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

Structural foundation for the role of enterococcal PrgB in conjugation, biofilm formation, and virulence

Type 4 Secretion Systems are a main driver for the spread of antibiotic resistance genes and virulence factors in bacteria. In Gram-positives, these secretion systems often rely on surface adhesins to enhance cellular aggregation and mating-pair formation. One of the best studied adhesins is PrgB from the conjugative plasmid pCF10 of Enterococcus faecalis, which has been shown to play major roles in conjugation, biofilm formation, and importantly also in bacterial virulence. Since prgB orthologs exist on a large number of conjugative plasmids in various different species, this makes PrgB a model protein for this widespread virulence factor. After characterizing the polymer adhesin domain of PrgB previously, we here report the structure for almost the entire remainder of PrgB, which reveals that PrgB contains four immunoglobulin (Ig)-like domains. Based on this new insight, we re-evaluate previously studied variants and present new in vivo data where specific domains or conserved residues have been removed. For the first time, we can show a decoupling of cellular aggregation from biofilm formation and conjugation in prgB mutant phenotypes. Based on the presented data, we propose a new functional model to explain how PrgB mediates its different functions. We hypothesize that the Ig-like domains act as a rigid stalk that presents the polymer adhesin domain at the right distance from the cell wall.

Probiotic and functional potential of lactic acid bacteria isolated from pulque and evaluation of their safety for food applications

Pulque is a traditional Mexican non-distilled alcoholic beverage to which several beneficial functions are attributed, mainly associated with gastrointestinal health, which can be explained by the presence of probiotic bacteria in its microbiota. Therefore, the objective of this work was to evaluate the safety, probiotic activity, and functional characteristics of seven strains of lactic acid bacteria (LAB) isolated from pulque using the probiotic strain Lactobacillus acidophilus NCFM as control. The LAB isolates were identified by 16S rRNA sequencing and MALDI Biotyper® MS as belonging to three different Lactobacillaceae genera and species: Lactiplantibacillus plantarum, Levilactobacillus brevis and Lacticaseibacillus paracasei. Most strains showed resistance to gastric juice, intestinal juice and lysozyme (10 mg/L). In addition, all strains exhibited bile salt hydrolase (BSH) activity and antibacterial activity against the pathogenic strain Listeria monocytogenes. Additionally, cell surface characteristics of LAB were evaluated, with most strains showing good hydrophobicity, auto-aggregation, and co-aggregation towards enteropathogenic Escherichia coli and L. monocytogenes. In terms of safety, most of the strains were sensitive to the tested antibiotics and only the Lact. paracasei UTMB4 strain amplified a gene related to antibiotic resistance (mecA). The strains Lact. plantarum RVG2 and Lact. plantarum UTMB1 presented γ-hemolytic activity, and the presence of the virulence-related gene agg was identified only in UTMB1 strain. Regarding functional characterization, the tested bacteria showed good β-galactosidase activity, antioxidant activity and cholesterol reduction Based on principal component analysis (PCA) and heat mapping, and considering the strain Lact. acidophilus NCFM as the probiotic reference, the strains Lacticaseibacillus paracasei UTMB4, Lactiplantibacillus plantarum RVG4 and Levilactobacillus brevis UTMB2 were selected as the most promising probiotic strains. The results of this study highlighted the probiotic, functional and safety traits of LAB strains isolated from pulque thus supporting the health benefits attributed to this ancestral beverage.

Enterococcal biofilm - a nidus for antibiotic resistance transfer?

Enterococci, important agents of hospital acquired infection, are listed on the WHO list of multi-drug resistant pathogens commonly encountered in hospital acquired infections are now of increasing importance, due to the development of strains resistant to multiple antibiotics. Enterococci are also important microorganisms in the environment and their presence is frequently used as an indicator of faecal pollution. Their success is related to their ability to survive within a broad range of habitats and the ease by which they acquire mobile genetic elements, including plasmids, from other bacteria. The enterococci are frequently present within a bacterial biofilm which provides stability and protection to the bacterial population along with an opportunity for a variety of bacterial interactions. Enterococci can accept extrachromosomal DNA both from within its own species and from other bacterial species and this is enhanced by the proximity of the donor and recipient strains. It is this exchange of genetic material that makes the role of biofilm such an important aspect of the success of enterococci. There remain many questions regarding the most suitable model systems to study enterococci in biofilm and regarding the transfer of genetic material including antibiotic resistance in these biofilms. This review focuses on some important aspects of biofilm in the context of horizontal gene transfer (HGT) in enterococci.

