Enterococcus, Streptococcus gallolyticus group (formerly Streptococcus bovis)

OVERVIEW: What every clinician needs to know

Pathogen name and classification

Enterococci are facultatively anaerobic gram-positive cocci from the family Enterococcaceae. It is worth noting that these organisms were previously classified as belonging to the genus “Streptococcus” but were placed in their own genus, “Enterococcus,” after genetic analysis (DNA-DNA reassociation analysis and 16S rRNA sequencing) showed they were too different to be grouped with the streptococci. Although more than 15 species have been associated with human infections, the vast majority are caused by two species: Enterococcus faecalis and E. faecium.

The Streptococcus bovis-group of organisms belongs to the “Streptococcaceae” family, and are gram-positive, catalase-negative cocci usually visualized as short chains in Gram stains. The taxonomy of these organisms has been changed, and the former S. bovis biotypes have been renamed Streptococcus gallolyticus subsp. gallolyticus (formerly, S. bovis biotype I), S. infantarius subsp. infantarius (S. bovis biotype II/1), and S. gallolyticus subsp. pasteurianus (S. bovis biotype II/2). They are typically referred as S. gallolyticus, S. infantarius and S. pasteurianus, respectively.

What is the best treatment?

The most active β-lactams include the amino-penicillins (ampicillin) followed by penicillin, piperacillin, and carbapenems (imipenem is the most active, but meropenem and doripenem also have good activity in vitro against ampicillin-susceptible enterococci, although breakpoints have not been established). High-dose ampicillin (up to 30g/day) could be considered (in combination) for isolates with minimal inhibitory concentrations (MICs) of ampicillin greater than or equal to 16 to less than or equal to 64μg/ml.

For severe enterococcal infections caused by E. faecalis and ampicillin-susceptible E. faecium (i.e., endocarditis and other endovascular infections), monotherapy with ampicillin is not recommended, since cure rates are not optimal and the compound alone may not be bactericidal for many enterococcal strains. The addition of an aminoglycoside (gentamicin or streptomycin) can achieve effective synergistic bactericidal therapy if the organism does not exhibit high-level resistance to the aminoglycoside. The use of aminoglycosides other than gentamicin and streptomycin is discouraged, since it is difficult to predict their synergistic activity against clinical isolates of E. faecalis or E. faecium. Additionally, the combination of ampicillin with ceftriaxone or cefotaxime has also been shown to be synergistic for E. faecalis due to differential saturation of penicillin binding proteins.

Vancomycin is a cell-wall active agent that has good activity against many enterococci and is the usual alternative to ampicillin to treat enterococcal infections (if susceptible). Synergistic bactericidal therapy is also achieved with aminoglycosides, and the combination could be considered for the treatment of severe infections, including endocarditis and other endovascular infections, when ampicillin cannot be used.

The only US Food and Drug Administration (FDA)-approved antibiotics to treat vancomycin-resistant E. faecium are quinupristin-dalfopristin (Q/D) and linezolid. However, linezolid is not bactericidal against enterococci, and its use as first-line therapy in severe endovascular infections is not recommended because of reports of therapeutic failures. Linezolid may be clinically useful in the treatment of enterococcal meningitis because of favorable pharmacokinetics. Q/D is a mixture of streptogramin antibiotics with in vitro bactericidal activity against many E. faecium (but not E. faecalis); however, the bactericidal effect may be compromised in isolates exhibiting macrolide resistance. Combining Q/D with other in vitro active agent(s) should be considered if this compound is used for the treatment of E. faecium enterococcal endovascular infections.

Daptomycin is bactericidal in vitro against ampicillin- and vancomycin-resistant E. faecalis and E. faecium. This compound is frequently used to treat vancomycin-resistant enterococci (VRE). However, the use of this antibiotic at FDA-approved doses (4 and 6mg/kg) has been associated with therapeutic failures and emergence of resistance during therapy. Therefore, higher doses (8-12mg/kg) and/or combination with other in vitro active agents (i.e., β-lactams, tigecycline, and aminoglycosides) should be considered when daptomycin is used against enterococci.

Tigecycline is a glycylcycline antibiotic with good in vitro activity against enterococci. However, its bacteriostatic activity and the low blood levels achieved at usual doses in humans preclude the use of this compound as monotherapy for the treatment of severe enterococcal infections. Tigecycline may offer promise as part of combination therapy with other agents.

