Characterization of plasmids determining hemolysin and bacteriocin production in Streptococcus faecalis 5952

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.

Genetic diversity among Enterococcus faecalis

Enterococcus faecalis, a ubiquitous member of mammalian gastrointestinal flora, is a leading cause of nosocomial infections and a growing public health concern. The enterococci responsible for these infections are often resistant to multiple antibiotics and have become notorious for their ability to acquire and disseminate antibiotic resistances. In the current study, we examined genetic relationships among 106 strains of E. faecalis isolated over the past 100 years, including strains identified for their diversity and used historically for serotyping, strains that have been adapted for laboratory use, and isolates from previously described E. faecalis infection outbreaks. This collection also includes isolates first characterized as having novel plasmids, virulence traits, antibiotic resistances, and pathogenicity island (PAI) components. We evaluated variation in factors contributing to pathogenicity, including toxin production, antibiotic resistance, polymorphism in the capsule (cps) operon, pathogenicity island (PAI) gene content, and other accessory factors. This information was correlated with multi-locus sequence typing (MLST) data, which was used to define genetic lineages. Our findings show that virulence and antibiotic resistance traits can be found within many diverse lineages of E. faecalis. However, lineages have emerged that have caused infection outbreaks globally, in which several new antibiotic resistances have entered the species, and in which virulence traits have converged. Comparing genomic hybridization profiles, using a microarray, of strains identified by MLST as spanning the diversity of the species, allowed us to identify the core E. faecalis genome as consisting of an estimated 2057 unique genes.

Screening of virulence determinants in Enterococcus faecium strains isolated from breast milk

In a previous study, the authors isolated lactic acid bacteria from breast milk of healthy mothers. Since some of the identified isolates belonged to the species Enterococcus faecium, the objective of this work was to evaluate their safety. The enterococcal strains were screened by polymerase chain reaction (PCR) and Southern hybridization for the presence of virulence determinants. The potential of the strains to acquire plasmids by conjugation was investigated by screening for genes involved in conjugation processes. Parallel, phenotypic assays were performed. Presence of genes conferring resistance to vancomycin was assessed by PCR. PCR amplifications and Southern hybridizations revealed that all the strains were clear of the majority of potential virulence determinants. None of the strains showed gelatinase activity, hemolysin production, or aggregation phenotype, and none carried the vanA or vanB genes. These findings suggest that milk of healthy mothers may be a source of avirulent E faecium isolates to the newborns.

Virulence, phenotype and genotype characteristics of endodontic Enterococcus spp

BACKGROUND/AIMS

Enterococci have been implicated in persistent root canal infections but their role in the infection process remains unclear. This study investigated the virulence, phenotype and genotype of 33 endodontic enterococcal isolates.

METHODS

Phenotypic tests were conducted for antibiotic resistance, clumping response to pheromone, and production of gelatinase, hemolysin and bacteriocin. Genotype analysis involved polymerase chain reaction amplification of virulence determinants encoding aggregation substances asa and asa373, cytolysin activator cylA, gelatinase gelE, gelatinase-negative phenotype ef1841/fsrC, adherence factors esp and ace, and endocarditis antigen efaA. Physical DNA characterization involved pulsed-field gel electrophoresis of genomic DNA, and plasmid analysis.

RESULTS

Potential virulence traits expressed included production of gelatinase by Enterococcus faecalis (n=23), and response to pheromones in E. faecalis culture filtrate (n=16). Fourteen strains produced bacteriocin. Five strains were resistant to tetracycline and one to gentamicin, whereas all were susceptible to ampicillin, benzylpenicillin, chloramphenicol, erythromycin, fusidic acid, kanamycin, rifampin, streptomycin and vancomycin. Polymerase chain reaction products encoding efaA, ace, and asa were detected in all isolates; esp was detected in 20 isolates, cylA in six isolates, but asa373 was never detected. The gelatinase gene (gelE) was detected in all isolates of E. faecalis (n=31) but not in Enterococcus faecium (n=2); a 23.9 kb deletion sequence corresponding to the gelatinase-negative phenotype was detected in six of the eight E. faecalis isolates that did not produce gelatinase. Pulsed-field gel electrophoresis and plasmid analyses revealed genetic polymorphism with clonal types evident. Plasmid DNA was detected in 25 strains, with up to four plasmids per strain and a similar (5.1 kb) plasmid occurring in 16 isolates.

CONCLUSIONS

Phenotypic and genotypic evidence of potential virulence factors were identified in endodontic Enterococcus spp., specifically production of gelatinase and response to pheromones.

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.

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

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

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.

Enterococci in milk and milk products

Enterococci are widely distributed in nature. They gain entry into milk and milk products through the water supply, equipment, and insanitary and unhygienic conditions of production and handling. They have been incriminated as direct or indirect agents of disease. The evidence concerning their involvement is only circumstantial. These reports are also disputed as the disease symptoms have not been experimentally induced in animal models. However, there is sufficient evidence to indicate that prolific growth of enterococci in foods may lead to formation of clinically significant levels of pressor amines. These amines are very thermostable and therefore remain active even after heat processing, which eliminates all viable streptococci. These pressor amines may be involved in the onset of migraine attacks and produce hypersensitive crises in psychiatric patients who are being treated with monoamine oxidase inhibitors for depression.

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.

