Identification of regions of the Streptococcus faecalis plasmid pCF-10 that encode antibiotic resistance and pheromone response functions

EF1025, a Hypothetical Protein From Enterococcus faecalis, Interacts With DivIVA and Affects Cell Length and Cell Shape

DivIVA plays multifaceted roles in Gram-positive organisms through its association with various cell division and non-cell division proteins. We report a novel DivIVA interacting protein in Enterococcus faecalis, named EF1025 (encoded by EF1025), which is conserved in Gram-positive bacteria. The interaction of EF1025 with DivIVAEf was confirmed by Bacterial Two-Hybrid, Glutathione S-Transferase pull-down, and co-immunoprecipitation assays. EF1025, which contains a DNA binding domain and two Cystathionine β-Synthase (CBS) domains, forms a decamer mediated by the two CBS domains. Viable cells were recovered after insertional inactivation or deletion of EF1025 only through complementation of EF1025 in trans. These cells were longer than the average length of E. faecalis cells and had distorted shapes. Overexpression of EF1025 also resulted in cell elongation. Immuno-staining revealed comparable localization patterns of EF1025 and DivIVAEf in the later stages of division in E. faecalis cells. In summary, EF1025 is a novel DivIVA interacting protein influencing cell length and morphology in E. faecalis.

Genetic and pathogenic difference between Streptococcus agalactiae serotype Ia fish and human isolates

Background

Streptococcus agalactiae (GBS) is a common pathogen to infect newborn, woman, the elderly, and immuno-compromised human and fish. 37 fish isolates and 554 human isolates of the GBS in 2007–2012 were investigated in serotypes, antibiotic susceptibility, genetic difference and pathogenicity to tilapia.

Results

PCR serotyping determined serotype Ia for all fish GBS isolates and only in 3.2 % (3–4.2 %) human isolates. For fish isolates, all consisted a plasmid less than 6 kb and belonged to ST7 type, which includes mainly pulsotypes I and Ia, with a difference in a deletion at the largest DNA fragment. These fish isolates were susceptible to all antimicrobials tested in 2007 and increased in non-susceptibility to penicillin, and resistance to clindamycin and ceftriaxone in 2011. Differing in pulsotype and lacking plasmid from fish isolates, human serotype Ia isolates were separated into eight pulsotypes II–IX. Main clone ST23 included pulsotypes II and IIa (50 %) and ST483 consisted of pulsotype III. Human serotype Ia isolates were all susceptible to ceftriaxone and penicillin and few were resistant to erythromycin, azithromycin, clindamycin, levofloxacin and moxifloxacine with the resistant rate of 20 % or less. Using tilapia to analyze the pathogenesis, fish isolates could cause more severe symptoms, including hemorrhage of the pectoral fin, hemorrhage of the gill, and viscous black and common scites, and mortality (>95 % for pulsotype I) than the human isolates (<30 %); however, the fish pulostype Ia isolate 912 with deletion caused less symptoms and the lowest mortality (<50 %) than pulsotype I isolates.

Conclusion

Genetic, pathogenic, and antimicrobial differences demonstrate diverse origin of human and fish serotype Ia isolates. The pulsotype Ia of fish serotype Ia isolates may be used as vaccine strains to prevent the GBS infection in fish.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0794-4) contains supplementary material, which is available to authorized users.

Investigating the mobilome in clinically important lineages of Enterococcus faecium and Enterococcus faecalis

BACKGROUND

The success of Enterococcus faecium and E. faecalis evolving as multi-resistant nosocomial pathogens is associated with their ability to acquire and share adaptive traits, including antimicrobial resistance genes encoded by mobile genetic elements (MGEs). Here, we investigate this mobilome in successful hospital associated genetic lineages, E. faecium sequence type (ST)17 (n=10) and ST78 (n=10), E. faecalis ST6 (n=10) and ST40 (n=10) by DNA microarray analyses.

RESULTS

The hybridization patterns of 272 representative targets including plasmid backbones (n=85), transposable elements (n=85), resistance determinants (n=67), prophages (n=29) and clustered regularly interspaced short palindromic repeats (CRISPR)-cas sequences (n=6) separated the strains according to species, and for E. faecalis also according to STs. RCR-, Rep_3-, RepA_N- and Inc18-family plasmids were highly prevalent and with the exception of Rep_3, evenly distributed between the species. There was a considerable difference in the replicon profile, with rep 17/pRUM , rep 2/pRE25 , rep 14/EFNP1 and rep 20/pLG1 dominating in E. faecium and rep 9/pCF10 , rep 2/pRE25 and rep 7 in E. faecalis strains. We observed an overall high correlation between the presence and absence of genes coding for resistance towards antibiotics, metals, biocides and their corresponding MGEs as well as their phenotypic antimicrobial susceptibility pattern. Although most IS families were represented in both E. faecalis and E. faecium, specific IS elements within these families were distributed in only one species. The prevalence of IS256-, IS3-, ISL3-, IS200/IS605-, IS110-, IS982- and IS4-transposases was significantly higher in E. faecium than E. faecalis, and that of IS110-, IS982- and IS1182-transposases in E. faecalis ST6 compared to ST40. Notably, the transposases of IS981, ISEfm1 and IS1678 that have only been reported in few enterococcal isolates were well represented in the E. faecium strains. E. faecalis ST40 strains harboured possible functional CRISPR-Cas systems, and still resistance and prophage sequences were generally well represented.

CONCLUSIONS

The targeted MGEs were highly prevalent among the selected STs, underlining their potential importance in the evolution of hospital-adapted lineages of enterococci. Although the propensity of inter-species horizontal gene transfer (HGT) must be emphasized, the considerable species-specificity of these MGEs indicates a separate vertical evolution of MGEs within each species, and for E. faecalis within each ST.

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.

