Biosynthesis of modified peptidoglycan precursors by vancomycin-resistantEnterococcus faecium

ddcP, pstB, and excess D-lactate impact synergism between vancomycin and chlorhexidine against Enterococcus faecium 1,231,410

Vancomycin-resistant enterococci (VRE) are important nosocomial pathogens that cause life-threatening infections. To control hospital-associated infections, skin antisepsis and bathing utilizing chlorhexidine is recommended for VRE patients in acute care hospitals. Previously, we reported that exposure to inhibitory chlorhexidine levels induced the expression of vancomycin resistance genes in VanA-type Enterococcus faecium. However, vancomycin susceptibility actually increased for VanA-type E. faecium in the presence of chlorhexidine. Hence, a synergistic effect of the two antimicrobials was observed. In this study, we used multiple approaches to investigate the mechanism of synergism between chlorhexidine and vancomycin in the VanA-type VRE strain E. faecium 1,231,410. We generated clean deletions of 7 of 11 pbp, transpeptidase, and carboxypeptidase genes in this strain (ponA, pbpF, pbpZ, pbpA, ddcP, ldt_fm, and vanY). Deletion of ddcP, encoding a membrane-bound carboxypeptidase, altered the synergism phenotype. Furthermore, using in vitro evolution, we isolated a spontaneous synergy escaper mutant and utilized whole genome sequencing to determine that a mutation in pstB, encoding an ATPase of phosphate-specific transporters, also altered synergism. Finally, addition of excess D-lactate, but not D-alanine, enhanced synergism to reduce vancomycin MIC levels. Overall, our work identified factors that alter chlorhexidine and vancomycin synergism in a model VanA-type VRE strain.

Chlorhexidine Induces VanA-Type Vancomycin Resistance Genes in Enterococci

Chlorhexidine is a bisbiguanide antiseptic used for infection control. Vancomycin-resistantE. faecium(VREfm) is among the leading causes of hospital-acquired infections. VREfm may be exposed to chlorhexidine at supra- and subinhibitory concentrations as a result of chlorhexidine bathing and chlorhexidine-impregnated central venous catheter use. We used RNA sequencing to investigate how VREfm responds to chlorhexidine gluconate exposure. Among the 35 genes upregulated ≥10-fold after 15 min of exposure to the MIC of chlorhexidine gluconate were those encoding VanA-type vancomycin resistance (vanHAX) and those associated with reduced daptomycin susceptibility (liaXYZ). We confirmed thatvanAupregulation was not strain or species specific by querying other VanA-type VRE. VanB-type genes were not induced. ThevanHpromoter was found to be responsive to subinhibitory chlorhexidine gluconate in VREfm, as was production of the VanX protein. UsingvanHreporter experiments withBacillus subtilisand deletion analysis in VREfm, we found that this phenomenon is VanR dependent. Deletion ofvanRdid not result in increased chlorhexidine susceptibility, demonstrating thatvanHAXinduction is not protective against chlorhexidine. As expected, VanA-type VRE is more susceptible to ceftriaxone in the presence of sub-MIC chlorhexidine. Unexpectedly, VREfm is also more susceptible to vancomycin in the presence of subinhibitory chlorhexidine, suggesting that chlorhexidine-induced gene expression changes lead to additional alterations in cell wall synthesis. We conclude that chlorhexidine induces expression of VanA-type vancomycin resistance genes and genes associated with daptomycin nonsusceptibility. Overall, our results indicate that the impacts of subinhibitory chlorhexidine exposure on hospital-associated pathogens should be further investigated in laboratory studies.

Peptidoglycan precursor pools associated with MraY and FtsW deficiencies or antibiotic treatments

Blocking peptidoglycan synthesis in Escherichia coli with moenomycin or vancomycin led to the accumulation of UDP-MurNAc-pentapeptide and of its immediate upstream precursors, whereas with cephaloridine or penicillin G the pool of UDP-MurNAc-pentapeptide decreased. With MraY and FtsW deficiencies the decrease of UDP-MurNAc-pentapeptide was accompanied by an increase of the upstream nucleotide precursors and the appearance of UDP-MurNAc-tetrapeptide.

