Acquired Vancomycin Resistance Research Articles

Restoring vancomycin activity against resistant Enterococcus faecalis using a transcription factor decoy as a vanA operon-inhibitor.

Vancomycin-resistant enterococci (VRE) represent a public health threat due to the few available treatments. Such alarm has triggered worldwide initiatives to develop effective antimicrobial compounds and novel delivery and therapeutic strategies. vanA operon is responsible for most cases of acquired vancomycin resistance in enterococci. Development of a transcription factor decoy (TFD) system as a vanA gene transcription-inhibitor. Vancomycin MIC was determined in the presence of TFD-lipoplexes. Additionally, the effect of TFD-lipoplexes on the expression level of the vanA gene and the growth pattern of E. faecalis was evaluated. The haemolytic activity of the developed TFD-lipoplexes and their cytotoxicity were examined. TFD-lipoplexes efficiency in treating vancomycin-resistant E. faecalis (VREF) infection was tested in vivo using a systemic mice infection model. A reduction in vancomycin MIC against VRE from 256 mg/L (resistant) to 16 mg/L (intermediate susceptible), in the presence of TFD-lipoplexes, was recorded. The developed TFD-lipoplexes lacked any effect on E. faecalis growth and significantly reduced the transcription level of the vanA gene by about 3-fold. In an initial evaluation of the safety of TFD-lipoplexes, they were found not to be overtly haemolytic to human blood or cytotoxic to human skin fibroblast cells. The co-administration of TFD-lipoplexes and vancomycin efficiently eradicated VREF infection in vivo. The developed TFD-lipoplexes successfully restored vancomycin activity against VREF. They offer a safe effective unconventional therapy against this stubborn organism and present a revolution in gene therapy that can be applied to other resistance-encoding genes in various organisms.

Prevalence and Molecular Characterization of Vancomycin Variable Enterococcus faecium Isolated From Clinical Specimens.

Vancomycin variable Enterococcus (VVE) bacteria are phenotypically susceptible to vancomycin, but they harbor the vanA gene. We aimed to ascertain the prevalence of VVE among clinically isolated vancomycin-susceptible Enterococcus faecium (VSE) isolates, as well as elucidate the molecular characteristics of the vanA gene cluster within these isolates. Notably, we investigated the prevalence and structure of the vanA gene cluster of VVE. Between June 2021 and May 2022, we collected consecutive, non-duplicated vancomycin-susceptible Enterococcus faecium (VSE) samples. Real-time PCR was performed to detect the presence of vanA, vanB, and vanC. Overlapping PCR with sequencing and whole-genome sequencing were performed for structural analysis. Sequence types (STs) were determined by multilocus sequence typing. Exposure testing was performed to assess the ability of the isolates to acquire vancomycin resistance. Among 282 VSE isolates tested, 20 isolates (7.1%) were VVE. Among them, 17 isolates had partial deletions in the IS1216 or IS1542 sequences in vanS (N=10), vanR (N=5), or vanH (N=2). All VVE isolates belonged to the CC17 complex and comprised five STs, namely ST17 (40.0%), ST1421 (25.0%), ST80 (25.0%), ST787 (5.0%), and ST981 (5.0%). Most isolates were related to three hospital-associated clones (ST17, ST1421, and ST80). After vancomycin exposure, 18 of the 20 VVEs acquired vancomycin resistance. Considering the high reversion rate, detecting VVE by screening VSE for vanA is critical for appropriate treatment and infection control.

Bacteriophage and antibiotic combination therapy for recurrent Enterococcus faecium bacteremia.

Phage therapy is an emerging therapeutic approach for treating bacterial infections that do not respond to traditional antibiotics. The addition of phage therapy to systemic antibiotics to treat a patient with recurrent E. faecium infections that were non-responsive to antibiotics alone resulted in fewer hospitalizations and improved the patient's quality of life. Combination phage and antibiotic therapy reduced E. faecium and VRE abundance in the patient's stool. Eventually, an anti-phage antibody response emerged that was able to neutralize phage activity, which may have limited clinical efficacy. This study demonstrates the potential of phages as an additional option in the antimicrobial toolbox for treating invasive enterococcal infections and highlights the need for further investigation to ensure phage therapy can be deployed for maximum clinical benefit.

