Vancomycin-variable Enterococcus faecium: in vivo emergence of vancomycin resistance in a vancomycin-susceptible isolate

Genetic analysis of vancomycin-variable Enterococcus faecium clinical isolates in Italy

PURPOSE

To investigate the occurrence of vancomycin-variable enterococci (VVE) in a hospital in central Italy.

METHODS

vanA positive but vancomycin-susceptible Enterococcus faecium isolates (VVE-S) were characterized by antibiotic susceptibility tests, molecular typing (PFGE and MLST), and WGS approach. The reversion of VVE-S to a resistant phenotype was assessed by exposure to increasing vancomycin concentrations, and the revertant isolates were used in filter mating experiments. qPCR was used to analyze the plasmid copy number.

RESULTS

Eleven putative VVE-S were selected. WGS revealed two categories of vanA cluster plasmid located: the first type showed the lack of vanR, the deletion of vanS, and an intact vanH/vanA/vanX cluster; the second type was devoid of both vanR and vanS and showed a deletion of 544-bp at the 5'-end of the vanH. Strains (n = 7) carrying the first type of vanA cluster were considered VVE-S and were able to regain a resistance phenotype (VVE-R) in the presence of vancomycin, due to a 44-bp deletion in the promoter region of vanH/vanA/vanX, causing its constitutive expression. VVE-R strains were not able to transfer resistance by conjugation, and the resistance phenotype was unstable: after 11 days of growth without selective pressure, the revertants were still resistant but showed a lower vancomycin MIC. A higher plasmid copy number in the revertant strains was probably related to the resistance phenotype.

CONCLUSION

We highlight the importance of VVE transition to VRE under vancomycin therapy resulting in a potential failure treatment. We also report the first-time identification of VVE-S isolates pstS-null belonging to ST1478.

Vancomycin-Resistant Enterococcus faecium and the emergence of new Sequence Types associated with Hospital Infection

Enterococcus faecium is a major cause of vancomycin-resistant enterococcal (VRE) infection. New variants of the pathogen have emerged and become dominant in healthcare settings. Two such examples, vanB ST796 and vanA ST1421 sequence types, originally arose in Australia and proceeded to cause VRE outbreaks in other countries. Of concern is the detection of a vancomycin variable enterococcal (VVE) variant of ST1421 in Europe that exhibits a vancomycin-susceptible phenotype but which can revert to resistant in the presence of vancomycin. The recent application of genome sequencing for increasing our understanding of the evolution and spread of VRE is also explored here.

Transiently silent acquired antimicrobial resistance: an emerging challenge in susceptibility testing

Acquisition and expression of antimicrobial resistance (AMR) mechanisms in bacteria are often associated with a fitness cost. Thus, evolutionary adaptation and fitness cost compensation may support the advance of subpopulations with a silent resistance phenotype when the antibiotic selection pressure is absent. However, reports are emerging on the transient nature of silent acquired AMR, describing genetic alterations that can change the expression of these determinants to a clinically relevant level of resistance, and the association with breakthrough infections causing treatment failures. This phenomenon of transiently silent acquired AMR (tsaAMR) is likely to increase, considering the overall expansion of acquired AMR in bacterial pathogens. Moreover, the augmented use of genotypic methods in combination with conventional phenotypic antimicrobial susceptibility testing (AST) will increasingly enable the detection of genotype and phenotype discrepancy. This review defines tsaAMR as acquired antimicrobial resistance genes with a corresponding phenotype within the wild-type distribution or below the clinical breakpoint for susceptibility for which genetic alterations can mediate expression to a clinically relevant level of resistance. References to in vivo resistance development and therapeutic failures caused by selected resistant subpopulations of tsaAMR in Gram-positive and Gram-negative pathogens are given. We also describe the underlying molecular mechanisms, including alterations in the expression, reading frame or copy number of AMR determinants, and discuss the clinical relevance concerning challenges for conventional AST.

