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Vancomycin-Resistant Enterococci: Colonization, Infection, Detection, and Treatment

  • Ali Zirakzadeh
    Affiliations
    Division of General Internal Medicine, Mayo Clinic College of Medicine, Rochester, Minn
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  • Robin Patel
    Correspondence
    Individual reprints of this article are not available. Address correspondence to Robin Patel, MD, Division of Infectious Diseases, Mayo Clinic College of Medicine, 200 First St SW, Rochester, MN 55905
    Affiliations
    Division of Infectious Diseases and Division of Clinical Microbiology, Mayo Clinic College of Medicine, Rochester, Minn
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      Vancomycin-resistant enterococci (VRE) are becoming a major concern in medical practice. Their increased prevalence and their ability to transfer vancomycin resistance to other bacteria (including methicillin-resistant Staphylococcus aureus) have made them a subject of close scrutiny and intense investigation. Colonization is usually acquired by susceptible hosts in an environment with a high rate of patient colonization with VRE (eg, intensive care units, oncology units). Vancomycin-resistant enterococci can survive in the environment for prolonged periods (>1 week), can contaminate almost any surface, and can be passed from one patient to another by health care workers. Whether VRE colonization leads to infection depends on the health status of the patient. Whereas immunocompetent patients colonized with VRE are at low risk for infection, weakened hosts (patients with hematologic disorders, transplant recipients, or severely ill patients) have an increased likelihood of developing infection following colonization. Quinupristin-dalfopristin and linezolid are among the anti-infective agents that have recently become available to treat infection caused by VRE. Other antimicrobials are currently under development. Molecular techniques such as polymerase chain reaction and standard culture studies are being used to detect VRE colonization, infection, and outbreaks.
      ICU (intensive care unit), MRSA (methicillin-resistant Staphylococcus aureus), PCR (polymerase chain reaction), VRE (vancomycin-resistant enterococci)
      The first isolates of high-level vancomycin-resistant enterococci (VRE) were reported from the United Kingdom in the late 1980s.
      • Uttley AH
      • Collins CH
      • Naidoo J
      • George RC
      Vancomycin-resistant enterococci [letter].
      Since then, rates of VRE colonization and infection have risen steadily.
      • Martone WJ
      Spread of vancomycin-resistant enterococci: why did it happen in the United States?.
      In the United States, hemodialysis patients have a 10% prevalence rate of colonization with VRE.
      • Burrell LJ
      • Grabsch EA
      • Padiglione AA
      • Grayson ML
      Prevalence of colonisation with vancomycin-resistant enterococci (VRE) among haemodialysis outpatients in Victoria: implications for screening [letter].
      A recent multicenter epidemiological study showed that 28% of enterococci cultured from 25 North American intensive care units (ICUs) were resistant to vancomycin.
      • Streit JM
      • Jones RN
      • Sader HS
      • Fritsche TR
      Assessment of pathogen occurrences and resistance profiles among infected patients in the intensive care unit: report from the SENTRY Antimicrobial Surveillance Program (North America, 2001).
      More alarmingly, a recent study from a large academic hospital ICU revealed a 9.5% incidence rate of co-colonization or coinfection with VRE and methicillin-resistant Staphylococcus aureus (MRSA).
      • Warren DK
      • Nitin A
      • Hill C
      • Fraser VJ
      • Kollef MH
      Occurrence of cocolonization or co-infection with vancomycin-resistant enterococci and methicillin-resistant Staphylococcus aureus in a medical intensive care unit.
      The consequences of such co-colonization have already been realized. In 2003, physicians in Michigan reported the first case of vancomycin resistance transferred from VRE to MRSA in a patient with a wound colonized by both organisms.
      • Chang S
      • Sievert DM
      • Hageman JC
      • Vancomycin-Resistant Staphylococcus aureus Investigative Team
      • et al.
      Infection with vancomycin-resistant Staphylococcus aureus containing the vanA resistance gene.
      Since then, 3 other cases of vancomycin-resistant MRSA have been reported.
      • Streit JM
      • Sader HS
      • Fritsche TR
      • Jones RN
      Dalbavancin activity against selected populations of antimicrobial-resistant Gram-positive pathogens.
      Clearly, clinicians need to be aware of the importance of VRE. This review provides an overview of VRE, available treatments, and current detection techniques.

