Induction signals for vancomycin resistance encoded by the vanA gene cluster in Enterococcus faecium

Regulation of Resistance in Vancomycin-Resistant Enterococci: The VanRS Two-Component System

Vancomycin-resistant enterococci (VRE) are a serious threat to human health, with few treatment options being available. New therapeutics are urgently needed to relieve the health and economic burdens presented by VRE. A potential target for new therapeutics is the VanRS two-component system, which regulates the expression of vancomycin resistance in VRE. VanS is a sensor histidine kinase that detects vancomycin and in turn activates VanR; VanR is a response regulator that, when activated, directs expression of vancomycin-resistance genes. This review of VanRS examines how the expression of vancomycin resistance is regulated, and provides an update on one of the field’s most pressing questions: How does VanS sense vancomycin?

Membrane Sensor Histidine Kinases: Insights from Structural, Ligand and Inhibitor Studies of Full-Length Proteins and Signalling Domains for Antibiotic Discovery

There is an urgent need to find new antibacterial agents to combat bacterial infections, including agents that inhibit novel, hitherto unexploited targets in bacterial cells. Amongst novel targets are two-component signal transduction systems (TCSs) which are the main mechanism by which bacteria sense and respond to environmental changes. TCSs typically comprise a membrane-embedded sensory protein (the sensor histidine kinase, SHK) and a partner response regulator protein. Amongst promising targets within SHKs are those involved in environmental signal detection (useful for targeting specific SHKs) and the common themes of signal transmission across the membrane and propagation to catalytic domains (for targeting multiple SHKs). However, the nature of environmental signals for the vast majority of SHKs is still lacking, and there is a paucity of structural information based on full-length membrane-bound SHKs with and without ligand. Reasons for this lack of knowledge lie in the technical challenges associated with investigations of these relatively hydrophobic membrane proteins and the inherent flexibility of these multidomain proteins that reduces the chances of successful crystallisation for structural determination by X-ray crystallography. However, in recent years there has been an explosion of information published on (a) methodology for producing active forms of full-length detergent-, liposome- and nanodisc-solubilised membrane SHKs and their use in structural studies and identification of signalling ligands and inhibitors; and (b) mechanisms of signal sensing and transduction across the membrane obtained using sensory and transmembrane domains in isolation, which reveal some commonalities as well as unique features. Here we review the most recent advances in these areas and highlight those of potential use in future strategies for antibiotic discovery. This Review is part of a Special Issue entitled “Interactions of Bacterial Molecules with Their Ligands and Other Chemical Agents” edited by Mary K. Phillips-Jones.

Antimicrobial resistance (AMR) nanomachines-mechanisms for fluoroquinolone and glycopeptide recognition, efflux and/or deactivation

In this review, we discuss mechanisms of resistance identified in bacterial agents Staphylococcus aureus and the enterococci towards two priority classes of antibiotics-the fluoroquinolones and the glycopeptides. Members of both classes interact with a number of components in the cells of these bacteria, so the cellular targets are also considered. Fluoroquinolone resistance mechanisms include efflux pumps (MepA, NorA, NorB, NorC, MdeA, LmrS or SdrM in S. aureus and EfmA or EfrAB in the enterococci) for removal of fluoroquinolone from the intracellular environment of bacterial cells and/or protection of the gyrase and topoisomerase IV target sites in Enterococcus faecalis by Qnr-like proteins. Expression of efflux systems is regulated by GntR-like (S. aureus NorG), MarR-like (MgrA, MepR) regulators or a two-component signal transduction system (TCS) (S. aureus ArlSR). Resistance to the glycopeptide antibiotic teicoplanin occurs via efflux regulated by the TcaR regulator in S. aureus. Resistance to vancomycin occurs through modification of the D-Ala-D-Ala target in the cell wall peptidoglycan and removal of high affinity precursors, or by target protection via cell wall thickening. Of the six Van resistance types (VanA-E, VanG), the VanA resistance type is considered in this review, including its regulation by the VanSR TCS. We describe the recent application of biophysical approaches such as the hydrodynamic technique of analytical ultracentrifugation and circular dichroism spectroscopy to identify the possible molecular effector of the VanS receptor that activates expression of the Van resistance genes; both approaches demonstrated that vancomycin interacts with VanS, suggesting that vancomycin itself (or vancomycin with an accessory factor) may be an effector of vancomycin resistance. With 16 and 19 proteins or protein complexes involved in fluoroquinolone and glycopeptide resistances, respectively, and the complexities of bacterial sensing mechanisms that trigger and regulate a wide variety of possible resistance mechanisms, we propose that these antimicrobial resistance mechanisms might be considered complex 'nanomachines' that drive survival of bacterial cells in antibiotic environments.