Mechanism of action of oritavancin and related glycopeptide antibiotics

Oritavancin (LY333328) is a semisynthetic glycopeptide antibiotic having excellent bactericidal activity against glycopeptide-susceptible and -resistant Gram-positive bacteria. Oritavancin is the N-alkyl-p-chlorophenylbenzyl derivative of chloroeremomycin (LY264826) and is currently in phase III clinical trials for use in Gram-positive infections. Studies show that oritavancin and related alkyl glycopeptides inhibit bacterial cell wall formation by blocking the transglycosylation step in peptidoglycan biosynthesis in a substrate-dependent manner. As with other glycopeptide antibiotics, including vancomycin, the effects of oritavancin on cell wall synthesis are attributable to interactions with dipeptidyl residues of peptidoglycan precursors. Unlike vancomycin, however, oritavancin is strongly dimerized and can anchor to the cytoplasmic membrane, the latter facilitated by its alkyl side chain. Cooperative interactions derived from dimerization and membrane anchoring in situ can be of sufficient strength to enable binding to either dipeptidyl or didepsipeptidyl peptidoglycan residues of vancomycin-susceptible and -resistant enterococci, respectively. This review describes the antibacterial activity of oritavancin, and examines the evidence supporting the proposed mechanism of action for this agent and related analogs.

Synthesis and biological evaluation of analogues of bacterial lipid I

Bacterial Lipid I analogues containing different anomeric groups at the muramic acid moiety were synthesized and screened in MurG enzyme assays run in the presence and absence of cell wall membranes. The results obtained in this study help elucidate the role of the lipid diphosphate in the recognition of Lipid I by MurG.

Antibacterial activity of synthetic analogues based on the disaccharide structure of moenomycin, an inhibitor of bacterial transglycosylase

Moenomycin is a natural product glycolipid that inhibits the growth of a broad spectrum of Gram-positive bacteria. In Escherichia coli, moenomycin inhibits peptidoglycan synthesis at the transglycosylation stage, causes accumulation of cell-wall intermediates, and leads to lysis and cell death. However, unlike Esc. coli, where 5-6 log units of killing are observed, 0-2 log units of killing occurred when Gram-positive bacteria were treated with similar multiples of the MIC. In addition, bulk peptidoglycan synthesis in intact Gram-positive cells was resistant to the effects of moenomycin. In contrast, synthetic disaccharides based on the moenomycin disaccharide core structure were identified that were bactericidal to Gram-positive bacteria, inhibited cell-wall synthesis in intact cells, and were active on both sensitive and vancomycin-resistant enterococci. These disaccharide analogues do not inhibit the formation of N:-acetylglucosamine-ss-1, 4-MurNAc-pentapeptide-pyrophosphoryl-undecaprenol (lipid II), but do inhibit the polymerization of lipid II into peptidoglycan in Esc. coli. In addition, cell growth was required for bactericidal activity. The data indicate that synthetic disaccharide analogues of moenomycin inhibit cell-wall synthesis at the transglycosylation stage, and that their activity on Gram-positive bacteria differs from moenomycin due to differential targeting of the transglycosylation process. Inhibition of the transglycosylation process represents a promising approach to the design of new antibacterial agents active on drug-resistant bacteria.