Antimicrobial resistance among canine enterococci in the northeastern United States, 2007-2020.

Antimicrobial resistance (AMR) is a growing and complex One Health concern worldwide, threatening the practice of human and veterinary medicine. Although dogs are a potential reservoir of multidrug-resistant bacteria, there are very few surveillance studies on AMR from the canine population in the United States. Here, we assessed the antimicrobial susceptibility patterns, identified temporal resistance and minimum inhibitory concentration trends, and described associations between resistance phenotypes among canine clinical enterococci in the northeastern United States. Through a large-scale retrospective study design, we collected species identification, minimum inhibitory concentration, and clinical data from 3,659 canine enterococci isolated at the Cornell University Animal Health Diagnostic Center between 2007 and 2020. We used the Mann-Kendall test, Sen's slope, multivariable logistic regression, and survival analysis models to detect the presence of a significant trend in resistance over the study period. Enterococcus faecalis was the most prevalent species (67.1% of isolates), followed by Enterococcus faecium (20.4%). We found high levels of AMR among enterococci to almost all the tested antimicrobials, particularly E. faecium. The lowest percentage of resistance was to vancomycin and chloramphenicol. Multidrug resistance was common (80% of E. faecium and 33% of E. faecalis) and 31 isolates were extensively drug resistant. Multidrug resistance among E. faecium increased over time, but not in E. faecalis. Resistance to penicillins, enrofloxacin, and rifampin increased during the study period, but resistance to tetracyclines is on a downward trajectory compared to AMR data from the last decade. Emerging vancomycin-resistant E. faecalis (0.3%) and E. faecium (0.8%) infections in the canine population are of great concern to both human and animal health. One E. faecium isolate with acquired vancomycin resistance was identified in 2017 and four vancomycin-resistant enterococci isolates were identified in 2020. There is a crucial need to make rational prescribing decisions on the prudent use of antimicrobials and improve the quality of care for patients, especially when empirical antimicrobial treatment for enterococcal infection is common.

Heterogeneous vancomycin resistance in Staphylococcus aureus does not predict development of vancomycin resistance upon vancomycin pressure

It is unclear whether Staphylococcus aureus with heterogeneous intermediate vancomycin resistance (hVISA) can develop vancomycin resistance faster than vancomycin-susceptible S. aureus (VSSA) strains. We compared the kinetics of vancomycin MIC increase for 15 days of sustained in vitro vancomycin exposure for clinical hVISA (n = 12) and VSSA (n = 24) isolates, as well as for reference strains Mu3 (hVISA) and ATCC 29213 (VSSA). Clinical isolates were categorized as hVISA using the population analysis profile method. MICs were monitored for 15 days and the rate of MIC increase under exposure, for each strain, was evaluated in a linear regression model relative to time. All isolates acquired vancomycin resistance upon exposure. Vancomycin MICs increased faster for VSSA compared with hVISA isolates (P < 0.01). The hVISA phenotype does not correspond to an enhanced adaptation potential to in vitro vancomycin pressure.

