High efficiency introduction of plasmid DNA into glycine treated Enterococcus faecalis by electroporation

The SrtA Sortase of Streptococcus agalactiae is required for cell wall anchoring of proteins containing the LPXTG motif, for adhesion to epithelial cells, and for colonization of the mouse intestine

Streptococcus agalactiae (group B streptococcus [GBS]) is the leading cause of neonatal pneumonia, sepsis, and meningitis. An in silico genome analysis indicated that GBS strain NEM316 encodes 35 proteins containing an LPXTG motif which are thought to be covalently linked to the peptidoglycan by an enzyme called sortase. The role of these cell wall-anchored proteins in GBS pathogenesis was evaluated on a global level by inactivating the srtA gene. This gene encodes the major sortase SrtA that anchors most of the LPXTG-containing proteins. We chose the C5a peptidase (ScpB) and Alp2, an abundant immunogenic protein, as prototypical LPXTG-containing proteins. As expected, the SrtA knockout mutant was unable to anchor the C5a peptidase (ScpB) and Alp2 to the cell wall. Complementation with plasmid-borne srtA inserted into the chromosome restored the correct surface localization of both ScpB and Alp2. Interestingly, the SrtA mutant was impaired for binding to the major extracellular matrix components fibronectin and fibrinogen and displayed a significant reduction in adherence to human (A549, HeLa, and Caco-2) and murine (L2) epithelial cells compared to the parental wild-type strain. Surprisingly, the inactivation of srtA had no effect on the virulence of the type III strain of GBS in a neonatal rat model (measured by the 50% lethal dose and lung colonization) but strongly impaired the capacity of the strain to colonize the intestines of gnotobiotic mice in a competition assay. These results demonstrate that LPXTG-containing proteins are involved in cell adhesion and GBS persistence in vivo.

CovS/CovR of group B streptococcus: a two-component global regulatory system involved in virulence

In this study, we carried out a detailed structural and functional analysis of a Streptococcus agalactiae (GBS) two-component system which is orthologous to the CovS/CovR (CsrS/CsrR) regulatory system of Streptococcus pyogenes. In GBS, covR and covS are part of a seven gene operon transcribed from two promoters that are not regulated by CovR. A DeltacovSR mutant was found to display dramatic phenotypic changes such as increased haemolytic activity and reduced CAMP activity on blood agar. Adherence of the DeltacovSR mutant to epithelial cells was greatly increased and analysis by transmission electron microscopy revealed the presence at its surface of a fibrous extracellular matrix that might be involved in these intercellular interactions. However, the DeltacovSR mutant was unable to initiate growth in RPMI and its viability in human normal serum was greatly impaired. A major finding of this phenotypic analysis was that the CovS/CovR system is important for GBS virulence, as a 3 log increase of the LD(50) of the mutant strain was observed in the neonate rat sepsis model. The pleiotropic phenotype of the DeltacovSR mutant is in full agreement with the large number of genes controlled by CovS/CovR as seen by expression profiling analysis, many of which encode potentially secreted or cell surface-associated proteins: 76 genes are repressed whereas 63 were positively regulated. CovR was shown to bind directly to the regulatory regions of several of these genes and a consensus CovR recognition sequence was proposed using both DNase I footprinting and computational analyses.

Systematic inactivation and phenotypic characterization of two-component signal transduction systems of Enterococcus faecalis V583

The ability of enterococci to adapt and respond to different environmental stimuli, including the host environment, led us to investigate the role of two-component signal transduction in the regulation of Enterococcus faecalis physiology. Using a bioinformatic approach, we previously identified 17 two-component systems (TCS), consisting of a sensory histidine kinase and the cognate response regulator, as well as an additional orphan response regulator (L. E. Hancock and M. Perego, J. Bacteriol. 184:5819-5825, 2002). In an effort to identify the potential function of each TCS in the biology of E. faecalis clinical isolate strain V583, we constructed insertion mutations in each of the response regulators. We were able to inactivate 17 of 18 response regulators, the exception being an ortholog of YycF, previously shown to be essential for viability in a variety of gram-positive microorganisms. The biological effects of the remaining mutations were assessed by using a number of assays, including antibiotic resistance, biofilm formation, and environmental stress. We identified TCS related to antibiotic resistance and environmental stress and found one system which controls the initiation of biofilm development by E. faecalis.

The Enterococcus faecalis fsr two-component system controls biofilm development through production of gelatinase

Bacterial growth as a biofilm on solid surfaces is strongly associated with the development of human infections. Biofilms on native heart valves (infective endocarditis) is a life-threatening disease as a consequence of bacterial resistance to antimicrobials in such a state. Enterococci have emerged as a cause of endocarditis and nosocomial infections despite being normal commensals of the gastrointestinal and female genital tracts. We examined the role of two-component signal transduction systems in biofilm formation by the Enterococcus faecalis V583 clinical isolate and identified the fsr regulatory locus as the sole two-component system affecting this unique mode of bacterial growth. Insertion mutations in the fsr operon affected biofilm formation on two distinct abiotic surfaces. Inactivation of the fsr-controlled gene gelE encoding the zinc-metalloprotease gelatinase was found to prevent biofilm formation, suggesting that this enzyme may present a unique target for therapeutic intervention in enterococcal endocarditis.

Role of class A penicillin-binding proteins in PBP5-mediated beta-lactam resistance in Enterococcus faecalis

Peptidoglycan polymerization complexes contain multimodular penicillin-binding proteins (PBP) of classes A and B that associate a conserved C-terminal transpeptidase module to an N-terminal glycosyltransferase or morphogenesis module, respectively. In Enterococcus faecalis, class B PBP5 mediates intrinsic resistance to the cephalosporin class of beta-lactam antibiotics, such as ceftriaxone. To identify the glycosyltransferase partner(s) of PBP5, combinations of deletions were introduced in all three class A PBP genes of E. faecalis JH2-2 (ponA, pbpF, and pbpZ). Among mutants with single or double deletions, only JH2-2 DeltaponA DeltapbpF was susceptible to ceftriaxone. Ceftriaxone resistance was restored by heterologous expression of pbpF from Enterococcus faecium but not by mgt encoding the monofunctional glycosyltransferase of Staphylococcus aureus. Thus, PBP5 partners essential for peptidoglycan polymerization in the presence of beta-lactams formed a subset of the class A PBPs of E. faecalis, and heterospecific complementation was observed with an ortholog from E. faecium. Site-directed mutagenesis of pbpF confirmed that the catalytic serine residue of the transpeptidase module was not required for resistance. None of the three class A PBP genes was essential for viability, although deletion of the three genes led to an increase in the generation time and to a decrease in peptidoglycan cross-linking. As the E. faecalis chromosome does not contain any additional glycosyltransferase-related genes, these observations indicate that glycan chain polymerization in the triple mutant is performed by a novel type of glycosyltransferase. The latter enzyme was not inhibited by moenomycin, since deletion of the three class A PBP genes led to high-level resistance to this glycosyltransferase inhibitor.

Molecular analysis of the Enterococcus faecalis serotype 2 polysaccharide determinant

We previously described a 15-kb genetic cluster consisting of 11 open reading frames (cps2A to cps2K) of Enterococcus faecalis FA2-2 that is responsible for the production of the serotype 2 capsular polysaccharide. By using transcriptional fusions to a promoterless lacZ gene, we identified two independent promoters related to the expression of the polysaccharide. Both transcription initiation sites were mapped by primer extension. Reverse transcription-PCR (RT-PCR) demonstrated the transcriptional linkage of genes present in both transcripts. Real-time RT-PCR quantification of transcripts revealed maximum transcription during log phase growth, an observation confirmed by promoter fusion studies. The heterologous expression of this pathway in Escherichia coli caused reactivity with E. faecalis type 2 antiserum, thus demonstrating the essential role of this pathway in the synthesis of the type-specific polysaccharide.

