Identification of aggregation substances of Enterococcus faecalis cells after induction by sex pheromones. An immunological and ultrastructural investigation

Enterococcal Endocarditis: Hiding in Plain Sight

Enterococcus faecalis is a major opportunistic bacterial pathogen of increasing clinical relevance. A substantial body of experimental evidence suggests that early biofilm formation plays a critical role in these infections, as well as in colonization and persistence in the GI tract as a commensal member of the microbiome in most terrestrial animals. Animal models of experimental endocarditis generally involve inducing mechanical valve damage by cardiac catheterization prior to infection, and it has long been presumed that endocarditis vegetation formation resulting from bacterial attachment to the endocardial endothelium requires some pre-existing tissue damage. Here we review both historical and contemporary animal model studies demonstrating the robust ability of E. faecalis to directly attach and form stable microcolony biofilms encased within a bacterially-derived extracellular matrix on the undamaged endovascular endothelial surface. We also discuss the morphological similarities when these biofilms form on other host tissues, including when E. faecalis colonizes the GI epithelium as a commensal member of the normal vertebrate microbiome - hiding in plain sight where it can serve as a source for systemic infection via translocation. We propose that these phenotypes may allow the organism to persist as an undetected infection in asymptomatic individuals and thus provide an infectious reservoir for later clinical endocarditis.

Pheromone peptide cOB1 from native Enterococcus faecalis forms amyloid-like structures: A new paradigm for peptide pheromones

Pheromone peptides are an important component of bacterial quorum-sensing system. The pheromone peptide cOB1 (VAVLVLGA) of native commensal Enterococcus faecalis has also been identified as an antimicrobial peptide (AMP) and reported to kill the prototype clinical isolate strain of E. faecalis V583. In this study, the pheromone peptide cOB1 has shown to form amyloid-like structures, a characteristic which is never reported for a pheromone peptide so far. With in silico analysis, the peptide was predicted to be highly amyloidogenic. Further, under experimental conditions, cOB1 formed aggregates displaying characteristics of amyloid structures such as bathochromic shift in Congo red absorbance, enhancement in thioflavin T fluorescence, and fibrillar morphology under transmission electron microscopy. This novel property of pheromone peptide cOB1 may have some direct effects on the binding of the pheromone to the receptor cells and subsequent conjugative transfer, making this observation more important for the therapeutics, dealing with the generation of virulent and multidrug-resistant pathogenic strains.

Identification of a peptide-pheromone that enhances Listeria monocytogenes escape from host cell vacuoles

Listeria monocytogenes is a Gram-positive facultative intracellular bacterial pathogen that invades mammalian cells and escapes from membrane-bound vacuoles to replicate within the host cell cytosol. Gene products required for intracellular bacterial growth and bacterial spread to adjacent cells are regulated by a transcriptional activator known as PrfA. PrfA becomes activated following L. monocytogenes entry into host cells, however the signal that stimulates PrfA activation has not yet been defined. Here we provide evidence for L. monocytogenes secretion of a small peptide pheromone, pPplA, which enhances the escape of L. monocytogenes from host cell vacuoles and may facilitate PrfA activation. The pPplA pheromone is generated via the proteolytic processing of the PplA lipoprotein secretion signal peptide. While the PplA lipoprotein is dispensable for pathogenesis, bacteria lacking the pPplA pheromone are significantly attenuated for virulence in mice and have a reduced efficiency of bacterial escape from the vacuoles of nonprofessional phagocytic cells. Mutational activation of PrfA restores virulence and eliminates the need for pPplA-dependent signaling. Experimental evidence suggests that the pPplA peptide may help signal to L. monocytogenes its presence within the confines of the host cell vacuole, stimulating the expression of gene products that contribute to vacuole escape and facilitating PrfA activation to promote bacterial growth within the cytosol.

Virulence Plasmids of Nonsporulating Gram-Positive Pathogens

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

An ABC transporter is required for secretion of peptide sex pheromones in Enterococcus faecalis

Enterococci are leading causes of hospital-acquired infection in the United States and continue to develop resistances to new antibiotics. Many Enterococcus faecalis isolates harbor pheromone-responsive plasmids that mediate horizontal transfer of even large blocks of chromosomal genes, resulting in hospital-adapted strains over a quarter of whose genomes consist of mobile elements. Pheromones to which the donor cells respond derive from lipoprotein signal peptides. Using a novel bacterial killing assay dependent on the presence of sex pheromones, we screened a transposon mutant library for functions that relate to the production and/or activity of the effector pheromone. Here we describe a previously uncharacterized, but well-conserved, ABC transporter that contributes to pheromone production. Using three distinct pheromone-dependent mating systems, we show that mutants defective in expressing this transporter display a 5- to 6-order-of-magnitude reduction in conjugation efficiency. In addition, we demonstrate that the ABC transporter mutant displays an altered biofilm architecture, with a significant reduction in biofilm biomass compared to that of its isogenic parent, suggesting that pheromone activity also influences biofilm development. The conservation of this peptide transporter across the Firmicutes suggests that it may also play an important role in cell-cell communication in other species within this important phylum.

Enterococcus faecalis ranks as one of the leading causes of hospital-associated infections. Strains possessing resistance to multiple antibiotics are becoming all too common in clinical settings. Pheromone-responsive plasmids play an important role in harboring and disseminating these antibiotic resistance genes. Here we have identified a novel ABC transporter that is responsible for the secretion of peptide pheromones, which enables communication between cells to mediate plasmid transfer. We have also shown that this transporter is important for biofilm formation, providing a strong rationale for its use as a viable therapeutic target which could be targeted to curb infection, as well as the spread of existing drug resistance.

Tales of conjugation and sex pheromones: A plasmid and enterococcal odyssey

This review covers highlights of the author's experience becoming and working as a plasmid biologist. The account chronicles a progression from studies of ColE1 DNA in Escherichia coli to Gram-positive bacteria with an emphasis on conjugation in enterococci. It deals with gene amplification, conjugative transposons and sex pheromones in the context of bacterial antibiotic resistance.