Genetics of glycopeptide resistance in enterococci

Glycopeptide resistance in enterococci is phenotypically and genotypically heterogeneous. The genes responsible for inducible resistance to high levels of vancomycin and teicoplanin (VanA phenotype) are carried by the 10,851-bp Tn1546 transposon. Transposition of Tn1546 into self-transferable plasmids and subsequent transfer by conjugation appears to be responsible for the dissemination of this type of resistance. Nine polypeptides are encoded by Tn1546 that belong to five functional groups: transposition functions (ORF1 and ORF2), regulation of resistance gene expression (VanR and VanS), synthesis of depsipeptide D-Ala-D-lactate (VanH and VanA), hydrolysis of D-Ala-D-Ala-containing peptidoglycan precursors (VanX and VanY), and low-level teicoplanin resistance (VanZ). VanB-type resistance (various levels of resistance to vancomycin and susceptibility to teicoplanin) is also due to production of D-Ala-D-Lac. The VanB ligase of VanB-type strains is structurally and functionally similar to VanA. The vanB gene was found on composite transposon Tn1547, which, in turn, was part of larger conjugative chromosomally located elements (90 to 250 kb). In contrast to acquired VanA- and VanB-type resistance, VanC-type resistance (low level of resistance to vancomycin and susceptibility to teicoplanin) is an intrinsic property of motile enterococci. Resistance in these species is due to synthesis of dipeptide D-Ala-D-Ser by VanC ligases leading to production of cell wall precursors with reduced vancomycin affinity.

Plasmid-mediated vanB glycopeptide resistance in enterococci

Enterococcus faecium, which was highly resistant to vancomycin (MIC 256 mg/liter), but susceptible to teicoplanin (MIC 2 mg/liter), caused two distinct episodes of infection on a renal unit in the United Kingdom. Pulsed field gel electrophoresis (PFGE) indicated that a single strain caused the first episode, while the second episode, which occurred 1 year later, involved multiple strains, all of which were distinct from the original strain. Vancomycin resistance in all but one of these strains was mediated by transferable plasmids that carried the vanB glycopeptide resistance gene. Transfer either of resistance plasmids or the vanB resistance determinant itself to different strains occurred during the second episode. Plasmid-mediated vanB resistance has not been widely documented. A retrospective study of a reference collection revealed two other vanB-encoding plasmids from an E. faecalis and an E. faecium referred from two further UK centers. Although restriction analysis indicated no similarity between the plasmids from the three different centers, all contained a 2.1-kb EcoRV fragment that hybridized with a probe for the vanB gene. This suggests that there has been dissemination of a conserved glycopeptide resistance determinant, of which vanB is a part.

Peptidoglycan structure of Lactobacillus casei, a species highly resistant to glycopeptide antibiotics

The structure of the peptidoglycan of Lactobacillus casei ATCC 393, a species highly resistant to glycopeptide antibiotics, was examined. After digestion, 23 muropeptides were identified; monomers represented 44.7% of all muropeptides, with monomer tetrapeptides being the major ones. Fifty-nine percent of the peptidoglycan was O-acetylated. The cross-bridge between D-alanine and L-lysine consisted of one asparagine, although aspartate could be found in minor quantities. Since UDP-MurNAc-tetrapeptide-D-lactate is the normal cytoplasmic precursor found in this species, monomer tetrapeptide-lactate was expected to be found. However, such a monomer was found only after exposure to penicillin, suggesting that penicillin-sensitive D,D-carboxypeptidases were very active in normal growing cells.

Presence of UDP-N-acetylmuramyl-hexapeptides and -heptapeptides in enterococci and staphylococci after treatment with ramoplanin, tunicamycin, or vancomycin

Analyses of the peptidoglycan nucleotide precursor contents of enterococci and staphylococci treated with ramoplanin, tunicamycin, or vancomycin were carried out by high-pressure liquid chromatography coupled with mass spectrometry (MS). In all cases, a sharp increase in the UDP-N-actetylmuramoyl-pentapeptide or -pentadepsipeptide pool was observed. Concomitantly, new peptidoglycan nucleotide peptides of higher molecular masses with hexa- or heptapeptide moieties were identified: UDP-MurNAc-pentapeptide-Asp or pentadepsipeptide-Asp in enterococci and UDP-MurNAc-pentapeptide-Gly or -Ala and UDP-MurNAc-pentapeptide-Gly-Gly or -Ala-Gly in staphylococci. These new compounds are derivatives of normal UDP-MurNAc-pentapeptide or -pentadepsipeptide precursors with the extra amino acid(s) linked to the lysine epsilon-amino group as established by various analytical procedures (MS, MS-MS fragmentation, chemical analysis, and digestion with R39 D,D carboxypeptidase). Except for tunicamycin-treated cells, it was not possible to ascertain whether these unusual nucleotides were formed by direct addition of the amino acids to UDP-MurNAc-pentapeptide (or -pentadepsipeptide) or whether they arose by reverse reactions from lipid I intermediates to which the amino acids had been added.