Enterococcus faecalis plasmid pAD1-encoded Fst toxin affects membrane permeability and alters cellular responses to lantibiotics

Fst is a peptide toxin encoded by the par toxin-antitoxin stability determinant of Enterococcus faecalis plasmid pAD1. Intracellular overproduction of Fst resulted in simultaneous inhibition of all cellular macromolecular synthesis concomitant with cell growth inhibition and compromised the integrity of the cell membrane. Cells did not lyse or noticeably leak intracellular contents but had specific defects in chromosome partitioning and cell division. Extracellular addition of synthetic Fst had no effect on cell growth. Spontaneous Fst-resistant mutants had a phenotype consistent with changes in membrane composition. Interestingly, overproduction of Fst sensitized cells to the lantibiotic nisin, and Fst-resistant mutants were cross-resistant to nisin and the pAD1-encoded cytolysin.

Cytolysin gene expression in Enterococcus faecalis is regulated in response to aerobiosis conditions

Here we investigate the expression of cylL(L)and cylL(S), the genes that encode the structural subunits of the cytolysin/haemolysin of Enterococcus faecalis, in response to aerobiosis conditions. Haemolysis assays of E. faecalis strains cultured under aerobic and anaerobic conditions revealed three different haemolytic phenotypes, one of which exhibited greater haemolysis under anaerobic conditions than under aerobic conditions, and was shown to be associated with the presence of the cyl genes. Reporter gene studies revealed that cylL(L) L(S) promoter activity was significantly greater (up to 8.6-fold) under anaerobic compared to aerobic conditions throughout batch growth, demonstrating that these genes are regulated in response to the degree of aerobiosis. Band shift assays confirmed the binding of a protein factor to the region between 202 and 37 bp upstream of the cylL(L)start codon, and a higher level of binding was observed with anaerobically derived cell-free extracts than with extracts of aerobically grown cells. This is the first report of an oxygen-regulated virulence factor in E. faecalis (that is distinct from the quorum-sensing regulatory system reported previously), and may be of in vivo relevance for the bacterium in biofilms and other environments characterised by oxygen gradients.

Axe-Txe, a broad-spectrum proteic toxin-antitoxin system specified by a multidrug-resistant, clinical isolate of Enterococcus faecium

Enterococcal species of bacteria are now acknowledged as leading causes of bacteraemia and other serious nosocomial infections. However, surprisingly little is known about the molecular mechanisms that promote the segregational stability of antibiotic resistance and other plasmids in these bacteria. Plasmid pRUM (24 873 bp) is a multidrug resistance plasmid identified in a clinical isolate of Enterococcus faecium. A novel proteic-based toxin-antitoxin cassette identified on pRUM was demonstrated to be a functional segregational stability module in both its native host and evolutionarily diverse bacterial species. Induced expression of the toxin protein (Txe) of this system resulted in growth inhibition in Escherichia coli. The toxic effect of Txe was alleviated by co-expression of the antitoxin protein, Axe. Homologues of the axe and txe genes are present in the genomes of a diversity of Eubacteria. These homologues (yefM-yoeB) present in the E. coli chromosome function as a toxin-antitoxin mechanism, although the Axe and YefM antitoxin components demonstrate specificity for their cognate toxin proteins in vivo. Axe-Txe is one of the first functional proteic toxin-antitoxin systems to be accurately described for Gram-positive bacteria.

Role of the Streptococcus agalactiae ClpP serine protease in heat-induced stress defence and growth arrest

The main causes of microbial death after heat exposure are not well understood. Here, it is shown that the heat-shock protein ClpP plays a major role in heat-induced growth arrest in Streptococcus agalactiae. A mutant lacking the ClpP protease was more sensitive to the inhibitory effects of heat, salt and oxidative stress than the isogenic wild-type strain. During growth arrest, this mutant displayed important modifications of its total protein content, including a decreased level of essential metabolic enzymes such as the alcohol dehydrogenase. Analysis of protein carbonylation demonstrated that the ClpP protease plays a role in preventing accelerated protein oxidation. Higher levels of oxidized DnaK, a key modulator of the heat-shock regulon, were observed in the ClpP mutant and these were increased following heat shock. Accumulation of oxidized/inactivated DnaK might explain why the ClpP mutant was unable to properly synthesize DNA and proteins, and why it exhibited an aberrant cell morphology. Even though ClpP plays a minor role in the virulence of S. agalactiae in a murine infection model, the data presented here point to the importance of ClpP in oxidative stress defence in preventing heat-induced cell alterations.

Construction and characterization of transposon insertion mutations in Corynebacterium diphtheriae that affect expression of the diphtheria toxin repressor (DtxR)

Transcription of the bacteriophage-borne diphtheria toxin gene tox is negatively regulated, in response to intracellular Fe(2+) concentration, by the chromosomally encoded diphtheria toxin repressor (DtxR). Due to a scarcity of tools, genetic analysis of Corynebacterium diphtheriae has primarily relied on analysis of chemically induced and spontaneously occurring mutants and on the results of experiments with C. diphtheriae genes cloned in Escherichia coli or analyzed in vitro. We modified a Tn5-based mutagenesis technique for use with C. diphtheriae, and we used it to construct the first transposon insertion libraries in the chromosome of this gram-positive pathogen. We isolated two insertions that affected expression of DtxR, one 121 bp upstream of dtxR and the other within an essential region of the dtxR coding sequence, indicating for the first time that dtxR is a dispensable gene in C. diphtheriae. Both mutant strains secrete diphtheria toxin when grown in medium containing sufficient iron to repress secretion of diphtheria toxin by wild-type C. diphtheriae. The upstream insertion mutant still produces DtxR in decreased amounts and regulates siderophore secretion in response to iron in a manner similar to its wild-type parent. The mutant containing the transposon insertion within dtxR does not produce DtxR and overproduces siderophore in the presence of iron. Differences in the ability of the two mutant strains to survive oxidative stress also indicated that the upstream insertion retained slight DtxR activity, whereas the insertion within dtxR abolished DtxR activity. This is the first evidence that DtxR plays a role in protecting the cell from oxidative stress.

Biochemical and genetic characterization of the vanC-2 vancomycin resistance gene cluster of Enterococcus casseliflavus ATCC 25788

The vanC-2 cluster of Enterococcus casseliflavus ATCC 25788 consisted of five genes (vanC-2, vanXY(C-2), vanT(C-2), vanR(C-2), and vanS(C-2)) and shared the same organization as the vanC cluster of E. gallinarum BM4174. The proteins encoded by these genes displayed a high degree of amino acid identity to the proteins encoded within the vanC gene cluster. The putative D,D-dipeptidase-D,D-carboxypeptidase, VanXY(C-2), exhibited 81% amino acid identity to VanXY(C), and VanT(C-2) displayed 65% amino acid identity to the serine racemase, VanT. VanR(C-2) and VanS(C-2) displayed high degrees of identity to VanR(C) and VanS(C), respectively, and contained the conserved residues identified as important to their function as a response regulator and histidine kinase, respectively. Resistance to vancomycin was expressed inducibly in E. casseliflavus ATCC 25788 and required an extended period of induction. Analysis of peptidoglycan precursors revealed that UDP-N-acetylmuramyl-L-Ala-delta-D-Glu-L-Lys-D-Ala-D-Ser could not be detected until several hours after the addition of vancomycin, and its appearance coincided with the resumption of growth. The introduction of additional copies of the vanT(C-2) gene, encoding a putative serine racemase, and the presence of supplementary D-serine in the growth medium both significantly reduced the period before growth resumed after addition of vancomycin. This suggested that the availability of D-serine plays an important role in the induction process.