A sampling survey of enterococci within pasteurized, fermented dairy products and their virulence and antibiotic resistance properties

Globally, fermented foods (FFs), which may be traditional or industrially-produced, are major sources of nutrition. In the traditional practice, the fermentation process is driven by communities of virtually uncharacterized microflora indigenous to the food substrate. Some of these flora can have virulent or antibiotic resistance properties, posing risk to consumers. Others, such as Enterococcus faecalis and Enterococcus faecium, may also be found in such foods. Enterococci that harbor antibiotic resistance or virulence factors can cycle among animals, food, humans and the environment, thereby transferring these harmful properties at the gene level to harmless commensals in the food matrix, animals and humans. In this work, several microbial isolates obtained from different FF sources were analyzed for their identity and virulence and/or antibiotic resistance properties. For identification aiming at enterococci, isolates that were Gram-positive and catalase- and oxidase-negative were subjected to multiple tests including for growth in broth containing 6.5% NaCl, growth and hydrolytic activity on medium containing bile-esculin, hemolytic activity on blood agar, and growth at 45°C and survival after incubation at 60°C for 30 min. Furthermore, the isolates were tested for susceptibility/resistance to a select group of antibiotics. Finally, the isolates were molecularly-characterized with respect to species identity and presence of virulence-encoding genes by amplification of target genes. Most sources contained enterococci, in addition to most of them also containing Gram-negative flora. Most of these also harbored virulence factors. Several isolates were also antibiotic-resistant. These results strongly suggest attention should be given to better control presence of such potentially pathogenic species.

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.

Identification of a Carboxy-Terminal Glutamine-Rich Domain in Agrobacterium tumefaciens Coupling Protein VirD4 Required for Recognition of T-Strand DNA and Not VirE2 as a Substrate for Transfer to Plant Cells

Agrobacterium tumefaciens transfers DNA and proteins to a plant cell inciting crown gall tumor disease on most plants. VirD4 targets the DNA and protein substrates to a type IV secretion (T4S) apparatus for translocation into the plant cell. Several bacteria with VirD4 homologs use T4S for intercellular export of microbial macromolecules to eukaryotic and prokaryotic hosts. How the VirD4 proteins recognize the diverse substrates is not well understood. To identify functional domains of A. tumefaciens pTiA6 VirD4, we introduced random 19-codon and targeted 10-codon insertions throughout the coding region. Analysis of 21 mutants showed that only the carboxy-terminal end of VirD4 is tolerant of an insertion. Sequence comparison of VirD4 proteins of Agrobacterium spp. and their close relative, Rhizobium etli, showed that these proteins contain a highly conserved C-terminal end, but the immediate upstream regions share no discernible sequence similarity. The conserved region sequence is rich in the amino acid glutamine (6/13 Q). Using site-specific and deletion mutagenesis, we demonstrated that the conserved Q-rich region is required for VirD4 function and for the specific recognition of VirD2-linked T-strand DNA as a substrate for translocation to plants. The Q-rich region is not required for the transfer of a second A. tumefaciens substrate, VirE2, to plants or a promiscuous Escherichia coli IncQ plasmid to another A. tumefaciens strain. We identified Q-rich sequences at or near the C terminus of several VirD4 homologs, including the E. coli F plasmid TraD. In F TraD, the Q-rich sequence maps to a region required specifically for the conjugative transfer of the F plasmid.

Exploiting biofilm phenotypes for functional characterization of hypothetical genes in Enterococcus faecalis

Enterococcus faecalis is a commensal organism as well as an important nosocomial pathogen, and its infections are typically linked to biofilm formation. Nearly 25% of the E. faecalis OG1RF genome encodes hypothetical genes or genes of unknown function. Elucidating their function and how these gene products influence biofilm formation is critical for understanding E. faecalis biology. To identify uncharacterized early biofilm determinants, we performed a genetic screen using an arrayed transposon (Tn) library containing ~2000 mutants in hypothetical genes/intergenic regions and identified eight uncharacterized predicted protein-coding genes required for biofilm formation. We demonstrate that OG1RF_10435 encodes a phosphatase that modulates global protein expression and arginine catabolism and propose renaming this gene bph (biofilm phosphatase). We present a workflow for combining phenotype-driven experimental and computational evaluation of hypothetical gene products in E. faecalis, which can be used to study hypothetical genes required for biofilm formation and other phenotypes of diverse bacteria.

Enterococcal Genetics

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

Planktonic Interference and Biofilm Alliance between Aggregation Substance and Endocarditis- and Biofilm-Associated Pili in Enterococcus faecalis

Most bacteria express multiple adhesins that contribute to surface attachment and colonization. However, the network and relationships between the various adhesins of a single bacterial species are less well understood. Here, we examined two well-characterized adhesins in Enterococcus faecalis, aggregation substance and endocarditis- and biofilm-associated pili, and found that they exhibit distinct functional contributions depending on the growth stage of the bacterial community. Pili interfere with aggregation substance-mediated clumping and plasmid transfer under planktonic conditions, whereas the two adhesins structurally complement one another during biofilm development. This study advances our understanding of how E. faecalis, a ubiquitous member of the human gut microbiome and an opportunistic pathogen, uses multiple surface structures to evolve and thrive.

Like many bacteria, Enterococcus faecalis encodes a number of adhesins involved in colonization or infection of different niches. Two well-studied E. faecalis adhesins, aggregation substance (AS) and endocarditis- and biofilm-associated pili (Ebp), both contribute to biofilm formation on abiotic surfaces and in endocarditis, suggesting that they may be expressed at the same time. Because different regulatory pathways have been reported for AS and Ebp, here, we examined if they are coexpressed on the same cells and what is the functional impact of coexpression on individual cells and within a population. We found that while Ebp are only expressed on a subset of cells, when Ebp and AS are expressed on the same cells, pili interfere with AS-mediated clumping and impede AS-mediated conjugative plasmid transfer during planktonic growth. However, when the population density increases, horizontal gene transfer rates normalize and are no longer affected by pilus expression. Instead, at higher cell densities during biofilm formation, Ebp and AS differentially contribute to biofilm development and structure, synergizing to promote maximal biofilm formation.

