Comparative analysis of 18 sex pheromone plasmids from Enterococcus faecalis: detection of a new insertion element on pPD1 and implications for the evolution of this plasmid family

A new flavor of entry exclusion in ICE elements provides a selective advantage for the element and its host

Entry exclusion has been described in many bacterial conjugation systems, but their molecular mechanisms are not well understood. In the current issue, Avello et al. describe a new exclusion system in the conjugative element ICEBs1. They identify the yddJ gene as the functional exclusion gene and its target as the protein product of the conG gene. They provide evidence for a possible mechanism and for the contribution of the system to reduce fitness costs of ICE expression.

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.

Harnessing bacteriocin biology as targeted therapy in the GI tract

Recently, our laboratory demonstrated that bacteriocins produced by commensal enterococci provide an advantage in niche maintenance in the highly competitive environment of the gastrointestinal (GI) tract. Bacterial production of bacteriocins is a conserved defense strategy to help establish an ecological niche. Bacteriocin-encoding genes in enterococci are often carried on mobile genetic elements, including conjugative plasmids, enabling the transfer of such traits to other community members in a shared niche. Use of a novel mouse model for enterococcal colonization of the GI tract allowed us to investigate enterococcal dynamics and the role of enterococcal bacteriocins in the mouse GI tract. We examined the role of bacteriocin-21, carried on the pPD1 plasmid, in enterococcal colonization of the gut. We discovered that Enterococcus faecalis (EF) harboring pPD1 effectively colonizes the GI tract by using Bac-21 to eliminate its competition. In our study, we also present evidence for active conjugation in the GI tract, a strategy EF uses to enhance the number of bacteriocin producers in a given niche and eliminate bacteriocin-susceptible populations. Using an engineered strain of EF that is capable of producing Bac-21 but impaired in its conjugation ability, we were able to reduce pre-existing colonization by vancomycin-resistant enterococci in the mouse gut. Thus, our results suggest a novel therapeutic strategy to de-colonize antibiotic-resistant enterococci from the GI tract of patients and thereby prevent the emergence of resistant enterococcal infections that are otherwise difficult, or impossible, to treat.

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.

Regulatory circuits controlling enterococcal conjugation: lessons for functional genomics

The regulatory system controlling pheromone-induced plasmid transfer in Enterococcus faecalis is the most thoroughly studied genetic system of this species. Transcription initiation from the target promoter is controlled by a pheromone receptor/repressor protein whose activity is determined by its interaction with two peptide signaling molecules that compete for the same binding site, but have opposing effects on the activity of the receptor protein. For the system to function as a sensitive and robust biological switch, several additional levels of post-transcriptional regulation are also required. Expression of important functions encoded within the enterococcal core genome may also be controlled by multilayered regulatory circuitry. The pheromone system may serve as a useful paradigm to guide comprehensive functional genomic analysis of E. faecalis.

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.

Pheromone-responsive conjugative vancomycin resistance plasmids in Enterococcus faecalis isolates from humans and chicken feces

The drug resistances and plasmid contents of a total of 85 vancomycin-resistant enterococcus (VRE) strains that had been isolated in Korea were examined. Fifty-four of the strains originated from samples of chicken feces, and 31 were isolated from hospital patients in Korea. Enterococcus faecalis KV1 and KV2, which had been isolated from a patient and a sample of chicken feces, respectively, were found to carry the plasmids pSL1 and pSL2, respectively. The plasmids transferred resistances to vancomycin, gentamicin, kanamycin, streptomycin, and erythromycin to E. faecalis strains at a high frequency of about 10(-3) per donor cell during 4 hours of broth mating. E. faecalis strains containing each of the pSL plasmids formed clumps after 2 hours of incubation in broth containing E. faecalis FA2-2 culture filtrate (i.e., the E. faecalis sex pheromone), and the plasmid subsequently transferred to the recipient strain in a 10-min short mating in broth, indicating that the plasmids are responsive to E. faecalis pheromones. The pSL plasmids did not respond to any of synthetic pheromones for the previously characterized plasmids. The pheromone specific for pSL plasmids has been designated cSL1. Southern hybridization analysis showed that specific FspI fragments from each of the pSL plasmids hybridized with the aggregation substance gene (asa1) of the pheromone-responsive plasmid pAD1, indicating that the plasmids had a gene homologous to asa1. The restriction maps of the plasmids were identical, and the size of the plasmids was estimated to be 128.1 kb. The plasmids carried five drug resistance determinants for vanA, ermB, aph(3'), aph(6'), and aac(6')/aph(2'), which encode resistance to vancomycin, erythromycin, kanamycin, streptomycin, and gentamicin/kanamycin, respectively. Nucleotide sequence analyses of the drug resistance determinants and their flanking regions are described in this report. The results described provide evidence for the exchange of genetic information between human and animal (chicken) VRE reservoirs and suggest the potential for horizontal transmission of multiple drug resistance, including vancomycin resistance, between farm animals and humans via a pheromone-responsive conjugative plasmid.