Hydrodynamics of the VanA-type VanS histidine kinase: an extended solution conformation and first evidence for interactions with vancomycin

VanA-type resistance to glycopeptide antibiotics in clinical enterococci is regulated by the VanS_AR_A two-component signal transduction system. The nature of the molecular ligand that is recognised by the VanS_A sensory component has not hitherto been identified. Here we employ purified, intact and active VanS_A membrane protein (henceforth referred to as VanS) in analytical ultracentrifugation experiments to study VanS oligomeric state and conformation in the absence and presence of vancomycin. A combination of sedimentation velocity and sedimentation equilibrium in the analytical ultracentrifuge (SEDFIT, SEDFIT-MSTAR and MULTISIG analysis) showed that VanS in the absence of the ligand is almost entirely monomeric (molar mass M = 45.7 kDa) in dilute aqueous solution with a trace amount of high molar mass material (M ~ 200 kDa). The sedimentation coefficient s suggests the monomer adopts an extended conformation in aqueous solution with an equivalent aspect ratio of ~(12 ± 2). In the presence of vancomycin over a 33% increase in the sedimentation coefficient is observed with the appearance of additional higher s components, demonstrating an interaction, an observation consistent with our circular dichroism measurements. The two possible causes of this increase in s – either a ligand induced dimerization and/or compaction of the monomer are considered.

Resistance Mechanisms, Epidemiology, and Approaches to Screening for Vancomycin-Resistant Enterococcus in the Health Care Setting

Infections attributable to vancomycin-resistant Enterococcus (VRE) strains have become increasingly prevalent over the past decade. Prompt identification of colonized patients combined with effective multifaceted infection control practices can reduce the transmission of VRE and aid in the prevention of hospital-acquired infections (HAIs). Increasingly, the clinical microbiology laboratory is being asked to support infection control efforts through the early identification of potential patient or environmental reservoirs. This review discusses the factors that contribute to the rise of VRE as an important health care-associated pathogen, the utility of laboratory screening and various infection control strategies, and the available laboratory methods to identify VRE in clinical specimens.

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.

Discovery of novel cell wall-active compounds using P ywaC, a sensitive reporter of cell wall stress, in the model gram-positive bacterium Bacillus subtilis

The emergence of antibiotic resistance in recent years has radically reduced the clinical efficacy of many antibacterial treatments and now poses a significant threat to public health. One of the earliest studied well-validated targets for antimicrobial discovery is the bacterial cell wall. The essential nature of this pathway, its conservation among bacterial pathogens, and its absence in human biology have made cell wall synthesis an attractive pathway for new antibiotic drug discovery. Herein, we describe a highly sensitive screening methodology for identifying chemical agents that perturb cell wall synthesis, using the model of the Gram-positive bacterium Bacillus subtilis. We report on a cell-based pilot screen of 26,000 small molecules to look for cell wall-active chemicals in real time using an autonomous luminescence gene cluster driven by the promoter of ywaC, which encodes a guanosine tetra(penta)phosphate synthetase that is expressed under cell wall stress. The promoter-reporter system was generally much more sensitive than growth inhibition testing and responded almost exclusively to cell wall-active antibiotics. Follow-up testing of the compounds from the pilot screen with secondary assays to verify the mechanism of action led to the discovery of 9 novel cell wall-active compounds.

