Supported by an unrestricted educational grant from Glaxo Wellcome Inc

Volume IV, Issue 3 - April 1998

Enterococcal Resistance

Enterococci are gram-positive cocci that grow in chains in broth media and clinical specimens. They are indistinguishable microscopically from streptococci and were originally classified as group D streptococci under the old Lancefield classification. However, enterococci are genetically quite different from true streptococci and, for that reason, been classified as a separate genus (the genus enterococcus). This genus now contains more than a dozen species but only a relatively small number of these are important as human pathogens. A recent study of bloodstream isolates of enterococci in the United States (US) confirms that E. faecalis are still the most frequent cause of enterococcal infections in man, followed by E. faecium (Table 1). The data in Table 1 document a clear-cut decrease in the overall percentage of infections caused by E. faecalis and a marked increase in those caused by E. faecium compared with similar figures from a decade or so previously. This clearly reflects the emergence of multiresistant strains of E. faecium as major nosocomial pathogens in US hospitals.

Enterococci are part of the normal flora of man and animals. They are usually thought to be harmless commensals and preparations containing enterococci (especially E. faecium) are available in health food stores in Europe and elsewhere as "probiotics" because they are thought to have health-promoting properties as part of normal gut flora. Under appropriate circumstances, however, enterococci can cause serious infections in humans. Data on virulence factors, cellular receptors, colonization factors, and the like for enterococci are still rudimentary but it is clear these organisms do not produce potent exotoxins like staphylococci and group A streptococci, do not contain lipopolysaccharide endotoxins (although the lipoteichoic acids in their outer cell wall can elicit cytokine responses in mammalian species), and do not produce enzymes known to facilitate tissue invasion. Despite this, enterococci have steadily grown to be important pathogens, especially among hospitalized patients. In recent surveys of US nosocomial infections, enterococci have consistently ranked in among the top 3 or 4 most frequent pathogens causing significant hospital-acquired infections. The most common nosocomial infections produced by these organisms are urinary tract infections (UTIs), followed by intraabdominal and pelvic infections (where they usually are present as part of a mixed abdominal flora). These organisms also cause surgical wound infections and are not infrequently found as part of the mixed flora in decubitus ulcers and diabetic foot infections. Enterococcal bacteremia is most frequently related to ascending UTIs or to abdominal or pelvic sepsis. In addition, we are now seeing more frequent evidence that intravenous and intraarterial lines may serve as portals of entry into the bloodstream for enterococci, especially in immunocompromised patients. Endocarditis and meningitis are less common but more serious infections caused by the enterococcus. Meningitis may be secondary to bacteremic spread or may complicate neurosurgical procedures or penetrating wounds into the cerebrospinal fluid. These organisms rarely, if ever, cause primary cellulitis or respiratory tract infections. Enterococci are becoming increasingly important as pathogens of immunocompromised patients, especially those undergoing liver or bone marrow transplantation, neutropenic cancer patients, and patients with chronic renal failure on dialysis.

In many of the above settings, the frequency of enterococcal infections appears to be increasing. Since these organisms are not particularly virulent, another reason for this phenomenon must be sought. It is relatively easily found. Enterococci have a remarkable ability to survive in an environment of heavy antibiotic use such as hospital intensive care units. Indeed, it is the resistance of these organisms to multiple antimicrobial agents that makes them such feared opponents (Table 2). Enterococci begin with intrinsic resistance to most ß-lactam antibiotics because they contain penicillin binding proteins (PBPs), especially low molecular weight PBPs such as PBP 5, which enable them to synthesize cell wall components even in the presence of modest concentrations of most ß-lactam antibiotics. They also exhibit low levels of "intrinsic" resistance to aminoglycosides and lincosamides such lincomycin and clindamycin. Although most enterococci are susceptible to co-trimoxazole in vitro, this combination does not work in vivo because enterococci are able to incorporate pre-formed folic acid which enables them to bypass the inhibition of folate synthesis produced by co-trimoxazole.

In addition to the above, E. faecium that have acquired high-level resistance to ß-lactams and ß-lactamase-producing strains of E. faecalis have also been described. Resistance to macrolides, lincosamides, streptogramins, tetracyclines, fluoroquinolones, rifampin, aminoglycosides (high level), and glycopeptides has also been acquired over the past 50 years by enterococci. The fact that these organisms are normal inhabitants of the gastrointestinal (GI) tract brings them in close contact with billions of bacteria that may contain resistance genes that can be transferred to enterococci via mechanisms such as conjugation in which plasmids or conjugative transposons are exchanged. There are clear-cut data demonstrating that enterococci have also acquired resistance genes from gram-positive organisms such as staphylococci which are not part of the normal GI tract flora.