Nitrofurantoin and fosfomycin are good alternatives for the treatment of uncomplicated urinary tract infections (UTI). Fosfomycin is FDA approved for vancomycin-susceptible E. faecalis. High-dose ampicillin may be useful in the treatment of uncomplicated UTI even in organisms exhibiting higher minimal inhibitory concentrations (MICs; up to 128 μg/ml and perhaps higher), since concentrations of ampicillin in the urine are usually high enough to achieve a therapeutic effect.

The preferred anti-infective agents for the treatment of S. gallolyticus, S. infantarius, and S. pasteurianus are the β-lactams. The American Heart Association recommends the use of ceftriaxone or penicillin G for 4 weeks or the combination of ceftriaxone and gentamicin for 2 weeks for endocarditis caused by organisms highly susceptible to penicillin (MIC <0.12μg/ml). The use of the 2-week regimen should be avoided in patients with cardiac or extracardiac abscesses or in those with impaired renal function (creatinine clearance <20μg/ml).

For endocarditis caused by S. bovis-group organisms with intermediate susceptibility to penicillin (MIC >0.12 to <0.5μg/ml), the American Heart Association recommends the use of ceftriaxone or the combination of penicillin or ceftriaxone for 4 weeks plus gentamicin for the first 2 weeks. For isolates with MICs greater than 0.5μg/ml, a regimen including penicillin or ceftriaxone plus gentamicin should be extended for 4 to 6 weeks.

For patients unable to tolerate a β-lactam, vancomycin is the preferred agent.

The most pressing issues regarding antibiotic resistance in modern-day enterococci involve the emergence of resistance to ampicillin, vancomycin, and high-level resistance (HLR) to aminoglycosides.

Ampicillin resistance is rare in E. faecalis (apart from a few outbreaks of β-lactamase producing isolates reported in the United States and Argentina in the 1980s). Similarly, vancomycin resistance is uncommon in clinical isolates of E. faecalis, although it appears to be increasing.

HLR to gentamicin and/or streptomycin in both E. faecalis and E. faecium appears to be increasing in many parts of the world and precludes the use of the combination of ampicillin and aminoglycosides to obtain bactericidal activity. E. faecium is intrinsically more resistant to tobramycin than E. faecalis, and this aminoglycoside should not be used for E. faecium.

Most E. faecium isolates recovered from hospitals in the United States are now resistant to both ampicillin (with MIC >64μg/ml) and vancomycin. The emergence and dissemination of clinical isolates of vancomycin-resistant enterococci (VRE) pose important therapeutic challenges for clinicians.

Linezolid and Q/D resistance has been well characterized in enterococci, including in outbreak settings.

Emergence of resistance to daptomycin during therapy appears to be an important problem of the use of this antibiotic during therapy.

Resistance to β-lactams is rare in S. bovis-group of organisms. Some isolates may have penicillin minimal inhibitory concentrations greater than 0.5μg/ml, in which case therapy should be extended to 4 to 6 weeks. It is important to note that the vanB gene cluster conferring high-level resistance to vancomycin has been described in S. bovis-group isolates.

Ampicillin resistance in E. faecium is mediated by the production and/or overexpression of a penicillin-binding protein (PBP) with reduced affinity for penicillin (designated PBP5-R).

Ampicillin resistance in E. faecalis can be mediated by the production of a plasmid-borne β-lactamase; such isolates are only rarely reported.

Vancomycin resistance in enterococci is due to the acquisition of gene clusters (often carried on transposable elements) encoding proteins whose biochemical activity results in the replacement of the last amino acid of peptidoglycan precursors (which is D-alanine) by D-lactate (high-level resistance to vancomycin and teicoplanin, VanA phenotype) or D-serine (low-level resistance to vancomycin [e.g., VanC phenotype]). Additionally, some of these proteins prevent the synthesis of (or destroy) D-alanine-D-alanine ending peptidoglycan precursors that bind vancomycin readily. The expression of these clusters is tightly regulated both in vitro and in vivo using genes encoding two-component regulatory systems.

High-level resistance to streptomycin is usually due to mutations in the 30S subunit of the ribosome, which is the target of the aminoglycoside antibiotics, and/or streptomycin modifying nucleotidyl transferases.

High-level resistance to gentamicin is most often due to the presence of a gene encoding a bifunctional enzyme that both phosphorylates and acetylates the aminoglycoside molecule (designated Aac(6’)-Ie-Aph(2″) enzyme.) This enzyme produces resistance to synergism with all aminoglycosides except streptomycin. Other enzymes that phosphorylate gentamicin also exist.

E. faecium has intrinsic higher levels of resistance to aminoglycosides this abolishes the synergism between cell wall active agents and several aminoglycosides, such as tobramycin, amikacin, netilmicin, and sisomicin. Therefore, these compounds should be avoided for the treatment of E. faecium infections.