In vivo studies suggest that induction of VanS-dependent vancomycin resistance requires binding of the drug to D-Ala-D-Ala termini in the peptidoglycan cell wall

VanRS two-component regulatory systems are key elements required for the transcriptional activation of inducible vancomycin resistance genes in bacteria, but the precise nature of the ligand signal that activates these systems has remained undefined. Using the resistance system in Streptomyces coelicolor as a model, we have undertaken a series of in vivo studies which indicate that the VanS sensor kinase in VanB-type resistance systems is activated by vancomycin in complex with the d-alanyl-d-alanine (d-Ala-d-Ala) termini of cell wall peptidoglycan (PG) precursors. Complementation of an essential d-Ala-d-Ala ligase activity by constitutive expression of vanA encoding a bifunctional d-Ala-d-Ala and d-alanyl-d-lactate (d-Ala-d-Lac) ligase activity allowed construction of strains that synthesized variable amounts of PG precursors containing d-Ala-d-Ala. Assays quantifying the expression of genes under VanRS control showed that the response to vancomycin in these strains correlated with the abundance of d-Ala-d-Ala-containing PG precursors; strains producing a lower proportion of PG precursors terminating in d-Ala-d-Ala consistently exhibited a lower response to vancomycin. Pretreatment of wild-type cells with vancomycin or teicoplanin to saturate and mask the d-Ala-d-Ala binding sites in nascent PG also blocked the transcriptional response to subsequent vancomycin exposure, and desleucyl vancomycin, a vancomycin analogue incapable of interacting with d-Ala-d-Ala residues, failed to induce van gene expression. Activation of resistance by a vancomycin-d-Ala-d-Ala PG complex predicts a limit to the proportion of PG that can be derived from precursors terminating in d-Ala-d-Lac, a restriction also enforced by the bifunctional activity of the VanA ligase.

A vancomycin photoprobe identifies the histidine kinase VanSsc as a vancomycin receptor

Inducible resistance to the glycopeptide antibiotic vancomycin requires expression of vanH, vanA and vanX, controlled by a two-component regulatory system consisting of a receptor histidine kinase, VanS, and a response regulator, VanR. The identity of the VanS receptor ligand has been debated. Using a synthesized vancomycin photoaffinity probe, we show that vancomycin directly binds Streptomyces coelicolor VanS (VanSsc) and this binding is correlated with resistance and required for vanH, vanA and vanX gene expression.

Vancomycin resistance VanS/VanR two-component systems

Vancomycin is a member of the glycopeptide class of antibiotics. Vancomycin resistance (van) gene clusters are found in human pathogens such as Enterococcus faecalis, Enterococcus faecium and Staphylococcus aureus, glycopeptide-producing actinomycetes such as Amycolotopsis orientalis, Actinoplanes teichomyceticus and Streptomyces toyocaensis and the nonglycopeptide producing actinomycete Streptomyces coelicolor. Expression of the van genes is activated by the VanS/VanR two-component system in response to extracellular glycopeptide antibiotic. Two major types of inducible vancomycin resistance are found in pathogenic bacteria; VanA strains are resistant to vancomycin itself and also to the lipidated glycopeptide teicoplanin, while VanB strains are resistant to vancomycin but sensitive to teicoplanin. Here we discuss the enzymes the van genes encode, the range of different VanS/VanR two-component systems, the biochemistry of VanS/VanR, the nature of the effector ligand(s) recognised by VanS and the evolution of the van cluster.

Validation for high-throughput screening of a VanRS-based reporter gene assay for bacterial cell wall inhibitors

AIMS

The present study was undertaken to validate, for antibiotic discovery, a reporter gene assay based on a Bacillus subtilis strain expressing the Enterococcusfaecium vanRS genes and a vanH-lacZ fusion, which produced beta-galactosidase activity in the presence of cell wall inhibitors (CWI) and lysozyme.

METHODS AND RESULTS

The reporter assay was miniaturized, automated and validated with antibiotics and tested against portions of chemical and microbial extract libraries. The assay is simple, fast and reproducible and can detect all CWI, sometimes at concentrations lower than those necessary to inhibit bacterial growth. However, some membrane-interfering compounds also generate comparable signals. While most CWI elicit a signal that is transcription-dependent and abolished in an osmoprotective medium, transcription is not required for beta-galactosidase activity brought about by the membrane-interfering compounds.

CONCLUSIONS

At least two distinct mechanisms appear to lead to enzymatic activity in the reporter strain. Effective counterscreens can be designed to discard the undesired classes of compounds.

SIGNIFICANCE AND IMPACT OF THE STUDY

Extensive validation is required before introducing a reporter assay in high-throughput screening. However, the ease of operation and manipulation makes the reporter assays powerful tools for antibiotic discovery.