Inhibition of peptidoglycan biosynthesis in vancomycin-susceptible and -resistant bacteria by a semisynthetic glycopeptide antibiotic

LY191145 is a p-chlorobenzyl derivative of LY264826 (A82846B) with activity against both vancomycin-susceptible and -resistant enterococci. Incorporation of L-[14C]lysine into peptidoglycan of intact vancomycin-susceptible and -resistant Enterococcus faecium was inhibited by LY191145 (50% inhibitory concentrations of 1 and 5 microgram/ml, respectively). Inhibition was accompanied by accumulation of UDP-muramyl-peptide precursors in the cytoplasm. This agent inhibited late-stage steps in peptidoglycan biosynthesis in permeabilized E. faecium when either the UDP-muramyl-pentapeptide precursor from vancomycin-susceptible E. faecium or the UDP-muramyl-pentadepsipeptide precursor from vancomycin-resistant E. faecium was used as a substrate. Inhibition of late-stage steps led to accumulation of an N-acetyl-[14C]glucosamine-labeled lipid intermediate indicative of inhibition of the transglycosylation step. Inhibition of peptidoglycan polymerization without affecting cross-linking in a particulate membrane-plus-wall-fragment assay from Aerococcus viridans was consistent with this explanation. The fact that inhibition of peptidoglycan biosynthesis by LY191145 was not readily antagonized by an excess of free acyl-D-alanyl-D-alanine or acyl-D-alanyl-D-lactate ligands indicates that the manner in which this compound inhibits transglycosylation may not be identical to that of vancomycin.

Knockout of the two ldh genes has a major impact on peptidoglycan precursor synthesis in Lactobacillus plantarum

Most bacteria synthesize muramyl-pentapeptide peptidoglycan precursors ending with a D-alanyl residue (e.g., UDP-N-acetylmuramyl-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala). However, it was recently demonstrated that other types of precursors, notably D-lactate-ending molecules, could be synthesized by several lactic acid bacteria. This particular feature leads to vancomycin resistance. Vancomycin is a glycopeptide antibiotic that blocks cell wall synthesis by the formation of a complex with the extremity of peptidoglycan precursors. Substitution of the terminal D-alanine by D-lactate reduces the affinity of the antibiotic for its target. Lactobacillus plantarum is a lactic acid bacterium naturally resistant to vancomycin. It converts most of the glycolytic pyruvate to L- and D-lactate by using stereospecific enzymes designated L- and D-lactate dehydrogenases, respectively. In the present study, we show that L. plantarum actually synthesizes D-lactate-ending peptidoglycan precursors. We also report the construction of a strain which is deficient for both D- and L-lactate dehydrogenase activities and which produces only trace amounts of D- and L-lactate. As a consequence, the peptidoglycan synthesis pathway is drastically affected. The wild-type precursor is still present, but a new type of D-alanine-ending precursor is also synthesized in large quantities, which results in a highly enhanced sensitivity to vancomycin.

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.

Bacterial resistance to vancomycin: five genes and one missing hydrogen bond tell the story

A plasmid-borne transposon encodes enzymes and regulator proteins that confer resistance of enterococcal bacteria to the antibiotic vancomycin. Purification and characterization of individual proteins encoded by this operon has helped to elucidate the molecular basis of vancomycin resistance. This new understanding provides opportunities for intervention to reverse resistance.