Resistance to linezolid has been well characterized in enterococci and most commonly involves mutations in genes encoding domain V of the 23S rRNA. Since enterococci possess multiple copies of these genes, the level of resistance increases with the number of mutated alleles. The G2576A substitution is the most common mutation observed in clinical isolates. The presence of the cfr gene encoding a methyl transferase that methylates A2305 position of the 23S rRNA has now been reported in a clinical human isolate of enterococci.

The common presence of the erm genes encoding rRNA methylases in clinical isolates of E. faecium causes resistance to quinupristin and appears to decrease the bactericidal activity of Q/D. Additionally, genes that either hydrolyze quinupristin (vgb genes) or modify dalfopristin (vat genes) have been associated with Q/D resistance in enterococci.

The emergence of daptomycin resistance in enterococci during therapy is an important problem. Recent data suggest that the mechanism of resistance involves mutations in a three-component regulatory system involved in modulating the cell envelope stress response to antibiotics (designated LiaFSR), coupled with amino acid substitutions in enzymes predicted to function in cell membrane phospholipid metabolism (glycerophosphodiester phosphodiesterase and cardiolipin synthase).

The mechanism of resistance in E. faecalis appears to involve diversion of the antibiotic molecule from the septum (the main target of daptomycin) to other cell membrane areas). Additionally, mutations in the liaFSR genes are associated with daptomycin tolerance (lack of killing) in daptomycin-susceptible E. faecium isolates.

Penicillin tolerance in S. gallolyticus, S. infantarium, and S. pasteurianus has been associated with altered affinity of the penicillin binding proteins for the β-lactam antibiotic.

High-level resistance to vancomycin in S. gallolyticus-group of organisms is associated with the acquisition of the vanB gene cluster, which results in the replacement of D-alanine of peptidoglycan precursors for D-lactate and concomitant destruction of D-alanine-D-alanine ending precursors.

Resistance to ampicillin in E. faecium is readily detected by conventional MIC determination (either automated systems, broth, or agar dilution tests) as recommended by the Clinical Laboratory Standards Institute (CLSI).

Conversely, penicillinase in E. faecalis is difficult to detect using standard MIC determination. If ampicillin resistance is suspected in this species, a high inoculum MIC (using 107CFU/mL) could be performed. Since the mechanism involves the production of a penicillinase found in staphylococci, the use of disks impregnated with nitrocefin is also useful to detect ampicillin resistance (the disk turns reddish/pink if the organism produces penicillinase).

HLR to gentamicin can be detected by plating the isolate on brain heart infusion (BHI) agar plates containing 500μg/ml of gentamicin. The presence of one or more colonies on these plates after overnight incubation at 37°C indicates the presence of HLR to gentamicin.

HLR to streptomycin can be detected by plating the isolate on BHI agar plates containing 2,000μg/ml of streptomycin. The presence of one or more colonies on these plates after overnight incubation at 37°C indicates the presence of HLR to streptomycin. Alternatively, growth of the isolate after 18 hours at 37°C in BHI broth containing 1,000μg/ml of streptomycin can be used.

High-level vancomycin resistance is readily detected by standard MIC determination in all enterococcal species. Isolates that exhibit lower levels of vancomycin resistance (MIC between 8 and 16μg/ml) are sometimes reported as susceptible by automated methods and can cause therapeutic failures. Disk diffusion methods are discouraged, since diffusion of the glycopeptide molecule into the agar plate is not optimal and erroneous results may be recorded. Polymerase chain reaction (PCR)-based methods are available to detect vancomycin resistance genes from clinical isolates, but they have not been validated in clinical settings. The presence of the vanC gene is characteristic of E. gallinarum and E. casseliflavus.

E-tests are now available to detect resistance to antibiotics, such as linezolid, Q/D, and daptomycin.

Determination of standard MICs of penicillin should be performed when treating endocarditis caused by S. gallolyticus-group organisms. The MIC values are important to guide duration of therapy.

A new therapy for E. faecalis endovascular infections is the combination of ceftriaxone plus ampicillin. The use of both β-lactams appears to have a synergistic effect because of differential saturation of the cell wall synthesis enzymes of E. faecalis. Possible alternative therapies for severe infections include daptomycin plus ampicillin or another active agent.

Vancomycin (if susceptible) is the agent of choice for ampicillin-resistant enterococci.