Isolation of VanB-type Enterococcus faecalis strains from nosocomial infections: first report of the isolation and identification of the pheromone-responsive plasmids pMG2200, Encoding VanB-type vancomycin resistance and a Bac41-type bacteriocin, and pMG2201, encoding erythromycin resistance and cytolysin (Hly/Bac)

Eighteen identical VanB-type Enterococcus faecalis isolates that were obtained from different hospitalized patients were examined for their drug resistance and plasmid DNAs. Of the 18 strains, 12 strains exhibited resistance to erythromycin (Em), gentamicin (Gm), kanamycin (Km), tetracycline (Tc), and vancomycin (Van) and produced cytolysin (Hly/Bac) and a bacteriocin (Bac) active against E. faecalis strains. Another six of the strains exhibited resistance to Gm, Km, Tc, and Van and produced a bacteriocin. Em and Van resistance was transferred individually to E. faecalis FA2-2 strains at a frequency of about 10(-4) per donor cell by broth mating. The Em-resistant transconjugants and the Van-resistant transconjugants harbored a 65.7-kbp plasmid and a 106-kbp plasmid, respectively. The 106-kbp and 65.7-kbp plasmids isolated from the representative E. faecalis NKH15 strains were designated pMG2200 and pMG2201, respectively. pMG2200 conferred vancomycin resistance and bacteriocin activity on the host strain and responded to the synthetic pheromone cCF10 for pCF10, while pMG2201 conferred erythromycin resistance and cytolysin activity on its host strain and responded to the synthetic pheromone cAD1 for pAD1. The complete DNA sequence of pMG2200 (106,527 bp) showed that the plasmid carried a Tn1549-like element encoding vanB2-type resistance and the Bac41-like bacteriocin genes of pheromone-responsive plasmid pYI14. The plasmid contained the regulatory region found in pheromone-responsive plasmids and encoded the genes prgX and prgQ, which are the key negative regulatory elements for plasmid pCF10. pMG2200 also encoded TraE1, a key positive regulator of plasmid pAD1, indicating that pMG2200 is a naturally occurring chimeric plasmid that has a resulting prgX-prgQ-traE1 genetic organization in the regulatory region of the pheromone response. The functional oriT region and the putative relaxase gene of pMG2200 were identified and found to differ from those of pCF10 and pAD1. The putative relaxase of pMG2200 was classified as a member of the MOB(MG) family, which is found in pheromone-independent plasmid pHTbeta of the pMG1-like plasmids. This is the first report of the isolation and characterization of a pheromone-responsive highly conjugative plasmid encoding vanB resistance.

Characterization of the sequence specificity determinants required for processing and control of sex pheromone by the intramembrane protease Eep and the plasmid-encoded protein PrgY

Conjugative transfer of the Enterococcus faecalis plasmid pCF10 is induced by the peptide pheromone cCF10 when recipient-produced cCF10 is detected by donors. cCF10 is produced by proteolytic processing of the signal sequence of a chromosomally encoded lipoprotein (CcfA). In donors, endogenously produced cCF10 is carefully controlled to prevent constitutive expression of conjugation functions, an energetically wasteful process, except in vivo, where endogenous cCF10 induces a conjugation-linked virulence factor. Endogenous cCF10 is controlled by two plasmid-encoded products; a membrane protein PrgY reduces pheromone levels in donors, and a secreted inhibitor peptide iCF10 inhibits the residual endogenous pheromone that escapes PrgY control. In this study we genetically determined the amino acid specificity determinants within PrgY, cCF10, and the cCF10 precursor that are necessary for cCF10 processing and for PrgY-mediated control. We showed that amino acid residues 125 to 241 of PrgY are required for specific recognition of cCF10 and that PrgY recognizes determinants within the heptapeptide cCF10 sequence, supporting a direct interaction between PrgY and mature cCF10. In addition, we found that a regulated intramembrane proteolysis (RIP) family pheromone precursor-processing protein Eep recognizes amino acids N-terminal to cCF10 in the signal sequence of CcfA. These results support a model where Eep directly targets pheromone precursors for RIP and PrgY interacts directly with the mature cCF10 peptide during processing. Despite evidence that both PrgY and Eep associate with cCF10 in or near the membrane, results presented here indicate that these two proteins function independently.

Specific control of endogenous cCF10 pheromone by a conserved domain of the pCF10-encoded regulatory protein PrgY in Enterococcus faecalis

Conjugative transfer of Enterococcus faecalis plasmid pCF10 is induced by the heptapeptide pheromone cCF10. cCF10 produced by plasmid-free recipient cells is detected by pCF10-containing donor cells, which respond by induction of plasmid-encoded transfer functions. The pCF10-encoded membrane protein PrgY is essential to prevent donor cells from responding to endogenously produced pheromone while maintaining the ability to respond to pheromone from an exogenous source; this function has not been identified in any nonenterococcal prokaryotic signaling system. PrgY specifically inhibited endogenous cCF10 and cPD1 (a pheromone that induces transfer of closely related plasmid pPD1) but not cAD1 (which is specific for less-related plasmid pAD1). Ectopic expression of PrgY in plasmid-free recipient cells reduced pheromone activity in culture supernatants and reduced the ability of these cells to acquire pCF10 by conjugation but did not have any effect on the interaction of these cells with exogenously supplied cCF10. The cloned prgY gene could complement a pCF10 prgY null mutation, and complementation was used to identify point mutations impairing PrgY function. Such mutations also abolished the inhibitory effect of PrgY expression in recipients on pheromone production and on acquisition of pCF10. Most randomly generated point mutations identified in the genetic screen mapped to a predicted extracellular domain in the N terminus of PrgY that is conserved in a newly identified family of related proteins from disparate species including Borrelia burgdorferi, Archaeoglobus fulgidus, Arabidopsis thaliana, and Homo sapiens. The combined genetic and physiological data suggest that PrgY may sequester or inactivate cCF10 as it is released from the membrane.

Enterococcus faecalis divIVA: an essential gene involved in cell division, cell growth and chromosome segregation

Enterococcus faecalis divIVA (divIVAEf) is an essential gene implicated in cell division and chromosome segregation. This gene was disrupted by insertional inactivation creating E. faecalis JHSR1, which was viable only when a wild-type copy of divIVAEf was expressed in trans, confirming the essentiality of the gene. The absence of DivIVAEf in E. faecalis JHSR1 inhibited proper cell division, which resulted in abnormal cell clusters possessing enlarged cells of altered shape instead of the characteristic diplococcal morphology of enterococci. The lower viability of the divIVAEf mutant is caused by improper nucleoid segregation and impaired septation within the numerous cells generated in each cluster. Overexpression of DivIVAEf in Escherichia coli KJB24 resulted in enlarged cells with disrupted cell division, suggesting that this round E. coli mutant strain could be used as an indicator for functionality of DivIVAEf. A Bacillus subtilis divIVA mutant was not complemented by DivIVAEf, indicating that this protein does not recognize DivIVA-specific target sites in B. subtilis, or that it does not interact with other proteins of the cell division machinery of this micro-organism. DivIVAEf also failed to complement a Streptococcus pneumoniae divIVA mutant, supporting the phylogenetic distance between Enterococcus and Streptococcus. Our results indicate that DivIVA is a species-specific multifunctional protein implicated in cell division and chromosome segregation in E. faecalis.