Properties of Enterococcus faecalis plasmid pAD1, a member of a widely disseminated family of pheromone-responding, conjugative, virulence elements encoding cytolysin

The 60-kb pAD1 represents a large and widely disseminated family of conjugative, pheromone-responding, virulence plasmids commonly found in clinical isolates of Enterococcus faecalis. It encodes a hemolysin/bacteriocin (cytolysin) shown to contribute to virulence in animal models, and the related bacteriocin is active against a wide variety of Gram-positive bacteria. This review summarizes what is currently known about the molecular biology of pAD1, including aspects of its cytolytic, UV-resistance, replication, maintenance, and conjugative properties.

Extracellular gelatinase of Enterococcus faecalis destroys a defense system in insect hemolymph and human serum

We isolated Enterococcus faecalis from the body fluids of dead larvae of the greater wax moth, Galleria mellonella. Extracellular gelatinase (GelE) and serine protease (SprE), both of which are considered putative virulence factors of E. faecalis, were purified from the culture supernatant of E. faecalis. In an attempt to elucidate their virulence mechanisms, purified GelE and SprE were injected into hemolymph of G. mellonella and evaluated with regard to their effects on the immune system of insect hemolymph. As a result, it was determined that E. faecalis GelE degraded an inducible antimicrobial peptide (Gm cecropin) which is known to perform a critical role in host defense during the early phase of microbial infection. The results obtained from the G. mellonella-E. faecalis infection model compelled us to assess the virulence activity of GelE against the complement system in human serum. E. faecalis GelE hydrolyzed C3a and also mediated the degradation of the alpha chain of C3b, thereby inhibiting opsonization and the formation of the membrane attack complex resultant from the activation of the complement cascade triggered by C3 activation. In contrast, E. faecalis SprE exhibited no virulence effect against the immune system of insect hemolymph or human serum tested in this study.

Hemagglutinating and hemolytic activities of Enterococcus faecalis strains isolated from different human clinical sources

A total of 95 Enterococcus faecalis strains isolated from different human clinical sources were investigated for hemagglutinating activities and hemolysin (Hly) production in the presence of erythrocytes from a wide range of species. MRHA (mannose-resistant hemagglutination) activity was found in all clinical strains tested in this study. MRHA of E. faecalis strains isolated from different sources was most frequently observed with human (both group O and A) and guinea pig erythrocytes. None of the strains agglutinated horse erythrocytes in the presence of 1% alpha-D-mannose. It should be emphasized that our data indicate the absence of a relationship between sources and MRHA. In contrast, all 95 strains investigated in this report were negative for MSHA (mannose-sensitive hemagglutination) activity. Regarding hemolysin production, it was seen that E. faecalis, and particularly urinary strains, preferably lysed horse erythrocytes. On the other hand, none of the 95 clinical strains tested in this study showed hemolytic activity against bovine and sheep erythrocytes. In general, these results show that E. faecalis strains isolated from different clinical sources possessed a diversity of hemagglutinins and a limited repertoire of hemolysin activities.

Immunogold labeling of chromosomes for scanning electron microscopy: a closer look at phosphorylated histone H3 in mitotic metaphase chromosomes of Hordeum vulgare

High-resolution detection of phosphorylated histone H3 at serine 10 in mitotic barley chromosomes for scanning electron microscopy was shown using a novel application of indirect immunogold labeling with Nanogold. This method permits localization and quantification of signals in a three-dimensional context. Because the chromosome structure is well preserved, characterization of binding sites (chromomeres, parallel matrix fibers, solenoids), currently in the realm of nanometer decades, is possible. Quantification and three-dimensional localization of labels is possible with stereoscopic analysis. Limitations of the method pertain to the challenges in preservation of chromosome ultrastructure, accessibility of immunoreactants into the fixed chromatin and unspecific labeling. The differences between silver and gold enhancement and the current status of labeling efficiency are addressed.

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

OBJECTIVE

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

DESIGN

Prospective controlled animal study.

SETTING

Animal experimental laboratory at a university medical center.

SUBJECTS

Fifty-two female Balb/c mice.

INTERVENTIONS

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

MEASUREMENTS AND MAIN RESULTS

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

CONCLUSIONS

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

Transfer origins in the conjugative Enterococcus faecalis plasmids pAD1 and pAM373: identification of the pAD1 nic site, a specific relaxase and a possible TraG-like protein

The Enterococcus faecalis conjugative plasmids pAD1 and pAM373 encode a mating response to the peptide sex pheromones cAD1 and cAM373 respectively. Sequence determination of both plasmids has recently been completed with strong similarity evident over many of the structural genes related to conjugation. pAD1 has two origins of transfer, with oriT1 being located within the repA determinant, whereas the more efficiently utilized oriT2 is located between orf53 and orf57, two genes found in the present study to be essential for conjugation. We have found a similarly located oriT to be present in pAM373. oriT2 corresponds to about 285 bp based on its ability to facilitate mobilization by pAD1 when ligated to the shuttle vector pAM401; however, it was not mobilized by pAM373. In contrast, a similarly ligated fragment containing the oriT of pAM373 did not facilitate mobilization by pAD1 but was efficiently mobilized by pAM373. The oriT sites of the two plasmids each contained a homologous large inverted repeat (spanning about 140 bp) adjacent to a series of non-homologous short (6 bp) direct repeats. A hybrid construction containing the inverted repeat of pAM373 and direct repeats of pAD1 was mobilized efficiently by pAD1 but not by pAM373, indicating a significantly greater degree of specificity is associated with the direct repeats. Mutational (deletion) analyses of the pAD1 oriT2 inverted repeat structure suggested its importance in facilitating transfer or perhaps ligation of the ends of the newly transferred DNA strand. Analyses showed that Orf57 (to be called TraX) is the relaxase, which was found to induce a specific nick in the large inverted repeat inside oriT; the protein also facilitated site-specific recombination between two oriT2 sites. Orf53 (to be called TraW) exhibits certain structural similarities to TraG-like proteins, although there is little overall homology.