Enterococci and streptococci

Besides Staphylococcus aureus, other Gram-positive bacteria have become multidrug-resistant and cause therapeutic problems, particularly amongst hospitalised patients. The acquisition of vancomycin resistance by strains of Enterococcus faecium and Enterococcus faecalis is of particular concern and has resulted in treatment failures. Some of the infections caused by these bacteria do respond to treatment with new antibiotics that have been released in the last few years, however more options are required as not all enterococci are inherently susceptible and resistance is beginning to emerge amongst those that were susceptible. Resistance to commonly used antibiotics is also emerging in Streptococcus spp., particularly to the tetracyclines and macrolides. In both genera, multiresistant strains spread between patients and between hospitals. In the laboratory, these bacteria show considerable susceptibility to tigecycline, with little propensity to develop resistance, indicating that tigecycline could assume an important role in controlling infections caused by these Gram-positive bacteria.

Modes and modulations of antibiotic resistance gene expression

Since antibiotic resistance usually affords a gain of function, there is an associated biological cost resulting in a loss of fitness of the bacterial host. Considering that antibiotic resistance is most often only transiently advantageous to bacteria, an efficient and elegant way for them to escape the lethal action of drugs is the alteration of resistance gene expression. It appears that expression of bacterial resistance to antibiotics is frequently regulated, which indicates that modulation of gene expression probably reflects a good compromise between energy saving and adjustment to a rapidly evolving environment. Modulation of gene expression can occur at the transcriptional or translational level following mutations or the movement of mobile genetic elements and may involve induction by the antibiotic. In the latter case, the antibiotic can have a triple activity: as an antibacterial agent, as an inducer of resistance to itself, and as an inducer of the dissemination of resistance determinants. We will review certain mechanisms, all reversible, that bacteria have elaborated to achieve antibiotic resistance by the fine-tuning of the expression of genetic information.

The vancomycin resistance VanRS two-component signal transduction system of Streptomyces coelicolor

We took advantage of the vancomycin-dependent phenotype of Streptomyces coelicolor femX null mutants to isolate a collection of spontaneous, drug-independent femX suppressor mutants that expressed the vancomycin-resistance (van) genes constitutively. All of the suppressor mutations were in vanS but, unexpectedly, many were predicted to be loss-of-function mutations. Confirming this interpretation, a constructed vanS deletion mutation also resulted in constitutive expression of the van genes, suggesting that VanS negatively regulated VanR function in the absence of drug. In contrast, a vanS pta ackA triple mutant, which should not be able synthesize acetyl phosphate, failed to express the van genes, whereas a pta ackA double mutant showed wild-type, regulated induction of the van genes. These results suggest that in the absence of vancomycin, acetyl phosphate phosphorylates VanR, and VanS acts as a phosphatase to suppress the levels of VanR approximately P. On exposure to vancomycin, VanS activity switches from a phosphatase to a kinase and vancomycin resistance is induced. In S. coelicolor, the van genes are induced by both vancomycin and the glycopeptide A47934, whereas in Streptomyces toyocaensis (the A47934 producer) resistance is induced by A47934 but not by vancomycin. We exploited this distinction to replace the S. coelicolor vanRS genes with the vanRS genes from S. toyocaensis. The resulting strain acquired the inducer profile of S. toyocaensis, providing circumstantial evidence that the VanS effector ligand is the drug itself, and not an intermediate in cell wall biosynthesis that accumulates as result of drug action. Consistent with this suggestion, we found that non-glycopeptide inhibitors of the late steps in cell wall biosynthesis such as moenomycin A, bacitracin and ramoplanin were not inducers of the S. coelicolor VanRS system, in contrast to results obtained in enterococcal VanRS systems.

Community-acquired vancomycin-resistant Enterococcus faecium: a case report from Malaysia

Vancomycin-resistant enterococci (VRE) are formidable organisms renowned for their ability to cause infections with limited treatment options and their potential for transferring resistance genes to other Gram-positive bacteria. Usually associated with nosocomial infections, VRE are rarely reported as a cause of community-acquired infection. Presented here is a case of community-acquired infection due to vancomycin-resistant Enterococcus faecium. The patient had been applying herbal leaves topically to his cheek to treat a buccal space abscess, resulting in a burn of the overlying skin. From pus aspirated via the skin a pure culture of E. faecium was grown that was resistant to vancomycin with a MIC of >256 microg ml-1 by the E test and resistant to teicoplanin by disc diffusion, consistent with the VanA phenotype. The organism was suspected of contaminating the leaf and infecting the patient via the burnt skin. This case highlights the need for further studies on the community prevalence of VRE among humans and animals to define unrecognized silent reservoirs for VRE, which may pose a threat to public health.

