins involved in cell-to-cell adhesion or conjugal DNA transfer. One ORF encodes a 96 kDa protein, SkARP1, that is highly similar to SARP1 adhesin involved in attachment of Spiroplasma citri to insect vector gut membrane. Five ORFs encode proteins similar to TraE and Mob in walled bacteria, and to ORFs found in the integrative, conjugative element (ICEF) of Mycoplasma fermentans, respectively. Presence of domains similar to proteins of the Type IV secretion system in pathogenic bacteria suggests that spiroplasma possesses a related translocation system. Plasmid pSKU146 also contains two identical oriT regions each containing a nick sequence characteristic of the IncP conjugative plasmid family, as well as a 58 bp palindromic sequence, palSK1. Features in pSKU146 suggest that the plasmid functions as a mobile genetic element in conjugative transmission of spiroplasma pathogenicity-related genes.

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.

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.

Controlled expression of CluA in Lactococcus lactis and its role in conjugation

CluA is a 136 kDa surface-bound protein encoded by the chromosomally located sex factor of Lactococcus lactis MG1363 and is associated with cell aggregation linked to high-frequency transfer of the sex factor. To further investigate the involvement of CluA in these phenomena, the cluA gene was cloned on a plasmid, downstream from the lactococcal nisA promoter. In a sex-factor-negative MG1363 derivative, nisin-controlled CluA expression resulted in aggregation, despite the absence of the other genes of the sex factor. Therefore, CluA is the only sex factor component responsible for aggregation. The direct involvement of CluA in the establishment of cell-to-cell contact for aggregate formation was observed by electron microscopy using immunogold-labelled CluA antibodies. Inactivation of cluA in an MG1363 background led to a dramatic decrease in sex factor conjugation frequency compared to the parental strain. Increasing levels of CluA expressed in trans in the cluA-inactivated donor strain facilitated a gradual restoration of conjugation frequency, reaching that of the parental strain. In conclusion, CluA is essential for efficient sex factor transfer in conjugation of L. lactis.

Virulence factors and in vitro adherence of Enterococcus strains to urinary catheters

The ability to adhere in vitro to urinary catheters and the presence of enterococcal virulence factors was determined in 30 Enterococcus urinary isolates (12 E. faecalis, 12 E. faecium, 3 E. casseliflavus, 3 E. gallinarum). Silicone, siliconized latex and polyvinyl chloride (PVC) were examined by sonication quantitative culture technique and scanning electron microscope. As compared to E. faecalis and E. faecium, E. casseliflavus and E. gallinarum displayed lower adhesion to all synthetic materials. All the tests performed showed higher adherence of all tested strains to siliconized latex and silicone than to PVC. Biofilmforming ability was observed in 5 E. faecalis but in none of the remaining strains. The gene coding enterococcal surface protein (Esp) was detected in 7 E. faecalis and 6 E. faecium strains. Gelatinase was found in 1 E. faecalis, 2 E. faecium and hemolysins were found in 6 E. faecalis and 1 E. faecium strains. All E. casseliflavus and E. gallinarum strainswere negative for these traits. Hydrophobic type of cell surface (measured by its affinity for n-hexadecane) was shown in a few isolates. Bacterial adherence was not significantly associated with the above pathogenic factors.

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.

Immunogenicity and protective efficacy of orally administered recombinant Lactococcus lactis expressing surface-bound HIV Env

This study investigates whether genetically modified orally administered Lactococcus lactis (L lactis) could be used as an HIV vaccine. L lactis is immunogenic and extremely safe when delivered orally. We created a recombinant L lactis vector expressing the envelope protein of HIV on its cell surface. Oral immunization with this vector induced high levels of HIV-specific serum IgG and fecal IgA antibodies. Cell-mediated immune responses also were generated in both the regional lymph nodes and the spleen. Dendritic cells are readily infected by L lactis and appear to play a potential role in mediating the development of these immune responses. The protective efficacy of this vaccine strategy was demonstrated by challenging mice intraperitoneally with an HIV Env-expressing vaccinia virus. Their viral loads were 350-fold lower than those of control mice. These findings support the further development of L lactis-based HIV vaccines.