Construction of an integration-proficient vector based on the site-specific recombination mechanism of enterococcal temperate phage phiFC1

The genome of temperate phage phiFC1 integrates into the chromosome of Enterococcus faecalis KBL 703 via site-specific recombination. In this study, an integration vector containing the attP site and putative integrase gene mj1 of phage phiFC1 was constructed. A 2,744-bp fragment which included the attP site and mj1 was inserted into a pUC19 derivative containing the cat gene to construct pEMJ1-1. E. faecalis KBL 707, which does not contain the bacteriophage but which has a putative attB site within its genome, could be transformed by pEMJ1-1. Southern hybridization, PCR amplification, and DNA sequencing revealed that pEMJ1-1 was integrated specifically at the putative attB site within the E. faecalis KBL 707 chromosome. This observation suggested that the 2,744-bp fragment carrying mj1 and the attP site of phage phiFC1 was sufficient for site-specific recombination and that pEMJ1-1 could be used as a site-specific integration vector. The transformation efficiency of pEMJ1-1 was as high as 6 x 10(3) transformants/microg of DNA. In addition, a vector (pATTB1) containing the 290-bp attB region was constructed. pATTB1 was transformed into Escherichia coli containing a derivative of the pET14b vector carrying attP and mj1. This resulted in the formation of chimeric plasmids by site-specific recombination between the cloned attB and attP sequences. The results indicate that the integration vector system based on the site-specific recombination mechanism of phage phiFC1 can be used for genetic engineering in E. faecalis and in other hosts.

The capsular polysaccharide of Enterococcus faecalis and its relationship to other polysaccharides in the cell wall

With the goal of identifying and characterizing traits of Enterococcus faecalis that play key roles in human disease, we identified an operon specifying synthesis of a capsular carbohydrate of the type most commonly expressed by clinical isolates. This surface-exposed carbohydrate consists of glycerol phosphate, glucose, and galactose residues, and its biosynthesis is encoded by a determinant that includes 11 ORFs. Insertional inactivation of genes in this pathway yielded mutants with enhanced susceptibility to phagocytic killing in vitro and compromised in the ability to persist in regional lymph nodes in vivo.

D-Ala:D-Ala ligase gene flanking the vanC cluster: evidence for presence of three ligase genes in vancomycin-resistant Enterococcus gallinarum BM4174

An open reading frame located 230 nucleotides downstream from the stop codon of vanS(c) and in the opposite direction relative to the other genes of the vanC cluster was identified in Enterococcus gallinarum BM4174. This gene (designated ddl2) encoded a protein of 343 amino acids that had significant predicted structural similarity to D-Ala:D-Ala ligases and displayed 33 and 35% amino acid identity to VanC-1 and the previously reported partial sequence of Ddl from E. gallinarum, respectively. Biochemical characterization by thin-layer chromatography confirmed that Ddl2 is a D-Ala:D-Ala ligase with no detectable D-Ala:D-Ser ligase activity. The vancomycin dependence of Enterococcus faecalis BM4320 (ddl mutant) was lost on electroporation of a plasmid construct expressing ddl2 constitutively. The latter strain was able to grow in the absence of vancomycin, and peptidoglycan precursor analysis under the same conditions indicated the synthesis of pentapeptide[D-Ala] as the main precursor, confirming the activity of Ddl2 in vivo. Expression of ddl and ddl2 in BM4174 was tested by reverse transcription-PCR: results suggested that both D-Ala:D-Ala ligases were expressed concomitantly. Our findings indicate that vancomycin-resistant E. gallinarum BM4174 is likely to express one D-Ala:D-Ser and two D-Ala:D-Ala ligase genes.

Regulation of D-alanyl-lipoteichoic acid biosynthesis in Streptococcus agalactiae involves a novel two-component regulatory system

The dlt operon of gram-positive bacteria comprises four genes (dltA, dltB, dltC, and dltD) that catalyze the incorporation of D-alanine residues into the lipoteichoic acids (LTAs). In this work, we characterized the dlt operon of Streptococcus agalactiae, which, in addition to the dltA to dltD genes, included two regulatory genes, designated dltR and dltS, located upstream of dltA. The dltR gene encodes a 224-amino-acid putative response regulator belonging to the OmpR family of regulatory proteins. The dltS gene codes for a 395-amino-acid putative histidine kinase thought to be involved in the sensing of environmental signals. The dlt operon of S. agalactiae is mainly transcribed from the P(dltR) promoter, which directs synthesis of a 6.5-kb transcript encompassing dltR, dltS, dltA, dltB, dltC, and dltD, and from a weaker promoter, P(dltA), which is located in the 3' extremity of dltS. We demonstrate that P(dltR), but not P(dlA), is activated by DltR in the presence of DltS in D-Ala-deficient LTA mutants resulting from insertional inactivation of the dltA gene, which encodes the cytoplasmic D-alanine-D-alanyl carrier ligase DltA. Expression of the dlt operon does not require DltR and DltS, since the basal activity of P(dltR) is high, being 20-fold that of the constitutive promoter P(aphA-3) which directs synthesis of the kanamycin resistance gene aphA-3 in various gram-positive bacteria. We hypothesize that the role of DltR and DltS in the control of expression of the dlt operon is to maintain the level of D-Ala esters in LTAs at a constant and appropriate value whatever the environmental conditions. The DltA(-) mutant displayed the ability to form clumps in standing culture and exhibited an increased susceptibility to the cationic antimicrobial polypeptide colistin.

Minimizing potential resistance: the molecular view

The major contribution of molecular biology to the study of antibiotic resistance has been the elucidation of nearly all biochemical mechanisms of resistance and the routes for dissemination of genetic information among bacteria. In this review, we consider the potential contribution of molecular biology to counteracting the evolution of resistant bacteria. In particular, we emphasize the fact that fundamental approaches have had direct practical effects on minimizing potential resistance: by improving interpretation of resistance phenotypes, by providing more adequate human therapy, by fostering more prudent use of antibiotics, and by allowing the rational design of new drugs that evade existing resistance mechanisms or address unexploited targets.

Contribution of Mn-cofactored superoxide dismutase (SodA) to the virulence of Streptococcus agalactiae

Superoxide dismutases convert superoxide anions to molecular oxygen and hydrogen peroxide, which, in turn, is metabolized by catalases and/or peroxidases. These enzymes constitute one of the major defense mechanisms of cells against oxidative stress and hence play a role in the pathogenesis of certain bacteria. We previously demonstrated that group B streptococci (GBS) possess a single Mn-cofactored superoxide dismutase (SodA). To analyze the role of this enzyme in the pathogenicity of GBS, we constructed a sodA-disrupted mutant of Streptococcus agalactiae NEM316 by allelic exchange. This mutant was subsequently cis complemented by integration into the chromosome of pAT113/Sp harboring the wild-type sodA gene. The SOD specific activity detected by gel analysis in cell extracts confirmed that active SODs were present in the parental and complemented strains but absent in the sodA mutant. The growth rates of these strains in standing cultures were comparable, but the sodA mutant was extremely susceptible to the oxidative stress generated by addition of paraquat or hydrogen peroxide to the culture medium and exhibited a higher mutation frequency in the presence of rifampin. In mouse bone marrow-derived macrophages, the sodA mutant showed an increased susceptibility to bacterial killing by macrophages. In a mouse infection model, after intravenous injection the survival of the sodA mutant in the blood and the brain was markedly reduced in comparison to that of the parental and complemented strains whereas only minor effects on survival in the liver and the spleen were observed. These results suggest that SodA plays a role in GBS pathogenesis.