Enterococcus faecalis surface proteins determine its adhesion mechanism to bile drain materials

An important step in infections associated with biliary drains is adhesion of micro-organisms to the surface. In this study the role of three surface proteins of Enterococcus faecalis (enterococcal surface protein, aggregation substances 1 and 373) in the adhesion to silicone rubber, fluoro-ethylene-propylene and polyethylene was examined. Four isogenic E. faecalis strains with and without aggregation substances and one strain expressing enterococcal surface protein were used. The kinetics of enterococcal adhesion to the materials was measured in situ in a parallel plate flow chamber. Initial deposition rates were similar for all strains, whereas the presence of surface proteins increased the total number of adhering bacteria. Nearest neighbour analysis demonstrated that enterococci expressing the whole sex-pheromone plasmid encoding aggregation substances 1 or 373 adhered in higher numbers through mechanisms of positive cooperativity, which means that adhesion of bacteria enhances the probability of adhesion of other bacteria near these bacteria. Enterococci with the enterococcal surface protein did not adhere through this mechanism. These findings indicate that the surface proteins of E. faecalis play a key role in the adhesion to bile drains and bile drain associated infections.

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

Aggregation substance proteins encoded by the sex pheromone plasmid family of Enterococcus faecalis have been shown previously to contribute to the formation of a stable mating complex between donor and recipient cells and have been implicated in the virulence of this increasingly important nosocomial pathogen. In an effort to characterize the protein further, prgB, the gene encoding the aggregation substance Asc10 on pCF10, was cloned in a vector containing the nisin-inducible nisA promoter and its two-component regulatory system. Expression of aggregation substance after nisin addition to cultures of E. faecalis and the heterologous bacteria Lactococcus lactis and Streptococcus gordonii was demonstrated. Electron microscopy revealed that Asc10 was presented on the cell surfaces of E. faecalis and L. lactis but not on that of S. gordonii. The protein was also found in the cell culture supernatants of all three species. Characterization of Asc10 on the cell surfaces of E. faecalis and L. lactis revealed a significant increase in cell surface hydrophobicity upon expression of the protein. Heterologous expression of Asc10 on L. lactis also allowed the recognition of its binding ligand (EBS) on the enterococcal cell surface, as indicated by increased transfer of a conjugative transposon. We also found that adhesion of Asc10-expressing bacterial cells to fibrin was elevated, consistent with a role for the protein in the pathogenesis of enterococcal endocarditis. The data demonstrate that Asc10 expressed under the control of the nisA promoter in heterologous species will be an useful tool in the detailed characterization of this important enterococcal conjugation protein and virulence factor.

A pAD1-encoded small RNA molecule, mD, negatively regulates Enterococcus faecalis pheromone response by enhancing transcription termination

pAD1 is a 60-kb hemolysin-bacteriocin plasmid in Enterococcus faecalis that encodes a conjugative mating response to a peptide sex pheromone, cAD1, secreted by plasmid-free bacteria. The pheromone response is regulated by two proteins: TraE1, which positively regulates all or most conjugative structural genes, and TraA, which negatively regulates traE1. TraA binds to pAD1 DNA at the iad (encoding the inhibitor peptide iAD1) promoter but is released upon binding to imported pheromone. This leads to enhanced transcription through two closely spaced downstream terminators (t1 and t2) into traE1. TraE1 is believed to then upregulate itself from a site located within t2; thus, a small amount of transcription through t1-t2 could lead to overall induction. It is important therefore that the t1-t2 terminators be tightly controlled to keep the response shut down in the absence of pheromone. A small (200-nucleotide) RNA molecule designated mD is encoded just upstream of t1 by a determinant (traD) oriented in the direction opposite to that of transcripts utilizing t1. mD is expressed at high levels in the uninduced state, but it decreases significantly upon induction. Here we present results of genetic studies relating to the activity of t1-t2 and show that mD strongly enhances transcriptional termination at t1. The mD activity is shown to influence transcription well downstream and can affect the determinant for aggregation substance asa1. The phenomenon is specific in that there is no effect of mD on the unrelated pheromone-responding plasmids pPD1 and pCF10.

Identification and characterization of a determinant (eep) on the Enterococcus faecalis chromosome that is involved in production of the peptide sex pheromone cAD1

Plasmid-free strains of Enterococcus faecalis secrete a peptide sex pheromone, cAD1, which specifically induces a mating response by donors carrying the hemolysin plasmid pAD1 or related elements. A determinant on the E. faecalis OG1X chromosome has been found to encode a 46.5-kDa protein that plays an important role in the production of the extracellular cAD1. Wild-type E. faecalis OG1X cells harboring a plasmid chimera carrying the determinant exhibited an eightfold enhanced production of cAD1, and plasmid-free cells carrying a mutated chromosomal determinant secreted undetectable or very low amounts of the pheromone. The production of other pheromones such as cPD1, cOB1, and cCF10 was also influenced, although there was no effect on the pheromone cAM373. The determinant, designated eep (for enhanced expression of pheromone), did not include the sequence of the pheromone. Its deduced product (Eep) contains apparent membrane-spanning sequences; conceivably it is involved in processing a pheromone precursor structure or in some way regulates expression or secretion.