Role of the Enterococcus faecalis GelE protease in determination of cellular chain length, supernatant pheromone levels, and degradation of fibrin and misfolded surface proteins

Gelatinase (GelE), a secreted Zn-metalloprotease of Enterococcus faecalis, has been implicated as a virulence factor by both epidemiological data and animal model studies. Expression of gelE is induced at a high cell density by the fsr quorum-sensing system. In the present study, GelE was shown to be responsible for the instability of a number of Asc10 (aggregation substance) mutant proteins, implying that GelE functions to clear the bacterial cell surface of misfolded proteins. Disruption of GelE production led to increased cell chain length of E. faecalis, from a typical diplococcus morphology to chains of 5 to 10 cells. This function of GelE was also exhibited when the protein was expressed in Streptococcus pyogenes. GelE-expressing E. faecalis strains were more autolytic, suggesting that GelE affects chain length through activation of an autolysin. GelE was also essential for degradation of polymerized fibrin. GelE expression reduced the titer of cCF10, the peptide pheromone that induces conjugation of pCF10, and pCF10 had increased conjugation into non-GelE-expressing strains. These new functions attributed to GelE suggest that it acts to increase the dissemination of E. faecalis in high-density environments.

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.

Identification and characterization of genes encoding sex pheromone cAM373 activity in Enterococcus faecalis and Staphylococcus aureus

The sex pheromone cAM373 of Enterococcus faecalis and the related staph-cAM373 of Staphylococcus aureus were found to correspond to heptapeptides located within the C-termini of the signal sequences of putative prelipoproteins. The deduced mature forms of the lipoproteins share no detectable homology and presumably serve unrelated functions in the cells. The chromosomally encoded genetic determinants for production of the pheromones have been identified and designated camE (encoding cAM373) and camS (encoding staph-cAM373). Truncated and full-length clones of camE were generated in Escherichia coli, in which cAM373 activity was expressed. In E. faecalis, insertional inactivation in the middle of camE had no detectable phenotypic effects on the pheromone system. Establishment of an in frame translation stop codon within the signal sequence resulted in reduction of cAM373 activity to 3% of normal levels. The camS determinant has homologues in Staphylococcus epidermidis, Bacillus subtilis and Listeria monocytogenes; however, corresponding heptapeptides present within those sequences do not resemble staph-cAM373 closely. The particular significance of staph-cAM373 as a potential intergeneric inducer of transfer-proficient genetic elements is discussed.

Production and targeting of the Brucella abortus antigen L7/L12 in Lactococcus lactis: a first step towards food-grade live vaccines against brucellosis

Brucella abortus is a facultative intracellular gram-negative bacterial pathogen that infects humans and animals by entry mainly through the digestive tract. B. abortus causes abortion in pregnant cattle and undulant fever in humans. The immunogenic B. abortus ribosomal protein L7/L12 is a promising candidate antigen for the development of oral live vaccines against brucellosis, using food-grade lactic acid bacteria (LAB) as a carrier. The L7/L12 gene was expressed in Lactococcus lactis, the model LAB, under the nisin-inducible promoter. Using different signals, L7/L12 was produced in cytoplasmic, cell-wall-anchored, and secreted forms. Cytoplasmic production of L7/L12 gave a low yield, estimated at 0.5 mg/liter. Interestingly, a secretable form of this normally cytoplasmic protein via fusion with a signal peptide resulted in increased yield of L7/L12 to 3 mg/liter; secretion efficiency (SE) was 35%. A fusion between the mature moiety of the staphylococcal nuclease (Nuc) and L7/L12 further increased yield to 8 mg/liter. Fusion with a synthetic propeptide (LEISSTCDA) previously described as an enhancer for heterologous protein secretion in L. lactis (Y. Le Loir, A. Gruss, S. D. Ehrlich, and P. Langella, J. Bacteriol. 180:1895-1903, 1998) raised the yield to 8 mg/liter and SE to 50%. A surface-anchored L7/L12 form in L. lactis was obtained by fusing the cell wall anchor of Streptococcus pyogenes M6 protein to the C-terminal end of L7/L12. The fusions described allow the production and targeting of L7/L12 in three different locations in L. lactis. This is the first example of a B. abortus antigen produced in a food-grade bacterium and opens new perspectives for alternative vaccine strategies against brucellosis.

ccfA, the genetic determinant for the cCF10 peptide pheromone in Enterococcus faecalis OG1RF

The nosocomial pathogen Enterococcus faecalis has a unique pheromone-inducible conjugative mating system. Conjugative transfer of the E. faecalis plasmid pCF10 is specifically induced by the cCF10 peptide pheromone (LVTLVFV). Genomic sequence information has recently allowed the identification of putative structural genes coding for the various enterococcal pheromones (D. B. Clewell et al., Mol. Microbiol. 35:246-247, 2000). The cCF10 pheromone sequence LVTLVFV was found within an open reading frame designated ccfA, encoding a putative lipoprotein precursor. Several other pheromone sequences were found in similar locations within other predicted lipoproteins. CcfA shows significant sequence relatedness to the Escherichia coli protein YidC, an inner membrane protein translocase, as well as to a large number of homologs identified in gram-positive and in gram-negative bacteria. Analysis of the deduced CcfA amino acid sequence suggested that mature cCF10 peptide could be formed from the proteolytic degradation of its signal peptide. Expression of the cloned ccfA gene with an inducible expression vector dramatically increased cCF10 production by E. faecalis and also resulted in cCF10 production by Lactococcus lactis, a non-pheromone producer. Site-directed mutagenesis of the ccfA sequence encoding the cCF10 peptide confirmed that ccfA was a functional genetic determinant for cCF10.