Analysis of the prevalence of tetracycline resistance genes in clinical isolates of Enterococcus faecalis and Enterococcus faecium in a Japanese hospital

Prevalence of seven tetracycline resistance (TC(R)) genes--tet(L), tet(M), tet(K), tet(O), tet(S), tet(T), and tet(U)--which are known to be distributed to gram-positive cocci was analyzed for 224 Enterococcus faecalis and 46 Enterococcus faecium clinical isolates obtained in a Japanese hospital. Any of the TC(R) genes was detected in 75.9% of all the enterococcal strains. The tet(M) was detected at highest rates in both E. faecalis (75.0%) and E. faecium (69.6%), followed by tet(L), which was harbored in 6.7% of E. faecalis isolates and 30.4% of E. faecium isolates. The tet(O), tet(S), and tet(T) were detected in E. faecalis at low frequencies mostly associated with tet(M), while tet(K) and tet(U) were not detected. Nucleotide sequences of tet(S) from E. faecalis strains were identical to that reported in Listeria monocytogenes. Sequences of tet(O) from two E. faecalis strains were almost identical to each other and also to those from Streptococcus pneumoniae, Streptococcus pyogenes, Streptococcus mutans, Campylobacter jejuni, and Campylobacter coli, although minor sequence divergence was observed. The tet(T), which had been reported only in Streptococcus pyogenes, was found in five E. faecalis strains. Sequence of the enterococcal tet(T) differed from that of S. pyogenes by only four nucleotides (four amino acids) and showed high sequence identity (99.8%, amino acid level). Enterococcal strains with any one TC(R) gene or those with two TC(R) genes showed generally similar MICs of tetracyclines, and no evident difference in resistance level was observed.

Characterization of transferable tetracycline resistance genes inEnterococcus faecalisisolated from raw food

The prevalence of tetracycline resistance, and of specific genetic determinants for this resistance was investigated in 1003 strains of Enterococcus faecalis isolated from various raw food products originating from five categories including chicken meat, other poultry meat, beef, pork, and 'other'. For the 238 resistant isolates identified, the ability to transfer the resistant phenotype to a given recipient in vitro was investigated. New and interesting observations were that the tet(L) resistance determinant was more readily transferred than tet(M), and that the presence of Tn916-like elements known to encode tet(M) did not correlate with increased transferability of the resistant phenotype.

Enterococcal peptide sex pheromones: synthesis and control of biological activity

The enterococcal pheromone-inducible plasmids such as pCF10 represent a unique class of mobile genetic elements whose transfer functions are induced by peptide sex pheromones. These pheromones are excreted by potential recipient cells and detected by plasmid-containing donor cells at the cell surface, where the pheromone is imported and signals induction of the plasmid transfer system. Pheromone is processed from a chromosomally encoded lipoprotein and excreted by both the donor and recipient cells, but a carefully controlled detection system prevents a response to self-pheromone while still allowing an extremely sensitive response to exogenous pheromone.

Evolution of genomic islands by deletion and tandem accretion by site-specific recombination: ICESt1-related elements from Streptococcus thermophilus

The 34 734-bp integrative and potentially conjugative element (putative ICE) ICESt1 has been previously found to be site-specifically integrated in the 3' end of the fda locus of Streptococcus thermophilus CNRZ368. Four types of genomic islands related to ICESt1 are integrated in the same position in seven other strains of S. thermophilus. One of these elements, ICESt3, harbours conjugation and recombination modules closely related to those of ICESt1 and excises by site-specific recombination. Two other types of elements, CIME19258 and CIME302, are flanked by site-specific attachment sites closely related to attL and attR of ICESt1 and ICESt3, whereas Delta CIME308 only possesses a putative attR site; none of these three elements carry complete conjugation and recombination modules. ICESt1 contains a functional internal recombination site, attL', that is almost identical to attL of CIME19258. The recombination between attL' and attR of ICESt1 leads to the excision of the expected circular molecule (putative ICE); a cis-mobilizable element (CIME) flanked by an attL site and an attB' site remains integrated into the 3' end of fda. Furthermore, sequences that could be truncated att sites were found within ICESt1, ICESt3 and CIME302. All together, these data suggest that these genomic islands evolved by deletion and tandem accretion of ICEs and CIMEs resulting from site-specific recombination. A model for this evolution is proposed and its application to other genomic islands is discussed.

Transfer of the pheromone-inducible plasmid pCF10 among Enterococcus faecalis microorganisms colonizing the intestine of mini-pigs

A new animal model, the streptomycin-treated mini-pig, was developed in order to allow colonization of defined strains of Enterococcus faecalis in numbers sufficient to study plasmid transfer. Transfer of the pheromone-inducible pCF10 plasmid between streptomycin-resistant strains of E. faecalis OG1 was investigated in the model. The plasmid encodes resistance to tetracycline. Numbers of recipient, donor, and transconjugant bacteria were monitored by selective plating of fecal samples, and transconjugants were subsequently verified by PCR. After being ingested by the mini-pigs, the recipient strain persisted in the intestine at levels between 10(6) and 10(7) CFU per g of feces throughout the experiment. The donor strain, which carried different resistance markers but was otherwise chromosomally isogenic to the recipient strain, was given to the pigs 3 weeks after the recipient strain. The donor cells were initially present in high numbers (10(6) CFU per g) in feces, but they did not persist in the intestine at detectable levels. Immediately after introduction of the donor bacteria, transconjugant cells appeared and persisted in fecal samples at levels between 10(3) and 10(4) CFU per g until the end of the experiment. These observations showed that even in the absence of selective tetracycline pressure, plasmid pCF10 was transferred from ingested E. faecalis cells to other E. faecalis organisms already present in the intestinal environment and that the plasmid subsequently persisted in the intestine.