For ampicillin-resistant E. faecium isolates, high-doses (up to 30g) of ampicillin can be considered if the MIC is less than or equal to 64μg/ml, usually combined with an aminoglycoside (if not highly resistant). Ampicillin monotherapy (9-12g/day) could be considered for uncomplicated UTIs caused by these isolates, since the concentration of ampicillin in urine should reach levels greater than 1,000μg/ml with normal renal function.

The agents with good in vitro activity against ampicillin and vancomycin-resistant E. faecium infections include linezolid, Q/D (both are FDA approved for VRE), daptomycin, and tigecycline. The latter two are FDA approved only for vancomycin-susceptible E. faecalis.

Linezolid is FDA approved for the treatment of VRE infections. However, this antibiotic is bacteriostatic, and the efficacy against VRE has been questioned in the treatment of endovascular infections. This compound has good pharmacokinetic profile in the central nervous system (CNS; including cerebrospinal fluid [CSF]) and may offer a benefit for the treatment of CNS infections (particularly meningitis) caused by VRE. Prolonged treatment courses with linezolid may increase the risks of toxicity, including thrombocytopenia, peripheral neuropathies, and optic neuritis. Caution should be used when linezolid is prescribed along with serotonin uptake inhibitors and monoamine oxidase inhibitors, since the combination may increase the risk of the serotonin syndrome.

Tedizolid is a new oxazolidinone antibiotic chemically related to linezolid with potent in vitro activity against all species of enterococci (including VRE). However, this drug is not approved for VRE and the clinical efficacy of tedizolid against MDR enterococci has not been studied.

Q/D is a bactericidal antibiotic active against E. faecium (but not E. faecalis) that is also approved by the FDA for the treatment of VRE infections. The use of Q/D monotherapy against VRE is discouraged because of reports of clinical failures. Combinations of Q/D and ampicillin (when the MIC of ampicillin is <64μg/mL) and of Q/D and doxycycline plus rifampin have been sporadically used with success in the treatment of VRE endocarditis. The occurrence of side effects, such as phlebitis, myalgias, arthralgias, and the need for a central line and several drug interactions make treatment difficult.

Daptomycin is a lipopeptide antibiotic with bactericidal activity against enterococci (all species), although it is not FDA approved for the treatment of VRE infections. The only enterococcal approval from the FDA for daptomycin is for the treatment of skin and soft tissue infections caused by vancomycin-susceptible E. faecalis. The potent in vitro bactericidal activity of daptomycin has made this antibiotic an interesting alternative to treat endovascular ampicillin and vancomycin-resistant enterococcal infections.

The Clinical Laboratory Standards Institute breakpoints for daptomycin are higher for enterococci than for staphylococci, and some experts suggest that, because of this situation and the high protein binding of the drug, enterococci should be treated with higher doses (e.g., 8-12mg/kg/day) than those approved for S. aureus bacteremia (6mg/kg) to try to avoid development of resistance during therapy. Moreover, it has been suggested that the antibiotic should be used in combination with other compounds with activity against enterococci. Thus, the combination of ampicillin and daptomycin is an attractive alternative for those E. faecalis infections in which aminoglycosides cannot be used due to resistance or toxicity.

The combination of daptomycin plus ampicillin and gentamicin, daptomycin plus gentamicin and rifampin and daptomycin plus tigecycline have been used successfully used in the treatment of isolated cases of vancomycin-resistant E. faecium endocarditis.

Ampicillin resistance due to penicillinase in E. faecalis can be overcome by the utilization of ampicillin with a beta-lactamase inhibitor (sulbactam). High doses (up to 12g) of ampicillin-sulbactam plus an aminoglycoside (or, presumably, ceftriaxone) are recommended in the treatment of serious E. faecalis infections with strains that produce penicillinase.

Tigecycline has good in vitro activity against enterococci, but, because of the low serum levels achieved with this drug, it is not recommended as monotherapy in serious enterococcal infections with bacteremia or endovascular infections. As mentioned, tigecycline has been used in combination with daptomycin in a few cases of endocarditis or meningitis caused by VRE. Resistance to tigecycline is rare and the mechanism is unknown.

Daptomycin, linezolid, tigecycline, and the new cephalosporins, ceftaroline and ceftobiprole, have very good in vitro activity against S. bovis-group organisms, but clinical experience with these compounds is limited.

Vancomycin is the therapy of choice for infections due to S. bovis-group of organisms when β-lactams cannot be used because of allergy or toxicity.

How do patients contract this infection, and how do I prevent spread to other patients?

  • Epidemiology

    The majority of enterococcal infections occur in the hospital environment. Infections that originate in the community are less common and usually include UTIs and endocarditis. There are no evident seasonal variations that affect the incidence of enterococcal infection.