IMPORTANCE Most bacteria express multiple adhesins that contribute to surface attachment and colonization. However, the network and relationships between the various adhesins of a single bacterial species are less well understood. Here, we examined two well-characterized adhesins in Enterococcus faecalis, aggregation substance and endocarditis- and biofilm-associated pili, and found that they exhibit distinct functional contributions depending on the growth stage of the bacterial community. Pili interfere with aggregation substance-mediated clumping and plasmid transfer under planktonic conditions, whereas the two adhesins structurally complement one another during biofilm development. This study advances our understanding of how E. faecalis, a ubiquitous member of the human gut microbiome and an opportunistic pathogen, uses multiple surface structures to evolve and thrive.

Mechanisms of bacterial multiresistance to antibiotics

Multiple drug resistance (MDR) to widening range of antibiotics emerging in increasing variety of pathogenic bacteria is a serious threat to the health of mankind nowadays. This is partially due to an uncontrolled usage of antibiotics not only in clinical practice, but also in various branches of agriculture. MDR is affected by two mechanisms: (1) accumulation of resistance genes as a result of intensive selection caused by antibiotics, and (2) active horizontal transfer of resistance genes. To unveil the reasons of bacterial multiresistance to antibiotics, it is necessary to understand the mechanisms of antibiotics action as well as the ways how either resistance to certain antibiotics emerge or resistance genes accumulate and transfer among bacterial strains. Current review is devoted to all these problems.

Influence of a subinhibitory concentration of vancomycin on the in vitro expression of virulence-related genes in the vancomycin-resistant Enterococcus faecalis

INTRODUCTION

Exposure to subinhibitory concentrations (SICs) of antimicrobials may alter the bacterial transcriptome.

METHODS

Here, we evaluated the expression of nine virulence-related genes in vancomycin-resistant enterococci (VRE) urinary tract infection isolates grown at SICs of vancomycin.

RESULTS

A Subinhibitory concentrations of vancomycin interferes with gene modulation, but does not affect the phenotype of a VRE strain in vitro .

CONCLUSIONS

Subinhibitory concentrations of vancomycin may regulate the expression of virulence factors in vivo or contribute to the selection of vancomycin-resistant strains.

Conjugation in Gram-Positive Bacteria

Conjugative transfer is the most important means of spreading antibiotic resistance and virulence factors among bacteria. The key vehicles of this horizontal gene transfer are a group of mobile genetic elements, termed conjugative plasmids. Conjugative plasmids contain as minimum instrumentation an origin of transfer (oriT), DNA-processing factors (a relaxase and accessory proteins), as well as proteins that constitute the trans-envelope transport channel, the so-called mating pair formation (Mpf) proteins. All these protein factors are encoded by one or more transfer (tra) operons that together form the DNA transport machinery, the Gram-positive type IV secretion system. However, multicellular Gram-positive bacteria belonging to the streptomycetes appear to have evolved another mechanism for conjugative plasmid spread reminiscent of the machinery involved in bacterial cell division and sporulation, which transports double-stranded DNA from donor to recipient cells. Here, we focus on the protein key players involved in the plasmid spread through the two different modes and present a new secondary structure homology-based classification system for type IV secretion protein families. Moreover, we discuss the relevance of conjugative plasmid transfer in the environment and summarize novel techniques to visualize and quantify conjugative transfer in situ.

Contribution of Cell Surface Hydrophobicity in the Resistance of Staphylococcus aureus against Antimicrobial Agents

Staphylococcus aureus is found in a wide variety of habitats, including human skin, where many strains are commensals that may be clinically significant or contaminants of food. To determine the physiological characteristics of resistant strain of Staphylococcus aureus against pediocin, a class IIa bacteriocin, a resistant strain was compared with wild type in order to investigate the contribution of hydrophobicity to this resistance. Additional clumping of resistant strain relative to wild type in light microscopy was considered as an elementary evidence of resistance attainment. A delay in log phase attainment was observed in resistant strain compared to the wild type strain. A significant increase in cell surface hydrophobicity was detected for resistant strain in both hexadecane and xylene indicating the contribution of cell surface hydrophobicity as adaptive reaction against antimicrobial agents.

Multiple roles for Enterococcus faecalis glycosyltransferases in biofilm-associated antibiotic resistance, cell envelope integrity, and conjugative transfer

The emergence of multidrug-resistant bacteria and the limited availability of new antibiotics are of increasing clinical concern. A compounding factor is the ability of microorganisms to form biofilms (communities of cells encased in a protective extracellular matrix) that are intrinsically resistant to antibiotics. Enterococcus faecalis is an opportunistic pathogen that readily forms biofilms and also has the propensity to acquire resistance determinants via horizontal gene transfer. There is intense interest in the genetic basis for intrinsic and acquired antibiotic resistance in E. faecalis, since clinical isolates exhibiting resistance to multiple antibiotics are not uncommon. We performed a genetic screen using a library of transposon (Tn) mutants to identify E. faecalis biofilm-associated antibiotic resistance determinants. Five Tn mutants formed wild-type biofilms in the absence of antibiotics but produced decreased biofilm biomass in the presence of antibiotic concentrations that were subinhibitory to the parent strain. Genetic determinants responsible for biofilm-associated antibiotic resistance include components of the quorum-sensing system (fsrA, fsrC, and gelE) and two glycosyltransferase (GTF) genes (epaI and epaOX). We also found that the GTFs play additional roles in E. faecalis resistance to detergent and bile salts, maintenance of cell envelope integrity, determination of cell shape, polysaccharide composition, and conjugative transfer of the pheromone-inducible plasmid pCF10. The epaOX gene is located in a variable extended region of the enterococcal polysaccharide antigen (epa) locus. These data illustrate the importance of GTFs in E. faecalis adaptation to diverse growth conditions and suggest new targets for antimicrobial design.