Effect of pheromone induction on transfer of the Enterococcus faecalis plasmid pCF10 in intestinal mucus ex vivo

The effect of synthetic sex pheromone on pheromone-inducible conjugation between the isogenic Enterococcus faecalis strains OG1RF and OG1SS was investigated in (i) Todd-Hewitt broth medium and (ii) intestinal mucus isolated from germ-free rats. In broth, the presence of synthetic pheromone cCF10 had no detectable effect on the transfer kinetics observed for the tetracycline resistance encoding plasmid pCF10. In mucus, presence of the same pheromone significantly increased the transfer efficiency observed during the first 2 h of conjugation, while the effect was less pronounced later in the experiment. We suggest that due to differences in diffusion rates and medium-binding of the pheromones, the effect of the synthetic cCF10 was immediately dominated by the effect of pheromones produced by the recipient E. faecalis strain in broth, while this happened later in mucus.

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

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

Mutagenesis in the post genomics era: tools for generating insertional mutations in the lactic acid bacteria

The increasing availability of whole genome sequences has increased the demand for effective tools to generate insertional mutations in the lactic acid bacteria (LAB). Several novel approaches, such as shuttle-, transposome- and intron-based mutagenesis methods, are possible additions to the existing repertoire of transposon- and recombination-based tools available for mutagenesis of LAB.

Dominant-negative mutants of prgX: evidence for a role for PrgX dimerization in negative regulation of pheromone-inducible conjugation

PrgX negatively regulates prgQ transcriptional readthrough in the pheromone-inducible enterococcal conjugative plasmid pCF10. We isolated and characterized 13 dominant-negative prgX mutants, all of which mapped in either the N- or the C-terminus of PrgX. In all mutants, the in vivo level of Qa RNA, an antisense RNA to prgQ RNA, was greatly reduced. When oligomerization of PrgX was tested with a phage lambda cI repressor fusion system, the oligomerization domain was found to be between amino acid residues 78 and 280. When histidine-tagged PrgX (His-PrgX) was purified by nickel column chromatography from a strain also expressing PrgX, PrgX was co-purified with His-PrgX. Although PrgX was expressed at a much higher level than His-PrgX, an approximately equal amount of PrgX was co-purified. Pheromone induction greatly decreased the co-purification of PrgX. Based on these data, we propose that both the N- and the C-terminal domains of PrgX are required for PrgX positive autoregulation and for the repression of prgQ transcription readthrough. In vivo, PrgX exists as a dimer, and dimerization is mediated by the central region of PrgX.

Improved vectors for nisin-controlled expression in gram-positive bacteria

A set of shuttle vectors, able to replicate in Escherichia coli and in gram-positive bacteria, containing a nisin-inducible promoter (PnisA) and genes encoding NisR and NisK, the two-component signaling mechanism for activating transcription from PnisA in the presence of nisin, was constructed. To test these vectors, Enterococcus faecalis pCF10 plasmid genes prgX, prgY, and prgZ, which respectively encode cytosolic, integral membrane, and cell surface proteins, were cloned downstream of PnisA. Increased protein expression, in the presence of nisin, was demonstrated by Western blot analysis.

Cell-associated pheromone peptide (cCF10) production and pheromone inhibition in Enterococcus faecalis

In Enterococcus faecalis, the peptide cCF10 acts as a pheromone, inducing transfer of the conjugative plasmid pCF10 from plasmid-containing donor cells to plasmid-free recipient cells. In these studies, it was found that a substantial amount of cCF10 associates with the envelope of the producing cell. Pheromone activity was detected in both wall and membrane fractions, with the highest activity associated with the wall. Experiments examining the effects of protease inhibitor treatments either prior to or following cell fractionation suggested the presence of a cell envelope-associated pro-cCF10 that can be processed to mature cCF10 by a maturase or protease. A pCF10-encoded membrane protein, PrgY, was shown to prevent self-induction of donor cells by reducing the level of pheromone activity in the cell wall fraction.

Identification of genetic determinants for the hemolytic activity of Streptococcus agalactiae by ISS1 transposition

Streptococcus agalactiae is a poorly transformable bacterium and studies of molecular mechanisms are difficult due to the limitations of genetic tools. Employing the novel pGh9:ISS1 transposition vector we generated plasmid-based mutant libraries of S. agalactiae strains O90R and AC475 by random chromosomal integration. A screen for mutants with a nonhemolytic phenotype on sheep blood agar led to the identification of a genetic locus harboring several genes that are essential for the hemolytic function and pigment production of S. agalactiae. Nucleotide sequence analysis of nonhemolytic mutants revealed that four mutants had distinct insertion sites in a single genetic locus of 7 kb that was subsequently designated cyl. Eight different open reading frames were identified: cylX, cylD, cylG, acpC, cylZ, cylA, cylB, and cylE, coding for predicted proteins with molecular masses of 11, 33, 26, 11, 15, 35, 32, and 78 kDa, respectively. The deduced amino acid sequence of the protein encoded by cylA harbors a conserved ATP-binding cassette (ABC) motif, and the predicted proteins encoded by cylA and cylB have significant similarities to the nucleotide binding and transmembrane proteins of typical ABC transporter systems. Transcription analysis by reverse transcription-PCR suggests that cylX to cylE are part of an operon. The requirement of acpC and cylZABE for hemolysin production of S. agalactiae was confirmed either by targeted mutagenesis with the vector pGh5, complementation studies with pAT28, or analysis of insertion elements in naturally occurring nonhemolytic mutants.

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.

Enterococcus faecalis pheromone binding protein, PrgZ, recruits a chromosomal oligopeptide permease system to import sex pheromone cCF10 for induction of conjugation

Conjugative transfer of the plasmid pCF10 by Enterococcus faecalis donor cells occurs in response to a peptide sex pheromone, cCF10, secreted by recipients. The plasmid-encoded cCF10 binding protein, PrgZ, is similar in sequence to binding proteins (OppAs) encoded by oligopeptide permease (opp) operons. Mutation of prgZ decreased the sensitivity of donor cells to pheromone, whereas inactivation of the chromosomal E. faecalis opp operon abolished response at physiological concentrations of pheromone. Affinity chromatography experiments demonstrated the interaction of the pheromone with several putative intracellular regulatory molecules, including an RNA molecule required for positive regulation of conjugation functions. These data suggest that processing of the pheromone signal involves recruitment of a chromosomal Opp system by PrgZ and that signaling occurs by direct interaction of internalized pheromone with intracellular effectors.

