Vancomycin-resistant enterococci

Overview
Enteroccocci are bacteria that are normally present in the human intestines and in the female genital tract and are often found in the environment. These bacteria can sometimes cause infections. Vancomycin is an antibiotic that is often used to treat infections caused by enterococci. In some instances, enterococci have become resistant to this drug and thus are called vancomycin-resistant enterococci (VRE). Most VRE infections occur in hospitals.

Epidemiology and Demographics
VRE Among ICU Patients, 1995-2004:

http://www.cdc.gov/ncidod/dhqp/pdf/ar/ICU_RESTrend1995-2004.pdf

Vancomycin-resistant enterococci (VRE), first reported in Europe in 1988, are emerging as a global threat to public health. The incidence of VRE infection and colonization among hospitalized patients has increased rapidly in the last 7 years. From 1989, the year VRE was first identified in the United States, through 1993, the proportion of enterococcal isolates resistant to vancomycin reported to the National Nosocomial Infections Surveillance System increased 20-fold. Infection with VRE may be associated with increased mortality, and no effective antimicrobial therapy is available for many VRE.

Multiple factors predispose a person to infection with VRE, but colonization precedes most infections. In the United States, nosocomial transmission of VRE from patient to patient has been emphasized. Although VRE introduced into hospitals by colonized patients from VRE-endemic settings has been reported, it is unclear how VRE is first introduced into most U.S. hospitals.

Although no data so far support significant acquisition and transmission of VRE outside the health-care setting in the United States, a growing number of reports from Europe suggest that colonization with VRE frequently occurs in the community. Reports from Europe also have suggested that VRE exist elsewhere in the environment, including animal feces and human foods of animal origin. Additional evidence supports the transmission of VRE to persons in contact with these sources, resulting in an increased human reservoir of VRE colonization. If VRE is frequently introduced into health-care settings from community sources, its control will require community-based initiatives, unlike measures used to control nosocomial pathogens.

This review summarizes the existing evidence for community acquisition and transmission of VRE outside the health-care setting in Europe, relates these findings to the epidemiology of VRE in the United States, discusses steps to stem community transmission of VRE, and identifies research needs.

Epidemiology:

Enterococci account for approximately 110,000 urinary tract infections, 25,000 cases of bacteremia, 40,000 wound infections, and 1,100 cases of endocarditis annually in the United States. Most infections occur in hospitals. Although several studies have suggested an increase in nosocomial infection rates for enterococci in recent years, National Nosocomial Infections Surveillance system data show little change in the percentage of enterococcal bloodstream (12% vs. 7%), surgical site (15% vs. 11%), and urinary tract (14% vs. 14%) infections over the past 2 decades. Adequate surveillance data prior to 1980 are not available. Enterococcal infection deaths have also been difficult to ascertain because severe comorbid illnesses are common; however, enterococcal sepsis is implicated in 7% to 50% of fatal cases. Several case-control and historical cohort studies show that death risk associated with antibiotic-resistant enterococcal bacteremia is severalfold higher than death risk associated with susceptible enterococcal bacteremia. This trend will likely increase as multiple drug resistant (MDR) isolates become more prevalent.

Colonization and infection with MDR enterococci occur worldwide. Early reports showed that in the United States, the percentage of nosocomial infections caused by VRE increased more than 20-fold (from 0.3% to 7.9%) between 1989 and 1993, indicating rapid dissemination. New database technologies, such as The Surveillance Network (TSN) Database-USA, now permit the assessment of resistance profiles according to species. TSN Database electronically collects and compiles data daily from more than 100 U.S. clinical laboratories, identifies potential laboratory testing errors, and detects emergence of resistance profiles and mechanisms that pose a public health threat (e.g., vancomycin-resistant staphylococci).