A Simple and Rapid Low-Cost Procedure for Detection of Vancomycin-Resistance Genes in Enterococci Reveals an Outbreak of Vancomycin-Variable Enterococcus faecium

The detection of resistance to vancomycin in enterococci cultured from patients is important for the treatment of individual patients and for the prevention of hospital transmission. Phenotypic antimicrobial resistance tests may fail to detect potential vancomycin-resistant enterococci. We have developed and tested a PCR based procedure for routine screening for vancomycin-resistance genes in clinical samples with enterococci. Primary cultures from diagnostic samples reported with growth of Enterococcus faecium or E. facalis were tested for vanA and vanB genes by real-time PCR without the isolation of specific bacteria. Up to ten samples were pooled and tested in each real-time PCR reaction, with subsequent individual testing of cultures from positive pools. In a one-month test period in 2017 vanA gene was detected in one out of 340 urine samples with vancomycin-susceptible enterococci reported from diagnostic culture. A second test period in 2018 included 357 urine samples, and vanA gene was detected in samples from eight patients. Subsequently, all urine samples reported with growth of E. faecium during a period of one year were tested. Fifty-eight individuals were identified with enterococci, carrying the vanA gene not previously detected. Routine molecular testing of primary culture material from patient samples may improve the detection of hospitalized patients carrying E. faecium with resistance genes to vancomycin.

Current status of multidrug-resistant bacteria

Background: The dissemination of multidrug-resistant (MDR) pathogen is of major public health concern as it leads to increased mortality rate, length of hospital stays, and medical expenses.Current Concepts: Korean Government enacted an Infectious Disease Control and Prevention Act in 2009, and 6 MDR bacteria including methicillin-resistant Staphylococcus aureus, vancomycin-resistant S. aureus (VRSA), vancomycin-resistant enterococci, multidrug-resistant Pseudomonas aeruginosa, multidrug-resistant Acinetobacter baumannii, and carbapenem-resistant Enterobacterales (CRE) have been legally declared as infectious diseases. According to the amendment of the Infectious Disease Control and Prevention Act in 2020, CRE and VRSA were classified as class 2 infectious diseases, and all cases of CRE and VRSA should be reported to the Korea Disease Control and Prevention Agency (KDCA). Methicillin-resistant S. aureus, vancomycin-resistant enterococci, multidrug-resistant P. aeruginosa, and multidrug-resistant A. baumannii were classified as class 4 infectious diseases, and cases that occur need to be monitored at KDCA-designated sentinel hospitals to prevent further dissemination.Discussion and Conclusion: In this review, the current antimicrobial resistance status of six types of MDR bacteria, legally declared as infectious diseases, was investigated.. The Korean government is operating national antimicrobial resistance surveillance systems such as the Korean Antimicrobial Resistance Monitoring System and Korean Global Antimicrobial Surveillance System, as a foundation for preventing the spread of antimicrobial resistance. Certain steps need to be taken, such as establishing a surveillance system for antimicrobial usages, strengthening antimicrobial stewardship and infection control systems, and developing new antimicrobials in order for us to achieve the national goal.

Diagnosing Antibiotic Resistance Using Nucleic Acid Enzymes and Gold Nanoparticles

The rapid and accurate detection of antimicrobial resistance is critical to limiting the spread of infections and delivering effective treatments. Here, we developed a rapid, sensitive, and simple colorimetric nanodiagnostic platform to identify disease-causing pathogens and their associated antibiotic resistance genes within 2 h. The platform can detect bacteria from different biological samples (i.e., blood, wound swabs) with or without culturing. We validated the multicomponent nucleic acid enzyme-gold nanoparticle (MNAzyme-GNP) platform by screening patients with central line associated bloodstream infections and achieved a clinical sensitivity and specificity of 86% and 100%, respectively. We detected antibiotic resistance in methicillin-resistant Staphylococcus aureus (MRSA) in patient swabs with 90% clinical sensitivity and 95% clinical specificity. Finally, we identified mecA resistance genes in uncultured nasal, groin, axilla, and wound swabs from patients with 90% clinical sensitivity and 95% clinical specificity. The simplicity and versatility for detecting bacteria and antibiotic resistance markers make our platform attractive for the broad screening of microbial pathogens.