200. Prospective Evaluation of the GenMark Dx ePlex® Blood Culture Identification Gram Positive Panel

BackgroundThe ePlex BCID Gram-Positive (GP) panel utilizes electrowetting technology to detect the most common causes of GP bacteremia (20 targets) and 4 antimicrobial resistance genes in positive blood culture bottles. Rapid detection of intrinsic vancomycin resistance and acquired resistance genes (mecA, mecC, vanA, vanB) enables early optimization of antimicrobial therapy whereas early detection of common contaminants decreases unnecessary antibiotic utilization and hospitalizations. MethodsIn this prospective study, we evaluated the performance of the BCID-GP panel compared to traditional standard of care culture and susceptibility testing with organism identification using the BioMerieux Vitek MS Matrix Assisted Laser Desorption Ionization (MALDI) Time of Flight mass spectrometry. Samples submitted for standard of care testing in Biomerieux BacT/Alert resin FA/FN blood culture bottles on the BacT/Alert VIRTUO automated blood culture system with GP bacteria on direct exam (n=100) were included. ResultsAll GP bacteria were represented on the BCID-GP panel, most tests 97/100 (97%) yielded valid results, 53 common skin contaminants (50 coagulase negative staphylococci (CNS), 2 Bacillus, 1 Corynebacterium) were identified, and 7/7 coinfections with Gram negative (GN) bacteria were detected by the Pan GN target and identified by the BCID-GN panel. Discordant analyses revealed a positive percent agreement (PPA) of 96/97 (99%) with 1 false negative CNS and a negative percent agreement (NPA) of 92/97 (94.8%) with 5 false positives for either S. epidermidis or Corynebacterium. Detection of vanA yielded a PPA of 4/4 and NPA of 9/9. mecA gene detection exhibited a PPA of 14/14 and NPA of 14/14 for S. aureus and a PPA of 31/32 (97%) and NPA of 16/16 for coagulase negative staphylococci with 1 false negative methicillin resistant S. epidermidis. ConclusionDetection of acquired vancomycin resistance (n=4) and absence of mecA gene detection in Staphylococcus species (n=30) represent opportunities for early optimization of antimicrobial therapy in 34/100 (34%) of samples. The BCID-GP panel provides rapid accurate detection of resistant isolates and common contaminants enabling high quality data driven optimization of antimicrobial therapy. Disclosures Todd P. McCarty, MD, Cidara (Grant/Research Support)GenMark (Grant/Research Support, Other Financial or Material Support, Honoraria for Research Presentation)T2 Biosystems (Consultant) Sixto M. Leal, Jr., MD, PhD, Abnova (Grant/Research Support)AltImmune (Grant/Research Support)Amplyx Pharmaceuticals (Grant/Research Support)Astellas Pharmaceuticals (Grant/Research Support)CNINE Dx (Grant/Research Support)GenMark Diagnostics (Grant/Research Support, Other Financial or Material Support, Honoraria- Research Presentation)IHMA (Grant/Research Support)IMMY Dx (Grant/Research Support)JMI/Sentry (Grant/Research Support)mFluiDx Dx (Grant/Research Support)SpeeDx Dx (Grant/Research Support)Tetraphase Pharmaceuticals (Grant/Research Support)

Rapid Detection of VanA/B-Producing Vancomycin-Resistant Enterococci Using Lateral Flow Immunoassay.

Vancomycin-resistant enterococci (VREs) have become one of the most important nosocomial pathogens worldwide, associated with increased treatment costs, prolonged hospital stays and high mortality. Rapid detection is crucial to reduce their spread and prevent infections and outbreaks. The lateral flow immunoassay NG-Test VanB (NG Biotech) was evaluated for the rapid detection of VanB-producing vancomycin-resistant enterococci (VanB-VREs) using 104 well-characterized enterococcal isolates. The sensitivity and specificity were both 100% when bacterial cells were grown in the presence of vancomycin used as a VanB inducer. The NG-Test VanB is an efficient, rapid and easy to implement assay in clinical microbiology laboratories for the confirmation of VanB-VREs from colonies. Together with the NG-Test VanA, they could replace the already existing tests available for the confirmation of acquired vancomycin resistance in enterococci, especially from selective media or from antibiograms, with 100% sensitivity and specificity. Rapid detection in less than 15 min will result in more efficient management of carriers and infected patients. In addition, these tests may be used for positive blood cultures, given a 3.5 h sub-culturing step on Chocolate agar PolyViteX in the presence of a 5-µg vancomycin disk, which is routinely performed in many clinical microbiology laboratories for every positive blood culture for subsequent MALDI-TOF identification of the growing bacteria.