The antisense RNA of the par locus of pAD1 regulates the expression of a 33-amino-acid toxic peptide by an unusual mechanism

The par stability determinant of the Enterococcus faecalis plasmid pAD1 is the first antisense RNA-regulated post-segregational killing system (PSK) identified in a Gram-positive organism. Par encodes two small, convergently transcribed RNAs, designated RNA I and RNA II, which are the toxin and antidote of the par PSK system respectively. RNA I encodes an open reading frame of 33 codons designated fst. The results presented here demonstrate that the peptide encoded by fst is the par toxin. The fst sequence was shown to be sufficient for cell killing, and removal of the final codon inactivated the toxin. In vitro translation reactions of purified RNA I transcript produced a product of the expected size for the fst-encoded peptide. This product was not produced when purified RNA II transcript was added to the translation reaction. Toeprint analysis demonstrated that purified RNA II was able to inhibit ribosome binding to RNA I. These data suggest that fst expression is regulated by RNA II via an antisense RNA mechanism. In vitro translation studies and toeprint analyses also indicated that fst expression is internally regulated by a stem-loop structure at the 5' end of RNA I. Removal of this structure resulted in better ribosome binding to RNA I and a 300-fold increase in production of the fst-encoded peptide. Finally, RNA II was shown to be less stable than RNA I in vivo, providing a basis for the selective expression of fst in plasmid-free cells.

Antisense RNA regulation of the pAD1 par post-segregational killing system requires interaction at the 5' and 3' ends of the RNAs

The par stability determinant of the Enterococcus faecalis plasmid pAD1 is the first antisense RNA-regulated post-segregational killing system (PSK) identified in a Gram-positive organism. Par encodes two small, convergently transcribed RNAs, designated RNA I and RNA II, which are the toxin and antidote of the par PSK system respectively. RNA I encodes an open reading frame for a 33-amino-acid toxin called Fst. Expression of fst is regulated post-transcriptionally by RNA II. In this paper, RNA II is shown to interact with RNA I by a unique antisense RNA mechanism. RNA I and RNA II contain complementary direct repeats at their 5' ends and a complementary transcriptional terminator stem-loop at their 3' ends. Deletion of the terminator or mutations within the terminator loop of RNA II severely reduced the rate of interaction in vitro. Mutations in the 5' direct repeats of RNA II prevented the RNAs from interacting in vitro. For these mutations in RNA II, complementary mutations in RNA I were shown to restore interaction. The reduced binding efficiency of the RNA II mutants was paralleled by the failure of these mutants to suppress par-mediated killing in vivo. These results indicate that regions at both the 5' and the 3' ends of the par transcripts are important for RNA I-RNA II interaction.

vanC cluster of vancomycin-resistant Enterococcus gallinarum BM4174

Glycopeptide-resistant enterococci of the VanC type synthesize UDP-muramyl-pentapeptide[D-Ser] for cell wall assembly and prevent synthesis of peptidoglycan precursors ending in D-Ala. The vanC cluster of Enterococcus gallinarum BM4174 consists of five genes: vanC-1, vanXY(C), vanT, vanR(C), and vanS(C). Three genes are sufficient for resistance: vanC-1 encodes a ligase that synthesizes the dipeptide D-Ala-D-Ser for addition to UDP-MurNAc-tripeptide, vanXY(C) encodes a D,D-dipeptidase-carboxypeptidase that hydrolyzes D-Ala-D-Ala and removes D-Ala from UDP-MurNAc-pentapeptide[D-Ala], and vanT encodes a membrane-bound serine racemase that provides D-Ser for the synthetic pathway. The three genes are clustered: the start codons of vanXY(C) and vanT overlap the termination codons of vanC-1 and vanXY(C), respectively. Two genes which encode proteins with homology to the VanS-VanR two-component regulatory system were present downstream from the resistance genes. The predicted amino acid sequence of VanR(C) exhibited 50% identity to VanR and 33% identity to VanR(B). VanS(C) had 40% identity to VanS over a region of 308 amino acids and 24% identity to VanS(B) over a region of 285 amino acids. All residues with important functions in response regulators and histidine kinases were conserved in VanR(C) and VanS(C), respectively. Induction experiments based on the determination of D,D-carboxypeptidase activity in cytoplasmic extracts confirmed that the genes were expressed constitutively. Using a promoter-probing vector, regions upstream from the resistance and regulatory genes were identified that have promoter activity.

Enterococcus faecalis bearing aggregation substance is resistant to killing by human neutrophils despite phagocytosis and neutrophil activation

Enterococcus faecalis aggregation substance (AS) mediates efficient bacterium-bacterium contact to facilitate plasmid exchange as part of a bacterial sex pheromone system. We have previously determined that AS promotes direct, opsonin-independent binding of E. faecalis to human neutrophils (PMNs) via complement receptor type 3 and other receptors on the PMN surface. We have now examined the functional consequences of this bacterium-host cell interaction. AS-bearing E. faecalis was phagocytosed and internalized by PMNs, as determined by deconvolution fluorescence microscopy. However, these bacteria were not killed by PMNs, and internalized bacteria excluded propidium iodide, indicating intact bacterial membranes. Resistance to killing occurred despite activation of PMNs, as indicated by an increase in both functional and total surface Mac-1 expression, shedding of L-selectin, and an increase in PMN extracellular superoxide and phagosomal oxidant production. Deconvolution fluorescence microscopy also revealed that phagosomes containing AS-bearing bacteria were markedly larger than phagosomes containing opsonized E. faecalis, suggesting that some modification of phagosomal maturation may be involved in AS-induced resistance to killing. PMN phagosomal pH was significantly higher after ingestion of nonopsonized AS-bearing E. faecalis than after that of opsonized bacteria. The novel ability of AS to promote intracellular survival of E. faecalis inside PMNs suggests that AS may be a virulence factor used by strains of E. faecalis.

Gene vanXYC encodes D,D -dipeptidase (VanX) and D,D-carboxypeptidase (VanY) activities in vancomycin-resistant Enterococcus gallinarum BM4174

VanX and VanY have strict D,D-dipeptidase and D,D-carboxypeptidase activity, respectively, that eliminates production of peptidoglycan precursors ending in D-alanyl-D-alanine (D-Ala-D-Ala) in glycopeptide-resistant enterococci in which the C-terminal D-Ala residue has been replaced by D-lactate. Enterococcus gallinarum BM4174 synthesizes peptidoglycan precursors ending in D-Ala-D-serine (D-Ala-D-Ser) essential for VanC-type vancomycin resistance. Insertional inactivation of the vanC-1 gene encoding the ligase that catalyses synthesis of D-Ala-D-Ser has a polar effect on both D, D-dipeptidase and D,D-carboxypeptidase activities. The open reading frame downstream from vanC-1 encoded a soluble protein designated VanXYC (Mr 22 318), which had both of these activities. It had 39% identity and 74% similarity to VanY in an overlap of 158 amino acids, and contained consensus sequences for binding zinc, stabilizing the binding of substrate and catalysing hydrolysis that are present in both VanX- and VanY-type enzymes. It had very low dipeptidase activity against D-Ala-D-Ser, unlike VanX, and no activity against UDP-MurNAc-pentapeptide[D-Ser], unlike VanY. The introduction of plasmid pAT708(vanC-1,XYC) or pAT717(vanXYC) into vancomycin-susceptible Enterococcus faecalis JH2-2 conferred low-level vancomycin resistance only when D-Ser was present in the growth medium. The peptidoglycan precursor profiles of E. faecalis JH2-2 and JH2-2(pAT708) and JH2-2(pAT717) indicated that the function of VanXYC was hydrolysis of D-Ala-D-Ala and removal of D-Ala from UDP-MurNAc-pentapeptide[D-Ala]. VanC-1 and VanXYC were essential, but not sufficient, for vancomycin resistance.