      VRE COLONIZATION

      Infection with VRE (described in more detail subsequently) typically follows vancomycin-resistant enterococcal colonization, predominantly of the gastrointestinal tract. Colonization, which does not result in symptoms, may last for long periods and may serve as a reservoir for the transmission of VRE to other patients. Within hospitals, widespread colonization with VRE may occur with a comparatively small number of documented infections. Therefore, tracking colonization with VRE through active surveillance in high-risk units is an important component of preventing further transmission. At the Mayo Clinic, we perform active surveillance on patients in the ICU and on hematology, oncology, and abdominal transplantation wards.
      Colonization is contingent on exposure to VRE and on being a “susceptible” host. With regard to exposure to VRE, the most important considerations are proximity to and duration of exposure to those already colonized with VRE. When the proportion of patients colonized with VRE on a particular ward (the so-called colonization pressure) is high (>50%), other risk factors for colonization (described subsequently) become less important.
      • Bonten MJ
      • Slaughter S
      • Ambergen AW
      • et al.
      The role of “colonization pressure” in the spread of vancomycin-resistant enterococci: an important infection control variable.
      “Susceptible hosts” are at high risk for VRE colonization.
      • Bonten MJ
      • Slaughter S
      • Ambergen AW
      • et al.
      The role of “colonization pressure” in the spread of vancomycin-resistant enterococci: an important infection control variable.
      These include patients who are severely ill and those receiving multiple and prolonged courses of antimicrobial agents. Colonization in these hosts often occurs in long-term care facilities and urban referral hospitals. Solid (especially abdominal) organ transplant recipients and hematology patients are at particularly increased risk for colonization with VRE. Health care workers and their household members are also at risk for VRE colonization.
      • Murray BE
      Vancomycin-resistant enterococcal infections.
      Most patients colonized with VRE will remain colonized for prolonged periods. A Mayo Clinic study that defined clearance as negative rectal VRE cultures on at least 3 consecutive tests obtained more than 1 week apart showed spontaneous decolonization in only 18 (34%) of 53 liver and kidney transplant recipients.
      • Patel R
      • Allen SL
      • Manahan JM
      • et al.
      Natural history of vancomycin-resistant enterococcal colonization in liver and kidney transplant recipients.
      Furthermore, VRE were detected on subsequent surveillance cultures from 2 of these previously decolonized patients.
      • Patel R
      • Allen SL
      • Manahan JM
      • et al.
      Natural history of vancomycin-resistant enterococcal colonization in liver and kidney transplant recipients.
      Spontaneous decolonization occurs infrequently, and little progress has been made in finding a pharmacological method to eliminate VRE from a colonized patient. Antimicrobials such as oral bacitracin, ramoplanin, and novobiocin have shown limited success in permanently eradicating VRE from these patients.
      • Murray BE
      Vancomycin-resistant enterococcal infections.
      • Wong MT
      • Kauffman CA
      • Standiford HC
      • et al.
      Ramoplanin VRE2 Clinical Study Group. Effective suppression of vancomycin-resistant Enterococcus species in asymptomatic gastrointestinal carriers by a novel glycolipo-depsipeptide, ramoplanin.
      • Rand KH
      • Houck H
      Daptomycin synergy with rifampicin and ampicillin against vancomycin-resistant enterococci.
      Controlling transmission of VRE in the health care environment can be challenging. Vancomycin-resistant enterococci are capable of prolonged (>1 week) survival in the environment and can be transferred from environmental sites to staff hands. Vancomycin-resistant enterococci have been isolated from virtually every site and every object in health care facilities, including monitoring devices (call bells, electrocardiographic monitors, pulse oximeters, glucose meters, stethoscopes, electronic thermometers, blood pressure cuffs, keyboards, wall-mounted control panels), furniture (telephones, air cushions, headboards, tables, chairs, bed rails), toilet seats, doors, floors, linens, and other medical equipment (ventilator tubing, pumps, wash bowls, automated medication dispensers, intravenous poles).