The origin of endodontic Enterococcus faecalis explored by comparison of virulence factor patterns and antibiotic resistance to that of isolates from stool samples, blood cultures and food

AIM

To elucidate the origin of Enterococcus faecalis isolated from secondary root canal infections and the possibility for a foodborne transmission by comparing them to strains recovered from food, blood and stool regarding putative virulence factors and antibiotic susceptibility profiles, where strains from common origin were hypothesized to harbour similar characteristics.

METHODOLOGY

A total of 108 E. faecalis strains recovered in the county of Stockholm, Sweden, were screened using PCR for putative virulence factors esp, cylA, gelE/gelatinase-negative phenotype (ef1841/fsrC), efaA, ace and asa1. The minimum inhibitory concentration (MIC) for ampicillin, piperacillin-tazobactam, imipenem, gentamicin, vancomycin, ciprofloxacin and linezolid was determined using the agar dilution method.

RESULTS

Next to strains from blood, the food isolates presented the highest average number of virulence determinants and were frequently enriched with asa1 coding for aggregation substance. None of the endodontic strains carried cylA, and the gelatinase-negative phenotype caused by a deletion dominated the group. Altogether, the most prevalent genes were gelE, efaA and ace, and a combination of them was equally present in approximately 80% of the strains from food, stool and root canals in comparison with 43.3% of the blood isolates. High-level resistance to ciprofloxacin and gentamicin was observed in 30% of the blood isolates, whereas the isolates from other origins, with single exceptions, were susceptible to all tested antibiotics.

CONCLUSIONS

Evidence for a foodborne transmission, explaining the high reported prevalence of E. faecalis in root filled teeth, could not be determined based on the similarities in virulence factor patterns and antibiotic susceptibility. The only linkage between isolates from food and root canals consisted of a shared common combination of the genes gelE, efaA and ace. The high occurrence of putative virulence traits in food isolates questions the safety of E. faecalis in food products.

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.

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.

Early adaptation to oxygen is key to the industrially important traits of Lactococcus lactis ssp. cremoris during milk fermentation

Background

Lactococcus lactis is the most used species in the dairy industry. Its ability to adapt to technological stresses, such as oxidative stress encountered during stirring in the first stages of the cheese-making process, is a key factor to measure its technological performance. This study aimed to understand the response to oxidative stress of Lactococcus lactis subsp. cremoris MG1363 at the transcriptional and metabolic levels in relation to acidification kinetics and growth conditions, especially at an early stage of growth. For those purposes, conditions of hyper-oxygenation were initially fixed for the fermentation.

Results

Kinetics of growth and acidification were not affected by the presence of oxygen, indicating a high resistance to oxygen of the L. lactis MG1363 strain. Its resistance was explained by an efficient consumption of oxygen within the first 4 hours of culture, leading to a drop of the redox potential. The efficient consumption of oxygen by the L. lactis MG1363 strain was supported by a coherent and early adaptation to oxygen after 1 hour of culture at both gene expression and metabolic levels. In oxygen metabolism, the over-expression of all the genes of the nrd (ribonucleotide reductases) operon or fhu (ferrichrome ABC transports) genes was particularly significant. In carbon metabolism, the presence of oxygen led to an early shift at the gene level in the pyruvate pathway towards the acetate/2,3-butanediol pathway confirmed by the kinetics of metabolite production. Finally, the MG1363 strain was no longer able to consume oxygen in the stationary growth phase, leading to a drastic loss of culturability as a consequence of cumulative stresses and the absence of gene adaptation at this stage.

Conclusions

Combining metabolic and transcriptomic profiling, together with oxygen consumption kinetics, yielded new insights into the whole genome adaptation of L. lactis to initial oxidative stress. An early and transitional adaptation to oxidative stress was revealed for L. lactis subsp. cremoris MG1363 in the presence of initially high levels of oxygen. This enables the cells to maintain key traits that are of great importance for industry, such as rapid acidification and reduction of the redox potential of the growth media.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1054) contains supplementary material, which is available to authorized users.

Conjugative type IV secretion systems in Gram-positive bacteria

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The conjugative transfer mechanism of broad-host-range, Enterococcus sex pheromone and Clostridium plasmids is reviewed.

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Comparisons with Gram-negative type IV secretion systems are presented.

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The current understanding of the unique Streptomyces double stranded DNA transfer mechanism is reviewed.