Multiple-antibiotic resistance of Enterococcus spp. isolated from commercial poultry production environments

The potential impact of food animals in the production environment on the bacterial population as a result of antimicrobial drug use for growth enhancement continues to be a cause for concern. Enterococci from 82 farms within a poultry production region on the eastern seaboard were isolated to establish a baseline of susceptibility profiles for a number of antimicrobials used in production as well as clinical environments. Of the 541 isolates recovered, Enterococcus faecalis (53%) and E. faecium (31%) were the predominant species, while multiresistant antimicrobial phenotypes were observed among all species. The prevalence of resistance among isolates of E. faecalis was comparatively higher among lincosamide, macrolide, and tetracycline antimicrobials, while isolates of E. faecium were observed to be more frequently resistant to fluoroquinolones and penicillins. Notably, 63% of the E. faecium isolates were resistant to the streptogramin quinupristin-dalfopristin, while high-level gentamicin resistance was observed only among the E. faecalis population, of which 7% of the isolates were resistant. The primary observations are that enterococci can be frequently isolated from the poultry production environment and can be multiresistant to antimicrobials used in human medicine. The high frequency with which resistant enterococci are isolated from this environment suggests that these organisms might be useful as sentinels to monitor the development of resistance resulting from the usage of antimicrobial agents in animal production.

Antibiotics that inhibit cell wall biosynthesis induce expression of the Bacillus subtilis sigma(W) and sigma(M) regulons

Bacillus subtilis encodes seven extracytoplasmic function (ECF) sigma factors. The sigma(W) regulon includes functions involved in detoxification and protection against antimicrobials, whereas sigma(M) is essential for growth at high salt concentrations. We now report that antibiotics that inhibit cell wall biosynthesis induce both sigma(W) and sigma(M) regulons as monitored using DNA microarrays. Induction of selected sigma(W)-dependent genes was confirmed using lacZ reporter fusions and Northern blot analysis. The ability of vancomycin to induce the sigma(W) regulon is dependent on both sigma(W) and the cognate anti-sigma, RsiW, but is independent of the transition state regulator AbrB. These results suggest that the membrane-localized RsiW anti-sigma(W) factor mediates the transcriptional response to cell wall stress. Our findings are consistent with the idea that one function of ECF sigma factors is to coordinate antibiosis stress responses and cell envelope homeostasis.

A signal transduction system in Streptomyces coelicolor that activates the expression of a putative cell wall glycan operon in response to vancomycin and other cell wall-specific antibiotics

We have investigated a signal transduction system proposed to allow Streptomyces coelicolor to sense and respond to changes in the integrity of its cell envelope. The system consists of four proteins, encoded in an operon: sigmaE, an RNA polymerase factor; CseA (formerly ORF202), a protein of unknown function; CseB, a response regulator; and CseC, a sensor histidine protein kinase with two predicted transmembrane helices (Cse stands for control of sigma E). To develop a sensitive bioassay for inducers of the sigE system, the promoter of the sigE operon (sigEp) was fused to a reporter gene conferring resistance to kanamycin. Antibiotics that acted as inducers of the sigE signal transduction system were all inhibitors of intermediate and late steps in peptidoglycan biosynthesis, including ramoplanin, moenomycin A, bacitracin, several glycopeptides and some beta-lactams. The cell wall hydrolytic enzyme lysozyme also acted as an inducer. These data suggest that the CseB-CseC signal transduction system may be activated by the accumulation of an intermediate in peptidoglycan biosynthesis or degradationa. A computer-based searching method was used to identify a sigmaE target operon of 12 genes (the cwg operon), predicted to specify the biosynthesis of a cell wall glycan. In low-Mg(2+) medium, transcription of the cwg operon was induced by vancomycin in a sigE-dependent manner but, in high-Mg(2+) medium, there was substantial cwg transcription in a sigE null mutant, and this sigE-independent activity was also induced by vancomycin. Based on these data, we propose a model for the regulation and function of the sigmaE signal transduction system.