Highlights Regarding the Use of Metallic Nanoparticles against Pathogens Considered a Priority by the World Health Organization

The indiscriminate use of antibiotics has facilitated the growing resistance of bacteria, and this has become a serious public health problem worldwide. Several microorganisms are still resistant to multiple antibiotics and are particularly dangerous in the hospital and nursing home environment, and to patients whose care requires devices, such as ventilators and intravenous catheters. A list of twelve pathogenic genera, which especially included bacteria that were not affected by different antibiotics, was released by the World Health Organization (WHO) in 2017, and the research and development of new antibiotics against these genera has been considered a priority. The nanotechnology is a tool that offers an effective platform for altering the physicalchemical properties of different materials, thereby enabling the development of several biomedical applications. Owing to their large surface area and high reactivity, metallic particles on the nanometric scale have remarkable physical, chemical, and biological properties. Nanoparticles with sizes between 1 and 100 nm have several applications, mainly as new antimicrobial agents for the control of microorganisms. In the present review, more than 200 reports of various metallic nanoparticles, especially those containing copper, gold, platinum, silver, titanium, and zinc were analyzed with regard to their anti-bacterial activity. However, of these 200 studies, only 42 reported about trials conducted against the resistant bacteria considered a priority by the WHO. All studies are in the initial stage, and none are in the clinical phase of research.

Alternative vanHAX promoters and increased vanA-plasmid copy number resurrect silenced glycopeptide resistance in Enterococcus faecium

Background

Vancomycin variable enterococci (VVE) are van-positive isolates with a susceptible phenotype that can convert to a resistant phenotype during vancomycin selection.

Objectives

To describe a vancomycin-susceptible vanA-PCR positive ST203 VVE Enterococcus faecium isolate (VVESwe-S) from a liver transplantation patient in Sweden which reverted to resistant (VVESwe-R) during in vitro vancomycin exposure.

Methods

WGS analysis revealed the genetic differences between the isolates. Expression of the van-operon was investigated by qPCR. Fitness and stability of the revertant were investigated by growth measurements, competition and serial transfer.

Results

The VVESwe-R isolate gained high-level vancomycin (MIC >256 mg/L) and teicoplanin resistance (MIC = 8 mg/L). VVESwe-S has a 5′-truncated vanR activator sequence and the VVESwe-R has in addition acquired a 44 bp deletion upstream of vanHAX in a region containing alternative putative constitutive promoters. In VVESwe-R the vanHAX-operon is constitutively expressed at a level comparable to the non-induced prototype E. faecium BM4147 strain. The vanHAX operon of VVESwe is located on an Inc18-like plasmid, which has a 3–4-fold higher copy number in VVESwe-R compared with VVESwe-S. Resistance has a low fitness cost and the vancomycin MIC of VVESwe-R decreased during in vitro serial culture without selection. The reduction in MIC was associated with a decreased vanA-plasmid copy number.

Conclusions

Our data support a mechanism by which vancomycin-susceptible VVE strains may revert to a resistant phenotype through the use of an alternative, constitutive, vanR-activator-independent promoter and a vanA-plasmid copy number increase.

Surveillance of vancomycin-resistant enterococci reveals shift in dominating clones and national spread of a vancomycin-variable vanA Enterococcus faecium ST1421-CT1134 clone, Denmark, 2015 to March 2019

We describe clonal shifts in vanA Enterococcus faecium isolates from clinical samples obtained from patients in Denmark from 2015 to the first quarter (Q1) of 2019. During Q1 2019, the vancomycin-variable enterococci (VVE) ST1421-CT1134 vanA E. faecium became the most dominant vanA E. faecium clone and has spread to all five regions in Denmark. Among 174 E. faecium isolates with vanA, vanB or vanA/vanB genes in Q1 2019, 44% belonged to this type.

Tandem amplification of the vanM gene cluster drives vancomycin resistance in vancomycin-variable enterococci

BACKGROUND

Vancomycin-variable enterococci (VVE) are a potential risk factor for vancomycin resistance gene dissemination and clinical treatment failure. vanM has emerged as a new prevalent resistance determinant among clinical enterococci in China. A total of 54 vancomycin-susceptible enterococci (VSE) isolates carrying incomplete vanM gene clusters were isolated in our previous study.

OBJECTIVES

To determine the potential of vanM-carrying VSE to develop vancomycin resistance and investigate the mechanism of alteration of the resistance phenotype.

METHODS

Fifty-four vanM-positive VSE strains were induced in vitro by culturing in increasing concentrations of vancomycin. Genetic changes between three parent VVE strains and their resistant variants were analysed using Illumina and long-read sequencing technologies, quantitative PCR and Southern blot hybridization. Changes in expression level were determined by quantitative RT-PCR.