Potency of vancomycin against Mycobacterium tuberculosis in the hollow fiber system model

ObjectivesTo determine whether an inhaled vancomycin formulation resulting in high intrapulmonary 24-h area under the concentration–time curve (AUC0–24) could be optimised for tuberculosis treatment. We also explored vancomycin synergy and antagonism with d-cycloserine and benzylpenicillin. MethodsWe determined MICs of two Mycobacterium tuberculosis (Mtb) laboratory strains (H37Ra and H37Rv) and two drug-susceptible and nine multidrug resistant clinical strains. Second, in the hollow fiber system model of TB [HFS-TB] using Mtb H37Ra strain, we recapitulated vancomycin intrapulmonary pharmacokinetics of eight doses administered twice daily over 28 days, mimicking a 6-h half-life. Using the HFS-TB, vancomycin was tested in combination with d-cycloserine and benzylpenicillin to determine synergy or antagonism between drugs targeting the same pathway. ResultsVancomycin MICs were 12 and 48 mg/L in drug-susceptible clinical isolates but >96 mg/L in all MDR isolates.In the HFS-TB, vancomycin killed 3.9 ± 0.6 log10 CFU/mL Mtb. The EC50 was calculated as AUC0–24/MIC of 184.6 ± 106.5. Compared with day 0, 1.0 and 2.0 log10 CFU/mL kill was achieved by AUC0–24/MIC of 168 and 685, respectively. Acquired vancomycin resistance developed to all vancomycin doses tested in the HFS-TB. In the HFS-TB, vancomycin was antagonistic to benzylpenicillin, which works downstream to glycopeptides in peptidoglycan synthesis, but synergistic with d-cycloserine, which inhibits upstream d-Ala-d-Ala ligase and alanine racemase. ConclusionOur proof-of-concept studies show that vancomycin optimal exposure target for Mtb kill could be achieved via inhalational drug delivery. Addition of drugs synergistic with vancomycin, e.g. d-cycloserine, may lower the vancomycin concentrations required to kill Mtb.

Phylogenomic analysis of vancomycin-resistant Pediococcus acidilactici and P. pentosaceus isolated from the environment

This study provides a phylogenomic analysis of P. acidilactici and P. pentosaceus highlighting antibiotic resistance genes, plasmids and pathogenicity. The whole genomes of isolates of P. acidilactici and P. pentosaceus isolates were sequenced. Additional P. acidilactici and P. pentosaceus genome sequences were downloaded from the National Center for Biotechnology Information (NCBI). The phylogenomic tree inferred through the CSI Phylogeny indicates two distinct species of P. acidilactici and P. pentosaceus . Resistome analysis revealed vancomycin resistance genes and aminoglycosides ( aph(3′)-III and aac(6′)-Ii ). Intrinsic vancomycin resistance operons VanL , vanRM and vanTG genes were present in 95.65% (n = 22) of all genomes, whereas the acquired vancomycin resistance gene vanHAX was only found in one isolated genome. In this study, we elucidated for the first time the resistome, plasmids and pathogenicity of complete genomes of Pediococcus acidilactici and P. pentosaceus from NCBI and our environmentally isolated strains (RS11, RS13, RW6 and RW12).

An operon encoding enzymes for synthesis of a putative extracellular carbohydrate attenuates acquired vancomycin resistance in Streptomyces coelicolor.

Actinomycete bacteria use polyprenol phosphate mannose as a lipid-linked sugar donor for extra-cytoplasmic glycosyl transferases that transfer mannose to cell envelope polymers, including glycoproteins and glycolipids. Strains of Streptomyces coelicolor with mutations in the gene ppm1, encoding polyprenol phosphate mannose synthase, and in pmt, encoding a protein O-mannosyltransferase, are resistant to phage ϕC31 and have greatly increased susceptibility to some antibiotics, including vancomycin. In this work, second-site suppressors of the vancomycin susceptibility were isolated. The suppressor strains fell into two groups. Group 1 strains had increased resistance to vancomycin, teicoplanin and β-lactams, and had mutations in the two-component sensor regulator system encoded by vanSR, leading to upegulation of the vanSRJKHAX cluster. Group 2 strains only had increased resistance to vancomycin and these mostly had mutations in sco2592 or sco2593, genes that are derepressed in the presence of phosphate and are likely to be required for the synthesis of a phosphate-containing extracellular polymer. In some suppressor strains the increased resistance was only observed in media with limited phosphate (mimicking the phenotype of wild-type S. coelicolor), but two strains, DT3017_R21 (ppm1-vanR-) and DT3017_R15 (ppm1- sco2593-), retained resistance on media with high phosphate content. These results support the view that vancomycin resistance in S. coelicolor is a trade-off between mechanisms that confer resistance and at least one that interferes with resistance mediated through the sco2594-sco2593-sco2592 operon.