A novel means of self-protection, unrelated to toxin activation, confers immunity to the bactericidal effects of the Enterococcus faecalis cytolysin

Enterococcus faecalis has become a pervasive clinical problem due to the emergence of resistance to most antibiotics. The cytolysin of E. faecalis is a novel bacterial toxin that contributes to the severity of disease. It consists of two structural subunits, which together possess both hemolytic and bactericidal activity. Both toxin subunits are encoded in a complex operon frequently harbored on pheromone-responsive plasmids. E. faecalis strains lacking such plasmids are susceptible to the bactericidal effects of the cytolysin. A novel cytolysin immunity determinant at the 3' end of the pAD1 cytolysin operon is described in the present study. Deletion analysis and specific mutagenesis isolated the immunity function to a single open reading frame. Specific mutagenesis experiments demonstrate that cytolysin immunity is unrelated to cytolysin activator (CylA) expression as previously proposed. Cytolysin immunity is, however, encoded on the same transcript as and 3' to CylA, and previous associations between immunity and CylA can be ascribed to the polar behavior of Tn917 insertion.

Atypical genetic locus associated with constitutive production of enterocin B by Enterococcus faecium BFE 900

A purified bacteriocin produced by Enterococcus faecium BFE 900 isolated from black olives was shown by Edman degradation and mass spectrometric analyses to be identical to enterocin B produced by E. faecium T136 from meat (P. Casaus, T. Nilsen, L. M. Cintas, I. F. Nes, P. E. Hernández, and H. Holo, Microbiology 143:2287-2294, 1997). The structural gene was located on a 2.2-kb HindIII fragment and a 12.0-kb EcoRI chromosomal fragment. The genetic characteristics and production of EntB by E. faecium BFE 900 differed from that described so far by the presence of a conserved sequence like a regulatory box upstream of the EntB gene, and its production was constitutive and not regulated. The 2.2-kb chromosomal fragment contained the hitherto undetected immunity gene for EntB in an atypical orientation that is the reverse of that of the structural gene. Typical transport and other genes associated with bacteriocin production were not detected on the 12.0-kb chromosomal fragment containing the EntB structural gene. This makes the EntB genetic system different from most other bacteriocin systems, where transport and possible regulatory genes are clustered. EntB was subcloned and expressed by the dedicated secretion machinery of Carnobacterium piscicola LV17A. The structural gene was amplified by PCR, fused to the divergicin A signal peptide, and expressed by the general secretory pathway in Enterococcus faecalis ATCC 19433.

Cell wall-anchored CshA polypeptide (259 kilodaltons) in Streptococcus gordonii forms surface fibrils that confer hydrophobic and adhesive properties

It has been shown previously that inactivation of the cshA gene, encoding a major cell surface polypeptide (259 kDa) in the oral bacterium Streptococcus gordonii, generates mutants that are markedly reduced in hydrophobicity, deficient in binding to oral Actinomyces species and to human fibronectin, and unable to colonize the oral cavities of mice. We now show further that surface fibrils 60.7 +/- 14.5 nm long, which are present on wild-type S. gordonii DL1 (Challis) cells, bind CshA-specific antibodies and are absent from the cell surfaces of cshA mutants. To more precisely determine the structural and functional properties of CshA, already inferred from insertional-mutagenesis experiments, we have cloned the entire cshA gene into the replicative plasmid pAM401 and expressed full-length CshA polypeptide on the cell surface of heterologous Enterococcus faecalis JH2-2. Enterococci expressing CshA exhibited a 30-fold increase in cell surface hydrophobicity over E. faecalis JH2-2 carrying the pAM401 vector alone and 2.4-fold-increased adhesion to human fibronectin. CshA expression in E. faecalis also promoted cell-cell aggregation and increased the ability of enterococci to bind Actinomyces naeslundii cells. Electron micrographs of negatively stained E. faecalis cells expressing CshA showed peritrichous surface fibrils 70.3 +/- 9.1 nm long that were absent from control E. faecalis JH2-2(pAM401) cells. The fibrils bound CshA-specific antibodies, as detected by immunoelectron microscopy, and the antibodies inhibited the adhesion of E. faecalis cells to fibronectin. The results demonstrate that the CshA polypeptide is the structural and functional component of S. gordonii adhesive fibrils, and they provide a molecular basis for past correlations of surface fibril production, cell surface hydrophobicity, and adhesion in species of oral "sanguis-like" streptococci.

Single-cell analysis of glycopeptide resistance gene expression in teicoplanin-resistant mutants of a VanB-type Enterococcus faecalis

The vanB gene cluster confers resistance to vancomycin but not to the related antibiotic teicoplanin, as the VanRB SB two-component regulatory system triggers expression of the glycopeptide resistance genes only in response to vancomycin. The VanRB regulator activates promoters PRB and PYB for transcription of the regulatory (vanRB SB) and resistance (vanYB WHB BXB) genes respectively. The gfpmut1 gene encoding a green fluorescent protein was fused to PYB to analyse promoter activation in single cells by fluorescence microscopy and flow cytometry. Characterization of 17 teicoplanin-resistant mutants indicated that amino acid substitutions on either side of the VanSB autophosphorylation site led to a constitutive phenotype. Substitutions in the membrane-associated domain resulted in a gain of function, as they allowed induction by teicoplanin. A vanSB null mutant expressed gfpmut1 at various levels under non-inducing conditions, and the majority of the bacteria were not fluorescent. Bacteria grown in the presence of vancomycin or teicoplanin were homogeneously fluorescent. The increase in the number of fluorescent bacteria resulted from induction in negative cells rather than from selection of a resistant subpopulation, indicating that VanRB was activated by cross-talk. Transglycosylase inhibition was probably the stimulus for the heterologous kinase, as moenomycin was also an inducer.

Characterization of dihydrofolate reductase genes from trimethoprim-susceptible and trimethoprim-resistant strains of Enterococcus faecalis

Enterococci are usually susceptible in vitro to trimethoprim; however, high-level resistance (HLR) (MICs, >1,024 microg/ml) has been reported. We studied Enterococcus faecalis DEL, for which the trimethoprim MIC was >1,024 microg/ml. No transfer of resistance was achieved by broth or filter matings. Two different genes that conferred trimethoprim resistance when they were cloned in Escherichia coli (MICs, 128 and >1,024 microg/ml) were studied. One gene that coded for a polypeptide of 165 amino acids (MIC, 128 microg/ml for E. coli) was identical to dfr homologs that we cloned from a trimethoprim-susceptible E. faecalis strain, and it is presumed to be the intrinsic E. faecalis dfr gene (which causes resistance in E. coli when cloned in multiple copies); this gene was designated dfrE. The nucleotide sequence 5' to this dfr gene showed similarity to thymidylate synthetase genes, suggesting that the dfr and thy genes from E. faecalis are located in tandem. The E. faecalis gene that conferred HLR to trimethoprim in E. coli, designated dfrF, codes for a predicted polypeptide of 165 amino acids with 38 to 64% similarity with other dihydrofolate reductases from gram-positive and gram-negative organisms. The nucleotide sequence 5' to dfrF did not show similarity to the thy sequences. A DNA probe for dfrF hybridized under high-stringency conditions only to colony lysates of enterococci for which the trimethoprim MIC was >1,024 microg/ml; there was no hybridization to plasmid DNA from the strain of origin. To confirm that this gene causes trimethoprim resistance in enterococci, we cloned it into the integrative vector pAT113 and electroporated it into RH110 (E. faecalis OG1RF::Tn916DeltaEm) (trimethoprim MIC, 0.5 microg/ml), which resulted in RH110 derivatives for which the trimethoprim MIC was >1, 024 microg/ml. These results indicate that dfrF is an acquired but probably chromosomally located gene which is responsible for in vitro HLR to trimethoprim in E. faecalis.