      • Bonten MJ
      • Hayden MK
      • Nathan C
      • et al.
      Epidemiology of colonisation of patients and environment with vancomycin-resistant enterococci.
      • Bonten MJ
      • Willems R
      • Weinstein RA
      Vancomycin-resistant enterococci: why are they here, and where do they come from?.
      Despite the use of contact precautions in a recent study, health care workers in an ICU spread VRE from a contaminated to an uncontaminated part of a patient's room in nearly 1 in 10 encounters.
      • Duckro AN
      • Blom DW
      • Lyle EA
      • Weinstein RA
      • Hayden MK
      Transfer of vancomycin-resistant enterococci via health care worker hands.
      Equally difficult is the maintenance of appropriate infection control measures. Nevertheless, during the course of caring for patients colonized with VRE, a change of gloves is necessary after contact with material that may contain high concentrations of the bacteria, such as stool, urine, bedpans, soiled linen, rectal thermometers, toilets, or other infected body sites. Strict isolation techniques should be used at all times: gloves and gown should be donned before entering and removed before exiting the patient's room. Hands should be sanitized immediately thereafter. Clothing and ungloved hands should not contact environmental surfaces potentially contaminated with VRE such as doorknobs or privacy curtains in the patient's room. Dedicated use of noncritical items (stethoscopes, thermometers) for individual patients decreases the risk of transmission of VRE from patient to patient. If such devices are used for other patients, they must first be thoroughly cleaned and disinfected.
      • Ostrowsky BE
      • Trick WE
      • Sohn AH
      • et al.
      Control of vancomycin-resistant Enterococcus in health care facilities in a region.
      A more thorough list of recommendations for preventing the spread of VRE is available through the Centers for Disease Control and Prevention.
      • Hospital Infection Control Practices Advisory Committee
      Recommendations for preventing the spread of vancomycin resistance: recommendations of the Hospital Infection Control Practices Advisory Committee (HICPAC).
      Some authorities advocate equally stringent infection control procedures for patients at high risk for colonization.
      • Trick WE
      • Weinstein RA
      • DeMarais PL
      • et al.
      Colonization of skilled-care facility residents with antimicrobial-resistant pathogens.
      At the Mayo Clinic, we practice universal gloving for all hematology, blood and marrow transplant, and solid organ transplant patients. Furthermore, we perform twice-weekly mandatory polymerase chain reaction (PCR) testing on all patients admitted to those services.
      Despite inconveniences, increased health care professional workloads, and bigger up-front costs, infection control measures decrease the spread of VRE and thus are cost-effective. A recent University of Maryland mathematical model showed that active surveillance in the ICU reduced VRE transmission by a projected 39%.
      • Perencevich EN
      • Fisman DN
      • Lipsitch M
      • Harris AD
      • Morris Jr, JG
      • Smith DL
      Projected benefits of active surveillance for vancomycin-resistant enterococci in intensive care units.
      Another recent study showed annual savings of more than $400,000 as a result of gown use in a facility with a high prevalence rate of VRE.
      • Puzniak LA
      • Gillespie KN
      • Leet T
      • Kollef M
      • Mundy LM
      A cost-benefit analysis of gown use in controlling vancomycin-resistant Enterococcus transmission: is it worth the price?.
      Thus, we recommend active surveillance for hospital populations at high risk (as previously described) for colonization with VRE.