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

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

Pheromone-regulated expression of sex pheromone plasmid pAD1-encoded aggregation substance depends on at least six upstream genes and a cis-acting, orientation-dependent factor

Conjugative transfer of Enterococcus faecalis-specific sex pheromone plasmids relies on an adhesin, called aggregation substance, to confer a tight cell-to-cell contact between the mating partners. To analyze the dependence of pAD1-encoded aggregation substance, Asa1, on pheromone induction, a variety of upstream fragments were fused to an alpha-amylase reporter gene, amyL, by use of a novel promoter probe vector, pAMY-em1. For pheromone-regulated alpha-amylase activity, a total of at least six genes, traB, traC, traA, traE1, orfY, and orf1, are required: TraB efficiently represses asa1 (by a mechanism unrelated to its presumptive function in pheromone shutdown, since a complete shutdown is observed exclusively in the presence of traC); only traC can relieve traB-mediated repression in a pheromone-dependent manner. In addition to traB, traA is required but not sufficient for negative control. Mutational inactivation of traE1, orfY, or orf1, respectively, results in a total loss of alpha-amylase activity for constructs normally mediating constitutive expression. Inversion of a fragment covering traA, P(0), and traE1 without disrupting any gene or control element switches off amyL or asa1 expression, indicating the involvement of a cis-acting, orientation-dependent factor (as had been shown for plasmid pCF10). Unexpectedly, pAD1 represses all pAMY-em1 derivatives in trans, while its own pheromone-dependent functions are unaffected. The discrepancy between the new data and those of former studies defining TraE1 as a trans-acting positive regulator is discussed.

Sex pheromones and gene transfer in Enterococcus faecalis

Cell-density-dependent regulatory controls have been recognized in recent years to play major roles with regard to many microorganisms. In gram-negative bacteria very often N-acyl-homoserine lactones act as 'quorum-sensing' regulators, whilst gram-positive bacteria mainly use peptides to monitor their cell density. The so-called sex pheromone system of Enterococcus faecalis is just one example of the latter type of regulation. The system is a complex one; in this communication, I will discuss in particular the potential role of the peptides to also act as virulence factors.

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.

Efficient transfer of the pheromone-independent Enterococcus faecium plasmid pMG1 (Gmr) (65.1 kilobases) to Enterococcus strains during broth mating

Plasmid pMG1 (65.1 kb) was isolated from a gentamicin-resistant Enterococcus faecium clinical isolate and was found to encode gentamicin resistance. EcoRI restriction of pMG1 produced five fragments, A through E, with molecular sizes of 50.2, 11.5, 2.0, 0.7, and 0.7 kb, respectively. The clockwise order of the fragments was ACDEB. pMG1 transferred at high frequency to Enterococcus strains in broth mating. pMG1 transferred between Enterococcus faecalis strains, between E. faecium strains, and between E. faecium and E. faecalis strains at a frequency of approximately 10(-4) per donor cell after 3 h of mating. The pMG1 transfers were not induced by the exposure of the donor cell to culture filtrates of plasmid-free E. faecalis FA2-2 or an E. faecium strain. Mating aggregates were not observed by the naked eye during broth mating. Small mating aggregates of several cells in the broth matings were observed by microscopy, while no aggregates of donor cells which had been exposed to a culture filtrate of E. faecalis FA2-2 or an E. faecium strain were observed, even by microscopy. pMG1 DNA did not show any homology in Southern hybridization with that of the pheromone-responsive plasmids and broad-host-range plasmids pAMbeta1 and pIP501. These results indicate that there is another efficient transfer system in the conjugative plasmids of Enterococcus and that this system is different from the pheromone-induced transfer system of E. faecalis plasmids.