Identification of chromosomal mobile element conferring high-level vancomycin resistance in Enterococcus faecium

A clinical isolate of Enterococcus faecium that contains a chromosomally encoded vanA gene cluster, Tn1546::IS1251, transferred vancomycin resistance to the plasmid-free strain Enterococcus faecalis JH2-2 during filter matings. Hybridization of a vanHAXY probe to SmaI restriction-digested genomic DNA separated by pulsed-field gel electrophoresis showed that the vanA gene cluster was located on a 40-kb fragment in the original donor strain and on fragments of different sizes (150 to 450 kb) in the transconjugants. No hybridization to vanA gene cluster probes was obtained with plasmid DNA preparations from the donor or transconjugants. These results suggested that in each case, the van genes had integrated into the recipient chromosome. The transconjugants in turn could act as donors of vancomycin resistance, and resistance was transferable to a Rec- recipient. The results of restriction analyses and DNA hybridizations of genomic DNA from the donor and transconjugants were consistent with the transfer of a mobile element that includes the 12.3-kb Tn1546::IS1251 gene cluster and at least 13 kb of additional DNA. This element has been tentatively designated Tn5482. DNA sequence analysis of a fragment predicted to contain the left end of Tn5482 revealed two insertion sequence-like elements: IS1216V and an apparently truncated IS3-like element. Restriction mapping and DNA hybridization patterns of the van gene clusters of three additional clinical isolates from New York City showed an element similar to Tn5482. Transfer of Tn5482 and related elements may be involved in dissemination of vancomycin resistance.

Targeted disruption of the PD78 gene (traF) reduces pheromone-inducible conjugal transfer of the bacteriocin plasmid pPD1 in Enterococcus faecalis

Bacterial sex pheromone, cPD1, induces sexual aggregation of Enterococcus faecalis harboring the bacteriocin plasmid, pPD1, and enables pPD1 to transfer at high frequency in a liquid culture. PD78 is a cPD1-inducible cell surface protein encoded by pPD1. The PD78 gene, traF, was disrupted by homologous recombination between pPD1 and an artificial vector having a deletion in the middle portion of traF. The disruption of traF did not affect the cPD1-inducible aggregation but reduced the transfer frequency of pPD1 to 2% of the wild-type level.

Genetic analysis of a region of the Enterococcus faecalis plasmid pCF10 involved in positive regulation of conjugative transfer functions

The prgB gene encodes the surface protein Asc10, which mediates cell aggregation resulting in high-frequency conjugative transfer of the pheromone-inducible tetracycline resistance plasmid pCF10 in Enterococcus faecalis. Previous Tn5 insertional mutagenesis and sequencing analysis of a 12-kb fragment of pCF10 indicated that a region containing prgX, -Q, -R, -S, and -T, located 3 to 6 kb upstream of prgB, is required to activate the expression of prgB. Complementation studies showed that the positive regulatory region functions in cis in an orientation-dependent manner (J. W. Chung and G. M. Dunny, Proc. Natl. Acad. Sci. USA 89:9020-9024, 1992). In order to determine the involvement of each gene in the activation of prgB, Tn5 insertional mutagenesis and exonuclease III deletion analyses of the regulatory region were carried out. The results indicate that prgQ and -S are required for the expression of prgB, while prgX, -R, and -T are not required. Western blot (immunoblot) analysis of these mutants shows that prgQ is also essential for the expression of prgA (encoding the surface exclusion protein Sec10), which is located between prgB and the positive-control region. Complementation analysis demonstrates that a cis-acting regulatory element is located in the prgQ region and that pCF10 sequences in an untranslated region 3' from prgQ are an essential component of the positive-control system. Analyses of various Tn5 insertions in pCF10 genes suggest that transcription reading into this transposon is terminated in E. faecalis but that outward-reading transcripts may initiate from within the ends of Tn5 or from the junction sequences.

Transcriptional analysis of a region of the Enterococcus faecalis plasmid pCF10 involved in positive regulation of conjugative transfer functions

The prgB gene encodes aggregation substance (Asc10) which is essential for transfer of the pheromone-inducible conjugative plasmid pCF10 in Enterococcus faecalis. The prgQ and prgS regions, located 4 kb upstream of prgB, are required for the expression of prgB. Complementation studies indicated that the prgQ region functions in cis and in an orientation-dependent manner relative to the prgB gene (J. W. Chung and G. M. Dunny, Proc. Natl. Acad. Sci. USA 89:9020-9024, 1992). Analysis of transcriptional fusions in this study, using a promoterless lacZ gene in several locations between prgQ and prgB, confirmed that the prgQ region does not carry a promoter for the expression of prgB and that prgB does not comprise an operon with prgA (which encodes the surface exclusion protein Sec10), the gene immediately upstream from prgB. Northern (RNA) blot analysis demonstrated that two distinct transcripts (Qs RNA and QL RNA), much larger than the prgQ gene, were expressed in the prgQ region. QS RNA was produced constitutively, whereas QL RNA was produced inducibly by pheromone. The lack of any other open reading frame in QL RNA and significant sequence complementarity between the 3' end of QL RNA and the promoter region of prgB suggested that the functional products of the prgQ region might be RNA molecules rather than proteins. A mutation in prgS completely abolished the production of QL RNA. A model for transcriptional activation of prgB is presented.

Transposon mutagenesis of Clostridium acetobutylicum P262: isolation and characterization of solvent deficient and metronidazole resistant mutants

An efficient transposon mutagenesis system using conjugative transposons Tn916 and Tn925::Tn917 was established for Clostridium acetobutylicum P262, an industrial strain which has proved difficult to manipulate genetically. Transposon insertions occurred at several different locations to produce a variety of mutants. An oligosporogenous mutant deficient in acetone and butanol production, and two sporulation-deficient and metronidazole resistant mutants were characterized with respect to differentiation and solvent production. Tn925::Tn917 inserted near a string of adenosine residues and transposon insertion was often multiple.