      VRE INFECTION

      Infection with VRE usually develops in patients colonized with the bacteria,
      • Chavers LS
      • Moser SA
      • Benjamin WH
      • et al.
      Vancomycin-resistant enterococci: 15 years and counting.
      with the ratio of infected-to-colonized patients dependent on the specific patient population. It is highest in hematology patients and organ transplant recipients and approaches zero in healthier (immunocompetent) populations.
      • Chavers LS
      • Moser SA
      • Benjamin WH
      • et al.
      Vancomycin-resistant enterococci: 15 years and counting.
      • Orloff SL
      • Busch AM
      • Olyaei AJ
      • et al.
      Vancomycin-resistant Enterococcus in liver transplant patients.
      • Zaas AK
      • Song X
      • Tucker P
      • Perl TM
      Risk factors for development of vancomycin-resistant enterococcal bloodstream infection in patients with cancer who are colonized with vancomycin-resistant enterococci.
      • Kapur D
      • Dorsky D
      • Feingold JM
      • et al.
      Incidence and outcome of vancomycin-resistant enterococcal bacteremia following autologous peripheral blood stem cell transplantation.
      Portals of entry for VRE typically include the urinary tract, intra-abdominal (eg, gastrointestinal tract, biliary tree) or pelvic sources, wounds (surgical wounds, decubitus ulcers), and intravascular catheters.
      • Bonten MJ
      • Willems R
      • Weinstein RA
      Vancomycin-resistant enterococci: why are they here, and where do they come from?.
      At the Mayo Clinic, most cases of VRE infection occur in abdominal organ transplant recipients and hematology patients.
      Urinary tract infections caused by VRE include cystitis, pyelonephritis, prostatitis, and perinephric abscess; most such infections are nosocomial and associated with urinary instrumentation. However, evidence of VRE in the urine in the absence of other findings (urgency, fever, flank pain, systemic symptoms) may have limited clinical importance, representing only asymptomatic bacteriuria and not requiring antimicrobial treatment.
      • Wong AH
      • Wenzel RP
      • Edmond MB
      Epidemiology of bacteriuria caused by vancomycin-resistant enterococci—a retrospective study.
      Patients with VRE have a high prevalence of skin colonization,
      • Bonten MJ
      • Willems R
      • Weinstein RA
      Vancomycin-resistant enterococci: why are they here, and where do they come from?.
      which may result in colonization of intravascular catheters and subsequent intravascular catheter-related sepsis. In liver transplant recipients, the most common types of VRE infection include intra-abdominal infections associated with biliary leaks, stenoses, or obstruction; hepatic or perihepatic abscesses; stenosis or thrombosis of the hepatic artery; or perforated viscera.
      • Newell KA
      • Millis JM
      • Arnow PM
      • et al.
      Incidence and outcome of infection by vancomycin-resistant Enterococcus following orthotopic liver transplantation.
      Bacteremia is common and typically derives from the aforementioned infections. In kidney and pancreas transplant recipients, bacteremia typically derives from wound or urinary tract infections.
      • Joels CS
      • Matthews BD
      • Sigmon LB
      • et al.
      Clinical characteristics and outcomes of surgical patients with vancomycin-resistant enterococcal infections.
      Neutropenic patients colonized with VRE are at risk for VRE bacteremia as a result of gut translocation, central venous catheter infections, or urinary tract infections.
      • Zaas AK
      • Song X
      • Tucker P
      • Perl TM
      Risk factors for development of vancomycin-resistant enterococcal bloodstream infection in patients with cancer who are colonized with vancomycin-resistant enterococci.
      • Yao YM
      • Yu Y
      • Sheng ZY
      • et al.
      Role of gut-derived endotoxaemia and bacterial translocation in rats after thermal injury: effects of selective decontamination of the digestive tract.
      • Gregory Jr, JJ
      • Small TN
      • Papadopoulos E
      • et al.
      Influence of vancomycin-resistant Enterococcus (VRE) on the outcome of recipients of bone marrow transplants [abstract].
      Meningitis, endocarditis, pleural space, and skin or soft tissue infections have also been reported.
      • Bishara J
      • Robenshtok E
      • Weinberger M
      • Yeshurun M
      • Sagie A
      • Pitlik S
      Infective endocarditis in renal transplant recipients.
      • Shaikh ZH
      • Peloquin CA
      • Ericsson CD
      Successful treatment of vancomycin-resistant Enterococcus faecium meningitis with linezolid: case report and literature review.
      Risk factors for VRE bacteremia include hemodialysis; organ transplantation; receipt of corticosteroids, chemotherapy, or parenteral nutrition; surgery; severe illness; long-term antibiotic administration; indwelling urinary catheters; neutropenia; and mucositis.
      • Patel R
      Clinical impact of vancomycin-resistant enterococci.
      Mortality rates for patients with VRE bacteremia vary depending on the population at risk. Recipients of autologous peripheral blood stem cell transplants have been shown to have mortality rates as low as 10%.
      • Kapur D
      • Dorsky D
      • Feingold JM
      • et al.
      Incidence and outcome of vancomycin-resistant enterococcal bacteremia following autologous peripheral blood stem cell transplantation.
      Patients with endocarditis caused by VRE have been reported to have mortality rates higher than 30%,
      • Murray BE
      Vancomycin-resistant enterococcal infections.
      those with solid tumors have death rates higher than 50%,
      • Zaas AK
      • Song X
      • Tucker P
      • Perl TM
      Risk factors for development of vancomycin-resistant enterococcal bloodstream infection in patients with cancer who are colonized with vancomycin-resistant enterococci.
      and some studies of critically ill and liver transplant patients have shown more than 70% mortality.
      • Chavers LS
      • Moser SA
      • Benjamin WH
      • et al.
      Vancomycin-resistant enterococci: 15 years and counting.
      • El-Khoury J
      • Fishman JA
      Linezolid in the treatment of vancomycin-resistant Enterococcus faecium in solid organ transplant recipients: report of a multicenter compassionate-use trial.