Surface proteins of gram-positive bacteria and mechanisms of their targeting to the cell wall envelope

The cell wall envelope of gram-positive bacteria is a macromolecular, exoskeletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to interact with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins and enzymes be properly targeted to the cell wall envelope. Two basic mechanisms, cell wall sorting and targeting, have been identified. Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More than 100 proteins that possess cell wall-sorting signals, including the M proteins of Streptococcus pyogenes, protein A of Staphylococcus aureus, and several internalins of Listeria monocytogenes, have been identified. Cell wall targeting involves the noncovalent attachment of proteins to the cell surface via specialized binding domains. Several of these wall-binding domains appear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins that are targeted to the cell surface include muralytic enzymes such as autolysins, lysostaphin, and phage lytic enzymes. Other examples for targeted proteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (internalin B) and Streptococcus pneumoniae (PspA) infections. In this review we describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surface proteins.

Molecular mechanism of peptide-specific pheromone signaling in Enterococcus faecalis: functions of pheromone receptor TraA and pheromone-binding protein TraC encoded by plasmid pPD1

Conjugative transfer of the Enterococcus faecalis plasmid pPD1 is activated by cPD1, one of several peptide sex pheromones secreted by plasmid-free recipient cells, and is blocked by a donor-produced peptide inhibitor, iPD1. Using a tritiated pheromone, [3H]cPD1, we investigated how pPD1-harboring donor cells receive these peptide signals. Donor cells rapidly incorporated [3H]cPD1. The cell extract but not the membrane fraction of the donor strain exhibited significant [3H]cPD1-binding activity. On the basis of these data and those of tracer studies, it was demonstrated that cPD1 was internalized, where it bound to a high-molecular-weight compound. The cell extract of a strain carrying the traA-bearing multicopy plasmid (pDLHH21) also exhibited high [3H]cPD1-binding activity. A recombinant TraA exhibited a dissociation constant of 0.49 +/- 0.08 nM against [3H]cPD1. iPD1 competitively inhibited [3H]cPD1 binding to TraA, whereas pheromones and inhibitors relating to other plasmid systems did not. These results show that TraA is a specific intracellular receptor for cPD1 and that iPD1 acts as an antagonist for TraA. A strain carrying the traC-bearing multicopy plasmid (pDLES23) exhibited significant [3H]cPD1-binding activity. A strain carrying traC-disrupted pPD1 (pAM351CM) exhibited lower [3H] cPD1-binding activity as well as lower sensitivity to cPD1 than a wild-type donor strain. Some of the other pheromones and inhibitors inhibited [3H]cPD1 binding to the traC transformant like cPD1 and iPD1 did. These results show that TraC, as an extracellular less-specific pheromone-binding protein, supports donor cells to receive cPD1.

Does aggregation substance of Enterococcus faecalis contribute to development of endocarditis?

Aggregation substance (AS) of Enterococcus faecalis which is encoded by so-called sex pheromone plasmids enables the bacteria to bind to in vitro-cultured pig kidney tubular cells. It is reported that the presence of AS is not of pivotal importance for the ability of E. faecalis to cause infective endocarditis (EN). The lines of evidence for this are twofold: 1) sex pheromone plasmids and, therefore, the gene for AS were not present more often in epidemiologically unrelated strains of E. faecalis isolated from human cases of EN than in isolates from well-water (26 vs. 18%); 2) the presence of the adhesin did not correlate with the establishment of EN in an animal (rat) model. The data are discussed with respect to the specificity of interaction of AS with eukaryotic cells and the results of other studies.

Cloning and characterization of a region of Enterococcus faecalis plasmid pPD1 encoding pheromone inhibitor (ipd), pheromone sensitivity (traC), and pheromone shutdown (traB) genes

Bacteriocin plasmid pPD1 in Enterococcus faecalis encodes a mating response to recipient-produced sex pheromone cPD1. Once a recipient acquires pPD1, transconjugants apparently shut off cPD1 activity in broth culture and no longer behave as recipients for pPD1. This event is performed by synthesis of the pheromone inhibitor iPD1 and also by repression of cPD1 production, the so-called "pheromone shutdown." A 5.4-kb EcoRV-HincII segment of pPD1, which expressed iPD1 in Escherichia coli, was sequenced and found to be organized as traC-traB-traA-ipd; each open reading frame is analogous to that found in other pheromone plasmids, pAD1 and pCF10, and thus is designated in accordance with the nomenclature in pAD1. The ipd gene encodes a peptide consisting of 21 amino acids, in which the C-terminal eight residues correspond to iPD1. The putative TraC product has a strong similarity to oligopeptide-binding proteins found in other bacterial species, as do pheromone-binding proteins of pCF10 and pAD1. A strain carrying traC-disrupted pPD1 required a concentration of cPD1 fourfold higher than that needed by the wild-type strain for induction of sexual aggregation. These results suggest that the TraC product contributes to pheromone sensitivity as a pheromone-binding protein. A strain transformed with traB-disrupted pPD1 produced a high level of cPD1 similar to that produced by plasmid-free recipients and underwent self-induction. Thus, the TraB product contributes to cPD1 shutdown.

The sex pheromone system of Enterococcus faecalis. More than just a plasmid-collection mechanism?