Data collected by the TSN Database between 1995 and September 1, 1997 were analyzed to determine whether the earlier increase in vancomycin resistance was unique to vancomycin, whether it represented a continuing trend, and whether speciation is quantifiably important in analyzing this trend. E. faecalis resistance to ampicillin and vancomycin is uncommon; little change in resistance prevalence occurred from 1995 to 1997. In contrast, E. faecium vancomycin and ampicillin resistance increased alarmingly. In 1997, 771 (52%) of 1,482 of E. faecium isolates exhibited vancomycin resistance, and 1,220 (83%) of 1,474 isolates exhibited ampicillin resistance. E. faecium resistance notwithstanding, E. faecalis remained by far the most commonly encountered of the two enterococcal species in TSN Database. E. faecalis to E. faecium total isolates were approximately 4:1, blood isolates 3:1, and urine isolates 5:1. This observation underscores important differences in the survival strategies and likelihood of therapeutic success, critical factors usually obscured by lumping the organisms together as Enterococcus species or enterococci. Widespread emergence and dissemination of ampicillin and vancomycin resistance in E. faecalis would significantly confound the current therapeutic dilemma. There is little reason to suspect that vancomycin and ampicillin resistances only provide selective advantage for the species faecium and not faecalis. The relative absence of these resistances in E. faecalis may simply reflect a momentary lack of penetrance and equilibration of the traits. Because of these important differences between the two species, meaningful surveillance of enterococcal resistance must include species identification.

Although exact modes of nosocomial transmission for MDR enterococci are difficult to prove, molecular microbiologic and epidemiologic evidence strongly suggest spread between patients, probably on the hands of health-care providers or medical devices, and between hospitals by patients with prolonged intestinal colonization. At least 16 outbreaks of MDR enterococci have been reported since 1989; all but two were due to E. faecium. This disparity, particularly in view of the higher numbers of clinical E. faecalis isolates, may reflect a reporting bias due to the novelty of the combinations of resistance that occur in E. faecium. When isolates from outbreaks of MDR enterococci have been analyzed by genetic fingerprints, more than half involve clonally related isolates.

Prior treatment with antibiotics is common in nearly all patients colonized or infected with MDR enterococci. Clindamycin, cephalosporin, aztreonam, ciprofloxacin, aminoglycoside, and metronidazole use is equally or more often associated with colonization or infection with MDR enterococci than vancomycin use. Other risk factors include prolonged hospitalization; high severity of illness score; intraabdominal surgery; renal insufficiency; enteral tube feedings; and exposure to specific hospital units, nurses, or contaminated objects and surfaces within patient-care areas.

Risk Factors
The following persons are at an increased risk becoming infected with VRE:


 * Persons who have been previously treated with vancomycin and combinations of other antibiotics such as penicillin and gentamicin.
 * Persons who are hospitalized, particularly when they receive antibiotic treatment for long periods of time.
 * Persons with weakened immune systems such as patients in Intensive Care Units, or in cancer or transplant wards.
 * Persons who have undergone surgical procedures such as abdominal or chest surgery.
 * Persons with medical devices that stay in for some time such as urinary catheters or central intravenous catheters.

Screening
How should clinical laboratory personnel screen for VRE?

Screening for VRE can be accomplished in a number of ways. For inoculating peri-rectal/anal swabs or stool specimens directly, one method uses bile esculin azide agar plates containing 6 µg/ml of vancomycin. Black colonies should be identified as an enterococcus to species level and further confirmed as vancomycin resistant by an MIC method before reporting as VRE.

Vancomycin resistance can be determined for enterococcal colonies available in pure culture by inoculating a suspension of the organism onto a commercially available brain heart infusion agar (BHIA) plate containing 6 µg/ml vancomycin. The National Committee for Clinical Laboratory Standards (NCCLS) recommends performing a vancomycin MIC test and also motility and pigment production tests to distinguish species with acquired resistance (vanA and vanB) from those with vanC intrinsic resistance.

When should clinical laboratory personnel screen for VRE?