      VRE TREATMENT

      Although no currently available antimicrobial agent can eradicate VRE colonization, several treatment options exist for VRE infection. Most isolates of VRE are resistant to penicillin and ampicillin; however, in unusual cases in which such agents are active, they can be useful therapeutic options. Several choices for current, and possibly future, treatment of infection with VRE are described subsequently. Antimicrobial susceptibility testing is recommended to verify the activity of any agent being used to treat VRE.

       Quinupristin-Dalfopristin

      In late 1999, quinupristin-dalfopristin became the first antimicrobial agent available for the treatment of vancomycin-resistant Enterococcus faecium infection. Although E faecium (the most common vancomycin-resistant Enterococcus species) is susceptible to quinupristin-dalfopristin, most Enterococcus faecalis isolates and many other non-E faecium species are intrinsically resistant to this antimicrobial agent. Quinupristin-dalfopristin, a streptogramin, targets the bacterial 50S ribosome, thereby inhibiting protein synthesis. Although uncommon, resistance can develop through modification of the target binding site, enzymatic inactivation, and/or efflux. The most common adverse effects are arthralgias and/or myalgias, which can be debilitating and have limited widespread use of the agent.
      • Winston DJ
      • Emmanouilides C
      • Kroeber A
      • et al.
      Quinupristin/dalfopristin therapy for infections due to vancomycin-resistant Enterococcus faecium.
      • Linden PK
      • Moellering Jr, RC
      • Wood CA
      • et al.
      Treatment of vancomycin-resistant Enterococcus faecium infections with quinupristin/dalfopristin.

       Linezolid

      Linezolid, the first of a new class of antimicrobial agents termed oxazolidinones, became available in 2000. It can be administered orally or intravenously and, unlike quinupristin-dalfopristin, has activity against both E faecium and non-E faecium species (E faecalis, Enterococcus casseliflavus, Enterococcus gallinarum). The oral formulation has excellent bioavailability. Currently, linezolid is the only oral agent approved by the Food and Drug Administration for treatment of infections caused by VRE. Linezolid inhibits ribosomal protein synthesis but at a different site from other agents that target the ribosome (chloramphenicol, macrolides, lincosamides, streptogramin, aminoglycosides, tetracycline). Consequently, existing mechanisms of resistance to these agents do not confer cross-resistance to linezolid.
      Linezolid is the anti-VRE drug used most commonly at the Mayo Clinic. However, its myelosuppressive adverse effects, especially thrombocytopenia, may limit its use in some patients. In addition, because linezolid is a weak monoamine oxidase inhibitor, a diet low in tyramine (as instructed in the package insert) is generally recommended while taking the medication. Linezolid has some potentially important drug-drug interactions, and careful review of the patient's medical regimen, in consultation with a pharmacist, is recommended before it is prescribed.
      • Smith PF
      • Birmingham MC
      • Noskin GA
      • et al.
      Safety, efficacy and pharmacokinetics of linezolid for treatment of resistant Gram-positive infections in cancer patients with neutropenia.
      • Stevens DL
      • Dotter B
      • Madaras-Kelly K
      A review of linezolid: the first oxazolidinone antibiotic.
      Linezolid has several characteristics that were initially perceived as important in preventing the emergence of resistance. It is a synthetic agent, and preexisting resistance similar to that seen with natural antibacterial agents (penicillin, vancomycin, etc) was therefore considered unlikely. In addition, because oxazolidinones inhibit protein synthesis by binding to domain V of ribosomal RNA, which is encoded by genes (ribosomal RNA gene) present in multiple copies (4 copies in E faecalis and 5-6 copies in E faecium), selection of mutational resistance was expected to require mutations in multiple copies of 23S ribosomal DNA, a hypothetically unlikely event.
      Unfortunately, cases of linezolid-resistant enterococci have emerged and spread nosocomially. In 2001, for example, 7 clinical isolates of linezolid-resistant, vancomycin-resistant E faecium were reported from the Mayo Clinic.
      • Herrero IA
      • Issa NC
      • Patel R
      Nosocomial spread of linezolid-resistant, vancomycin-resistant Enterococcus faecium [letter].
      The linezolid-resistant, vancomycin-resistant E faecium strain was identified in a liver transplant recipient whose course was complicated by vancomycin-resistant E faecium intra-abdominal infection treated with linezolid therapy. The strain was transmitted nosocomially to 6 other patients despite strict isolation of the index case, the use of private rooms, and universal gloving by health care workers before entering patients' rooms.
      • Herrero IA
      • Issa NC
      • Patel R
      Nosocomial spread of linezolid-resistant, vancomycin-resistant Enterococcus faecium [letter].