    Enterococci are normal commensals of the gastrointestinal tract, and it is thought that acquisition of enterococci occurs from food and water.

    The most important environmental factors that influence the acquisition of antibiotic-resistant enterococci and subsequent infections involve those related to the hospital setting.

    Among the conditions that favor the colonization and infection by enterococci (particularly VRE) are: administration of broad spectrum antibiotics, having a hospital roommate colonized or infected with VRE, staying in a room previously occupied by a patient infected or colonized with VRE, direct contact with patients infected or colonized with VRE, contact with health-care workers whose hands are colonized with VRE, and contact with hospital surfaces or equipment contaminated with VRE. Moreover, enterococci are resistant to a wide variety of disinfectants, which favors their persistence in the hospital environment.

    There is no specific environmental condition associated with gastrointestinal colonization of S. Gallolyticus-group organisms; although hosts with colon cancer or polyps are known to develop bacteremia with these organisms.

    In the United States, enterococci are isolated from about 12% of health-care associated infections reported to the National Healthcare Safety Network (2006-2007). Thus, enterococci ranked second to staphylococci as the most frequent organisms recovered from nosocomial infections. In Europe, enterococci are less frequently associated with hospital infections, accounting for about 3.5% of isolates recovered from intensive care units. Enterococci are also less frequent causes of nosocomial infections in Latin America, accounting for 3% of nosocomial infections in recent surveillances.

    S. gallolyticus-group of organisms account for approximately 24% of streptococcal endocarditis. A recent meta-analysis indicated that S. gallolyticus is highly associated with colonic malignancy. Patients who have endocarditis caused by this organism should be screened for the presence of this neoplasm and colonic polyps.

    The incidence of infection with E. faecium, which are frequently ampicillin and vancomycin resistant, has increased in the last few decades. Indeed, E. faecium isolates are approaching E. faecalis in hospitals across the United States. This is in contrast to what was reported in the 1980s when 90 to 95% of clinical specimens were E. faecalis. The “untaming” of E. faecium may be related to the presence of specific genetic lineages with a higher occurrence of antimicrobial resistance, virulence, and/or colonization traits that increase their ability to reside in the gastrointestinal (GI) tract of hospitalized patients and subsequently produce disease.

    The prevalence of S. bovis-type organisms (most E. gallolyticus) causing endocarditis appears to be stable, and no evident increase in the number of cases has been documented.

    The transmission of enterococci occurs most often due to direct contact between the hands of health-workers, contaminated hospital devices, fomites or surfaces, and the patient. The main reservoir of multidrug resistant enterococci is the GI tract of hospitalized patients. Therefore, contact precautions, including gloves, gowns, and hand washing, are the most important measures to curtail the chain of transmission, particularly in patients in whom fecal secretions are increased (e.g., diarrhea, patients with colostomy, or other GI derivations). Additionally, room disinfection is also of paramount importance to avoid recurrent transmission of VRE to other incoming patients.

    There are no current vaccines available for enterococci.

    Prophylaxis is indicated to prevent enterococcal endocarditis only in specific circumstances. The current guidelines recommend the use of anti—-enterococcal prophylaxis in patients at high risk of endocarditis (with prosthetic cardiac valves, previous bacterial endocarditis, unrepaired cyanotic cardiac disease, completely repaired congenital heart defects within the first 6 months after the procedure, repaired congenital heart disease with residual defects, and cardiac transplant patients who developed valvulopathy) who undergo cystoscopy without known culture results. For patients who have an established genitourinary or intra-abdominal enterococcal infection, anti-enterococcal therapy may be indicated in patients with risk factors for endocarditis.

What host factors protect against this infection?

  • The first line of defense is the innate immune system. Toll-like receptors (TLR) play an important role in the recognition process mediated by TLR2 signaling via the myeloid differentiation primary-response gene 88 (MyD88). The effector molecule is the lectin RegIIIγ, which is produced by the intestinal epithelial cells and is capable of controlling the numbers of gram-positive organisms, including VRE. Antibiotics that reduce gram-negative bacterial populations (anaerobes and facultative bacteria) produce a reduction in the production of RegIIIγ (in mice) which correlates with increased numbers of VRE in the gastrointestinal tract. Neutrophils and macrophages are important mediators of the early response against enterococci. Indeed, neutropenic mice exhibited important delays in enterococcal clearance from peritoneal fluid, lung, blood, and liver.

    The complement system also plays a key role in the clearance of enterococci via opsonization of bacteria and promotion of phagocytosis, although some clinical isolates of enterococci are resistant to phagocytosis.