Glycopeptide antibiotic resistance

Glycopeptide antibiotics are integral components of the current antibiotic arsenal that is under strong pressures as a result of the emergence of a variety of resistance mechanisms over the past 15 years. Resistance has manifested itself largely through the expression of genes that encode proteins that reprogram cell wall biosynthesis and thus evade the action of the antibiotic in the enterococci, though recently new mechanisms have appeared that afford resistance and tolerance in the more virulent staphylococci and streptococci. Overcoming glycopeptide resistance will require innovative approaches to generate new antibiotics or otherwise to inhibit the action of resistance elements in various bacteria. The chemical complexity of the glycopeptides, the challenges of discovering and successfully exploiting new targets, and the growing number of distinct resistance types all increase the difficulty of the current problem we face as a result of the emergence of glycopeptide resistance.

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.

Vancomycin-resistant enterococci

After they were first identified in the mid-1980s, vancomycin-resistant enterococci (VRE) spread rapidly and became a major problem in many institutions both in Europe and the United States. Since VRE have intrinsic resistance to most of the commonly used antibiotics and the ability to acquire resistance to most of the current available antibiotics, either by mutation or by receipt of foreign genetic material, they have a selective advantage over other microorganisms in the intestinal flora and pose a major therapeutic challenge. The possibility of transfer of vancomycin resistance genes to other gram-positive organisms raises significant concerns about the emergence of vancomycin-resistant Staphylococcus aureus. We review VRE, including their history, mechanisms of resistance, epidemiology, control measures, and treatment.

Impact of flavophospholipol and vancomycin on conjugational transfer of vancomycin resistance plasmids

The influence of vancomycin and flavophospholipol (FPL) on the transfer rate of conjugative plasmids harboring the vancomycin resistance operon vanA was determined in several clinical and animal isolates of Enterococcus faecium. FPL significantly inhibited the frequency of transfer of conjugative VanA plasmids up to 70-fold. Vancomycin had no significant effect on the transfer rate of VanA plasmids.

Vancomycin-resistant enterococci

After they were first identified in the mid-1980s, vancomycin-resistant enterococci (VRE) spread rapidly and became a major problem in many institutions both in Europe and the United States. Since VRE have intrinsic resistance to most of the commonly used antibiotics and the ability to acquire resistance to most of the current available antibiotics, either by mutation or by receipt of foreign genetic material, they have a selective advantage over other microorganisms in the intestinal flora and pose a major therapeutic challenge. The possibility of transfer of vancomycin resistance genes to other gram-positive organisms raises significant concerns about the emergence of vancomycin-resistant Staphylococcus aureus. We review VRE, including their history, mechanisms of resistance, epidemiology, control measures, and treatment.

Characterization of the Mycobacterium tuberculosis iniBAC promoter, a promoter that responds to cell wall biosynthesis inhibition

The cell wall provides an attractive target for antibiotics against Mycobacterium tuberculosis. Agents such as isoniazid and ethambutol that work by inhibiting cell wall biosynthesis are among the most highly effective antibiotics against this pathogen. Although considerable progress has been made identifying the targets for cell wall active antibiotics, little is known about the intracellular mechanisms that are activated as a consequence of cell wall injury. These mechanisms are likely to have an important role in growth regulation and in the induction of cell death by antibiotics. We previously discovered three isoniazid-induced genes (iniB, iniA, and iniC) organized in tandem on the M. tuberculosis genome. Here, we investigate the unique features of the putative iniBAC promoter. This promoter was specifically induced by a broad range of inhibitors of cell wall biosynthesis but was not inducible by other conditions that are toxic to mycobacteria via other mechanisms. Induction required inhibitory concentrations of antibiotics and could be detected only in actively growing cells. Analysis of the iniBAC promoter sequence revealed both a regulatory element upstream and a potential repressor binding region downstream of the transcriptional start site. The induction phenotype and structure of the iniBAC promoter suggest that a complex intracellular response occurs when cell wall biosynthesis is inhibited in M. tuberculosis and other mycobacteria.