Cloning and characterization of a region of the Enterococcus faecalis conjugative plasmid, pCF10, encoding a sex pheromone-binding function

In order to investigate the mechanism by which peptide sex pheromones induce expression of the conjugation functions of certain Enterococcus faecalis plasmids, a biological assay was developed to measure the ability of cells carrying the conjugative plasmid pCF10 to bind the sex pheromone cCF10. The data indicated that pCF10 endows its host E. faecalis cell with the ability to specifically remove (apparently by irreversible binding) cCF10 activity from culture medium. The pCF10 DNA encoding this ability was localized to a 3.4-kb segment within a region involved in negative control of expression of conjugal transfer functions. This segment also encoded ability to bind the pheromone inhibitor peptide iCF10. DNA sequencing revealed three open reading frames, which have been denoted prgW (pheromone responsive gene W), prgZ, and prgY. The deduced product of prgW resembled regulatory proteins from other bacteria and eucaryotes, with a very high degree of identity within a putative DNA-binding domain. The prgY gene actually extended into an adjacent region of pCF10 and could encode a protein with significant similarity to a protein called TraB, believed to be involved in shutdown of pheromone cAD1 production by cells carrying the pheromone-inducible hemolysin plasmid pAD1, according to F.Y. An and D.B. Clewell (Abstr. Gen. Meet. Am. Soc. Microbiol. 1992, H70, 1992). The prgZ gene product showed significant relatedness to binding proteins encoded by oligopeptide permease (opp) operons in gram-positive and gram-negative bacteria and is highly similar to a pAD1-encoded protein, TraC, which is believed to mediate sex pheromone cAD1 binding (K. Tanimoto, F. Y. An, and D. B. Clewell, submitted for publication). A Tn5 insertion into prgZ abolished cCF10 binding ability.

Tetracycline enhances Tn916-mediated conjugal transfer

Pregrowth of the donor on medium containing tetracycline increased conjugative transposition of Tn916 and the transposon-dependent mobilization of pC194 19- to 119-fold in matings between Bacillus subtilis and Bacillus thuringiensis subsp. israelensis. Tn916 and pC194 transferred independently under these conditions. When Enterococcus faecalis was the donor and B. thuringiensis subsp. israelensis the recipient, pregrowth in tetracycline increased the conjugative transposition frequency by approximately 15-fold. Tetracycline-enhanced conjugation appeared during matings as short as 3 h in length. Pregrowth in tetracycline did not enhance conjugation in Bacillus sphaericus x B. thuringiensis subsp. israelensis or B. thuringiensis subsp. israelensis x B. subtilis matings. Incorporation of tetracycline into the mating medium, at concentrations that did not inhibit growth of the B. thuringiensis subsp. israelensis recipient, resulted in conjugation frequencies similar to those obtained by pregrowth of the B. subtilis donors in antibiotic-containing medium. The data suggest stimulation of donor function by tetracycline.

Cis-acting, orientation-dependent, positive control system activates pheromone-inducible conjugation functions at distances greater than 10 kilobases upstream from its target in Enterococcus faecalis

The prgB gene encodes the surface protein, Asc10, which mediates cell aggregation, resulting in high-frequency conjugative transfer of the pheromone-inducible tetracycline-resistance plasmid pCF10 in Enterococcus faecalis. Messenger RNA analysis by Northern blot hybridization and primer extension indicates that prgB transcription is pheromone-inducible and monocistronic. Previous transposon mutagenesis and sequencing analysis of a 12-kilobase (kb) region of pCF10 indicated that several genes including prgR and prgS are required to activate expression of prgB. The distance (3-4 kb) between these regulatory genes and prgB suggested that the activation might function in trans. To test this, a promoterless lacZ gene fusion to prgB was constructed and cloned without some or all of the regulatory genes. Several restriction fragments of the regulatory region were cloned in a higher copy-number plasmid, and numerous complementation studies were carried out in E. faecalis. Complementation in trans was not observed in any of these experiments. However, when the regulatory region and target genes were cloned in different sites of the same plasmid, separated by as much as 12 kb, activation of prgB was observed. Interestingly, this activation occurred only when the regions were cloned in the same relative orientation in which they exist on wild-type pCF10. These results suggest that one or more regulatory molecules may bind to an upstream cis-acting site and track along the DNA to reach a target site to activate prgB transcription.

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

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

Conjugative transfer of Tn916 in Enterococcus faecalis: trans activation of homologous transposons

Tn916 [carries tet(M)] is a 16.4-kb conjugative transposon that can establish itself in multiple copies in Enterococcus faecalis. To study the interaction of coresident homologous transposons during conjugation, an E. faecalis mutant defective in homologous recombination was utilized for construction of strains harboring Tn916 delta E (a derivative in which erm is substituted for tet) on the chromosome and Tn916 on a nonconjugative plasmid. When these strains were used as donors, the two transposons were able to transfer independently; however, they were found to transfer and become coestablished in the recipient up to 50% of the time. In contrast, cotransfer of a plasmid marker located outside the transposon occurred at a frequency of no greater than 0.5%. Separate experiments showed that mobilization of the nonconjugative plasmids pAM401 and pVA749 by chromosome-borne copies of Tn916 occurred only at low frequencies (generally less than 2% cotransfer). The data imply that the initiation of transposition of Tn916 results in a trans activation that is specific for homologous transposons present in the same cell.

Characterization of the gentamicin resistance transposon Tn5281 from Enterococcus faecalis and comparison to staphylococcal transposons Tn4001 and Tn4031

In Enterococcus faecalis, the genetic determinant encoding gentamicin resistance (Gmr) on the conjugative plasmid pBEM10 previously has been shown to be on a mobile element. In the current study, this element, termed Tn5281, was shown to relocate in the absence of homologous recombination in E. faecalis UV202. On the basis of restriction endonuclease analysis and DNA-DNA hybridization studies, Tn5281 was shown to be similar, if not identical, to the Gmr transposons Tn4001 found in Australian isolates of Staphylococcus aureus and Tn4031 found in U.S. isolates of Staphylococcus epidermidis, since all three of these transposons have symmetrically located HindIII (2.5 kb apart), ClaI (slightly more than 2.5 kb apart), and HaeIII (3.9 kb apart) sites. Restriction endonuclease digestion patterns of Tn5281 generated with HincII, ScaI, and AluI were also consistent with Tn4001 and Tn4031. By using a probe specific for the external portion of the terminal inverted repeat of Tn4031, it was determined that each terminus of Tn5281 contained a 0.35-kb HaeIII fragment and a 0.7-kb HindIII-HaeIII fragment. The sizes of these fragments are identical to those found in the staphylococcal transposons, which is a further indication that inverted repeats like IS256 are present in Tn5281. A 1-kb HaeIII fragment in pBEM10 also hybridized with this probe, which indicates that Tn5281 in pBEM10 contains a double copy of the inverted repeat at one end.