  • Critically ill and/or immunosuppressed patients receiving antibiotics are the most susceptible to enterococcal infections. Specific populations include patients with solid and hematological malignancies (neutropenic patients are at increased risk); solid organ transplant recipients; patients receiving allogeneic blood and bone marrow transplants; patients receiving chemotherapeutic agents that disrupt the GI epithelium; hospitalized patients in critical care units (both medical and surgical); patients with indwelling catheters for intravenous (IV) access and urinary catheters, patients with diabetes; patients with chronic renal failure, particularly if on hemodialysis; and patients who develop Strongyloides stercoralis hyperinfection syndrome. In neutropenic patients, VRE bacteremia occurs primarily in those whose gastrointestinal tract is dominated by VRE.

    There is a strong association between bloodstream infection and/or endocarditis caused by S. gallolyticus-group organisms (S. gallolyticus subsp gallolyticus) and the presence of malignant of premalignant lesions of the colon.

What are the clinical manifestations of infection with this organism?

  • Urinary tract infections

    Enterococci are frequently isolated from the urine of hospitalized patients. However, it is sometimes difficult to differentiate between colonization and true infection. UTIs are usually associated with indwelling catheters, instrumentation, and manipulations of the genitourinary tract. Removal of the catheter may be sufficient to treat the infections. Outside of the hospital, enterococcal UTIs seem more common in older men, and an association with prostatitis and epididymitis is well documented.

  • Endocarditis

    A common presentation of enterococcal disease is endocarditis. Enterococci account for 5 to 20% of endocarditis, and are the second most frequent cause of nosocomial endocarditis after staphylococci. Both prosthetic and native valves may be involved, with many cases described in settings in which there are structural abnormalities of the cardiac valves. Endocarditis is also the most common presentation caused by members of the S. bovis-group, particularly S. gallolyticus subsp. gallolyticus.

  • Bloodstream infections

    Enterococci are currently one of the leading causes of nosocomial bacteremias, some of which may be related to contamination of a catheter site leading to positive blood cultures drawn from the catheter. Common origins of clinically important bloodstream infections include the genitourinary, GI, and biliary tracts, endocarditis, and indwelling catheters (both intravascular and urinary). Bacteremia usually affects seriously ill patients who have received antibiotics. An association between enterococcal bacteremia and Strongyloides stercoralis hyperinfection syndrome has been documented.

  • Meningitis

    Enterococci are rare causes of meningitis. Spontaneous meningitis has been described in seemingly healthy patients and in those with some degree of immunosuppression or CNS defects. Postoperative meningitis is usually seen in the setting of shunt devices or after chemotherapy. Enterococcal meningitis has also been associated with S. stercoralis hyperinfection and rarely as a complication of lumbar or ventricular tap and epidural anesthesia. Bacteremia can be seen in more than one-half of patients with enterococcal meningitis.

  • Intra-abdominal infections

    Clinical evidence suggests the role of enterococci in community onset intra-abdominal infection in immunocompetent patients is minor, and no specific anti-enterococcal therapy is indicated for these infections when intra-abdominal collections are adequately drained and antimicrobials with activity against gram-negative and anaerobes are utilized. However, enterococci are relevant pathogens in patients with nosocomial peritonitis and abdominal sepsis and anti-enterococcal therapy should be considered in critically ill patients with these conditions or in patients with multiple intra-abdominal infections who received a course of antibiotics with no anti-enterococcal activity.

    Enterococci have been described as causing spontaneous peritonitis in cirrhotic patients and patients undergoing peritoneal dialysis. Enterococci are also important causes of genitourinary tract infections in women of childbearing age, including salpingitis, endometritis, and pelvic abscesses.

  • Neonatal infections

    Enterococci have been implicated in several neonatal infections, including late-onset sepsis, bacteremias, UTIs, pneumonias, and meningitis. These organisms are part of the normal vaginal flora of adult women, and it is thought that infection may occur during delivery. Neonates with low birth weight, intravascular devices, necrotizing enterocolitis, and those who undergo bowel surgery seem to have an increased risk of developing enterococcal sepsis.

  • Skin and soft tissue infections

    Enterococci are frequently isolated from skin and soft tissue infections, although the pathogenic role in these situations is unclear. When enterococci are isolated from surgical site infections, they are usually accompanied by other microorganisms. Skin infection or colonization may be the portal of entry for deeper infections such as osteomyelitis.