Bacterial conjugation presents the most important means to spread antibiotic resistance and virulence factors among closely and distantly related bacteria. Conjugative plasmids are the mobile genetic elements mainly responsible for this task. All the genetic information required for the horizontal transmission is encoded on the conjugative plasmids themselves. Two distinct concepts for horizontal plasmid transfer in Gram-positive bacteria exist, the most prominent one transports single stranded plasmid DNA via a multi-protein complex, termed type IV secretion system, across the Gram-positive cell envelope. Type IV secretion systems have been found in virtually all unicellular Gram-positive bacteria, whereas multicellular Streptomycetes seem to have developed a specialized system more closely related to the machinery involved in bacterial cell division and sporulation, which transports double stranded DNA from donor to recipient cells. This review intends to summarize the state of the art of prototype systems belonging to the two distinct concepts; it focuses on protein key players identified so far and gives future directions for research in this emerging field of promiscuous interbacterial transport.

Interference in pheromone-responsive conjugation of a high-level bacitracin resistant Enterococcus faecalis plasmid of poultry origin

The current study reports on contact interference of a high-level bacitracin- resistant pheromone-responsive plasmid of Enterococcus faecalis strain 543 of poultry origin during conjugative transfer of bcr antimicrobial resistance genes using a polyclonal antiserum aggregation substance_44–560 (AS). After induction with pheromones produced by the recipient strain E. faecalis JH2-2, clumping of the donor E. faecalis strain 543 was observed as well as high transfer frequencies of bcr in short time broth mating. Filter mating assays from donor strain E. faecalis 543 to the recipient strain E. faecalis JH2-2 revealed conjugative transfer of asa1 (AS), bcrRAB and traB (negative regulator pheromone response) genes. The presence of these genes in transconjugants was confirmed by antimicrobial susceptibility testing, PCR, Southern hybridization and sequencing. A significant reduction in formation of aggregates was observed when the polyclonal anti-AS_44–560 was added in the pheromone-responsive conjugation experiments as compared to the induced state. Moreover, interference of anti-AS_44–560 antibodies in pheromone-responsive conjugation was demonstrated by a reduction in horizontal transfer of asa1 and bcr genes between E. faecalis strain 543 and E. faecalis JH2-2. Reducing the pheromone-responsive conjugation of E. faecalis is of interest because of its clinical importance in the horizontal transfer of antimicrobial resistance.

The expanding bacterial type IV secretion lexicon

The bacterial type IV secretion systems (T4SSs) comprise a biologically diverse group of translocation systems functioning to deliver DNA or protein substrates from donor to target cells generally by a mechanism dependent on establishment of direct cell-to-cell contact. Members of one T4SS subfamily, the conjugation systems, mediate the widespread and rapid dissemination of antibiotic resistance and virulence traits among bacterial pathogens. Members of a second subfamily, the effector translocators, are used by often medically-important pathogens to deliver effector proteins to eukaryotic target cells during the course of infection. Here we summarize our current understanding of the structural and functional diversity of T4SSs and of the evolutionary processes shaping this diversity. We compare mechanistic and architectural features of T4SSs from Gram-negative and -positive species. Finally, we introduce the concept of the 'minimized' T4SSs; these are systems composed of a conserved set of 5-6 subunits that are distributed among many Gram-positive and some Gram-negative species.

Speciation and frequency of virulence genes of Enterococcus spp. isolated from rainwater tank samples in Southeast Queensland, Australia

In this study, 212 Enterococcus isolates from 23 rainwater tank samples in Southeast Queensland (SEQ), Australia were identified to the species level. The isolates were also tested for the presence of 6 virulence genes associated with Enterococcus related infections. Among the 23 rainwater tank samples, 20 (90%), 10 (44%), 7 (30%), 5 (22%), 4 (17%), 2 (9%), and 1 (4%) samples yielded E. faecalis, E. mundtii, E. casseliflavus, E. faecium, E. hirae, E. avium, and E. durans, respectively. Among the 6 virulence genes tested, gelE and efaA were most prevalent, detected in 19 (83%) and 18 (78%) of 23 rainwater tank samples, respectively. Virulence gene ace was also detected in 14 (61%) rainwater tank samples followed by AS, esp (E. faecalis variant), and cylA genes which were detected in 3 (13%), 2 (9%), and 1 (4%) samples, respectively. In all, 120 (57%) Enterococcus isolates from 20 rainwater tank samples harbored virulence genes. Among these tank water samples, Enterococcus spp. from 5 (25%) samples harbored a single virulence gene and 15 (75%) samples were harboring two or more virulence genes. The significance of these strains in terms of health implications remains to be assessed. The potential sources of these strains need to be identified for the improved management of captured rainwater quality. Finally, it is recommended that Enterococcus spp. should be used as an additional fecal indicator bacterium in conjunction with E. coli for the microbiological assessment of rainwater tanks.

Transcriptional regulator PerA influences biofilm-associated, platelet binding, and metabolic gene expression in Enterococcus faecalis

Enterococcus faecalis is an opportunistic pathogen and a leading cause of nosocomial infections, traits facilitated by the ability to quickly acquire and transfer virulence determinants. A 150 kb pathogenicity island (PAI) comprised of genes contributing to virulence is found in many enterococcal isolates and is known to undergo horizontal transfer. We have shown that the PAI-encoded transcriptional regulator PerA contributes to pathogenicity in the mouse peritonitis infection model. In this study, we used whole-genome microarrays to determine the PerA regulon. The PerA regulon is extensive, as transcriptional analysis showed 151 differentially regulated genes. Our findings reveal that PerA coordinately regulates genes important for metabolism, amino acid degradation, and pathogenicity. Further transcriptional analysis revealed that PerA is influenced by bicarbonate. Additionally, PerA influences the ability of E. faecalis to bind to human platelets. Our results suggest that PerA is a global transcriptional regulator that coordinately regulates genes responsible for enterococcal pathogenicity.