The sex pheromone system of Enterococcus faecalis was discovered by observing a clumping reaction of E. faecalis strains during conjugative transfer of plasmids. It was found that only a special type of E. faecalis plasmids, the so-called sex pheromone plasmids, are transferred via this mechanism. Various experiments, especially by the group of D. B. Clewell, led to the formulation of a model describing how the sex pheromone system works. Small linear peptides, the so-called sex pheromones, are excreted by strains not possessing the corresponding sex pheromone plasmid. Donor strains harboring the plasmid do not produce the corresponding sex pheromone; they react to the presence of the peptide by production of a plasmid-encoded adhesin, the so-called aggregation substance. This adhesin allows contact between the non-motile mating partners; after conjugative transfer of the plasmid, the former recipient possesses and replicates the new plasmid. Thereby the population of E. faecalis strains is shifted to a high percentage of donor strains. This is especially true because a donor strain will still excrete sex pheromones corresponding to plasmids it does not harbor; therefore, such a strain can also function as recipient for other sex pheromone plasmids it does not possess. Various aspects of this unique plasmid collection mechanism have been studied during the last few years. The data indicate that, with the exception of pAM373, all sex pheromone plasmids possess one DNA region which is highly similar to and codes for the adhesin. It is also becoming more and more clear that regulatory functions/proteins are not conserved between different sex pheromone plasmids. Induction of adhesin synthesis needs the action of a regulatory cascade composed of unique features; at the moment we are just beginning to understand this cascade. By sequencing the first structural gene for one of those adhesins, we realized that the aggregation substance might act also as an adhesin for eucaryotic cells, probably by interaction with integrins. At least in the case of the in vitro cultured pig kidney tubulus cell line LLC-PK1 this idea could be verified. An interesting aspect of the sex pheromone system of E. faecalis is its evolution. I will discuss the idea that two different components, both of which well might contribute to virulence of the opportunistic pathogenic bacterium, were combined in the species E. faecalis to result in this unique plasmid collection system.

Cloning and molecular analysis of genes affecting expression of binding substance, the recipient-encoded receptor(s) mediating mating aggregate formation in Enterococcus faecalis

Transfer of the conjugative plasmid pCF10 in Enterococcus faecalis strains involves production of a plasmid-encoded aggregation substance on the surface of donor cells in response to stimulation by a pheromone secreted by recipient cells. Aggregation substance then facilitates attachment to recipient cells via a chromosomally encoded receptor, termed binding substance (BS). A BS mutant, strain INY3000, generated by random Tn916 insertions, was previously found to carry copies of the transposon at four unique sites (K. M. Trotter and G. M. Dunny, Plasmid 24:57-67, 1990). In the present study, DNA flanking the Tn916 insertions was used to complement the BS mutation of INY3000 following Tn916 excision from cloned chromosomal fragments. Complementation results showed that three of the four regions mutated in INY3000 play some role in BS expression. Tn5 mutagenesis and DNA sequence analysis of the complementing fragment from one of these regions indicated the presence of three genes (ebsA, ebsB, and ebsC) that affect BS expression. The ebsA and ebsB genes encode peptides likely to function in cell wall metabolism, whereas ebsC may encode a product that suppresses the function or expression of EbsB.

Characterization of the traC determinant of the Enterococcus faecalis hemolysin-bacteriocin plasmid pAD1: binding of sex pheromone

pAD1, a conjugative, 60-kb, hemolysin-bacteriocin plasmid in Enterococcus faecalis, encodes a mating response to a small peptide sex pheromone, cAD1, secreted by potential recipient bacteria. A gene, traC, encoding a 60.7-kDa protein with a typical amino terminal signal peptide, was identified within a region that appears to encode a product that binds to exogenous pheromone. A cloned segment of DNA containing traC resulted in specific binding of cells to synthetic cAD1. The putative traC product has strong similarity to a product of the E. faecalis plasmid pCF10 as well as oligopeptide binding proteins of Escherichia coli, Salmonella typhimurium, and Bacillus subtilis.

Sex pheromone plasmid pAD1-encoded aggregation substance of Enterococcus faecalis is positively regulated in trans by traE1

Sex-pheromone-plasmid-bearing strains of Enterococcus faecalis react with sex pheromone to induce the expression of an adhesin, the so-called aggregation substance, on their cell surface. Here we show that, by complementation studies, for sex-pheromone plasmid pAD1, expression of the structural gene asa1, coding for an aggregation substance, is mediated by a diffusible factor encoded on pAD1. We were able to demonstrate that a small open reading frame, traE1, is sufficient for transcription of the operon containing asa1. A model for expression of asa1 under the influence of the positive regulator is presented, which is supported by our observation that regulation involves an all-or-nothing induction phenomenon, leading to cells either fully expressing asa1 or not at all.

Identification and characterization of surface proteins from Staphylococcus saprophyticus

Staphylococcus saprophyticus, a well known cause of urinary tract infections, possesses several properties, such as hemagglutination of sheep erythrocytes, adherence to various cell types and production of urease, which may be virulence factors. In this contribution, we summarise the present knowledge about recently discovered surface proteins of Staphylococcus saprophyticus, a 95 kDa surface-associated protein (Ssp) and the hemagglutinin, a 160 kDa surface polypeptide. We describe culture conditions conducive to production of these surface proteins and discuss the molecular and clinical implications of our findings.

Regulation of the pAD1-encoded sex pheromone response in Enterococcus faecalis: expression of the positive regulator TraE1

pAD1 is a conjugative, 60-kb, hemolysin-bacteriocin plasmid in Enterococcus faecalis that encodes a mating response to a small peptide sex pheromone, cAD1, secreted by potential recipient bacteria. The response is regulated by a cluster of genes that includes a positive regulatory determinant, traE1, able to activate key structural genes involved in the conjugative process. A negative regulatory determinant, traA, affects the expression of traE1 and is sensitive to the pheromone signal. Between the two determinants is a gene, iad, which encodes a small peptide, iAD1, a competitive inhibitor of cAD1. The determinants (traE1-iad-traA) are oriented such that iad and traE1 are transcribed in the same direction, opposite that of traA. Transcription of iad and traA starts between these determinants and moves outward in each case. A recent report from our laboratory, dealing with transcriptional fusions in the traE1-iad region (L. T. Pontius and D. B. Clewell, J. Bacteriol. 174:3152-3160, 1992), indicated that traE1 expression may be dependent on transcriptional read-through of a terminator(s) between iad and traE1. The present report provides direct analyses of relevant RNA species before and during induction and shows that indeed transcriptional read-through from iad is important in the initial expression of traE1. However, the data show that once traE1 is activated, it can then be expressed independently, probably because of TraE1 activating its own promoter. This view is also supported by genetic complementation studies. In addition, DNA binding studies with TraA showed that the protein binds to the promoter of iad. Binding of TraA to the region between iad and traE1 could not be detected; however, the involvement of TraA in influencing transcription termination in this region is still not ruled out, since additional factors could be involved. A model for the regulation of the pheromone response is presented.