Effect of disruption of a gene encoding an autolysin of Enterococcus faecalis OG1RF

A mutant (TX5127) of Enterococcus faecalis OG1RF was generated by disruption mutagenesis of a previously described autolysin gene. TX5127 formed longer chains (2 to 10 cells per chain) than wild-type OG1RF (mainly single cells) during growth in broth even though it had a growth rate similar to that of the parental strain as measured by turbidity and cell count. Autolysin activity, as defined by the ability to lyse heat-killed Micrococcus lysodeikticus cells, was absent in TX5127, while this activity was easily detectable in OG1RF. However, disruption of this autolysin gene did not block the ability of TX5127 to hydrolyze E. faecalis cell walls compared to that of OG1RF. The autolysis rate of cells of TX5127 in 10 mM sodium phosphate buffer (pH 6.8) was slower than that of wild-type OG1RF. TX5127 also showed a decreased rate of lysis in the presence of penicillin, as measured by changes in the turbidity of the culture during 24 h of incubation at 37 degrees C and a slightly decreased effect of penicillin as measured by time-kill curves. The virulence of TX5127 was similar to that of OG1RF in the mouse peritonitis model, indicating that the autolysin of E. faecalis is not important for infection in this model.

Binding properties of Streptococcus gordonii SspA and SspB (antigen I/II family) polypeptides expressed on the cell surface of Lactococcus lactis MG1363

The oral bacterium Streptococcus gordonii expresses two cell wall-associated polypeptides, designated SspA (1,542 amino acid residues) and SspB (1,462 amino acid residues), that have 70% sequence identity. These polypeptides are members of the antigen I/II family of oral streptococcal adhesins and mediate the binding of streptococci to salivary glycoproteins, collagen, and other oral microorganisms such as Actinomyces naeslundii. To determine if SspA and SspB have differential binding properties, the coding sequences of the sspA and sspB genes were cloned into expression plasmid vector pTREX1-usp45LS to generate pTREX1-sspA and pTREX1-sspB, respectively, and the Ssp polypeptides were displayed on the cell surface of Lactococcus lactis MG1363. Lactococcal cells expressing similar levels of surface SspA or SspB polypeptide were then compared for their abilities to adhere to a range of antigen I/II polypeptide substrates. More than twice as many L. lactis cells expressing SspA bound to immobilized salivary agglutinin glycoprotein (SAG) as did L. lactis cells expressing SspB. In contrast, lactococci expressing SspB adhered twice as well as lactococci producing SspA to collagen type I and to Candida albicans. The binding of A. naeslundii to lactococci was only weakly enhanced by surface expression of Ssp polypeptides. L. lactis(pTREX1-sspB) cells bound in greater numbers to SAG than did Enterococcus faecalis JH2-2 cells expressing SspB from pAM401EB-5. The results suggest that SspA and SspB have markedly different binding affinities for their oral substrates and thus may function to promote site diversity in colonization by S. gordonii.

Conjugal transfer of plasmid DNA from Escherichia coli to enterococci: a method to make insertion mutations

Shuttle vector pAT18 was transferred by conjugation from Escherichia coli S17-1 to Enterococcus faecalis OG1RF and Enterococcus faecium SE34. Transfer was mediated by the transfer functions of plasmid RK2 in E. coli S17-1 and the origin of conjugal transfer (oriT) located on pAT18. The oriT sequence was then inserted into two plasmids to generate vectors pTEX5235 and pTEX5236. These two vectors cannot replicate in gram-positive bacteria and can be used to make insertion mutants in gram-positive bacteria. An internal sequence from an autolysin gene of E. faecalis OG1RF was cloned into pTEX5235 and transferred by conjugation from E. coli S17-1 to E. faecalis OG1RF. The plasmid was found to integrate into the chromosome of OG1RF by a single crossover event, resulting in a disrupted autolysin gene. A cosmid carrying the pyrimidine gene cluster from E. faecalis, with a transposon insertion in pyrC, was also transferred from E. coli S17-1 to E. faecalis OG1RF. After selection for the transposon, it was found to have recombined into the recipient chromosome by a double crossover between the cosmid and the chromosome of OG1RF. This resulted in a pyrC knockout mutant showing an auxotrophic phenotype.

Internalin of Listeria monocytogenes with an intact leucine-rich repeat region is sufficient to promote internalization

Listeria monocytogenes can use two different surface proteins, internalin (InlA) and InlB, to invade mammalian cells. The exact role of these invasiveness factors in vivo remains to be determined. In cultured cells, InlA is necessary to promote Listeria entry into human epithelial cells, such as Caco-2 cells, whereas InlB is necessary to promote Listeria internalization in several other cell types, including hepatocytes, fibroblasts, and epithelioid cells, such as Vero, HeLa, CHO, or Hep-2 cells. We have recently reported that the InlA receptor on Caco-2 cells is the cell adhesion molecule E-cadherin and demonstrated that nonpermissive fibroblasts become permissive for internalin-mediated entry when transfected with the gene coding for LCAM, the chicken homolog of the human E-cadherin gene. In this study, we demonstrate for the first time that the internalin protein alone is sufficient to promote internalization into cells expressing its receptor. Indeed, internalin confers invasiveness to both Enterococcus faecalis and internalin-coated latex beads. As shown by transmission electron microscopy, these beads were phagocytosed via a "zipper" mechanism similar to that observed during the internalin-E-cadherin-mediated entry of Listeria. Moreover, a functional analysis of internalin demonstrates that its amino-terminal region, encompassing the leucine-rich repeat (LRR) region and the inter-repeat (IR) region, is necessary and sufficient to promote bacterial entry into cells expressing its receptor. Several lines of evidence suggest that the LRR region would interact directly with E-cadherin, whereas the IR region would be required for a proper folding of the LRR region.

Regulation of transfer of the Enterococcus faecalis pheromone-responding plasmid pAD1: temperature-sensitive transfer mutants and identification of a new regulatory determinant, traD

The enterococcal, conjugative, cytolysin plasmid pAD1 confers a mating response to the peptide sex pheromone cAD1 secreted by plasmid-free strains of Enterococcus faecalis. Cells carrying pAM714, a pAD1::Tn917 derivative with wild-type conjugation properties, were mutagenized with ethyl methanesulfonate to obtain variants that were induced (in the absence of pheromone) to transfer plasmid DNA upon shifting from 32 to 42 degrees C. Of 31 such mutants generated, the results of analyses of 7 are presented in detail. All seven strains were thermosensitive in the E. faecalis host FA2-2; colony morphology, clumping, and DNA transfer correlated well with each other at the two temperatures. In the nonisogenic host E. faecalis OG1X, however, only one derivative (pAM2725) exhibited correlation of all three traits at both temperatures. Three (pAM2700, pAM2703, and pAM2717) clumped and had colonies characteristic of pheromone-induced cells at 32 degrees C but transferred plasmid DNA at a higher frequency only at the elevated temperature. The other three (pAM2708, pAM2709, and pAM2712) were derepressed at both temperatures for all three characteristics. Four of the mutations, including that of pAM2725, mapped within the traA determinant, whereas two mapped identically in a previously unnoted open reading frame (designated traD) putatively encoding a short (23-amino-acid) peptide downstream of the inhibitor peptide determinant iad and in the opposite orientation. One mutant could not be located in the regions sequenced. Studies showed that the traA and traD mutations could be complemented in trans with a DNA fragment carrying the corresponding regions.