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.

Analysis of functional domains of the Enterococcus faecalis pheromone-induced surface protein aggregation substance

Pheromone-inducible aggregation substance (AS) proteins of Enterococcus faecalis are essential for high-efficiency conjugation of the sex pheromone plasmids and also serve as virulence factors during host infection. A number of different functions have been attributed to AS in addition to bacterial cell aggregation, including adhesion to host cells, adhesion to fibrin, increased cell surface hydrophobicity, resistance to killing by polymorphonuclear leukocytes and macrophages, and increased vegetation size in an experimental endocarditis model. Relatively little information is available regarding the structure-activity relationship of AS. To identify functional domains, a library of 23 nonpolar 31-amino-acid insertions was constructed in Asc10, the AS encoded by the plasmid pCF10, using the transposons TnlacZ/in and TnphoA/in. Analysis of these insertions revealed a domain necessary for donor-recipient aggregation that extends further into the amino terminus of the protein than previously reported. In addition, insertions in the C terminus of the protein also reduced aggregation. As expected, the ability to aggregate correlates with efficient plasmid transfer. The results also indicated that an increase in cell surface hydrophobicity resulting from AS expression is not sufficient to mediate bacterial aggregation.

Antibodies to a surface-exposed, N-terminal domain of aggregation substance are not protective in the rabbit model of Enterococcus faecalis infective endocarditis

The aggregation substance (AS) surface protein from Enterococcus faecalis has been implicated as an important virulence factor for the development of infective endocarditis. To evaluate the role of antibodies specific for Asc10 (the AS protein from the conjugative plasmid pCF10) in protective immunity to infective endocarditis, an N-terminal region of Asc10 lacking the signal peptide and predicted to be surface exposed (amino acids 44 to 331; AS(44-331)) was cloned with a C-terminal histidine tag translational fusion and expressed from Escherichia coli. N-terminal amino acid sequencing of the purified protein revealed the correct sequence, and rabbit polyclonal antisera raised against AS(44-331) reacted specifically to Asc10 expressed from E. faecalis OG1SSp, but not to other proteins as judged by Western blot analysis. Using these antisera, flow cytometry analysis demonstrated that antibodies to AS(44-331) bound to a surface-exposed region of Asc10. Furthermore, antibodies specific for AS(44-331) were opsonic for E. faecalis expressing Asc10 in vitro but not for cells that did not express Asc10. New Zealand White rabbits immunized with AS(44-331) were challenged intravenously with E. faecalis cells constitutively expressing Asc10 in the rabbit model of experimental endocarditis. Highly immune animals did not show significant differences in clearance of organisms from the blood or spleen or in formation of vegetations on the aortic valve, in comparison with nonimmune animals. Although in vivo expression of Asc10 was demonstrated by immunohistochemistry, these experiments provide evidence that immunity to Asc10 does not play a role in protection from experimental infective endocarditis due to E. faecalis and may have important implications for the development of immunological approaches to combat enterococcal endocarditis.

Coevolutionary networks: a novel approach to understanding the relationships of humans with the infectious agents

Human organism is interpenetrated by the world of microorganisms, from the conception until the death. This interpenetration involves different levels of interactions between the partners including trophic exchanges, bi-directional cell signaling and gene activation, besides genetic and epigenetic phenomena, and tends towards mutual adaptation and coevolution. Since these processes are critical for the survival of individuals and species, they rely on the existence of a complex organization of adaptive systems aiming at two apparently conflicting purposes: the maintenance of the internal coherence of each partner, and a mutually advantageous coexistence and progressive adaptation between them. Humans possess three adaptive systems: the nervous, the endocrine and the immune system, each internally organized into subsystems functionally connected by intraconnections, to maintain the internal coherence of the system. The three adaptive systems aim at the maintenance of the internal coherence of the organism and are functionally linked by interconnections, in such way that what happens to one is immediately sensed by the others. The different communities of infectious agents that live within the organism are also organized into functional networks. The members of each community are linked by intraconnections, represented by the mutual trophic, metabolic and other influences, while the different infectious communities affect each other through interconnections. Furthermore, by means of its adaptive systems, the organism influences and is influenced by the microbial communities through the existence of transconnections. It is proposed that these highly complex and dynamic networks, involving gene exchange and epigenetic phenomena, represent major coevolutionary forces for humans and microorganisms.