Biochemical, immunological and ultrastructural characterization of aggregation substances encoded by Enterococcus faecalis sex-pheromone plasmids

The sex-pheromone system of Enterococcus faecalis can be viewed as a unique and highly efficient plasmid-collection mechanism. The contact needed for transfer of the conjugative sex-pheromone plasmids is mediated by an adhesin, called aggregation substance, which is encoded by these plasmids. We show here that for 17 of the 18 sex-pheromone plasmids (pAM373 being the exception) described to date, their adhesins are immunologically related to each other. In each case, we observed the presence of an N-terminal fragment of about 78 kDa in addition to the 137-kDa form of mature aggregation substance. The cross-reactions were different for the various plasmids. In the case of pPD1 the 78-kDa fragment reacted only weakly. The aggregation substance encoded by sex-pheromone plasmid pAD1 (Asa1) was characterized in detail. The conditions used for SDS/PAGE had a drastic influence on the migration behavior of mature aggregation substance and differently migrating, interconvertible forms were identified. Preliminary data indicate that Asa1 might be a glycoprotein. Antibodies were isolated which are directed against the N- and C-terminal parts of aggregation substance. They showed about the same reactivity on Western blots; however, only antibodies directed against the N-terminal part of the aggregation substance could inhibit the bacterial cell/cell contact. The reactions of the two antibody preparations with induced cells of E. faecalis was analyzed by transmission electron microscopy. The results indicated that especially the N-terminal part of aggregation substance is exposed on the cell surface of E. faecalis; the C-terminal part seems to be much less exposed.

Neutralizing antibodies against the peptide antibiotic AS-48: immunocytological studies

Antisera against the broad-spectrum peptide antibiotic AS-48 produced by Enterococcus faecalis were obtained from immunized rabbits. Appreciable antibody titers were obtained only after repeated immunization, suggesting a feeble antigenicity for AS-48. Upon incubation with AS-48, the antisera neutralized its bacteriolytic action on E. faecalis S-47, although the simultaneous addition of AS-48 and serum did not prevent lysis. Crude serum cross-reacted with outer envelope components of enterococci, although specific anti-AS-48 antibodies, purified by affinity chromatography, reacted only with AS-48-treated cells. Labelling with immunofluorescence and colloidal gold particles was carried out on sensitive and resistant bacterial species to determine the interaction of AS-48 with cell structures.

Identification of new sex pheromone plasmids in Enterococcus faecalis

We describe the identification of the following new sex pheromone plasmids in Enterococcus faecalis: a haemolysin-bacteriocin plasmid, pIP964; three R plasmids, pIP1017, pIP1438 and pIP1440; and two cryptic conjugative plasmids, pIP1141 and pMV120. The identification was based on the formation of cell aggregates on filter membranes during conjugation, on efficient transfer in broth matings, and on a positive clumping reaction of cells carrying these plasmids. In addition these plasmids hybridized with DNA probes specific for sex pheromone-induced structural genes encoding surface proteins required for conjugative transfer of the plasmids.

Sex pheromone plasmid pAD1-encoded surface exclusion protein of Enterococcus faecalis

During conjugative transfer of sex pheromone plasmids of Enterococcus faecalis a so-called surface exclusion protein reduces the frequency with which these plasmids are transferred to cells already possessing the same plasmid. We report here the DNA sequence of a 3.8 kb fragment of the sex pheromone plasmid pAD1 containing the structural gene sea1 for surface exclusion protein and a small open reading frame (ORF) upstream of sea1. Surface exclusion protein Sea1 was found to be highly homologous to the surface exclusion protein Sec10 encoded by the sex pheromone plasmid pCF10. Hybridization studies with DNA probes derived from the structural gene sea1 demonstrated that, with the exception of pAM373, all known sex pheromone plasmids carry a homologous gene. These studies also indicated that the genetic organization is similar in these plasmids, with the structural gene for surface exclusion protein being located 5' to that for aggregation substance.

Transcriptional control of sex-pheromone-inducible genes on plasmid pAD1 of Enterococcus faecalis and sequence analysis of a third structural gene for (pPD1-encoded) aggregation substance

The expression of several neighbouring genes on plasmid pAD1 that are necessary for conjugation depend on induction with sex pheromone cAD1. Analyses of transcripts by Northern blot hybridization demonstrated that the genes sea1 (encoding surface exclusion protein) and asa1 (encoding aggregation substance) are transcribed independently. Both genes are organized in different operons together with neighbouring open reading frames of unknown function. Several transcripts could be identified for sea1 and asa1. Their transcriptional start sites were determined by primer extension experiments, confirming the results of the Northern blot experiments. We also could identify sea1- and iad- (encoding an inhibitory peptide counteracting sex pheromone cAD1) specific transcripts which are expressed constitutively, but to a lower extent relative to induced conditions. In addition, we localized the asp1 gene coding for aggregation substance of sex pheromone plasmid pPD1 and determined its DNA sequence, which was found to be highly homologous to asa1 (aggregation substance gene of pAD1) and prgB (aggregation substance gene of pCF10). The structural genes were found to be organized more or less identically on the three sex-pheromone plasmids pAD1, pCF10, and pPD1, and to be highly conserved. Regions supposed to be of crucial importance for regulatory functions, however, were found to differ. We also could identify some conserved DNA motifs which might be potential target sites for transcriptional regulators. In combination these data allowed us to formulate a model for the regulation of sex-pheromone-inducible genes of plasmid pAD1. Its main statement is that only in the presence of cAD1 can the gene traE1 be transcribed. The positive regulatory factor TraE1 then can trigger expression of the structural genes sea1 and asa1.