Enterococcus faecalis hemolysin-bacteriocin plasmids belong to the same incompatibility group

Plasmid pair coexistence was studied both among nine Enterococcus faecalis hemolysin-bacteriocin (Hly-Bcn) plasmids, including pJH2, pAD1, pAM gamma 1, and pIP964, and between pIP964 and five R plasmids. Some of the Hly-Bcn plasmids used were derivatives encoding resistance to erythromycin or tetracycline. The Hly-Bcn plasmids were incompatible with each other; 40 to 100% displacement was observed bilaterally for eight pairs and unilaterally for one pair. In contrast, pIP964 stably coexisted with each of the R plasmids. Entry exclusion was associated with incompatibility for most of the Hly-Bcn plasmids. The nine Hly-Bcn plasmids harbored by E. faecalis form a distinct incompatibility (Inc) group, designated IncHly.

Transduction of the genetic determinant for streptolysin S in group A streptococci

The genetic determinant for streptolysin S production (SLS+) was successfully transduced to two naturally occurring nonhemolytic strains of group A streptococci (Streptococcus pyogenes), an M-type 18 strain associated with an outbreak of rheumatic fever and an M-negative variant of a type 49 strain isolated from a skin lesion. Attempts to transduce this determinant to a nonhemolytic M-type 68 strain and a nonhemolytic T-type 12 strain were not successful. Transduction was accomplished with a double temperature-sensitive mutant bacteriophage. Cellular antigenic characters and the phage sensitivity of the transductants remained unaltered. The donor strain also transduced streptomycin resistance well when the nonhemolytic type 49 strain was used as a recipient. There was no evidence of cotransduction of the determinants for streptolysin S and streptomycin resistance.

Plasmids, drug resistance, and gene transfer in the genus Streptococcus

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Evidence for a chromosome-borne resistance transposon (Tn916) in Streptococcus faecalis that is capable of "conjugal" transfer in the absence of a conjugative plasmid

Streptococcus faecalis strain DS16 harbors the conjugative hemolysin-bacteriocin plasmid pAD1 (35 megadaltons) and the nonconjugative R-plasmid pAD2 determining resistance to streptomycin, kanamycin, and erythromycin; a tetracycline resistance (Tetr) determinant is located on the chromosome. When strain DS16 was mated (on membrane filters) with the plasmid-free strain JH2-2, Tetr transconjugants could be obtained at a frequency of about 10(-6) per recipient. Analyses of transconjugants showed that some contained the Tetr determinant linked to pAD1. Subsequent studies showed that the Tetr determinant was located on a 10-megaldalton transposon, designated Tn916, which could insert into two hemolysin plasmids: pAM gamma 1 and pOB1. In addition, derivatives of DS16 devoid of pAD1 were capable of transferring Tetr to recipient strains. Transconjugants (plasmid-free) from such matings could subsequently act as donors in the transfer of Tetr. Both transposition and transfer were found to be rec independent.

Determination of the functions of hemolytic plasmid pHly152 of Escherichia coli

The alpha-hemolytic Escherichia coli strain PM152 harbors three transmissible plasmids, which have molecular weights of 65 X 10(6) (pA152), 41 X 10(6) pHly152), and 32 X 10(6) (pC152). Plasmids pHly152 and pC152 belong to incompatibility groups J2 and N, respectively. By transforming E. coli K-12 with isolated plasmids, we showed that the genetic determinant required for hemolysis was located entirely on plasmid pHly152, and a physical map of this plasmid was constructed. By transposon mutagenesis, a deoxyribonucleic acid segment of about 3.5 X 10(6) daltons was identified as being essential for hemolysis. Most of the EcoRI and HindIII fragments of the hemolytic plasmid pHly152 were cloned by using pACYC184 and RSF2124 as vectors. Two classes of Tn3-induced hemolysis-negative mutants could be complemented by recombinant plasmids carrying fragments from the hemolysis region of pHly152, whereas a third class could be restored to hemolytic activity only by recombination between the mutant plasmids and a suitable recombinant deoxyribonucleic acid. These data suggest that there are at least three clustered cistrons which are required for hemolysis. Other EcoRI and HindIII fragments of pHly152 were identified as being essential for replication, incompatibility, transfer, and restriction.

Induced cell aggregation and mating in Streptococcus faecalis: evidence for a bacterial sex pheromone

Recipient strains of Streptococcus faecalis produce a trypsin sensitive, heat resistant, nuclease resistant factor, designated clumping-inducing agent (CIA) which causes strains carrying certain conjugative plasmids to aggregate. RNA and protein synthesis but not DNA synthesis are required for aggregation to occur. Recipient filtrates that contain CIA activity also induce donors to mate at high frequencies. Introduction of a transferable plasmid into strains producing CIA dramatically reduces the amount of CIA activity produced by the strain but allows the strain to respond to exogenously added CIA. Our data suggest that CIA represents a bacterial sex hormone (pheromone).

"Conjugal" transfer of plasmid DNA among oral streptococci

The beta plasmid from Streptococcus faecalis strain DS5, which codes for resistance to erythromycin and lincomycin, was introduced into a Lancefield group F streptococcus, strain DR1501, by transformation. This strain, DR1501 (beta), was found to be an excellent donor of the beta plasmid and readily transferred the resistance markers to various lactic acid bacteria, including certain strains of S. mutans, S. sanguis, and S. salivarius. Evidence is presented indicating that the transfer of the beta plasmid is mediated by a mechanism that requires cell-to-cell contact. The transfer of plasmid DNA during conjugation has been confirmed by the isolation of beta plasmid from transconjugant clones and by their ability to then serve as donors of the erythromycin and lincomycin resistance markers.