Acceleration of Enterococcus faecalis biofilm formation by aggregation substance expression in an ex vivo model of cardiac valve colonization

Infectious endocarditis involves formation of a microbial biofilm in vivo. Enterococcus faecalis Aggregation Substance (Asc10) protein enhances the severity of experimental endocarditis, where it has been implicated in formation of large vegetations and in microbial persistence during infection. In the current study, we developed an ex vivo porcine heart valve adherence model to study the initial interactions between Asc10(+) and Asc10(-)E. faecalis and valve tissue, and to examine formation of E. faecalis biofilms on a relevant tissue surface. Scanning electron microscopy of the infected valve tissue provided evidence for biofilm formation, including growing masses of bacterial cells and the increasing presence of exopolymeric matrix over time; accumulation of adherent biofilm populations on the cardiac valve surfaces during the first 2-4 h of incubation was over 10-fold higher than was observed on abiotic membranes incubated in the same culture medium. Asc10 expression accelerated biofilm formation via aggregation between E. faecalis cells; the results also suggested that in vivo adherence to host tissue and biofilm development by E. faecalis can proceed by Asc10-dependent or Asc10-independent pathways. Mutations in either of two Asc10 subdomains previously implicated in endocarditis virulence reduced levels of adherent bacterial populations in the ex vivo system. Interference with the molecular interactions involved in adherence and initiation of biofilm development in vivo with specific inhibitory compounds could lead to more effective treatment of infectious endocarditis.

Biological diversity of prokaryotic type IV secretion systems

Type IV secretion systems (T4SS) translocate DNA and protein substrates across prokaryotic cell envelopes generally by a mechanism requiring direct contact with a target cell. Three types of T4SS have been described: (i) conjugation systems, operationally defined as machines that translocate DNA substrates intercellularly by a contact-dependent process; (ii) effector translocator systems, functioning to deliver proteins or other macromolecules to eukaryotic target cells; and (iii) DNA release/uptake systems, which translocate DNA to or from the extracellular milieu. Studies of a few paradigmatic systems, notably the conjugation systems of plasmids F, R388, RP4, and pKM101 and the Agrobacterium tumefaciens VirB/VirD4 system, have supplied important insights into the structure, function, and mechanism of action of type IV secretion machines. Information on these systems is updated, with emphasis on recent exciting structural advances. An underappreciated feature of T4SS, most notably of the conjugation subfamily, is that they are widely distributed among many species of gram-negative and -positive bacteria, wall-less bacteria, and the Archaea. Conjugation-mediated lateral gene transfer has shaped the genomes of most if not all prokaryotes over evolutionary time and also contributed in the short term to the dissemination of antibiotic resistance and other virulence traits among medically important pathogens. How have these machines adapted to function across envelopes of distantly related microorganisms? A survey of T4SS functioning in phylogenetically diverse species highlights the biological complexity of these translocation systems and identifies common mechanistic themes as well as novel adaptations for specialized purposes relating to the modulation of the donor-target cell interaction.

Multiple functional domains of Enterococcus faecalis aggregation substance Asc10 contribute to endocarditis virulence

Aggregation substance proteins encoded by sex pheromone plasmids increase the virulence of Enterococcus faecalis in experimental pathogenesis models, including infectious endocarditis models. These large surface proteins may contain multiple functional domains involved in various interactions with other bacterial cells and with the mammalian host. Aggregation substance Asc10, encoded by plasmid pCF10, is induced during growth in the mammalian bloodstream, and pCF10 carriage gives E. faecalis a significant selective advantage in this environment. We employed a rabbit model to investigate the role of various functional domains of Asc10 in endocarditis. The data suggested that the bacterial load of the infected tissue was the best indicator of virulence. Isogenic strains carrying either no plasmid, wild-type pCF10, a pCF10 derivative with an in-frame deletion of the prgB gene encoding Asc10, or pCF10 derivatives expressing other alleles of prgB were examined in this model. Previously identified aggregation domains contributed to the virulence associated with the wild-type protein, and a strain expressing an Asc10 derivative in which glycine residues in two RGD motifs were changed to alanine residues showed the greatest reduction in virulence. Remarkably, this strain and the strain carrying the pCF10 derivative with the in-frame deletion of prgB were both significantly less virulent than an isogenic plasmid-free strain. The data demonstrate that multiple functional domains are important in Asc10-mediated interactions with the host during the course of experimental endocarditis and that in the absence of a functional prgB gene, pCF10 carriage is actually disadvantageous in vivo.

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.

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.