The decision about who and when to screen for VRE is a facility-specific decision. CDC recommendations can assist in the determination of a screening strategy appropriate for health care facilities (Recommendations for Preventing the Spread of Vancomycin Resistance Recommendations of the Hospital Infection Control Practices Advisory Committee (HICPAC) MMWR 1995; 44(RR12):1-13). Infection control personnel at some healthcare facilities selectively screen newly admitted or high-risk patients (e.g., intensive care, oncology, and surgery patients) determined to be at greater risk for VRE colonization.

Why is the difference between colonization and infection important for VRE screening?

Infected patients carry VRE and show clinical signs or symptoms of disease. Colonized patients carry VRE but do not have clinical signs or symptoms of infection. This distinction is important in VRE screening. Patients are usually colonized in the gastrointestinal tract and occasionally in the urinary tract. VRE colony counts are similar in the stool of colonized or infected patients. If a hospital VRE rate is based solely on VRE isolated from clinical cultures (infected patients), the facility may be adequately reporting its infection rate, but may be underestimating the true burden (and therefore potential transmissibility) of VRE in the facility. Screening for patients colonized by VRE provides information about potential sources of illness. The goal of screening is to identify as many colonized patients as possible so that infection control measures can be implemented to decrease transmission and reduce the number of patients infected with VRE.

Pathophysiology & Etiology
VRE is usually passed to others by direct contact with stool, urine or blood containing VRE. It can also be spread indirectly via the hands of healthcare providers or on contaminated environmental surfaces. VRE usually is not spread through casual contact such as touching or hugging. VRE is not spread through the air by coughing or sneezing.

Implications:

If community transmission is important in the global spread of VRE, factors leading to its emergence in this setting must be examined, and measures must be taken to control transmission. In response to data linking the use of avoparcin with the emergence of VRE in the food chain and potential transmission to humans, Denmark (1995) and Germany (1996) imposed bans on the use of avoparcin at subtherapeutic doses in food animals for growth promotion. This action by two member states has been followed recently by a European Unionwide ban on avoparcin (41). Avoparcin has never been licensed for use in the United States or Canada because of its carcinogenic potential; illegal use, however, has been reported.

Feed additive manufacturers have resisted proposals to ban the use of avoparcin at subtherapeutic doses for food animal growth promotion in Europe. Opponents of the ban have suggested that the relatively low incidence of human VRE infections in European countries where avoparcin has been used for many years, compared with the high rate in the United States, where avoparcin is not used, is inconsistent with the hypothesis that avoparcin is a major factor in the emergence of VRE. However, profound differences may exist between the United States and European countries in the amount of glycopeptides used in health-care settings.

Vancomycin use in U.S. hospitals has increased dramatically in the past 10 to 15 years because of a variety of factors, including increases in the incidence of methicillin-resistant staphylococci, prosthetic device-related infections, Clostridium difficile colitis, and inappropriate use of the drug. Although vancomycin and other glycopeptide use in European health-care settings has not been similarly documented, this marked increase in human glycopeptide use is thought to be primarily a U.S. phenomenon. Because the use of vancomycin and other antimicrobial drugs is an important risk factor for human VRE infection, if glycopeptides were prescribed in European hospitals at levels common in U.S. hospitals, there might be an even greater incidence of VRE infections in Europe. Likewise, given the suspected greater use of glycopeptides in U.S. hospitals, if community carriage of VRE were to increase in the United States, there might be an even greater incidence of VRE infections in U.S. hospitals.

Although the exact role of human antimicrobial use in the transmission of VRE is not known, observations from an animal model (in which mice were orally administered VRE and became only transiently colonized unless simultaneously exposed to antimicrobials) support an important role for antimicrobial drugs in establishing persistent colonization. Antimicrobial drugs used in health-care settings may alter bowel flora, rendering patients more susceptible to colonization by VRE transmitted from other colonized or infected patients. Epidemiologic evidence of foodborne VRE transmission in the community suggests that antimicrobial drugs may predispose hospitalized patients to colonization with ingested VRE. Contamination of a patient's food may occur during consumption by a variety of mechanisms, including contamination with VRE from the hands of the patient or health-care worker. In areas where VRE is also found in the animal food supply, contamination may also occur during processing by contact with VRE from the bowel flora of the food animal.