Identification and characterization of a surface-associated protein (Ssp) of Staphylococcus saprophyticus

A 95-kDa protein was isolated from Staphylococcus saprophyticus 7108 grown on dialysis membranes placed on the surface of brain heart infusion agar. Strain CCM883 did not produce this protein. Ultrathin sections revealed the presence of very thin, tuftlike, 50- to 75-nm-long structures on the surface of strain 7108, whereas strain CCM883 was comparably smooth. The surface material could be removed by digestion with proteinase K, suggesting that the surface structures contain protein. High-resolution scanning electron microscopy showed a thick layer of surface material on strain 7108, whereas strain CCM883 appeared smooth. The 95-kDa protein was purified by Sephacryl S-300 chromatography, and an antiserum was raised in rabbits. This antiserum was used in immunogold labeling experiments, which showed that the protein is associated with the surface structures. Our experiments thus demonstrate the presence of a fibrillar protein on the surface of S. saprophyticus (Ssp for S. saprophyticus surface-associated protein).

Aggregation substance of Enterococcus faecalis mediates adhesion to cultured renal tubular cells

The sex pheromone system of Enterococcus faecalis is a unique, highly efficient plasmid collection mechanism for this species. A crucial role in this system is played by an adhesin called aggregation substance which enables the cell-cell contact between donor and recipient strains. The existence of the amino acid motif Arg-Gly-Asp-Ser in the adhesin prompted us to look for a possible binding of E. faecalis cells expressing aggregation substance to eucaryotic cells. We were able to show that the adhesin mediated binding to cultured renal tubular cells (porcine cell line LLC-PK1) via light microscopic, electron microscopic, and enzyme-linked immunosorbent assay-based studies. Synthesis of the adhesin was induced by some component(s) of serum. These data are interpreted to mean that aggregation substance is an adhesin mediating not only cell-cell contact between different E. faecalis strains but also binding of E. faecalis to eucaryotic cells, and therefore it might contribute to virulence.

Molecular and genetic analysis of a region of plasmid pCF10 containing positive control genes and structural genes encoding surface proteins involved in pheromone-inducible conjugation in Enterococcus faecalis

Exposure of Enterococcus faecalis cells carrying the tetracycline resistance plasmid pCF10 to the heptapeptide pheromone cCF10 results in an increase in conjugal transfer frequency by as much as 10(6)-fold. Pheromone-induced donor cells also express at least two plasmid-encoded surface proteins, the 130-kDa Sec 10 protein, which is involved in surface exclusion, and the 150-kDa Asc10 protein, which has been associated with the formation of mating aggregates. Previous subcloning and transposon mutagenesis studies indicated that the adjacent EcoRI c (7.5 kb) and e (4.5 kb) fragments of pCF10 encode the structural genes for these proteins and that the EcoRI c fragment also encodes at least two regulatory genes involved in activation of the expression of the genes encoding Asc10 and Sec10. In this paper, the results of physical and genetic analysis of this region of pCF10, along with the complete DNA sequences of the EcoRI c and e fragments, are reported. The results of the genetic studies indicate the location of the structural genes for the surface proteins and reveal important features of their transcription. In addition, we provide evidence here and in the accompanying paper (S. B. Olmsted, S.-M. Kao, L. J. van Putte, J. C. Gallo, and G. M. Dunny, J. Bacteriol. 173:7665-7672, 1991) for a role of Asc10 in mating aggregate formation. The data also reveal a complex positive control system that acts at distances of at least 3 to 6 kb to activate expression of Asc10. DNA sequence analysis presented here reveals the positions of a number of specific genes, termed prg (pheromone-responsive genes) in this region of pCF10. The genes mapped include prgA (encoding Sec10) and prgB (encoding Asc10), as well as four putative regulatory genes, prgX, -R, -S, and -T. Although the predicted amino acid sequences of Sec10 and Asc10 have some structural features in common with a number of surface proteins of gram-positive cocci, and the Asc10 sequence is highly similar to that of a similar protein encoded by the pheromone-inducible plasmid pAD1 (D. Galli, F. Lottspeich, and R. Wirth, Mol. Microbiol. 4:895-904, 1990), the regulatory genes show relatively little resemblance to any previously sequenced genes from either procaryotes or eucaryotes.

Role of the pheromone-inducible surface protein Asc10 in mating aggregate formation and conjugal transfer of the Enterococcus faecalis plasmid pCF10

The high transfer frequency of pheromone-inducible conjugative plasmids of Enterococcus faecalis in liquid culture is due in part to the formation of mating aggregates. These aggregates result from the interaction of two surface components, aggregation substance (AS), which is plasmid encoded, and the chromosomally encoded binding substance (BS). In the accompanying paper (S.-M. Kao, S. B. Olmsted, A. S. Viksnins, J.C. Gallo, G. M. Dunny, J. Bacteriol, 173:7650-7664, 1991), the sequence of the prgB gene encoding the AS molecule (Asc10) produced by pheromone-induced cells carrying plasmid pCF10 is presented. Here we report the results of genetic and immunological experiments which define the role of Asc10 in aggregation and plasmid transfer. These data indicate expression of AS on the surface of an E. faecalis cell and its binding to BS expressed on a second cell are required for the formation of a mating pair and the efficient transfer of pCF10 in liquid matings. However, the orientation of the receptors was not critical for transfer; ie., AS expressed on recipient cells could facilitate plasmid transfer via binding to BS on the donor. Our results suggest that additional (as yet unidentified) products are involved in forming the channel that ultimately serves to transfer the DNA, with AS-BS binding serving primarily to generate the initial attachment between cells. The putative prgC gene product, identified by DNA sequencing (data presented in the accompanying paper), could be involved in transfer events occurring subsequent to aggregation.