Current perspectives on glycopeptide resistance

In the last 5 years, clinical isolates of gram-positive bacteria with intrinsic or acquired resistance to glycopeptide antibiotics have been encountered increasingly. In many of these isolates, resistance arises from an alteration of the antibiotic target site, with the terminal D-alanyl-D-alanine moiety of peptidoglycan precursors being replaced by groups that do not bind glycopeptides. Although the criteria for defining resistance have been revised frequently, the reliable detection of low-level glycopeptide resistance remains problematic and is influenced by the method chosen. Glycopeptide-resistant enterococci have emerged as a particular problem in hospitals, where in addition to sporadic cases, clusters of infections with evidence of interpatient spread have occurred. Studies using molecular typing methods have implicated colonization of patients, staff carriage, and environmental contamination in the dissemination of these bacteria. Choice of antimicrobial therapy for infections caused by glycopeptide-resistant bacteria may be complicated by resistance to other antibiotics. Severe therapeutic difficulties are being encountered among patients infected with enterococci, with some infections being untreatable with currently available antibiotics.

Induction of vancomycin resistance in Enterococcus faecium by non-glycopeptide antibiotics

Bacitracin and other antibiotics that inhibit late stages in peptidoglycan biosynthesis induce vancomycin resistance in a high-level, inducibly vancomycin-resistant strain of Enterococcus faecium. Exposure to bacitracin led to synthesis of the lactate-containing UDP-MurNAc-pentadepsipeptide precursor required for vancomycin resistance. These findings indicate that inhibition of peptidoglycan biosynthesis can lead to induction of vancomycin resistance and raise the possibility that multiple signals may serve to induce resistance.

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.

Analysis of genes encoding D-alanine:D-alanine ligase-related enzymes in Leuconostoc mesenteroides and Lactobacillus spp

Degenerate oligodeoxyribonucleotides complementary to sequences encoding conserved amino acid (aa) motifs in D-alanine:D-alanine ligases (Ddl) were used to amplify approx. 600-bp fragments from glycopeptide-resistant strains of Leuconostoc mesenteroides (Lm), Lactobacillus plantarum, La. salivarius and La. confusus, and from a susceptible strain of La. leichmannii. Comparison of the deduced aa sequences of the PCR products revealed that the Ddl-related enzymes of resistant Lm and Lactobacillus spp. are more akin to each other (47-63% aa identity) than to that of susceptible La. leichmannii (33-37% aa identity), indicating that the Ddl-related enzymes in these intrinsically resistant species of Gram+ bacteria exhibit structural differences with those in susceptible species. The Ddl-related enzymes, VanA and VanB, implicated in acquired resistance to glycopeptides in enterococci, were not closely related to their counterparts in Lm and Lactobacillus spp., as they displayed only 26-32% aa identity.

Glycopeptide resistance mediated by enterococcal transposon Tn1546 requires production of VanX for hydrolysis of D-alanyl-D-alanine

Cloning and nucleotide sequencing indicated that transposon Tn1546 from Enterococcus faecium BM4147 encodes a 23,365 Da protein, VanX, required for glycopeptide resistance. The vanX gene was located downstream from genes encoding the VanA ligase and the VanH dehydrogenase which synthesize the depsipeptide D-alanyl-D-lactate (D-Ala-D-Lac). In the presence of ramoplanin, an Enterococcus faecalis JH2-2 derivative producing VanH, VanA and VanX accumulated mainly UDP-MurNAc-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Lac (pentadepsipeptide) and small amounts of UDP-MurNAc-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala (pentapeptide) in the ratio 49:1. Insertional inactivation of vanX led to increased synthesis of pentapeptide with a resulting change in the ratio of pentadepsipeptide: pentapeptide to less than 1:1. Expression of vanX in E. faecalis and Escherichia coli resulted in production of a D,D-dipeptidase that hydrolysed D-Ala-D-Ala. Pentadepsipeptide, pentapeptide and D-Ala-D-Lac were not substrates for the enzyme. These results establish that VanX is required for production of a D,D-dipeptidase that hydrolyses D-Ala-D-Ala, thereby preventing pentapeptide synthesis and subsequent binding of glycopeptides to D-Ala-D-Ala-containing peptidoglycan precursors at the cell surface.