RESULTS

Twenty-five of the 54 VSE strains carrying vanM became resistant upon vancomycin exposure. A significant increase in vanM copy number was observed ranging from 5.28 to 127.64 copies per cell in induced resistant VVE strains. The vanM transposon was identified as tandem repeats with IS1216E between them, and occurred in either the plasmid or the chromosome of resistant VVE cells. In addition, an increase in vanM expression was observed after resistance conversion in VVE.

CONCLUSIONS

This study identified tandem amplification of the vanM gene cluster as a new mechanism for vancomycin resistance in VVE strains, offering a competitive advantage for VVE under antibiotic pressure.

Characterization of vanM carrying clinical Enterococcus isolates and diversity of the suppressed vanM gene cluster

Here we report the prevalence of the suppressed vanM gene cluster as a reservoir of vancomycin resistance genes. Among 1284 clinical isolates of enterococci from four hospitals in Hangzhou, China, 55 isolates of Enterococcus faecium and one isolate of Enterococcus faecalis were screened positive for the vanM genotype. Antimicrobial susceptibility testing showed that 55 of the 56 vanM-positive isolates were susceptible to vancomycin and teicoplanin. Most of them (54/56) belonged to the main epidemic lineage CC17, mostly the ST78 type. The vanM gene clusters in the 55 vancomycin-susceptible isolates showed sequence diversity owing to different insertion locations of IS1216E. The vanM transposons could be classified into five types and they all carried two or more IS1216E elements, leading to complete or partial deletions of vanR, vanS, or vanX. Quantitative reverse transcription polymerase chain reaction showed that the expression level of vanM was significantly lower in the vancomycin-susceptible isolates than in the vancomycin-resistant isolate. Considering the prevalence of the vanM genotype and the potential for conversion to a resistant phenotype, vanM might act as an important determinant of glycopeptide resistance in the future. It is essential to strengthen the surveillance of vanM-containing enterococci to control the dissemination of vancomycin resistance.

Dissemination and genetic analysis of the stealthy vanB gene clusters of Enterococcus faecium clinical isolates in Japan

Background

VanB-type vancomycin (VAN) resistance gene clusters confer VAN resistances on Enterococcus spp. over a wide range of MIC levels (MIC = 4–1000 mg/L). However, the epidemiology and the molecular characteristics of the VAN susceptible VanB-type Enterococcus still remain unclear.

Results

We characterized 19 isolates of VanB-type Enterococcus faecium that might colonize in the gut and were not phenotypically resistant to VAN (MIC = 3 mg/L). They were obtained from two hospitals in Japan between 2009 and 2010. These isolates had the identical vanB gene cluster and showed same multilocus sequence typing (MLST) (ST78) and the highly related profiles in pulsed-field gel electrophoresis (PFGE). The vanB gene cluster was located on a plasmid, and was transferable to E. faecium and E. faecalis. Notably, from these VanB-type VREs, VAN resistant (MIC≥16 mg/L) mutants could appear at a frequency of 10^− 6–10^− 7/parent cell in vitro. Most of these revertants acquired mutations in the vanS_B gene, while the remainder of the revertants might have other mutations outside of the vanB gene cluster. All of the revertants we tested showed increases in the VAN-dependent expression of the vanB gene cluster, suggesting that the mutations affected the transcriptional activity and increased the VAN resistance. Targeted mutagenesis revealed that three unique nucleotide substitutions in the vanB gene cluster of these strains attenuated VAN resistance.

Conclusions

In summary, this study indicated that stealthy VanB-type E. faecium strains that have the potential ability to become resistance to VAN could exist in clinical settings.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1342-1) contains supplementary material, which is available to authorized users.