       Daptomycin

      A cyclic lipopeptide fermentation product of Streptomyces roseosporus, daptomycin became available in 2003. It rapidly kills gram-positive bacteria by disrupting multiple aspects of bacterial membrane function.
      • Rand KH
      • Houck H
      Daptomycin synergy with rifampicin and ampicillin against vancomycin-resistant enterococci.
      Although daptomycin has in vitro activity against VRE, publishedclinical data regarding its use for the treatment of infections caused by VRE, a non-Food and Drug Administration-approved indication, are minimal. Daptomycin should not be used to treat pneumonia because clinical trials have shown a high failure rate in this setting.
      • Jeu L
      • Fung HB
      Daptomycin: a cyclic lipopeptide antimicrobial agent.
      In addition, resistance to daptomycin has begun to emerge.
      • Long JK
      • Choueiri TK
      • Hall GS
      • Avery RK
      • Sekeres MA
      Daptomycin-resistant Enterococcus faecium in a patient with acute myeloid leukemia.
      • Munoz-Price LS
      • Lolans K
      • Quinn JP
      Emergence of resistance to daptomycin during treatment of vancomycin-resistant Enterococcus faecalis infection [letter].
      • Lewis II, JS
      • Owens A
      • Cadena J
      • Sabol K
      • Patterson JE
      • Jorgensen JH
      Emergence of daptomycin resistance in Enterococcus faecium during daptomycin therapy [published correction appears in Antimicrob Agents Chemother. 2005;49:2152].

       Other Agents

      Tigecycline, a broad-spectrum glycylcycline antimicrobial agent, became available in 2005. This novel tetracycline derivative has activity against gram-positive and gram-negative aerobic and anaerobic bacteria, including tetracycline-resistant isolates.
      • Rubinstein E
      • Vaughan D
      Tigecycline: a novel glycylcycline.
      Similar to daptomycin, this compound has in vitro activity against VRE; however, clinical data on the treatment of infections caused by VRE are lacking.
      Many VRE isolates are susceptible to nitrofurantoin, which has been used to treat VRE urinary tract infection but does not have useful activity in other VRE infections.
      Future options for treatment of VRE infection may include mannopeptimycins and dalbavancin. Mannopeptimycins, a novel class of glycopeptides, are semisynthetic antimicrobials isolated from Streptomyces hygroscopicus. Like vancomycin, they prevent transglycosylation of cell wall peptidoglycans. They are active against a wide variety of gram-positive bacteria, including VRE, and have been shown to be bactericidal in vivo.
      • Rice LB
      • Lakticova V
      • Helfand MS
      • Hutton-Thomas R
      In vitro antienterococcal activity explains associations between exposures to antimicrobial agents and risk of colonization by multiresistant enterococci.
      • Ruzin A
      • Singh G
      • Severin A
      • et al.
      Mechanism of action of the mannopeptimycins, a novel class of glycopeptide antibiotics active against vancomycin-resistant gram-positive bacteria.
      Dalbavancin, another semisynthetic glycopeptide, has gained attention because of its once-weekly dosing schedule. However, while active against VanB glycopeptide-resistant phenotype enterococci, dalbavancin has little activity against the more common VanA glycopeptide-resistant phenotype enterococci.
      • Streit JM
      • Sader HS
      • Fritsche TR
      • Jones RN
      Dalbavancin activity against selected populations of antimicrobial-resistant Gram-positive pathogens.