In vivo oligomerization of the F conjugative coupling protein TraD

Type IV secretory systems are a group of bacterial transporters responsible for the transport of proteins and nucleic acids directly into recipient cells. Such systems play key roles in the virulence of some pathogenic organisms and in conjugation-mediated horizontal gene transfer. Many type IV systems require conserved "coupling proteins," transmembrane polypeptides that are critical for transporting secreted substrates across the cytoplasmic membrane of the bacterium. In vitro evidence suggests that the functional form of coupling proteins is a homohexameric, ring-shaped complex. Using a library of tagged mutants, we investigated the structural and functional organization of the F plasmid conjugative coupling protein TraD by coimmunoprecipitation, cross-linking, and genetic means. We present direct evidence that coupling proteins form stable oligomeric complexes in the membranes of bacteria and that the formation of some of these complexes requires other F-encoded functions. Our data also show that different regions of TraD play distinct roles in the oligomerization process. We postulate a model for in vivo oligomerization and discuss the probable participation of individual domains of TraD in each step.

Sets of transposon-generated sequence-tagged mutants for structure-function analysis and engineering

Various genetic strategies are available for the isolation of small, in-frame insertional mutants. Here, we summarize some of the ways in which the resulting mutant libraries in particular genes have been used for the analysis of protein structure-function relationships and in engineering applications.

General mutagenesis of F plasmid TraI reveals its role in conjugative regulation

Bacteria commonly exchange genetic information by the horizontal transfer of conjugative plasmids. In gram-negative conjugation, a relaxase enzyme is absolutely required to prepare plasmid DNA for transit into the recipient via a type IV secretion system. Here we report a mutagenesis of the F plasmid relaxase gene traI using in-frame, 31-codon insertions. Phenotypic analysis of our mutant library revealed that several mutant proteins are functional in conjugation, highlighting regions of TraI that can tolerate insertions of a moderate size. We also demonstrate that wild-type TraI, when overexpressed, plays a dominant-negative regulatory role in conjugation, repressing plasmid transfer frequencies approximately 100-fold. Mutant TraI proteins with insertions in a region of approximately 400 residues between the consensus relaxase and helicase sequences did not cause conjugative repression. These unrestrictive TraI variants have normal relaxase activity in vivo, and several have wild-type conjugative functions when expressed at normal levels. We postulate that TraI negatively regulates conjugation by interacting with and sequestering some component of the conjugative apparatus. Our data indicate that the domain responsible for conjugative repression resides in the central region of TraI between the protein's catalytic domains.

The Tra domain of the lactococcal CluA surface protein is a unique domain that contributes to sex factor DNA transfer

CluA is a cell surface-presented protein that causes cell aggregation and is essential for a high-efficiency conjugation process in Lactococcus lactis. We know from previous work that in addition to promoting cell-to-cell contact, CluA is involved in sex factor DNA transfer. To define the CluA domains involved in aggregation and in transfer, we first performed random mutagenesis of the cluA gene using a modified mini-Tn7 element which generated five amino acid insertions located throughout the encoded protein. Thirty independent cluA insertion mutants expressing modified CluA proteins at the cell surface were isolated and characterized further. The level of aggregation of each mutant was determined. The cell binding capacity of CluA was affected strongly when the protein had a mutation in its N-terminal region, which defined an aggregation domain extending from amino acid 153 to amino acid 483. Of the cluA mutants that still exhibited aggregation, eight showed an attenuated ability to conjugate, and six mutations were located in a 300-amino-acid C-terminal region of the protein defining a transfer domain (Tra). This result was confirmed by a phenotypic analysis of an additional five mutants obtained using site-directed mutagenesis in which charged amino acids of the Tra domain were replaced by alanine residues. Two distinct functional domains of the CluA protein were defined in this work; the first domain is involved in cell binding specificity, and the Tra domain is probably involved in the formation of the DNA transport machinery. This is the first report of a protein involved in conjugation that actively contributes to DNA transfer and mediates contact between donor and recipient strains.

Development of a method for markerless genetic exchange in Enterococcus faecalis and its use in construction of a srtA mutant

Enterococcus faecalis is a gram-positive commensal bacterium of the gastrointestinal tract and an important opportunistic pathogen. Despite the increasing clinical significance of the enterococci, genetic analysis of these organisms has thus far been limited in scope due to the lack of advanced genetic tools. To broaden the repertoire of genetic tools available for manipulation of E.faecalis, we investigated the use of phosphoribosyl transferases as elements of a counterselection strategy. We report here the development of a counterselectable markerless genetic exchange system based on the upp-encoded uracil phosphoribosyl transferase of E. faecalis. Whereas wild-type E. faecalis is sensitive to growth inhibition by the toxic base analog 5-fluorouracil (5-FU), a mutant bearing an in-frame deletion of upp is resistant to 5-FU. When a cloned version of upp was ectopically introduced into the deletion mutant, sensitivity to 5-FU growth inhibition was restored, thereby providing the basis for a two-step integration and excision strategy for the transfer of mutant alleles to the enterococcal chromosome by recombination. This method was validated by the construction of a DeltasrtA mutant of E. faecalis and by the exchange of the surface protein Asc10, encoded on the pheromone-responsive conjugative plasmid pCF10, with a previously isolated mutant allele. Analysis of the DeltasrtA mutant indicated that SrtA anchors Asc10 to the enterococcal cell wall, facilitating the pheromone-induced aggregation of E. faecalis cells required for high-frequency conjugative plasmid transfer in liquid matings. The system of markerless exchange reported here will facilitate detailed genetic analysis of these important pathogens.