  • Bloodstream infections and endocarditis are the most common presentation of disease caused by S. bovis-group organisms. S. gallolyticus subsp. gallolyticus are much more frequently associated with endocarditis than the other species within the group (odds ratio 16.61 vs S. infantarium and S. pasteurianus). Conversely, S. pasteurianus has been associated more frequently with meningitis rather than with endocarditis.

What common complications are associated with infection with this pathogen?

  • Endocarditis is the most common complication of enterococcal bacteremia. The risk of endocarditis during an episode of E. faecalis bacteremia with more than two sets of blood cultures positive has been postulated to be approximately 6% but increases substantially in patients with valvulopathies.

  • Sepsis: Debilitated and critically ill patients may develop signs of systemic inflammatory response syndrome and shock during an episode of enterococcal bacteremia.

  • Embolization, particularly to the central nervous system, and cardiac failure are the two most severe complications of endocarditis caused by both enterococci and S. gallolyticus.

How should I identify the organism?

Enterococci are sturdy organisms and can survive some extreme environmental conditions, including high temperatures and elevated concentrations of salt. These organisms grow well using standard laboratory media. The most likely fluids to yield a positive culture include blood and urine; media selective for enterococci should be used for isolation from stools or rectal swabs.

S. gallolyticus, S. infantarium, and S. pasteurianus can also be readily recovered from most human specimens. The highest yield appears to be from blood.

  • Enterococci are readily visualized by standard light microscopy. Gram stain usually shows gram-positive oval cocci arranged in pairs and short chains, although some isolates may exhibit long chains. S. gallolyticus, S. infantarium, and S. pasteurianus are gram-positive cocci that usually grow in chains in liquid medium.

  • Enterococci grow well in standard laboratory media, except those, such as MacConkey agar, that are selective for gram-negative organisms. Media selective for enterococci versus most streptococci include bile salts and esculin. Most clinically relevant species grow well at 35°C to 37°C and do not require increased C02 for growth; enterococci also show marked temperature tolerance up to 42°C. Anaerobic conditions and the presence of C02 may enhance the growth of some isolates. S. gallolyticus, S. infantarium, and S. pasteurianus grow best on most blood agar media used to identify streptococci. The optimal temperatures are between 35 and 37°C, supplemental C02 or anaerobic conditions may enhance the growth of some species.

    Clinical specimens from sterile body sites can be plated on nonselective media that support the growth of enterococci including trypticase soy agar or broth, brain heart infusion (BHI) agar/broth, or any agar containing 5% blood. Selective media are sometimes used to enhance the isolation of enterococci in samples for nonsterile sites (e.g., stools) or when heavy contamination is suspected. These media usually contain bile salts, sodium azide, antibiotics, and/or indicator substances, such as esculin or tetrazolium.

    These components form the bases for the majority of selective media used commercially to detect vancomycin-resistant enterococci from stool samples in infected or colonized patients in the hospital.

    Streptococcus bovis-group of organisms grow well on blood agar medium. They are also able to grow in media that contain bile salts. However, they cannot tolerate growth in media containing 6.5% NaCl, an important difference between the S. bovis-group organisms and enterococci.

    Colonies from clinically important species are usually small, gray-looking punctiform convex colonies with clear and delineated margins. They usually do not display hemolysis on sheep blood agar, but some E. faecalis are able to lyse human, horse, and rabbit red blood cells.

    S. bovis-group of organisms has colony morphologies very similar to those of other streptococci and enterococci. Colonies are usually small, glistening white or gray. On blood agar, they are usually nonhemolytic or alpha-hemolytic.

    Once a catalase-negative, gram-positive coccus is identified, several biochemical tests can be used to differentiate enterococci from other organisms that may appear similar on Gram stain and culture. The most useful is the production of pyrrolidonylarylamidase (PYR), which differentiates enterococci from Leuconostoc spp. and the Streptococcus bovis-group. Enterococci, as well as some of the S. gallolyticus group, typically give a Lancefield group D reaction. Most clinical laboratories do not perform identification of enterococci at the species level. Resistance to ampicillin usually implies E. faecium since E. faecalis and other species are usually susceptible to ampicillin.

    A potentially useful test to differentiate E. faecalis from E. faecium is the breakdown of arabinose; E. faecium isolates are able to metabolize arabinose, and most E. faecalis give a negative arabinose test. Additionally, susceptibility to Q/D may help differentiate between E. faecalis and E. faecium. Since most E. faecalis are resistant to Q/D, whereas E. faecium isolates are usually susceptible to the antibiotic.

    Strains of Streptococcus gallolyticus subsp gallolytycus and S. infantarium give a Lancefield group-D reaction, whereas S. pasteurianus does not. Members of the S. bovis-group do not ferment sorbitol.