Wild corvid birds colonized with vancomycin-resistant Enterococcus faecium of human origin harbor epidemic vanA plasmids

The most prevalent type of acquired vancomycin resistance in Enterococcus faecium (VREfm) is encoded by the vanA transposon Tn1546, mainly located on transferable plasmids. vanA plasmids have been characterized in VREfm from a variety of sources but not wild birds. The aim of this study was to analyse the genetic context of VREfm strains recovered from wild corvid birds and to compare their plasmid and strain characteristics with human strains. To achieve that, 75 VREfm isolates, including strains from wild birds recovered during wide surveillance studies performed in Europe, Canada and the United States (2010−2013), and clinical and wastewater strains from Czech Republic, a region lacking data about vanA plasmids, were analysed. Their population structure, presence of major putative virulence markers and characterization of vanA transposons and plasmids were established. VREfm from wild birds were mainly associated with major human lineages (ST18 and ST78) circulating in hospitals worldwide and were enriched in putative virulence markers that are highly associated with clinical E. faecium from human infections. They also carried plasmids of the same families usually found in the clinical setting [RCR, small theta plasmids, RepA_N (pRUM/pLG1) and Inc18]. The clinically widespread IS1251-carrying Tn1546 type “F” was predominant and Tn1546-vanA was mainly located on pRUM/Axe-Txe (USA) and Inc18- or pLG1-like (Europe) plasmids. VREfm from hospitals and wastewaters carried Tn1546-vanA in different plasmid types including mosaic pRUM-Inc18 plasmids, not identified in wild birds. This is the first characterization of vanA plasmids obtained from wild birds. A similar plasmid pool seems to exist in different clonal E. faecium backgrounds of humans and wild birds. The isolation of VREfm strains from wild birds that belong to human E. faecium adapted lineages and carry virulence genes, Tn1546 and plasmid variants widespread in the clinical setting is of concern and highlight their role as potential drivers of the global dissemination of vancomycin resistance.

Vancomycin-resistant enterococci with vanA and vanB genes in Australian gulls.

This study is revealing the possible dissemination of vancomycin-resistant enterococci (VRE) from humans into the wildlife. We studied silver gulls (Chroicocephalus novaehollandiae) in Australia as potential carriers and reservoirs of VRE with acquired vancomycin resistance. In New South Wales (Australia), we have found two multi-resistant isolates belonging to Enterococcus faecium (sequence type 341, vanB genotype) and Enterococcus dispar (vanA genotype). Based on our knowledge, this is the first report of VRE in Australian wildlife.

Identification of vancomycin-susceptible major clones of clinical Enterococcus from Algeria.

The main objectives of this study were to characterize clinical strains of Enterococcus spp. isolated from Algerian inpatients and outpatients, to investigate their susceptibility to antibiotics and to analyse their phylogenetic relatedness. A total of 85 non-duplicate Enterococcus spp. isolates collected between 2010 and 2013 from various clinical samples, including urine, vaginal swab, pus, blood and semen, from Algerian inpatients (n=62) and outpatients (n=23) were identified using matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF/MS). Antibiotic susceptibility testing was performed using disk diffusion and Etest methods. Clonal relatedness was analysed using multilocus sequence typing (MLST). Enterococcus faecalis was the most predominant species (75.3%), followed by Enterococcus faecium (21.2%), Enterococcus gallinarum (2.4%) and Enterococcus casseliflavus (1.2%). High-level resistance to aminoglycosides was significantly more prevalent in hospitalized patients than in outpatients. None of the E. faecalis and E. faecium isolates were resistant to vancomycin. High genetic diversity was observed among the E. faecalis isolates, with the identification of a new clonal complex (CC256), as well as the detection of E. faecalis ST6 and E. faecium lineages ST17, ST18 and ST78 associated with hospital isolates. This is the first report of E. faecalis ST6 and E. faecium ST17 and ST18 in Algeria. Although acquired vancomycin resistance was not observed among the enterococcal strains, there is a continued need to monitor the level of antibiotic resistance among enterococci as well as the evolution of the E. faecalis/E. faecium ratio.