What are the types of vancomycin resistance in enterococci? There are the two types of vancomycin resistance in enterococci. The first type is intrinsic resistance. Isolates of Enterococcus gallinarum and E. casseliflavus/E. flavescens demonstrate an inherent, low-level resistance to vancomycin.

The second type of vancomycin resistance in enterococci is acquired resistance. Enterococci can become resistant to vancomycin by acquisition of genetic information from another organism. Most commonly, this resistance is seen in E. faecium and E. faecalis, but also has been recognized in E. raffinosus, E. avium, E. durans, and several other enterococcal species.

Several genes, including vanA, vanB, vanC, vanD, and vanE, contribute to resistance to vancomycin in enterococci.

Habitat and Microbiology:

Enterococci normally inhabit the bowel. They are found in the intestine of nearly all animals, from cockroaches to humans. Enterococci are readily recovered outdoors from vegetation and surface water, probably because of contamination by animal excrement or untreated sewage. In humans, typical concentrations of enterococci in stool are up to 108 CFU per gram. Although the oral cavity and vaginal tract can become colonized, enterococci are recovered from these sites in fewer than 20% of cases. The predominant species inhabiting the intestine varies. In Europe, the United States, and the Far East, Enterococcus faecalis predominates in some instances and E. faecium in others. Ecologic or microbial factors promoting intestinal colonization are obscure. Of 14 or more enterococcal species, only E. faecalis and E. faecium commonly colonize and infect humans in detectable numbers. E. faecalis is isolated from approximately 80% of human infections, and E. faecium from most of the rest. Infections to other enterococcal species are rare.

Enterococci are exceedingly hardy. They tolerate a wide variety of growth conditions, including temperatures of 10°C to 45°C, and hypotonic, hypertonic, acidic, or alkaline environments. Sodium azide and concentrated bile salts, which inhibit or kill most microorganisms, are tolerated by enterococci and used as selective agents in agar-based media. As facultative organisms, enterococci grow under reduced or oxygenated conditions. Enterococci are usually considered strict fermenters because they lack a Kreb's cycle and respiratory chain. E. faecalis is an exception since exogenous hemin can be used to produce d, b, and o type cytochromes. In a survey of 134 enterococci and related streptococci, only E. faecalis and Lactococcus lactis expressed cytochrome-like respiration. Cytochromes provide a growth advantage to E. faecalis during aerobic growth. E. faecalis cytochromes are only expressed under aerobic conditions in the presence of exogenous hemin and, therefore, may promote the colonization of inappropriate sites.

Enterococci are intrinsically resistant to many antibiotics. Unlike acquired resistance and virulence traits, which are usually transposon or plasmid encoded, intrinsic resistance is based in chromosomal genes, which typically are nontransferrable. Penicillin, ampicillin, piperacillin, imipenem, and vancomycin are among the few antibiotics that show consistent inhibitory, but not bactericidal, activity against E. faecalis. E. faecium are less susceptible to ß-lactam antibiotics than E. faecalis because the penicillin-binding proteins of the former have markedly lower affinities for the antibiotics. The first reports of strains highly resistant to penicillin began to appear in the 1980s.

Enterococci often acquire antibiotic resistance through exchange of resistance-encoding genes carried on conjugative transposons, pheromone-responsive plasmids, and other broad-host-range plasmids. The past two decades have witnessed the rapid emergence of MDR enterococci. High-level gentamicin resistance occurred in 1979 and was quickly followed by numerous reports of nosocomial infection in the 1980s. Simultaneously, sporadic outbreaks of nosocomial E. faecalis and E. faecium infection appeared with penicillin resistance due to ß-lactamase production; however, such isolates remain rare. Finally, MDR enterococci that had lost susceptibility to vancomycin were reported in Europe and the United States.