Prevalence of vancomycin-variable Enterococcus faecium (VVE) among vanA-positive sterile site isolates and patient factors associated with VVE bacteremia

Vancomycin-variable enterococci (VVE) are vanA-positive, vancomycin-susceptible enterococci with the ability to revert to a vancomycin-resistant phenotype on exposure to vancomycin. We sought to assess the prevalence of VVE and to determine clinical characteristics of patients infected with VVE. We prospectively collected Enterococcus faecium sterile site isolates from Toronto Invasive Bacterial Diseases Network hospitals from January 2015 to June 2016 and calculated VVE (defined as vanA-positive, vancomycin-susceptible isolates) prevalence among vanA-containing isolates. We performed chart reviews of VVE and vancomycin-resistant E. faecium (VRE) bacteremias identified from January 2012 to June 2016, and on a random sample of patients with bacteremia due to vanA/vanB-negative, vancomycin-susceptible enterococci (VSE) from January 2015 to June 2016. Clinical characteristics were compared and factors associated with mortality assessed. Because of the potential reversion from VVE to VRE, pulsed-field gel electrophoresis (PFGE) was performed for strains causing breakthrough bacteremia in order to identify relatedness among strains with different phenotypic resistance within the same patient. VVE comprised 47% (18/38) of vanA-positive isolates. The charts of 36 VRE, 25 VVE, and 79 VSE patients were reviewed. Central venous catheter associated bacteremia was more common in VVE (44%) and VRE patients (57%) than in VSE patients (28%) (P = 0.01). The Pitt bacteremia (OR 1.3, P = 0.002) and the Charlson score (OR 1.2, P = 0.008) were the only independent mortality predictors. PFGE of strains causing breakthrough bacteremia showed high within-patient clonality, irrespective of vanA-positivity or vancomycin-susceptibility. A substantial proportion of vanA-positive isolates are VVE and are therefore not detected with conventional selective culture methods. Bacteremia sources of patients with VVE are similar to those infected with VRE. We detected no association between VVE and 30-day mortality or breakthrough bacteremia.

A Silenced vanA Gene Cluster on a Transferable Plasmid Caused an Outbreak of Vancomycin-Variable Enterococci

We report an outbreak of vancomycin-variable vanA(+) enterococci (VVE) able to escape phenotypic detection by current guidelines and demonstrate the molecular mechanisms for in vivo switching into vancomycin resistance and horizontal spread of the vanA cluster. Forty-eight vanA(+) Enterococcus faecium isolates and one Enterococcus faecalis isolate were analyzed for clonality with pulsed-field gel electrophoresis (PFGE), and their vanA gene cluster compositions were assessed by PCR and whole-genome sequencing of six isolates. The susceptible VVE strains were cultivated in brain heart infusion broth containing vancomycin at 8 μg/ml for in vitro development of resistant VVE. The transcription profiles of susceptible VVE and their resistant revertants were assessed using quantitative reverse transcription-PCR. Plasmid content was analyzed with S1 nuclease PFGE and hybridizations. Conjugative transfer of vanA was assessed by filter mating. The only genetic difference between the vanA clusters of susceptible and resistant VVE was an ISL3-family element upstream of vanHAX, which silenced vanHAX gene transcription in susceptible VVE. Furthermore, the VVE had an insertion of IS1542 between orf2 and vanR that attenuated the expression of vanHAX Growth of susceptible VVE occurred after 24 to 72 h of exposure to vancomycin due to excision of the ISL3-family element. The vanA gene cluster was located on a transferable broad-host-range plasmid also detected in outbreak isolates with different pulsotypes, including one E. faecalis isolate. Horizontally transferable silenced vanA able to escape detection and revert into resistance during vancomycin therapy represents a new challenge in the clinic. Genotypic testing of invasive vancomycin-susceptible enterococci by vanA-PCR is advised.

Whole-genome sequencing reveals transmission of vancomycin-resistant Enterococcus faecium in a healthcare network

Background

Bacterial whole-genome sequencing (WGS) has the potential to identify reservoirs of multidrug-resistant organisms and transmission of these pathogens across healthcare networks. We used WGS to define transmission of vancomycin-resistant enterococci (VRE) within a long-term care facility (LTCF), and between this and an acute hospital in the United Kingdom (UK).

Methods

A longitudinal prospective observational study of faecal VRE carriage was conducted in a LTCF in Cambridge, UK. Stool samples were collected at recruitment, and then repeatedly until the end of the study period, discharge or death. Selective culture media were used to isolate VRE, which were subsequently sequenced and analysed. We also analysed the genomes of 45 Enterococcus faecium bloodstream isolates collected at Cambridge University Hospitals NHS Foundation Trust (CUH).