A phase variation event that activates conjugation functions encoded by the Enterococcus faecalis plasmid pAD1

Enterococcus faecalis cells carrying the conjugative plasmid pAD1 undergo several related changes when induced by the sex pheromone cAD1. Included are the production of novel surface proteins, the formation of cellular aggregates in broth cultures, the ability to transfer the plasmid at high frequency in broth matings, and the change from a soft to a "dry" colony morphology. Spontaneous, constitutively dry colony (Dryc) variants of E. faecalis (pAD1) were found to arise at a frequency of 10(-4)-10(-2). Dryc phase variants constitutively expressed aggregation and plasmid transfer functions typically expressed only under cAD1-inducing conditions. Reversion of Dryc variants to a cAD1-inducible phenotype (Dry+) occurred at a similar frequency. Tn917-lac mutagenesis of regions of pAD1 previously shown to be involved in plasmid transfer revealed that in Dry+ cells these regions were transcribed only when the inducer, cAD1, was present. In Dryc variants the regions were transcribed constitutively. A pAD1 miniplasmid containing determinants regulating cAD1 inducible plasmid transfer and a cAD1-inducible lacZ transcriptional fusion displayed phase variation in LacZ expression at a rate similar to the Dry+/Dryc phase variation. These results suggest that the site of mutation(s) resulting in the Dryc phenotype is within the regulation-related region of pAD1. Complementation tests showed that this region, when supplied in trans, complemented the Dryc phenotype. Phase variation affecting mating functions represents an alternative (pheromone independent) method of regulating pAD1 transfer.

Comparative analysis of Enterococcus faecalis sex pheromone plasmids identifies a single homologous DNA region which codes for aggregation substance

An analysis of the 11 known sex pheromone plasmids of Enterococcus faecalis was performed by DNA-DNA hybridization. Plasmids pAD1, pJH2, and pBEM10 turned out to be closely related, whereas pAM373 showed only weak homology with pAD1. A comparison of the hemolysin/bacteriocin determinants of pAD1, pJH2, and pOB1 revealed strong similarities at the DNA level. Our main finding was that one DNA region is conserved among all sex pheromone plasmids, with pAM373 again being an exception; for pAD1 this region was shown earlier to code for aggreagation substance. Detailed hybridization studies of the genes for this plasmid-coded adhesin, which is responsible for cell-cell contact during conjugative transfer via the so-called sex pheromone system of E. faecalis, support the idea of their common origin.

Sequence analysis of Enterococcus faecalis aggregation substance encoded by the sex pheromone plasmid pAD1

The location of the structural gene for aggregation substance on the sex pheromone plasmid pAD1 of Enterococcus faecalis was determined using an oligonucleotide deduced from the N-terminal amino acid sequence of the purified protein. The nucleotide sequence was determined for the corresponding region and two open reading frames (ORFs) could be identified. ORF1 codes for a small (Mr 13,160) acidic protein of unknown function. The gene for aggregation substance (named asa1) was found to code for a protein of 1296 amino acids (Mr 142,248). The protein has a signal peptide of 43 amino acids (the resulting Mr for mature aggregation substance is 137,429) and contains in its C-terminal region a proline-rich sequence, previously characterized as being involved in cell wall association, which is followed by a membrane anchor. The membrane anchor showed significant similarity to that of other Gram-positive organisms, but no other similarities to surface proteins from Gram-positive bacteria were found. In particular, no repeats on the DNA or protein level could be detected for pAD1-specific aggregation substance. The protein contains the amino acid motifs Arg-Gly-Asp-Ser and Arg-Gly-Asp-Val (once each), which, it is proposed, play a crucial role in adherence to eukaryotic cells.

Genetic functions and cell-cell interactions in the pheromone-inducible plasmid transfer system of Enterococcus faecalis

Pheromone-inducible plasmid transfer is a novel form of bacterial conjugation which has, to date, been observed only in Enterococcus (Streptococcus) faecalis. This process includes several important stages of interaction between the donor and recipient cell. The initial interaction is the transmission of a chemical signal from the recipient to the donor cell. Recent evidence has shown that the signal is in the form of a small hydrophobic peptide, which is capable of inducing a complex mating response in the donor cell at concentrations as low as 1-5 molecules per responder cell. Most E. faecalis strains produce multiple pheromones, each of which induces a response only in cells carrying a particular plasmid (or member of a family of related plasmids). Genetic functions ascribed to the pheromone response include: (i) cell-cell aggregation, which promotes initial close contact between mating cells; (ii) surface exclusion, which prevents plasmid transfer between aggregated donor cells; and (iii) highly efficient DNA transfer, which requires other unidentified functions in addition to aggregation. The first two processes appear to be mediated by proteinaceous surface antigens.

Movable genetic elements and antibiotic resistance in enterococci

The enterococci possess genetic elements able to move from one strain to another via conjugation. Certain enterococcal plasmids exhibit a broad host range among gram-positive bacteria, but only when matings are performed on solid surfaces. Other plasmids are more specific to enterococci, transfer efficiently in broth, and encode a response to recipient-produced sex pheromones. Transmissible non-plasmid elements, the conjugative transposons, are widespread among the enterococci and determine their own fertility properties. Drug resistance, hemolysin, and bacteriocin determinants are commonly found on the various transmissible enterococcal elements. Examples of the different systems are discussed in this review.

Sex pheromones and plasmid transfer in Enterococcus faecalis

Plasmid-free Enterococcus faecalis excrete peptides (sex pheromones) which specifically induce a mating response in strains harboring certain conjugative plasmids. The response is characterized by the synthesis of a "fuzzy" surface material, visible by electron microscopy, which is believed to facilitate the aggregation of donors and recipients. Transconjugants which receive a specific plasmid shut down the production of endogenous pheromone; however, they continue to produce pheromones specific for donors harboring different classes of plasmids. In this review, we summarize what is known about the biochemistry and genetics of this phenomenon. Some emphasis is given to the hemolysin plasmid pAD1 and the regulation of its conjugal transfer.