      GENETICS OF VANCOMYCIN RESISTANCE IN ENTEROCOCCI

      To date, 6 glycopeptide-resistant enterococcal phenotypes, VanA, VanB, VanC, VanD, VanE, and VanG, have been described. They can usually be distinguished on the basis of the level, inducibility, and transferability of resistance to vancomycin and teicoplanin (Table 1). The first 2 types are the most clinically relevant.
      Table 1Resistance to Glycopeptides in Enterococci
      From Patel R. Vancomycin-resistant enterococci in solid organ transplantation. Curr Opin Organ Transplant. 1999;4:271-280, with permission from Lippincott Williams & Wilkins.
      PhenotypeGenotype (resistance operon present)Vancomycin MIC (μg/mL)Teicoplanin MIC (μg/mL)ExpressionAbility to transfer resistanceSpecies
      VanAvanA64->100016-512InducibleYesEnterococcus faecium
      E faecalis
      E avium
      E gallinarum
      E durans
      E mundtii
      E casseliflavus
      E raffinosus
      E hirae
      VanBvanB4-10000.25-2InducibleYesE faecium
      E faecalis
      E gallinarum
      E durans
      VanCvanC-12-320.12-2ConstitutiveNoE gallinarum
      Inducible
      VanCvanC-22-320.12-2ConstitutiveNoE casseliflavus
      VanCvanC-32-320.12-2ConstitutiveNoE flavescens
      VanD
      * Rare, inducible = gene expression occurs in the presence of vancomycin; constitutive = continuous gene expression; MIC = minimum inhibitory concentration.
      vanD16-2562-64ConstitutiveNoE faecium
      E faecalis
      VanE
      * Rare, inducible = gene expression occurs in the presence of vancomycin; constitutive = continuous gene expression; MIC = minimum inhibitory concentration.
      vanE160.5InducibleNoE faecalis
      VanG
      * Rare, inducible = gene expression occurs in the presence of vancomycin; constitutive = continuous gene expression; MIC = minimum inhibitory concentration.
      vanG160.5InducibleNoE faecalis
      * Rare, inducible = gene expression occurs in the presence of vancomycin; constitutive = continuous gene expression; MIC = minimum inhibitory concentration.
      Vancomycin complexes with the d-alanyl-d-alanine termini of normal peptidoglycan cell wall precursors, thereby inhibiting cell wall synthesis (Figure 1). The genes associated with high-level vancomycin resistance in enterococci encode a ligase responsible for the synthesis of the depsipeptide d-alanyl-d-lactate. This depsipeptide is incorporated into the terminal portion of the peptidoglycan cell wall precursor, limiting vancomycin-peptidoglycan precursor binding (Figure 1).
      Figure thumbnail gr1
      Figure 1Mechanism of action of vancomycin and mechanisms of vancomycin resistance in enterococci with vanA-associated vancomycin resistance. V = vancomycin. From Patel R. Vancomycin-resistant enterococci in solid organ transplantation. Curr Opin Organ Transplant. 1999;4:271-280, with permission from Lippincott Williams & Wilkins.
      VanA-type glycopeptide resistance is characterized by acquired inducible resistance to both vancomycin and teicoplanin. It is mediated by transposon Tn1546 or closely related genetic elements. Tn1546 contains the vanA gene clusterthat encodes 8 polypeptides (Figure 2). This transposon may be located on plasmids or bacterial chromosomes.
      Figure thumbnail gr2
      Figure 2The vanA operon in Tn1546. From Patel R. Vancomycin-resistant enterococci in solid organ transplantation. Curr Opin Organ Transplant. 1999;4:271-280, with permission from Lippincott Williams & Wilkins.
      The transfer of high-level (VanA) vancomycin resistance from E faecalis to S aureus via Tn1546 was described recently.
      • Weigel LM
      • Clewell DB
      • Gill SR
      • et al.
      Genetic analysis of a high-level vancomycin-resistant isolate of Staphylococcus aureus.
      VanB-type glycopeptide resistance is characterized by acquired inducible resistance to various concentrations of vancomycin but typically not to teicoplanin. The vanB gene cluster, as described in E faecalis V583, has homology to the vanA gene cluster; it consists of genes encodingpolypeptides assigned to the regulation of glycopeptide resistance genes (vanRB and vanSB), synthesis of the depsipeptide d-alanyl-d-lactate (vanHB and vanB), and hydrolysis of precursors of normal peptidoglycan (vanXB and vanYB). The vanB sequence varies among different enterococcal isolates.