Results

Forty-five residents were recruited during a 6-month period in 2014, and 693 stools were collected at a frequency of at least 1 week apart. Fifty-one stool samples from 3/45 participants (7 %) were positive for vancomycin-resistant E. faecium. Two residents carried multiple VRE lineages, and one carried a single VRE lineage. Genome analyses based on single nucleotide polymorphisms (SNPs) in the core genome indicated that VRE carried by each of the three residents were unrelated. Participants had extensive contact with the local healthcare network. We found that VRE genomes from LTCF residents and hospital-associated bloodstream infection were interspersed throughout the phylogenetic tree, with several instances of closely related VRE strains from the two settings.

Conclusions

A proportion of LTCF residents are long-term carriers of VRE. Evidence for genetic relatedness between these and VRE associated with bloodstream infection in a nearby acute NHS Trust indicate a shared bacterial population.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-015-0259-7) contains supplementary material, which is available to authorized users.

Vancomycin-Variable Enterococcal Bacteremia

Vancomycin-variable enterococcus (VVE) is an emerging pathogen. VVE isolates initially appear phenotypically susceptible to vancomycin but possesses the vanA gene and can develop in vitro and in vivo resistance to vancomycin. We report a case of VVE bacteremia and describe how VVE poses diagnostic and therapeutic dilemmas.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Vancomycin-variable enterococci can give rise to constitutive resistance during antibiotic therapy

Vancomycin-resistant enterococci (VRE) are notorious clinical pathogens restricting the use of glycopeptide antibiotics in the clinic setting. Routine surveillance to detect VRE isolated from patients relies on PCR bioassays and chromogenic agar-based test methods. In recent years, we and others have reported the emergence of enterococcal strains harboring a "silent" copy of vancomycin resistance genes that confer a vancomycin-susceptible phenotype (vancomycin-susceptible enterococci [VSE]) and thus escape detection using drug sensitivity screening tests. Alarmingly, these strains are able to convert to a resistance phenotype (VSE→VRE) during antibiotic treatment, severely compromising the success of therapy. Such strains have been termed vancomycin-variable enterococci (VVE). We have investigated the molecular mechanisms leading to the restoration of resistance in VVE isolates through the whole-genome sequencing of resistant isolates, measurement of resistance gene expression, and quantification of the accumulation of drug-resistant peptidoglycan precursors. The results demonstrate that VVE strains can revert to a VRE phenotype through the constitutive expression of the vancomycin resistance cassette. This is accomplished through a variety of changes in the DNA region upstream of the resistance genes that includes both a deletion of a likely transcription inhibitory secondary structure and the introduction of a new unregulated promoter. The VSE→VRE transition of VVE can occur in patients during the course of antibiotic therapy, resulting in treatment failure. These VVE strains therefore pose a new challenge to the current regimen of diagnostic tests used for VRE detection in the clinic setting.

Outbreak of vancomycin-susceptible Enterococcus faecium containing the wild-type vanA gene

Accurate detection of vancomycin-resistant enterococci (VRE) is essential in preventing transmission in health care settings. Chromogenic media are widely used for screening VRE because of fast turnaround times (TAT) and high sensitivity. We report an outbreak of Enterococcus faecium bearing vanA yet susceptible to vancomycin (vancomycin-variable Enterococcus [VVE]). Between October 2009 to March 2011, clinical and screening specimens (n=14,747) were screened for VRE using VRE-selective medium and/or PCR. VVE isolates were genotyped to determine relatedness. Plasmids from these isolates were characterized by sequencing. Overall, 52 VVE isolates were identified, comprising 15% of all VRE isolates identified. Isolates demonstrated growth on Brilliance VRE agar (Oxoid) at 24 h of incubation but did not grow on brain heart infusion agar with 6 μg/ml vancomycin (Oxoid) or bile esculin azide agar with 6 μg/ml vancomycin (Oxoid) and were susceptible to vancomycin. Genotyping of 20 randomly selected VVE isolates revealed that 15/20 were identical, while 5 were highly related. PCR of the VVE transposon confirmed the presence of vanHAXY gene cluster; however, vanS (sensor) and vanR (regulator) genes were absent. The outbreak was controlled through routine infection control measures. We report an emergence of a fit strain of E. faecium containing vanA yet susceptible to vancomycin. Whether this new strain represents VRE has yet to be determined; however, unique testing procedures are required for reliable identification of VVE.