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

Aggregation substance (AS) is an Enterococcus faecalis surface protein that may contribute to virulence. Using a recently described system for controlled expression of AS in E. faecalis and the heterologous host Lactococcus lactis, experiments were designed to assess the effect of AS on bacterial internalization by HT-29 and Caco-2 enterocytes. AS expression was associated with increased internalization of E. faecalis by HT-29 enterocytes and of L. lactis by HT-29 and Caco-2 enterocytes. Compared to enterocytes cultivated under standard conditions, either cultivation in hypoxia or 1-h pretreatment of enterocytes with calcium-free medium resulted in increased internalization of both E. faecalis and L. lactis (with and without AS expression). Also, AS expression augmented these increases when E. faecalis was incubated with pretreated HT-29 enterocytes and when L. lactis was incubated with pretreated Caco-2 and HT-29 enterocytes. These data indicated that AS might facilitate E. faecalis internalization by cultured enterocytes.

Relationships between enterococcal virulence and antimicrobial resistance

Enterococci have become a vexing problem in clinical medicine because of their ability to infect patients who are typically receiving antibiotic therapy for unrelated underlying illness. Moreover, the infections have become extremely difficult to manage because of the accumulation of antibiotic resistances among enterococci. The ability of enterococci to cause disease is an intrinsic property of the organism or possibly subpopulations within enterococcal species. The probability of an infection's becoming established, however, is almost certainly in part a function of the enterococcal burden. By altering endogenous bacterial flora, antibiotic therapy promotes increased colonization by antibiotic-resistant organisms. Therefore, antibiotic resistance and intrinsic virulence both contribute to disease, but in separate and complementary ways. We review the virulence of enterococci, as distinct from the acquisition of antimicrobial resistance genes, and identify current gaps in our understanding of enterococcal virulence and the basis for disease.

Relationships between enterococcal virulence and antimicrobial resistance

Enterococci have become a vexing problem in clinical medicine because of their ability to infect patients who are typically receiving antibiotic therapy for unrelated underlying illness. Moreover, the infections have become extremely difficult to manage because of the accumulation of antibiotic resistances among enterococci. The ability of enterococci to cause disease is an intrinsic property of the organism or possibly subpopulations within enterococcal species. The probability of an infection's becoming established, however, is almost certainly in part a function of the enterococcal burden. By altering endogenous bacterial flora, antibiotic therapy promotes increased colonization by antibiotic-resistant organisms. Therefore, antibiotic resistance and intrinsic virulence both contribute to disease, but in separate and complementary ways. We review the virulence of enterococci, as distinct from the acquisition of antimicrobial resistance genes, and identify current gaps in our understanding of enterococcal virulence and the basis for disease.

Pathogenic Mechanisms of Enterococcal Endocarditis

Enterococci are gram-positive bacteria that are now established as major nosocomial pathogens and have become increasingly important in recent years due to the development and transmission of antibiotic resistance traits. These organisms commonly cause a variety of nosocomial infections, including surgical wound infections and urinary tract infections, as well as cardiovascular infections such as bacteremia and endocarditis. Infective endocarditis is a life-threatening microbial infection of the endothelial surface of the heart, which typically occurs on heart valve tissue. The enterococci are the third most common cause of infective endocarditis, and are becoming increasingly significant in this disease. In this review, we discuss the role of enterococci in infective endocarditis and focus on the current knowledge of enterococcal virulence mechanisms, with specific reference to this disease.

Regulation of intron function: efficient splicing in vivo of a bacterial group II intron requires a functional promoter within the intron

Conjugative transfer of the Lactococcus lactis plasmid pRS01 requires splicing of a group II intron, Ll.ltrB, for accurate translation of the mRNA for the exon gene ltrB. The protein product of ltrB is a conjugative relaxase, essential for pRS01 transfer. Using a molecular technique for the identification of transcription initiation sites in bacteria, a functional promoter within Ll.ltrB was identified upstream from the gene for the intron-encoded protein (IEP) LtrA. LtrA is required for efficient splicing of Ll.ltrB in vivo. Mutation of the ltrA promoter dramatically reduced the steady-state level of ltrA mRNA, LtrA, intron splicing and conjugation in L. lactis. These effects could be relieved by expression in trans of the ltrA gene cloned under the control of an inducible promoter. These results suggest that the ltrA mRNAs are translated inefficiently. We hypothesize that this bacterial intron, in contrast to previously studied group II introns in eukaryotes, requires a promoter within the intron to regulate ltrA expression and to produce an adequate level of the protein in the cell for efficient splicing.