The VanS sensor negatively controls VanR-mediated transcriptional activation of glycopeptide resistance genes of Tn1546 and related elements in the absence of induction

Transposon Tn1546 from Enterococcus faecium BM4147 encodes a histidine protein kinase (VanS) and a response regulator (VanR) that regulate transcription of the vanHAX operon encoding a dehydrogenase (VanH), a ligase (VanA), and a D,D-dipeptidase (VanX). These last three enzymes confer resistance to glycopeptide antibiotics by production of peptidoglycan precursors ending in the depsipeptide D-alanyl-D-lactate. Transcription of vanS and the role of VanS in the regulation of the vanHAX operon were analyzed by inserting a cat reporter gene into vanS. Transcription of cat and vanX was inducible by glycopeptides in partial diploids harboring vanS and vanS(omega)cat but was constitutive in strains containing only vanS(omega)cat. Promoters P(R) and P(H), located upstream from vanR and vanH, respectively, were cloned into a promoter probing vector to study transactivation by chromosomally encoded VanR and VanS. The promoters were inactive in the absence of vanR and vanS, inducible by glycopeptides in the presence of both genes, and constitutively activated by VanR in the absence of VanS. Thus, induction of the vanHAX operon involves an amplification loop resulting from binding of phospho-VanR to the P(R) promoter and increased transcription of the vanR and vanS genes. Full activation of P(R) and P(H) by VanR was observed in the absence of VanS, indicating that the sensor negatively controls VanR in the absence of glycopeptides, presumably by dephosphorylation. Activation of the VanR response regulator in the absence of VanS may involve autophosphorylation of VanR with acetyl phosphate or phosphorylation by a heterologous histidine protein kinase.

Specificity of induction of glycopeptide resistance genes in Enterococcus faecalis

Regulation of VanA- and VanB-type glycopeptide resistance in enterococci is mediated by related two-component regulatory systems (VanR-VanS and VanRB-VanSB). The transglycosylase inhibitors vancomycin, teicoplanin, and moenomycin induced synthesis of the VanX D,D-dipeptidase in a VanA-type Enterococcus faecalis harboring transposon Tn1546. Inhibitors of reactions immediately preceding (ramoplanin) or following (penicillin G and bacitracin) transglycosylation were not inducers. These results identify accumulation of membrane-bound lipid intermediate II as a potential signal for induction of VanA-type resistance. In E.faecalis BM4281 harboring a wild vanB genetic element, D,D-dipeptidase synthesis was only inducible by vancomycin. Induction of the production of the VanB ligase by vancomycin was required for growth of a vancomycin-dependent derivative of BM4281, since introduction of a plasmid coding for constitutive synthesis of the VanA ligase eliminated the requirement of glycopeptide for growth. Both vancomycin and teicoplanin were able to induce D,D-dipeptidase synthesis in BM4281 derivatives that were vancomycin and teicoplanin resistant or vancomycin and teicoplanin dependent. Acquisition of teicoplanin resistance in the latter types of strains was due to alteration in induction specificity associated with an increase in the sensitivity of the regulatory system to vancomycin. Thus, the wild VanRB-VanSB system is unable or not sensitive enough to sense teicoplanin, although mutations can lead to recognition of this antibiotic.

Functional analysis of the Enterococcus faecalis plasmid pAD1-encoded stability determinant par

The molecular organization and functional characteristics of the PAD1 replicon-encoded par stability determinant were examined. par encodes two convergently transcribed RNAS of approximately 210 and 65 nucleotides designated RNA I and RNA II, respectively. The sequence of RNA II is largely complementary to RNA I, suggesting that RNA II could regulate RNA I function as an anti-sense RNA. Results of functional studies are consistent with a role for par as a post-segregational killing system, the first to be identified in Gram-positive bacteria, with RNA I encoding the toxin and RNA II the antidote. These results include: (i) destabilization of par-containing replicons in the presence of a second complete par or the RNA II coding sequence in the same cell; (ii) par-dependent stabilization of a highly unstable vector at the expense of host-cell growth rate; and (iii) protection of cells from the toxic effects of overexpression of RNA I by RNA II supplied in trans.

Regulation of VanB-type vancomycin resistance gene expression by the VanS(B)-VanR (B) two-component regulatory system in Enterococcus faecalis V583

Acquired VanA- and VanB-type glycopeptide resistance in enterococci is due to synthesis of modified peptidoglycan precursors terminating in D-lactate. As opposed to VanA-type strains which are resistant to both vancomycin and teicoplanin, VanB-type strains remain teicoplanin susceptible. We have determined the sequence of a 7,160-bp DNA fragment associated with VanB-type resistance in Enterococcus faecalis V583 that contains seven open reading frames. The distal part encoded the VanH (B), VanB, and VanX (B) proteins that are highly similar to the putative VanH, VanA, and VanX proteins responsible for VanA-type resistance. Upstream from the structural genes for these proteins were the vanY(B) gene encoding a D,D-carboxypeptidase and an open reading frame vanW with an unknown function. The proximal part of the gene cluster coded for the apparent VanS(B)-VanR (B) two-component regulatory system. VanR (B) was related to response regulators of the OmpR subclass, and VanS (B) was related to membrane-associated histidine protein kinases. Analysis of transcriptional fusions with a reporter gene and promoter mapping indicated that the VanR B-VanS B two-component regulatory system activates a promoter located immediately downstream from the vanS B gene. Vancomycin, but not teicoplanin, was an inducer, which explains teicoplanin susceptibility of VanB-type enterococci.

A convenient and reproducible method to genetically transform bacteria of the genus Bifidobacterium

A protocol was developed for the introduction of foreign plasmid DNA into various Bifidobacterium strains. The method, which is applicable to all Bifidobacterium species tested so far, is based on electroporation of bacteria made competent by preincubation in electroporation buffer for several hours at 4 degrees C. Transformation of Bifidobacterium could be achieved with a plasmid vector originating from Bifidobacterium and with plasmid vectors from Corynebacterium, but not with vectors carrying replicons from Lactococcus or Lactobacillus.

Generation of auxotrophic mutants of Enterococcus faecalis

A 22-kb segment of chromosomal DNA from Enterococcus faecalis OG1RF containing the pyrimidine biosynthesis genes pyrC and pyrD was previously detected as complementing Escherichia coli pyrC and pyrD mutations. In the present study, it was found that the E. faecalis pyrimidine biosynthetic genes in this clone (designated pKV48) are part of a larger cluster resembling that seen in Bacillus spp. Transposon insertions were isolated at a number of sites throughout the cluster and resulted in loss of the ability to complement E. coli auxotrophs. The DNA sequences of the entire pyrD gene of E. faecalis and selected parts of the rest of the cluster were determined, and computer analyses found these to be similar to genes from Bacillus subtilis and Bacillus caldolyticus pyrimidine biosynthesis operons. Five of the transposon insertions were introduced back into the E. faecalis chromosome, and all except insertions in pyrD resulted in pyrimidine auxotrophy. The prototrophy of pyrD knockouts was observed for two different insertions and suggests that E. faecalis is similar to Lactococcus lactis, which has been shown to possess two pyrD genes. A similar analysis was performed with the purL gene from E. faecalis, contained in another cosmid clone, and purine auxotrophs were isolated. In addition, a pool of random transposon insertions in pKV48, isolated in E. coli, was introduced into the E. faecalis chromosome en masse, and an auxotroph was obtained. These results demonstrate a new methodology for constructing defined knockout mutations in E. faecalis.