Among several phenotypes for vancomycin-resistant enterococci, VanA (resistance to vancomycin and teicoplanin) and VanB (resistance to vancomycin alone) are most common. In the United States, VanA and VanB account for approximately 60% and 40% of vancomycin-resistant enterococci (VRE) isolates, respectively. Inducible genes encoding these phenotypes alter cell wall synthesis and render strains resistant to glycopeptides.

VanA and VanB types of resistance are primarily found among enterococci isolated from clinical, veterinary, and food specimens, but not other common intestinal or environmental bacteria. In the laboratory, however, these genes can be transferred from enterococci to other bacteria. For example, Staphylococcus aureus has been rendered vancomycin-resistant through apparent transfer of resistance from E. faecalis on the surface of membrane filters and on the skin of hairless obese mice, which indicates that there is no biologic barrier to the emergence of vancomycin-resistant S. aureus. Clinical isolates of highly vancomycin-resistant S. aureus have yet to be identified, although strains with reduced susceptibility to vancomycin have appeared. The mechanism of resistance for these strains remains undetermined but does not appear to involve genes associated with VanA or VanB phenotypes.

Laboratory Findings
What are typical vancomycin MICs (phenotypes) for various species of VRE?

E. faecium is the most frequently isolated species of VRE in hospitals and typically produces high vancomycin (>128 µg/ml) and teicoplanin (>16 µg/ml) minimum inhibitory concentrations (MICs). These isolates typically contain vanA genes. A vanB-containing isolate typically produces lower level resistance to vancomycin (MICs 16 to 64 µg/ml) and is susceptible to teicoplanin (MICs <1 µg/ml). Recently, a few vanD--containing isolates of E. faecium with a moderate level of resistance to vancomycin (MICs 64 to 128 µg/ml) and teicoplanin (MICs 4-8 µg/ml) have been reported, as has a novel vanE-containing E. faecalis.

E. gallinarum and E. casseliflavus/E. flavescens isolates are intrinsically resistant to vancomycin. These isolates contain vanC genes that typically produce vancomycin MICs of 2 to 16 µg/ml.

Is identification of VRE to species level important?

Yes. Identification of VRE to species level aids in confirming whether an isolate has intrinsic (vanC) or acquired resistance (vanA or vanB). Knowledge of the type of resistance is critical for infection control purposes. vanA and vanB genes are transferable and can spread from organism to organism. In contrast, vanC genes are not transferable, have been associated less commonly with serious infections, and have not been associated with outbreaks.

For species differentiation, motility and pigment tests are easily performed and are important tests to distinguish among species phenotypically. E. faecium and E. faecalis are non-motile, whereas E. gallinarum and E. casseliflavus/E. flavescens generally are motile. Most isolates of E. casseliflavus/E. flavescens have a distinct yellow pigment, which can be observed by collecting growth from an agar plate on a swab. In addition to motility and pigment tests, an organism's susceptibility profile also helps differentiate vanA and vanB isolates from vanC isolates.

Treatment
Most VRE infections can be treated with antibiotics other than vancomycin. The treatment of VRE is determined by laboratory testing to determine which antibiotics are effective. For persons who get VRE infections and have urinary catheters, removal of the catheter when it is no longer needed can help getting rid of the infection. People who are colonized (bacteria are present, but have no symptoms of an infection) with VRE do not usually need treatment.

Primary Prevention
If you or someone in your household has VRE, the following are some measures to prevent spread of VRE:


 * Always wash your hands thoroughly after using the bathroom and before preparing food. Clean your hands after close contact with persons who have VRE. Wash with soap and water (particularly when visibly soiled) or clean with alcohol-based hand cleaner.
 * Frequently clean areas of your home such as your bathroom that may become contaminated with VRE. Use a household disinfectant or a mixture of one-fourth cup bleach and one quart of water to clean those areas and surfaces that are touched frequently.
 * Wear gloves if you may come in contact with body fluids that may contain VRE, such as stool. Always wash your hands after removing gloves.
 * Be sure to tell any healthcare providers that you have VRE so that they are aware of your infection.

Acknowledgements
The content on this page was first contributed by: C. Michael Gibson, M.S., M.D.