One of the most interesting characteristics of enterococci is that the majority of them are "tolerant" to the activity of cell-wall-active antimicrobial agents. By this we mean that enterococci are inhibited but not killed by these agents. This property, too, is an acquired characteristic since enterococci from antibiotic-virgin populations are rapidly killed (and lysed) by penicillin. Enterococci quickly develop tolerance after exposure to as few as 5 doses (pulses) of penicillin. Because most enterococci are "tolerant" to cell-wall-active agents, penicillins or glycopeptides alone often fail to cure infections such as endocarditis and meningitis which require bactericidal therapy. This phenomenon has been known since the 1940s when it was first discovered that combinations of penicillin plus streptomycin produced bactericidal killing of enterococci. The mechanism of penicillin-aminoglycoside synergism has clearly been elucidated and it is related to the fact that cell-wall-active agents markedly enhance the intracellular uptake of aminoglycosides, allowing the latter to bind to enterococcal ribosomes where they exert a bactericidal action. Unfortunately, enterococci can develop high-level resistance to streptomycin via chromosomal mutation which alters ribosomal binding of streptomycin or via the acquisition of genes that encode a nucleotidyl transferase which inactivates streptomycin. Strains of enterococci with high level resistance to streptomycin are not necessarily highly resistant to gentamicin and other aminoglycosides and, in recent years, penicillin (or ampicillin) plus gentamicin has become the standard of therapy for enterococcal endocarditis, meningitis, and other serious infections requiring bactericidal therapy. Unfortunately, the l980s and 1990s have seen a marked worldwide increase in strains of enterococci with genes that encode a bifunctional phosphotransferase/acetyltransferase enzyme that inactivates gentamicin and all other currently available aminoglycosides except streptomycin. Such organisms are not killed synergistically by combinations of gentamicin plus cell-wall-active antibiotics. Moreover, there are a number of descriptions of failures of penicillin-aminoglycoside combinations to cure endocarditis due to such organisms.

Enterococci which will not be killed synergistically by penicillin-aminoglycoside combinations can be readily detected in the clinical microbiology laboratory by screening for high-level (minimal inhibitory concentration [MIC] 1000 µg/mL) resistance to streptomycin and or high-level (MIC 500 µg/mL) resistance to gentamicin. Another disturbing event has occurred among E. faecium strains in the US and elsewhere. Increasing resistance to penicillin and ampicillin in these organisms has clearly been documented and we are now seeing many strains of E. faecium that can not be inhibited or killed by concentrations penicillin or ampicillin of 250 µg/mL or greater! For such organisms, the only other viable therapeutic option is vancomycin (or teicoplanin in countries outside the US where it is available).

It was not surprising that considerable consternation greeted the first reports of the appearance of vancomycin-resistant enterococci (VRE) in Europe in the mid-1980s. Initially described in the United Kingdom, France, Germany, and Spain, these organisms have rapidly spread to the US, where they are becoming a significant clinical problem. Once the mechanism of vancomycin resistance in enterococci was elucidated, it became clear that development of resistance to glycopeptides represented a major genetic achievement in these organisms. Indeed, the segment of DNA encoding vancomycin resistance for the most common type of vancomycin resistance contains 9 separate genes, at least 5 of which are absolutely essential for the inducible production of vancomycin resistance. At the present time, 4 different phenotypes of vancomycin resistance are recognized opponents (Table 3). These are designated VanA, VanB, VanC, and VanD. The majority (70% to 80%) of VRE in the US exhibit the VanA phenotype and are highly resistant to both vancomycin and teicoplanin. Organisms of the VanB phenotype make up about 20% of resistant enterococci. The VanC phenotype is chromosomally mediated and is limited to E. gallinarum, E. casseliflavus, and E. flavescens. Thus far, only a single isolate with the VanD phenotype has been reported in the literature but at least 3 others are known to exist in the US..

The past decade has seen a remarkable increase in numbers of VRE in the US. In 1986 no vancomycin-resistant isolates were found in the US. There has been a steady increase in such isolates since then and by 1996, national surveys had demonstrated that 16% of all enterococci in the US were resistant to vancomycin. There is a marked difference in the prevalence of vancomycin resistance in strains of E. faecalis (3% to 5% of which are currently resistant to vancomycin) and E. faecium (46% of which are resistant to vancomycin) in the US. Initially, there was a marked predominance of VRE in isolates from intensive care units as opposed to other hospital sites. This has now evened out in more recent surveys, and VRE are clearly finding their way into nursing homes and the community as well. Risk factors for colonization or infection with VRE include both heavy use of antimicrobial agents (especially vancomycin, third-generation cephalosporins, and antimicrobial agents with activity against anaerobes) and a variety of nonantimicrobial factors including prior GI colonization with VRE, increased length of hospital stay, proximity to a case, care for a case by health care worker with GI colonization with VRE, and immunosuppression opponents (Table 4).

As VRE become more and more frequent in the US, reports of serious infections caused by these organisms are likewise increasing. Of greatest concern are strains of E. faecium that have high-level resistance to ß-lactams (including ampicillin and penicillin), high-level aminoglycoside resistance, and resistance to vancomycin. For such organisms there is essentially no effective treatment which yields bactericidal activity. Current recommendations for management of patients with VRE are given in Table 5.