First confirmed case of a vancomycin-resistant Enterococcus faecium with vanA phenotype from Brazil: isolation from a meningitis case in São Paulo

The importance of enterococci as a nosocomial etiologic agent is well documented; however, enterococci are also capable of causing a variety of community-acquired infections. Vancomycin resistance in a clinical Enterococcus isolate was first reported in 1986, and since then vancomycin-resistant enterococci (VRE) have been reported world-wide. This report describes a case of E. faecium with the VanA phenotype, isolated from meningitis in Sao Paulo, Brazil. Two E. faecium strains were isolated. One strain showed VanA phenotype, and the molecular characterization of the VanA gene was confirmed by polymerase chain reaction. The other strain was susceptible to vancomycin and teicoplanin. The authors would like to call the attention of the scientific community to this first identification of a VRE case in Sao Paulo, Brazil.

Induction of VanA vancomycin resistance genes in Enterococcus faecalis: use of a promoter fusion to evaluate glycopeptide and nonglycopeptide induction signals

To characterize induction of VanA resistance a plasmid was constructed in which the gene for firefly luciferase lucA was placed under the control of the promoter for the VanA resistance genes, the vanH promoter. This system afforded convenient quantitative measurement of induction of the VanA genes. Glycopeptide antibiotics and antibiotics representing 19 different mechanisms of action were evaluated for their ability to induce. Antibiotics that acted as inducers were all inhibitors of late steps of peptidoglycan synthesis. These included moenomycin, bacitracin, tunicamycin, ramoplanin and glycopeptides, but not penicillin or other beta-lactam antibiotics. Glycopeptide antibiotics were the most potent inducers. Both glycopeptides with little or no antimicrobial activity and semisynthetic glycopeptides active against VanA resistant enterococci were inducers. Overall, results suggest that an induction response may involve both an internal signal, such as precursor accumulation, and the glycopeptide molecule itself as a signal. The system may be useful as a screen for new antimicrobial agents.

Antibiotic usage in animals: impact on bacterial resistance and public health

Antibiotic use whether for therapy or prevention of bacterial diseases, or as performance enhancers will result in antibiotic resistant micro-organisms, not only among pathogens but also among bacteria of the endogenous microflora of animals. The extent to which antibiotic use in animals will contribute to the antibiotic resistance in humans is still under much debate. In addition to the veterinary use of antibiotics, the use of these agents as antimicrobial growth promoters (AGP) greatly influences the prevalence of resistance in animal bacteria and a poses risk factor for the emergence of antibiotic resistance in human pathogens. Antibiotic resistant bacteria such as Escherichia coli, Salmonella spp., Campylobacter spp. and enterococci from animals can colonise or infect the human population via contact (occupational exposure) or via the food chain. Moreover, resistance genes can be transferred from bacteria of animals to human pathogens in the intestinal flora of humans. In humans, the control of resistance is based on hygienic measures: prevention of cross contamination and a decrease in the usage of antibiotics. In food animals housed closely together, hygienic measures, such as prevention of oral-faecal contact, are not feasible. Therefore, diminishing the need for antibiotics is the only possible way of controlling resistance in large groups of animals. This can be achieved by improvement of animal husbandry systems, feed composition and eradication of or vaccination against infectious diseases. Moreover, abolishing the use of antibiotics as feed additives for growth promotion in animals bred as a food source for humans would decrease the use of antibiotics in animals on a worldwide scale by nearly 50%. This would not only diminish the public health risk of dissemination of resistant bacteria or resistant genes from animals to humans, but would also be of major importance in maintaining the efficacy of antibiotics in veterinary medicine.

Histidine kinases and two-component signal transduction systems

The phosphorylation of histidine is the first step in many signal transduction cascades in bacteria, yeast and higher plants. The transfer of a very reactive phosphoryl group from phosphorylated histidine kinase to an acceptor is an essential step in many cellular signaling responses.

Glycerol monolaurate inhibits induction of vancomycin resistance in Enterococcus faecalis

Glycerol monolaurate (GML) is a surfactant that has been found to inhibit the post-exponential phase activation of virulence factor production and the induction of beta-lactamase in Staphylococcus aureus. It has been suggested that signal transduction is the most probable target for GML (S. J. Projan, S. Brown-Skrobot, P. M. Schlievert, F. Vandenesch, and R. P. Novick, J. Bacteriol. 176:4204-4209, 1994). We found that GML suppresses growth of vancomycin-resistant Enterococcus faecalis on plates with vancomycin and blocks the induction of vancomycin resistance, which involves a membrane-associated signal transduction mechanism, either at or before initiation of transcription. Given the surfactant nature of GML and the results of previous experiments, we suggest that GML blocks signal transduction. In contrast, GML has no effect on the induction of erythromycin-inducible macrolide resistance in S. aureus, which does not involve signal transduction.