Mechanisms of, and barriers to, horizontal gene transfer between bacteria

Bacteria evolve rapidly not only by mutation and rapid multiplication, but also by transfer of DNA, which can result in strains with beneficial mutations from more than one parent. Transformation involves the release of naked DNA followed by uptake and recombination. Homologous recombination and DNA-repair processes normally limit this to DNA from similar bacteria. However, if a gene moves onto a broad-host-range plasmid it might be able to spread without the need for recombination. There are barriers to both these processes but they reduce, rather than prevent, gene acquisition.

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.

Comparison of the incidence of virulence determinants and antibiotic resistance between Enterococcus faecium strains of dairy, animal and clinical origin

Enterococci are part of the dominant microbiota of several dairy products. They are also present in the gut of humans and animals. Their presence in traditional raw milk cheeses is probably due to faecal contamination of milk during milking. Due to their importance as a cause of nosocomial infections, enterococci are acquiring increased significance. Such infections are becoming more and more difficult to treat as resistance to antibiotics increases. The aim of this investigation was to compare the potential virulence of Enterococcus faecium isolated from different ecological habitats and to establish if strains isolated from dairy products should really be considered as potential pathogens. In the present work, the antibiotic resistance pattern of 40 E. faecium strains isolated from dairy products, 26 E. faecium isolated from ewes' faeces and 28 clinical isolates of the same species was studied, and checks were made to see if known virulence determinants were present. Resistance to 12 different antibiotics commonly used in the treatment of human infections was tested using the broth microdilution method as described by the NCCLS. In addition, polymerase chain reaction (PCR) tests were carried out to see if genes for vancomycin resistance were present. The presence of the aggregation substance (AS) gene, the surface protein gene esp, the accessory colonisation factor ace, the Enterococcus faecalis endocarditis antigen efaA and the gelatinase gelE gene, which are involved in the virulence of enterococci, were also tested by PCR. The results of this study clearly indicate that E. faecium strains isolated from both cheese and sheep faeces are less pathogenic than those isolated from clinical samples. A similar pattern of resistance to antibiotics was observed in both dairy and animal strains. It was also found that there was difference in the kind of virulence determinants present in dairy and clinical isolates, while no virulence traits were found in sheep faeces strains. The results of this study suggest that E. faecium from traditional Sardinian raw milk cheeses should not be considered to be the main source of untreatable nosocomial enterococcal infections in humans in the island of Sardinia.

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

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

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

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

Conjugative plasmid transfer in gram-positive bacteria

Conjugative transfer of bacterial plasmids is the most efficient way of horizontal gene spread, and it is therefore considered one of the major reasons for the increase in the number of bacteria exhibiting multiple-antibiotic resistance. Thus, conjugation and spread of antibiotic resistance represents a severe problem in antibiotic treatment, especially of immunosuppressed patients and in intensive care units. While conjugation in gram-negative bacteria has been studied in great detail over the last decades, the transfer mechanisms of antibiotic resistance plasmids in gram-positive bacteria remained obscure. In the last few years, the entire nucleotide sequences of several large conjugative plasmids from gram-positive bacteria have been determined. Sequence analyses and data bank comparisons of their putative transfer (tra) regions have revealed significant similarities to tra regions of plasmids from gram-negative bacteria with regard to the respective DNA relaxases and their targets, the origins of transfer (oriT), and putative nucleoside triphosphatases NTP-ases with homologies to type IV secretion systems. In contrast, a single gene encoding a septal DNA translocator protein is involved in plasmid transfer between micelle-forming streptomycetes. Based on these clues, we propose the existence of two fundamentally different plasmid-mediated conjugative mechanisms in gram-positive microorganisms, namely, the mechanism taking place in unicellular gram-positive bacteria, which is functionally similar to that in gram-negative bacteria, and a second type that occurs in multicellular gram-positive bacteria, which seems to be characterized by double-stranded DNA transfer.

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.

Functional analysis of TraA, the sex pheromone receptor encoded by pPD1, in a promoter region essential for the mating response in Enterococcus faecalis

Conjugative transfer of a bacteriocin plasmid, pPD1, of Enterococcus faecalis is induced in response to a peptide sex pheromone, cPD1, secreted from plasmid-free recipient cells. cPD1 is taken up by a pPD1 donor cell and binds to an intracellular receptor, TraA. Once a recipient cell acquires pPD1, it starts to produce an inhibitor of cPD1, termed iPD1, which functions as a TraA antagonist and blocks self-induction in donor cells. In this study, we discuss how TraA transduces the signal of cPD1 to the mating response. Gel mobility shift assays indicated that TraA is bound to a traA-ipd intergenic region, which is essential for cPD1 response. DNase I footprinting analysis suggested the presence of one strong (tab1) and two weak (tab2 and tab3) TraA-binding sites in the intergenic region. Primer extension analysis implied that the transcriptional initiation sites of traA and ipd were located in the intergenic region. Northern analysis showed that cPD1 upregulated and downregulated transcription of ipd and traA, respectively. The circular permutation assay showed that TraA bent a DNA fragment corresponding to the tab1 region, and its angle was changed in the presence of cPD1 or iPD1. From these data, we propose a model that TraA changes the conformation of the tab1 region in response to cPD1 and upregulates the transcription of ipd, which may lead to expression of genes required for the mating response.