Enterococcus faecium resistente a glucopéptidos. Análisis del genotipo de resistencia, virulencia y líneas genéticas

ObjectiveThe objective of this study was to analyse the genotypic characteristics of all Enterococcus isolates with acquired vancomycin resistance (VRE) recovered in the Hospital Clinic (Barcelona, Spain) in a period of three years and two months. MethodsAll VRE isolated in the referred Hospital in the period January 2004-March 2007 were included in the study. The vancomycin resistance mechanism was investigated, as well as other antibiotic resistance mechanisms. Isolates were also typed by pulsed-field-gel-electrophoresis (PFGE) and multi-locus-sequence-typing (MLST). ResultsThirty-nine VRE were recovered, all being identified as E. faecium, representing 2% of total enterococci obtained in that period. Thirty-eight of them carried the vanA gene, and one isolate the vanB2 gene. The 39 VRE were classified into 13 different pulsotypes (A-M), with one main pulsotype, A, which included 13 isolates. The sequence type was identified by MLST in 24 VRE (with unrelated or closely-related PFGE patterns), and they were ascribed to the clonal complex CC17, but two classified as CC9. All VRE showed a multiresistance phenotype, including, in most cases ampicillin, ciprofloxacin, erythromycin, streptomycin, gentamicin, kanamycin and chloramphenicol, harbouring multiple antibiotic resistance genes. The presence of esp and/or hyl genes was identified in 37 VRE. ConclusionAll VRE, but one, showed the vanA genotype and they were mostly ascribed to the high-risk clonal complex CC17.

Characterization of a vancomycin intermediate-resistant Staphylococcus aureus isolated from a hospital.

A Vancomycin intermediate resistant strain of Staphylococcus aureus (S. aureus) labeled as CP2 (MIC 16 microg/ml) was isolated from an in-patient of local Cardiac Hospital of Karachi. CP2 showed typical characters of Vancomycin intermediate resistant S. aureus (VISA) i.e. high level of oxacillin resistance, thickened cell wall with rough surface and reduced autolytic activities associated with murein hydrolase (MH) enzyme. This strain may have acquired vancomycin resistance due to long term exposure to antibiotic during the treatment of the patient. Therefore, it implicates the importance of monitoring the usage and also to control of the abuse of antibiotics for prevention of any further proliferation of this type of notorious bugs.

Characterization of enterococci of animal and environmental origin using phenotypic methods and comparison with PCR based methods

The purpose of this study was to evaluate the discriminatory power of &lt;I&gt;ddl&lt;/I&gt;-PCR (D-alanin-D-alanin ligase PCR) and ITS-PCR (Internal Transcribed Spacer PCR) for accurate identification of enterococcal species in comparison with phenotypic assays. Results confirm previous published data that &lt;I&gt;ddl-&lt;/I&gt;PCR simple approach allows rapid identification of two the most frequently isolated spp. &lt;I&gt;E. faecalis&lt;/I&gt; and &lt;I&gt;E. faecium&lt;/I&gt;. When identification points towards other enterococci then that mentioned above, the ITS-PCR seems to be suitable complementary assay. Correct identification of enterococci to species level is important because of different spp. susceptibility to some clinically important antibiotics. For example, it is necessary to distinquish acquired vancomycin resistance from inherent one that is less epidemiologically important. Additionally, both methods can be valuable in epidemiological studies following enterococcus resistance gene transfer within human population.