Fortunately, all strains of glycopeptide-resistant E. faecalis in the US have thus far remained moderately susceptible to penicillin and ampicillin and these agents represent the drugs of choice for treating serious infections due to vancomycin-resistant E. faecalis. If the organisms are not highly resistant to streptomycin and/or gentamicin, one of these agents can be used in combination to produce a synergistic bactericidal effect. Most strains of E. faecalis (and E. faecium as well) remain susceptible to nitrofurantoin and this agent can be used for UTIs due to these organisms. Vancomycin-resistant strains of E. faecium for which the MICs of penicillin or ampicillin are 32 µg/mL or less may be treated with very high doses of penicillin or ampicillin (up to 300 mg/kg per day of ampicillin in patients with normal renal function). There are anecdotal reports of responses to such regimens in patients infected with strains of E. faecium for which the MIC of ampicillin was as high as 64 µg/mL. The addition of an aminoglycoside (if the organism does not exhibit high-level resistance to it) would obviously add to the efficacy of such a regimen. For glycopeptide-resistant strains of E. faecium exhibiting very high levels of resistance to penicillin, ampicillin, and aminoglycosides, there are few if any regimens demonstrated to be efficacious. Some of these strains remain susceptible to tetracyclines, erythromycin, chloramphenicol, fluoroquinolones, novobiocin, or rifampin and physicians have used these agents (usually combining 2 or 3 which exhibit in vivo efficacy) in an attempt to treat such infections. Parenthetically, the results of treatment with chloramphenicol, even for chloramphenicol-susceptible strains of enterococci, have been disappointing to date. An investigational streptogramin, quinupristin/dalfopristin (Synercid) is in the final stages of clinical investigation in the US. This agent has activity against E. faecium but not E. faecalis and has been used to treat a number of patients infected with multiresistant, vancomycin-resistant E. faecium. The overall results of clinical trials to date suggest that it is efficacious in about 70% of cases. There are, however, documented patients in whom vancomycin-resistant E. faecium bacteremia has persisted, despite the administration of quinupristin/dalfopristin. This agent is generally only bacteriostatic against E. faecium and, therefore, it should be used with caution in patients with endocarditis and/or meningitis. Despite these misgivings, there are several documented cures of both E. faecium endocarditis and E. faecium meningitis with quinupristin/dalfopristin. Adverse effects include thrombophlebitis as well as arthritis and arthralgias, which appear reversible once the drug is stopped. It is expected that this drug may be approved by the US Food and Drug Administration in early 1998. It is currently available on a compassionate use protocol from Rhone-Poulenc Rorer Pharmaceuticals in Collegeville, Pennsylvania.

Teicoplanin has been used in experimental animals and some patients infected with enterococci exhibiting the VanB phenotype. When it is used alone in infections due to VanB VRE, however, there has been emergence of teicoplanin resistance during therapy. It appears clear, therefore, that this agent should be used in combination with at least one other effective agent, preferably an aminoglycoside if the organisms do not exhibit high-level aminoglycoside resistance. Unfortunately, teicoplanin is not currently available, even for compassionate use, in the US.

There are anecdotal reports of successful use of multiple drug combinations including rifampin, ciprofloxacin, and gentamicin; combinations of cell-wall-active agents with vancomycin; and other combinations of agents in an attempt to manage infections caused by multiply resistant, glycopeptide-resistant enterococci. At present there are not enough clinical data available to evaluate the efficacy of any these regimens. Paradoxically, combinations of penicillin or ampicillin with vancomycin do exhibit bacteriostatic "synergy" against some strains of VRE. However, when such combinations have been used clinically and in experimental animals as well, organisms resistant to this synergism have arisen during therapy.

A number of experimental drugs are currently being evaluated for activity against VRE. Of these, 3 agents are currently undergoing clinical trials in various stages of development. These include the oxazolidinone Linezolid (Upjohn-Pharmacia), the everninomycin Zeracin (Schering), and the glycopeptide LY333328 (Eli Lilly & Company). It is currently too early to tell whether any of these agents will have great utility in the management of infections due to VRE.

Currently, our best approach to the problems posed by multiresistant enterococci is to do all in our power to limit antibiotic use in hospital settings, since it is clear that this is a major contributor to the emergence of multiresistant organisms such as these. In addition, identification and institution of appropriate infection control methods are important in our management of infections due to multiresistant enterococci. Unfortunately, infection control has not proven capable of solving the problem of VRE in US hospitals to date. More work in this area is clearly needed to solve the problem of multiresistant enterococci.

Suggested Reading

0. Principles and Practice of Infectious Disease (4th Ed). Churchill-Livingston, 1995:1826-1835.
1. Clin Microbiol Rev 1990;3:46-65.
2. J Infect Chemother 1997;3:1-4.
3. Antimicrob Agents Chemother 1991;35:2180-2184.
4. Am J Med 1997;102:284-293.
5. J Antimicrob Chemother. 1991; 28: 1-12.
6. Microbial Drug Resist 1996; 2:219-223.
7. Clin Infect Dis 1995;20:1126-1133.

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