      MOLECULAR TESTS FOR DETECTION OF VRE COLONIZATION

      At the Mayo Clinic, VRE colonization is identified by PCR on samples obtained from perianal, perirectal, or rectal swabs or from stool specimens. Bacterial DNA is extracted using the automated MagNA Pure instrument (Roche Diagnostics Corporation, Indianapolis, Ind). Then, the LightCycler instrument (Roche Diagnostics Corporation) is used to detect vanA and vanB using a rapid real-time PCR assay (Figure 3). This method is more sensitive and faster (~3.5 vs >72 hours) than culture for detecting VRE colonization.
      • Sloan LM
      • Uhl JR
      • Vetter EA
      • et al.
      Comparison of the Roche LightCycler vanA/vanB detection assay and culture for detection of vancomycin-resistant enterococci from perianal swabs.
      The assay detects the presence of genes associated with vancomycin resistance in enterococci, vanA and vanB. A positive result indicates colonization but not necessarily infection with VRE.
      Figure thumbnail gr3
      Figure 3Rapid real-time polymerase chain reaction assay using fluorescent resonance energy transfer (FRET) technology. Probes specific to van genes containing donor and reporter dyes are annealed to amplifying DNA. When these dyes are located in proximity to one another (as a result of probe hybridization) and are stimulated by an outside light source, light is emitted at a unique wavelength allowing specific identification of van genes. Reprinted from Infect Dis Clin North Am, Vol 15, Wolk D, Mitchell S, Patel R. Principles of molecular and microbiology testing methods, pp 1157-1204, copyright 2001, with permission from Elsevier.
      Molecular testing for vanA and vanB may not always detect VRE (ie, if the bacteria are present in very low quantities). Furthermore, organisms other than enterococci (such as enteric anaerobes) can carry these van genes, leading to false-positive results.
      • Ballard SA
      • Pertile KK
      • Lim M
      • Johnson PD
      • Grayson ML
      Molecular characterization of vanB elements in naturally occurring gut anaerobes.
      Therefore, some positive results may not actually represent the presence of VRE. The frequency with which this phenomenon occurs is unknown, but it is considered rare.
      In theory, PCR testing should decrease the spread of VRE by more rapid identification and earlier isolation of colonized patients. However, the cost-effectiveness of this practice is unknown because some PCR-positive patients have minimal bacterial burden and thus present a low risk for the spread of VRE.

      INDICATIONS FOR VRE CULTURES

      Conventional aerobic bacterial cultures are used to isolate VRE from clinical specimens such as blood for the diagnosis of VRE infection. This approach allows antimicrobial susceptibility testing for selection of appropriate antimicrobial treatment.
      Culture for VRE is also required to “fingerprint” isolates in outbreak investigations. Vancomycin-resistant enterococci are genetically diverse. Nosocomial outbreaks may be monoclonal, oligoclonal, or polyclonal; certain clones may establish themselves as endemic strains. Pulsed-field gel electrophoresis is used commonly to evaluate clonality. With use of this molecular technique, the DNA of the bacterium is “cut” into large fragments (>30 kilobase) using restriction enzymes. The fragments are then separated in a gel using electrophoresis with periodic reorientation of the electric field. Smaller DNA pieces move faster, larger pieces lag behind, and ultimately, the DNA separates into multiple bands on the gel. Band patterns from different isolates can then be compared to assess whether they are similar. Indistinguishable band patterns suggest the possibility that 2 isolates are clones of one another, while different patterns imply a more distant relationship.
      • Cheng AC
      • Murdoch DR
      • Harrell LJ
      • Barth Reller L
      Clinical profile and strain relatedness of recurrent enterococcal bacteremia.

      CONCLUSION

      Colonization with VRE occurs in susceptible hosts when colonization pressure is high. Real-time PCR analysis of perianal, perirectal, or rectal swab specimens allows for rapid detection of colonization. Infection, with associated high mortality rates, may follow colonization in high-risk individuals. Culture techniques are used to isolate VRE in order to diagnose infection, assess antimicrobial susceptibility, and identify clonality in the case of nosocomial outbreaks. Linezolid is the antimicrobial used most commonly to treat infection with VRE. Other antimicrobials such as quinupristin-dalfopristin, daptomycin, tigecycline, and nitrofurantoin are also prescribed. Currently, no accepted treatment for colonization has been determined.
      Knowledge about VRE is important for all health care professionals. With their increasing prevalence, capacity for prolonged survival in the environment, ability to over-come infection-control procedures, and capability of transferring vancomycin resistance to S aureus, VRE represent an important infectious disease threat.

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