PCR-RFLP of 16S ribosomal DNA to confirm the identification of Enterococcus gallinarum and Enterococcus casseliflavus isolated from clinical and food samples

This study aimed to confirm the identification of Enterococcus gallinarum and Enterococcus casseliflavus isolated from clinical and food samples by PCR-RFLP. Fifty-two strains identified by conventional biochemical exams were submitted to PCR amplification and digested with HinfI. Only 20 (38.5%) of the 52 strains showed a DNA pattern expected for E. gallinarum and E. casseliflavus. Analysis of the results of this study showed that E. gallinarum and E. casseliflavus are occasionally erroneously identified and confirmed the potential application of 16S rDNA analysis for accurate identification of these species. A correct identification is important to distinguish between intrinsic and acquired vancomycin resistance.

Complex Nature of Enterococcal Pheromone-Responsive Plasmids

Pheromone-responsive plasmids constitute a unique group of approximately 20 plasmids identified, as yet, only among enterococcal species. Several of their representatives, e.g. pAD1, pCF10, pPD1 and pAM373 have been extensively studied. These plasmids possess a sophisticated conjugation mechanism based on response to sex pheromones--small peptides produced by plasmid-free recipient cells. Detailed analysis of regulation and function of the pheromone response process revealed its great complexity and dual role--in plasmid conjugation and modulation of enterococcal virulence. Among other functional modules identified in pheromone plasmids, the stabilization/partition systems play a crucial role in stable maintenance of the plasmid molecule in host bacteria. Among them, the par locus of pAD1 is one of the exceptional RNA addiction systems. Pheromone-responsive plasmids contribute also to enterococcal phenotype being an important vehicle of antibiotic resistance in this genus. Both types of acquired vancomycin resistance determinants, vanA and vanB, as well many other resistant phenotypes, were found to be located on these plasmids. They also encode two basic agents of enterococcal virulence, i.e. aggregation substance (AS) and cytolysin. AS participates in mating-pair formation during conjugation but can also facilitate the adherence ofenterococci to human tissues during infection. The second protein, cytolysin, displays hemolytic activity and helps to invade eukaryotic cells. There are still many aspects of the nature of pheromone plasmids that remain unclear and more detailed studies are needed to understand their uniqueness and complexity.

Emergence and spread of vancomycin resistance among enterococci in Europe

Nowadays, six types of acquired vancomycin resistance in enterococci are known; however, only VanA and to a lesser extent VanB are widely prevalent. Various genes encode acquired vancomycin resistance and these are typically associated with mobile genetic elements which allow resistance to spread clonally and laterally. The major reservoir of acquired vancomycin resistance is Enterococcus faecium; vancomycin-resistant Enterococcus faecalis are still rare. Population analysis of E. faecium has revealed a distinct subpopulation of hospital-acquired strain types, which can be differentiated by molecular typing methods (MLVA, MLST) from human commensal and animal strains. Hospital-acquired E. faecium have additional genomic content (accessory genome) including several factors known or supposed to be virulence-associated. Acquired ampicillin resistance is a major phenotypic marker of hospital-acquired E. faecium in Europe and experience has shown that it often precedes increasing rates of VRE with a delay of several years. Several factors are known to promote VRE colonisation and transmission; however, despite having populations with similar predispositions and preconditions, rates of VRE vary all over Europe.

Acquired Vancomycin Resistance in Clinically Relevant Pathogens

Acquired resistance to vancomycin is an increasing problem in pathogenic bacteria. It is best studied and most prevalent among Enterococcus and still remains rare in other pathogenic bacteria. Different genotypes of vancomycin resistance, vanA-G, have been described. The different van gene clusters consist of up to nine genes encoding proteins of different functions; their interplay leads to an alternative cell wall precursor less susceptible to glycopeptide binding. Variants of vanA and vanB types are found worldwide, with vanA predominating; their reservoir is Enterococcus faecium. Within this species a subpopulation of hospital-adapted types exists that acquired van gene clusters and which is responsible for outbreaks of vancomycin-resistant enterococci all over the world. Acquisition of vanA by methicillin-resistant Staphylococcus aureus (MRSA) is worrisome and seven cases have been described. Nonsusceptibility to glycopeptides also occurs independently from van genes and is a growing therapeutic challenge, especially in MRSA.