Tetracycline resistance heterogeneity in Enterococcus faecium

The tetracycline (Tet) determinants, which encode resistance either to tetracyclines without minocycline (Tcr) or to tetracyclines including minocycline (Tcr-Mnr), of 30 wild-type clinical isolates of Enterococcus faecium were identified and localized. The Tet determinants were transferred by conjugation into a plasmid-free Enterococcus faecalis recipient at frequencies of 10(-6) to 10(-9) transconjugants per donor, as follows: Tcr, 6 strains; Tcr-Mnr, 14 strains; both Tcr and Tcr-Mnr, 6 strains; no detectable transfer, 4 strains. Classes L (Tcr phenotype) and M and O (Tcr-Mnr phenotype) of the Tet determinants were identified by DNA-DNA hybridization experiments. The Tet L determinant was plasmid-borne in 18 strains and was chromosomal in 2 strains. Tet M was chromosomal in 27 strains and plasmid-borne (pIP1534) in 1 strain; pIP1534 also carried Tet L. Tet M was located on Tn916-like elements in 22 strains and on a Tn916-modified element in 1 strain. Tet O was detected in only one strain in which it was plasmid-borne. Both Tet L and Tet M determinants were carried by 19 strains. One strain carried, in addition to chromosomal nonconjugative Tet L and Tet M determinants, a conjugative Tcr-Mnr marker which did not correspond to any Tet determinant tested in this study. These results attest to the genetic complexity of tetracycline resistance in E. faecium strains.

Improved electroporation and cloning vector system for gram-positive bacteria

A protocol for transformation of intact Enterococcus faecalis cells by electroporation was developed through a systematic examination of the effects of changes in various parameters, including (i) growth conditions; (ii) composition of the electroporation solution; (iii) electroporation conditions, such as field strength and resistance; (iv) size, concentration, and purity of DNA used for transformation; and (v) conditions used to select for transformants. Key features of this protocol include the use of exponential-phase cells grown in inhibitory concentrations of glycine and the use of an acidic sucrose electroporation solution. Frequencies of greater than 2 x 10(5) transformants per microgram of plasmid DNA were obtained for E. faecalis cells, whereas various strains of streptococci and Bacillus anthracis were transformed at frequencies of 10(3) to 10(4) transformants per microgram of plasmid DNA with the same protocol. A novel Escherichia coli-Streptococcus and Enterococcus shuttle cloning vector, pDL276, was constructed for use in conjunction with the electroporation system. This vector features a multiple cloning site region flanked by E. coli transcription termination sequences, a relatively small size (less than 7 kb), and a kanamycin resistance determinant expressed in both gram-positive and gram-negative hosts. Various enterococcal and streptococcal DNA sequences were cloned in E. coli (including sequences that could not be cloned on other vectors) and were returned to the original host by electroporation. The vector and electroporation system was also used to clone directly into E. faecalis.

Transfer of Tn925 and plasmids between Bacillus subtilis and alkaliphilic Bacillus firmus OF4 during Tn925-mediated conjugation

Conjugative transposon Tn925 was transferred to alkaliphilic Bacillus firmus OF4 during mating experiments, as monitored by the acquisition of tetracycline resistance at pH 7.5 and confirmed by Southern analysis of chromosomal DNA from transconjugants. Tetracycline resistance could not be demonstrated at pH 10.5, but transconjugants retained resistance upon growth at pH 7.5 after having grown for several generations at pH 10. When the Bacillus subtilis donor strain contained plasmids, either pUB110 or pTV1, in addition to Tn925, transfer of the plasmid to the alkaliphile occurred during conjugation, either together with or independently of the transfer of the transposon. The plasmids were stable in B. firmus OF4, expressing their resistance markers for kanamycin or chloramphenicol at pH 7.5 after growth of the transformants at high pH. Transconjugant B. firmus OF4, which carried Tn925, could serve as the donor in mating experiments with B. subtilis lacking the transposon. These studies establish a basis for initiation of genetic studies in this alkaliphilic Bacillus species, including the introduction of cloned genes and the use of transposon-mediated insertional mutagenesis.

The conjugative transposon Tn925: enhancement of conjugal transfer by tetracycline in Enterococcus faecalis and mobilization of chromosomal genes in Bacillus subtilis and E. faecalis

The effects of tetracycline on transfer of the conjugative, tetracycline-resistance transposon. Tn925, as well as the ability of the transposon to promote the transfer of chromosomal genes was examined in Enterococcus faecalis and Bacillus subtilis. To test for chromosomal transfer, multiply-marked strains of each organism, each carrying a single chromosomal copy of Tn925, were mated on filters with suitable recipient strains, under conditions where transformation and transduction were precluded. In both cases, transfer of a variety of chromosomal genes, at frequencies comparable to the frequency of Tn925 transfer, was detected readily. The presence of Tn925 in one of the members of the mating pair was absolutely required for chromosomal transfer, but transfer of Tn925 did not accompany every chromosomal transfer event. The results were consistent with a mating event resembling a type of cell fusion, allowing for extensive recombination between the genomes of the mating partners. Growth of Tn925-containing donor cells in the presence of tetracycline increased the transfer frequency of Tn925 by about tenfold in E. faecalis, but not in B. subtilis.