Transcriptional regulation of the Enterococcus faecium BM4147 vancomycin resistance gene cluster by the VanS-VanR two-component regulatory system in Escherichia coli K-12

An Escherichia coli K-12 model system was developed for studying the VanS-VanR two-component regulatory system required for high-level inducible vancomycin resistance in Enterococcus faecium BM4147. Our model system is based on the use of reporter strains with lacZ transcriptional and translational fusions to the PvanR or PvanH promoter of the vanRSHAX gene cluster. These strains also express vanR and vanS behind the native PvanR promoter, the arabinose-inducible ParaB promoter, or the rhamnose-inducible PrhaB promoter. Our reporter strains have the respective fusions stably recombined onto the chromosome in single copy, thereby avoiding aberrant regulatory effects that may occur with plasmid-bearing strains. They were constructed by using allele replacement methods or a conditionally replicative attP plasmid. Using these reporter strains, we demonstrated that (i) the response regulator VanR activates PvanH, but not PvanR, expression upon activation (phosphorylation) by the partner kinase VanS, the noncognate kinase PhoR, or acetyl phosphate, indicating that phospho-VanR (P-VanR) is a transcriptional activator; (ii) VanS interferes with activation of VanR by PhoR or acetyl phosphate, indicating that VanS also acts as a P-VanR phosphatase; and (iii) the conserved, phosphate-accepting histidine (H164) of VanS is required for activation (phosphorylation) of VanR but not for deactivation (dephosphorylation) of P-VanR. Similar reporter strains may be useful in new studies on these and other interactions of the VanS-VanR system (and other systems), screening for inhibitors of these interactions, and deciphering the molecular logic of the signal(s) responsible for activation of the VanS-VanR system in vivo. Advantages of using an E. coli model system for in vivo studies on VanS and VanR are also discussed.

Avoparcin used as a growth promoter is associated with the occurrence of vancomycin-resistant Enterococcus faecium on Danish poultry and pig farms

We determined the association between the use of the glycopeptide antibiotic avoparcin as a growth promoter and the occurrence of Enterococcus faecium (VREF) with high-level resistance to vancomycin (MIC > or = 64 micrograms ml-1) on poultry and pig farms. The investigations were conducted as retrospective cohort studies, where groups of farms exposed or not exposed to avoparcin between September 1994 and April 1995 were compared. In poultry, the association between the use of avoparcin and the occurrence of VREF was confounded by the use of broad-spectrum antibiotics, and the adjusted relative risk was 2.9 (1.4-5.9). In pigs, the association had a similar magnitude with a non-adjusted relative risk of 3.3 (0.9-12.3). The similar findings in the two studies provide evidence in favour of a causal association between the use of avoparcin and the occurrence of VREF on farms, and suggest that food animals constitute a potential reservoir of infection for VREF in humans.

Vancomycin-resistant enterococcus. Detection, epidemiology, and control measures

VRE have spread rapidly since their initial description in 1988. Although much has been learned about the epidemiology of VRE, further studies are needed to establish the reservoirs of the organism and the relative importance of various modes of transmission. There is considerable anecdotal evidence that nosocomial transmission of VRE can be thwarted by using measures such as those recommended by HICPAC, especially if they are implemented promptly after VRE have been introduced into hospitals.

A vancomycin-inducible lacZ reporter system in Bacillus subtilis: induction by antibiotics that inhibit cell wall synthesis and by lysozyme

We have constructed a Bacillus subtilis strain in which expression of a vanH::lacZ gene fusion is regulated by VanR and VanS of Enterococcus faecium. This construct allows a nonpathogenic bacterial strain to be used as a model system for studying regulation of vancomycin resistance. Antibiotics and enzymes that affect cell wall biosynthesis and stability were tested for the ability to induce lacZ expression. As a result, fosfomycin and D-cycloserine were added to the group of peptidoglycan synthesis inhibitors shown to induce expression from the vanH promoter. Induction by cell wall hydrolytic enzymes, as well as by antibiotics whose actions may lead to the accumulation of chemically different peptidoglycan precursors, raises the possibility that models that postulate induction by peptidoglycan [correction of peptidodoglycan] precursors are wrong.