Modification of peptidoglycan precursors is a common feature of the low-level vancomycin-resistant VANB-type Enterococcus D366 and of the naturally glycopeptide-resistant species Lactobacillus casei, Pediococcus pentosaceus, Leuconostoc mesenteroides, and Enterococcus gallinarum

The biochemical basis for the acquired or natural resistance of various gram-positive organisms to glycopeptides was studied by high-pressure liquid chromatographic analysis of their peptidoglycan UDP-MurNAc-peptide precursors. In all cases, resistance was correlated with partial or complete replacement of the C-terminal D-Ala-D-Ala-containing UDP-MurNAc-pentapeptide by a new precursor with a modified C terminus. Nuclear magnetic resonance analysis by sequential assignment showed that the new precursor encountered in Enterococcus faecium D366, a strain belonging to the VANB class, which expresses low-level resistance to vancomycin, was UDP-MurNAc-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-lactate, identical to that previously found in the VANA class, which expresses high-level resistance to vancomycin. High-pressure liquid chromatographic analyses, composition determinations, and digestion by R39 D,D-carboxypeptidase demonstrated the exclusive presence of the new precursor in Lactobacillus casei and Pediococcus pentosaceus, which are naturally highly resistant to glycopeptides. The low-level natural resistance of Enterococcus gallinarum to vancomycin was found to be associated with the synthesis of a new precursor identified as a UDP-MurNAc-pentapeptide containing a C-terminal D-serine. The distinction between low and high levels of resistance to glycopeptides appeared also to depend on the presence or absence of a substantial residual pool of a D-Ala-D-Ala-containing UDP-MurNAc-pentapeptide.

Vancomycin-resistant Leuconostoc mesenteroides and Lactobacillus casei synthesize cytoplasmic peptidoglycan precursors that terminate in lactate

The emergence of acquired high-level resistance among Enterococcus species has renewed interest in mechanisms of resistance to glycopeptide antibiotics in gram-positive bacteria. In Enterococcus faecalis and Enterococcus faecium, resistance is encoded by the van gene cluster and is due to the production of a peptidoglycan precursor terminating in D-alanyl-D-lactate, to which vancomycin does not bind. Most Leuconostoc and many Lactobacillus species are intrinsically resistant to high levels of glycopeptide antibiotics, but the mechanism of resistance has not been elucidated. To determine whether the mechanisms of resistance are similar in intrinsically resistant bacteria, cytoplasmic peptidoglycan precursors were isolated from Leuconostoc mesenteroides and Lactobacillus casei and analyzed by mass spectrometry, revealing structures consistent with UDP-N-acetylmuramyl-L-Ala-D-Glu-L-Lys-(L-Ala)-D-Ala-D-lactate and UDP-N-acetylmuramyl-L-Ala-D-Glu-L-Lys-D-Ala-D-lactate, respectively.

Characterization of Tn1546, a Tn3-related transposon conferring glycopeptide resistance by synthesis of depsipeptide peptidoglycan precursors in Enterococcus faecium BM4147

Sequence determination of the flanking regions of the vancomycin resistance van gene cluster carried by pIP816 in Enterococcus faecium BM4147 revealed similarity to transposons of the Tn3 family. Imperfect inverted repeats (36 of 38 bp) delineated a 10,851-bp element designated Tn1546. The 4-kb region located upstream from the vanR gene contained two open reading frames (ORF) transcribed in opposite directions. The deduced amino acid sequence of ORF1 (988 residues) displayed, respectively, 56 and 42% identity to those of the transposases of Tn4430 from Bacillus thuringiensis and of Tn917 from Enterococcus faecalis. The product of ORF2 (191 residues) was related to the resolvase of Tn917 (33% amino acid identity) and to the Res protein (48%) of plasmid pIP404 from Clostridium perfringens. Tn1546 transposed consecutively in Escherichia coli from plasmid pUC18 into pOX38 and from pOX38 into various sites of pBR329. Transposition was replicative, led to the formation of cointegrates, and produced a 5-bp duplication at the target site. Southern hybridization and DNA amplification revealed the presence of Tn1546-related elements in enterococci highly resistant to glycopeptides. Analysis of sequences surrounding these elements indicated that transposition plays a role in dissemination of the van gene cluster among replicons of human clinical isolates of E. faecium.