Natural transfer of conjugative transposon Tn916 between gram-positive and gram-negative bacteria

The conjugative streptococcal transposon Tn916 was found to transfer naturally between a variety of gram-positive and gram-negative eubacteria. Enterococcus faecalis hosting the transposon could serve as a donor for Alcaligenes eutrophus, Citrobacter freundii, and Escherichia coli at frequencies of 10(-6) to 10(-8). No transfer was observed with several phototrophic species. Mating of an E. coli strain carrying Tn916 yielded transconjugants with Bacillus subtilis, Clostridium acetobutylicum, Enterococcus faecalis, and Streptococcus lactis subsp. diacetylactis at frequencies of 10(-4) to 10(-6). Acetobacterium woodii was the only gram-positive organism tested that did not accept the transposon from a gram-negative donor. The results prove the ability of conjugative transposable elements such as Tn916 for natural cross-species gene transfer, thus potentially contributing to bacterial evolution.

Mutants of Enterococcus faecalis deficient as recipients in mating with donors carrying pheromone-inducible plasmids

From Enterococcus faecalis cells containing random chromosomal insertions of Tn916, strains resistant to a lytic phage were selected and tested for conjugal mating ability. The phage-resistant strains all showed decreased recipient ability (Con-) in broth matings with donors carrying pheromone-inducible plasmids. These strains were normal with respect to donor ability in broth matings and recipient ability in filter matings. The data suggest that the mutants are deficient in the binding substance receptor for the pheromone-induced donor aggregation substance. These mutants contained multiple insertions of Tn916, and none of the individual insertions from the mutant strains were capable of generating the phenotype. Analysis of cell envelope lipoteichoic acids and protein revealed changes in both associated with the Con- phenotype.

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

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

Transfer and expression of the tetracycline resistance transposon Tn925 in Acetobacterium woodii

The Enterococcus faecalis conjugative plasmid pCF10 was used to introduce Tn925 into Acetobacterium woodii by filter mating. Tetracycline resistance was transferred at frequencies of about 10(-6) per donor, but no plasmid DNA was found in the transconjugants. DNA hybridization analyses of HindIII-digested chromosomal DNA demonstrated the insertion of Tn925 at a variety of locations, whereas wild type DNA showed no hybridization at all. The transconjugants were used as donor in mating experiments with tetracycline-sensitive Bacillus subtilis. Transfer of tetracycline resistance was observed at frequencies of 10(-8) per recipient.

Vancomycin resistance is encoded on a pheromone response plasmid in Enterococcus faecium 228

In Enterococcus faecium 228, vancomycin resistance is encoded on a 55-kilobase conjugative plasmid, pHKK100. This plasmid was transferred with high frequency into susceptible strains of Enterococcus faecalis and conferred responses to pheromones produced by E. faecalis and Streptococcus sanguis. pHKK100 is the first plasmid described that mediates both vancomycin resistance and pheromone response.

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.

Plasmid promiscuity and chastity and its uses

Bacterial plasmids are obligate and intracellular genetic elements that replicate and are maintained autonomously from the chromosome. They are ubiquitous. Some of them are relatively more promiscuous than others. Plasmid genetic systems that contribute to relative promiscuity or chastity in naturally occurring plasmids are described and discussed. Both the promiscuity and the chastity of plasmid-based genetic systems have applications in bacterial molecular genetics, in the production of recombinant DNA products and in the breeding and use of desirable bacteria. The role of these systems in such applications is considered.

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.

Identification and cloning of the conjugative transfer region of Staphylococcus aureus plasmid pGO1

The conjugative transfer (tra) genes of a 52-kilobase (kb) staphylococcal plasmid, pGO1, were localized by deletion analysis and transposon insertional inactivation. All transfer-defective (Tra-) deletions and Tn551 or Tn917 transposon insertions occurred within a 14.5-kb BglII fragment. Deletions and insertions outside this fragment all left the plasmid transfer proficient (Tra+). The tra region was found to be flanked by directly repeated DNA sequences, approximately 900 base pairs in length, at either end. Clones containing the 14.5-kb BglII fragment (pGO200) and subclones from this fragment were constructed in Escherichia coli on shuttle plasmids and introduced into Staphylococcus aureus protoplasts. Protoplasts could not be transformed with pGO200E (pGO200 on the staphylococcal replicon, pE194) or subclones containing DNA at one end of the tra fragment unless pGO1 or specific cloned tra DNA fragments were present in the recipient cell. However, once stabilized by sequences present on a second replicon, each tra fragment could be successfully introduced alone into other plasmid-free S. aureus recipients by conjugative mobilization or transduction. In this manner, two clones containing overlapping fragments comprising the entire 14.5-kb BglII fragment were shown to complement each other. The low-frequency transfer resulted in transconjugants containing one clone intact, deletions of that clone, and recombinants of the two clones. The resulting recombinant plasmid (pGO220), which regenerated the tra region intact on a single replicon, transferred at frequencies comparable to those of pGO1. Thus, all the genes necessary and sufficient for conjugative transfer of pGO1 are contained within a 14.5-kb region of DNA.

Cloning and expression of genes encoding pheromone-inducible antigens of Enterococcus (Streptococcus) faecalis

Fragments, generated by restriction enzyme digestion, of the 58-kilobase Enterococcus (Streptococcus) faecalis tetracycline resistance plasmid pCF10 were cloned and introduced into Escherichia coli and E. faecalis to characterize the pheromone-inducible conjugation system encoded by this plasmid. Western blot (immunoblot) analyses revealed that a 130-kilodalton (kDa) antigen, identical to the Tra130 antigen shown previously to be involved in pCF10-mediated pheromone-inducible surface exclusion, was produced by both bacterial hosts carrying the recombinant plasmid pINY1825 (cloned EcoRI C fragment). Both bacterial hosts carrying pINY1825 also produced various amounts of immunologically related 118- to 125-kDa antigens (designated pre-Tra130) that resembled antigens produced by E. faecalis cells carrying pCF10. An additional 150-kDa antigen, Tra150, probably involved in pheromone-induced cellular aggregation, was produced by Escherichia coli and E. faecalis hosts carrying pINY1801 (cloned EcoRI C and E fragments). The coding sequences for the Tra150 and Tra130 antigens were further localized in the TRA region of pCF10 by transposon insertion mutagenesis. Western blot analyses of the recombinant strains, and of strains carrying derivatives of pCF10 or various recombinant plasmids containing Tn5 or Tn917 insertions, suggested that the portion of pCF10 comprising the tra3 through -6 segments (previously defined by Tn917 insertional mutagenesis) contained several genes that are involved in regulating the synthesis of Tra130 and Tra150.