Phase variation of Enterococcus faecalis pAD1 conjugation functions relates to changes in iteron sequence region

pAD1 (60 kb) is a conjugative, hemolysin/bacteriocin plasmid in Enterococcus faecalis. It confers a mating response to the peptide sex pheromone cAD1 produced by recipient (plasmid-free) cells, leading to highly efficient plasmid transfer in broth matings. Control of the physiological response to cAD1 can been overridden by a reversible phase variation event at frequencies on the order of 10(-4) to 10(-3) per cell per generation (L. T. Pontius and D. B. Clewell, Plasmid 26:172-185, 1991). The variant forms are designated Dryc and Dry+, which reflects the colony morphologies of cells whose conjugation functions are switched on and off, respectively. Here we show that Dryc variants exhibit a structural change in a region between repA and repB that contains two clusters of 8-bp iterons. The change involved a 31- or 32-bp increase in size of this region. In three or four independent variants examined, one of the iteron clusters increased in size from 13 to 17 iterons. When iteron DNA was placed on a multicopy plasmid and introduced into a wild-type pAD1 derivative, the Dryc phenotype was generated. Since traA, a key negative regulator of conjugation, bears several centrally located iteron-like sequences with the same orientation, we speculate that the protein(s) that normally binds iterons (possibly RepA and/or RepB) blocks traA transcription in Dryc variants.

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

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

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.

The vanZ gene of Tn1546 from Enterococcus faecium BM4147 confers resistance to teicoplanin

A five-gene cluster from Tn1546 confers resistance to the glycopeptide antibiotics vancomycin (Vm) and teicoplanin (Te) by synthesis of pentadepsipeptide peptidoglycan precursors terminating in D-lactate, which replaces D-alanine in the same position of precursors utilized by susceptible enterococci. Cloning and nucleotide sequencing indicated that Tn1546 contains an additional gene, designated vanZ, which confers low-level Te resistance, in the absence of the genes required for pentadepsipeptide synthesis. Analysis of cytoplasmic peptidoglycan precursors, accumulated in the presence of ramoplanin, showed that VanZ-mediated Te resistance does not involve incorporation of a substituent of D-alanine into the precursors.

Genetic structure of the Enterococcus faecalis plasmid pAD1-encoded cytolytic toxin system and its relationship to lantibiotic determinants

Pheromone-responsive conjugative plasmids are unique to the species Enterococcus faecalis. Many pheromone-responsive plasmids, including those frequently isolated from sites of infection, express a novel cytolysin that possesses both hemolytic and bacteriocin activities. Further, this cytolysin has been shown to be a toxin in several disease models. In the present study, nucleotide sequence determination, mutagenesis, and complementation analysis were used to determine the organization of the E. faecalis plasmid pAD1 cytolysin determinant. Four open reading frames are required for expression of the cytolysin precursor (cylLL, cylLS, cylM, and cylB). The inferred products of two of these open reading frames, CyILL and CyILS, constitute the cytolysin precursor and bear structural resemblance to posttranslationally modified bacteriocins termed lantibiotics. Similarities between the organization of the E. faecalis cytolysin determinant and expression units for lantibiotics exist, indicating that the E. faecalis cytolysin represents a new branch of this class and is the first known to possess toxin activity.

Contribution of VanY D,D-carboxypeptidase to glycopeptide resistance in Enterococcus faecalis by hydrolysis of peptidoglycan precursors

The vanR, vanS, vanH, vanA, and vanX genes of enterococcal transposon Tn1546 were introduced into the chromosome of Enterococcus faecalis JH2-2. Complementation of this portion of the van gene cluster by a plasmid encoding VanY D,D-carboxypeptidase led to a fourfold increase in the vancomycin MIC (from 16 to 64 micrograms/ml). Multicopy plasmids pAT80 (vanR vanS vanH vanA vanX) and pAT382 (vanR vanS vanH vanA vanX vanY) conferred similar levels of vancomycin resistance to JH2-2. The addition of D-alanine (100 mM) to the culture medium restored the vancomycin susceptibility of E. faecalis JH2-2/pAT80. The pentapeptide UDP-MurNAc-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala partially replaced pentadepsipeptide UDP-MurNAc-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Lac when the strain was grown in the presence of D-alanine. In contrast, resistance mediated by pAT382 was almost unaffected by the addition of the amino acid. Expression of the vanY gene of pAT382 resulted in the formation of the tetrapeptide UDP-MurNAc-L-Ala-gamma-D-Glu-L-Lys-D-Ala, indicating that a portion of the cytoplasmic precursors had been hydrolyzed. These results show that VanY contributes to glycopeptide resistance in conditions in which pentapeptide is present in the cytoplasm above a threshold concentration. However, the contribution of the enzyme to high-level resistance mediated by Tn1546 appears to be moderate, probably because hydrolysis of D-alanyl-D-alanine by VanX efficiently prevents synthesis of the pentapeptide.

Electrotransformation of Streptococcus agalactiae with plasmid DNA

A protocol for efficient electrotransformation of Streptococcus agalactiae (group B streptococcus) Lancefield's strain O90R (NTCT 9993) (an unencapsulated derivative of type Ia strain O90) was developed. The Escherichia coli-Streptococcus shuttle vector pDP28 (7.8 kb) carrying the ermB gene for resistance to erythromycin was used as donor DNA. Frozen 'electrocompetent' cells were prepared by repeated washes in 10% glycerol. A 50-microliters aliquot containing about 5 x 10(9) colony forming units of bacteria was subjected to the electric pulse. Optimal conditions for electrotransformation were determined using different media, harvesting cells at different points of the growth curve, and using different field strengths. The dose-response curve for transformation of S. agalactiae with pDP28 showed one-hit kinetics as donor DNA varied between 0.01 and 3 micrograms. The efficiency of electrotransformation for this range of amounts of donor DNA was 1.2 x 10(4) cfu micrograms-1. The transformation frequencies obtained with this electroporation protocol are high enough to allow both subcloning and shotgun cloning of streptococcal DNA in S. agalactiae.

Evidence that coupling sequences play a frequency-determining role in conjugative transposition of Tn916 in Enterococcus faecalis

The conjugative transposon Tn916 (encodes resistance to tetracycline), originally identified in Enterococcus faecalis, moves by an excision-insertion process in which the rate-limiting step is believed to be excision. Individual transposon-containing strains exhibit characteristic mating frequencies which range over several orders of magnitude; the basis of this phenomenon is addressed in the present study. We were able to generate independent single-copy insertions in identical target locations and with similar orientations within a plasmid hemolysin determinant (cylA); however, transposition from this site occurred at very different frequencies (10(-8) to 10(-4) per donor) depending on the individual isolate. DNA sequencing analyses showed that the coupling (junction) sequences differed between isolates and thus appeared to be responsible for differences in excision frequencies. Other experiments showed that inducible transcription into either end of the transposon had no significant effect on transfer.