    Enterococci doubling time is approximately 40 to 45 minutes in nonselective medium. The growth may vary in different strains. Early visible colonies are present in blood agar plates after overnight incubation.

    Culture and isolation of enterococci and S. bovis-group organisms depend on several factors, including prior treatment with antibiotics, presence of antimicrobials in the growth medium, the composition of the medium, and the characteristics of the infecting strain. In general, the isolation of enterococci and S. bovis organisms in nonselective media is highly sensitive.

  • PCR-based techniques have been used for species identification of enterococcal isolates. The most common target genes include ddl, encoding the D-alanine-D-alanine ligase, and sodA, encoding a superoxide dismutase. To the best of our knowledge, these PCR tests are not FDA approved.

  • Sequencing of the 16S rRNA gene is currently used for species identification of enterococci in research laboratories. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of whole cell protein profiles has been used for the differentiation of enterococcal species. A commercially available DNA probe kit (designated AccuProbe from Gen-Probe, Inc) has been shown to be useful but is not yet FDA approved. The use of fluorescence in situ hybridization (FISH) has been shown to be clinically useful for the identification of enterococci from positive blood culture bottles. The E. faecalis/other Enterococcus species PNA FISH (AdvanDx, Woburn, MA) is a peptide-nucleic acid FISH method cleared by the FDA.

  • Because of the change in taxonomy, identification at the species level may be important for the S. bovis-group of organisms, since there is a strong association between the isolation of this microorganism and colonic malignancies. The classification based on biochemical biotypes is not sensitive enough to differentiate the different species of the group. Therefore, 16S rRNA sequencing is the most reliable technique used for species identification of the S. bovis-group members.

How does this organism cause disease?

  • Enterococci do not produce potent toxins and seem to rely on an array of factors for in vivo pathogenesis.

    The secreted factors (exported outside of the cell and released) shown to contribute to virulence of enterococcal infections include the enterococcal cytolysin/hemolysin and two proteases, gelatinase and a serine protease.

    Bacterial surface proteins are likely involved in the early stages of infection. They interact with host proteins from the extracellular matrix (i.e., collagen, fibronectin, among many others) and eukaryotic cells. Some of the most important surface proteins known to be associated with enterococcal virulence include aggregation substance (AS), which is involved in cell to cell attachment, as well as plasmid transfer between enterococcal isolates; enterococcal surface proteins (Esp of E. faecalis and Espfm of E. faecium) shown to be important in biofilm formation and colonization of the urinary tract of mice; adhesions of collagen of enterococci (Ace of E. faecalis, Acm of E. faecium), which are important in the pathogenesis of endocarditis and urinary tract infection; and enterococcal pili, which are cell-wall anchored projections present in the surface of enterococci that appear to play a pivotal role in the pathogenesis of experimental endocarditis and UTIs.

    Other important enterococcal factors associated with in vivo pathogenesis include the E. faecalis Gls24 and E. faecium Gls 20 and 33 stress proteins, which appear to play an important role in the pathogenesis of endocarditis (E. faecalis) and peritonitis (E. faecium), as well as in bile resistance.

    The identification and characterization of large transferable plasmids in E. faecium suggests that these plasmids are important colonization and virulence factors of this species. A megaplasmid from a clinical strain of E. faecium (TX16) has been involved in the pathogenesis of experimental peritonitis.

    The type of disease produced by the S. gallolyticus subsp. gallolyticus appears associated with the ability to form biofilm in collagen-rich surfaces and traverse the intestinal epithelium without causing interleukin 8 or 1β responses. Indeed, an important percentage of clinical strains of S. bovis-group organisms bind collagen types I and IV. Genome analysis of a clinical strain of S. gallolyticus identified 11 potential proteins with the ability to bind extracellular-matrix proteins. One of these proteins, designated Acb, showed high affinity binding to immobilized collagen. Additionally, a pilus protein, designated Pil1, is involved in binding to collagen, biofilm formation, and affects virulence of experimental endocarditis.

  • Aggregation substance, produced by some strains of E. faecalis, facilitates the attachment of cells to each other, and it is thought to contribute to the pathogenesis of endocarditis by favoring cell-to-cell attachment.

    The cytolysin/hemolysin, often encoded on pheromone-responsive plasmids found in some E. faecalis strains (as are aggregation substance genes), likely contributes to virulence by lysing red blood cells and leukocytes.

    The proteases are thought to affect virulence by several mechanisms, including the breakdown of host tissues and modification of critical components of the immune system. GelE also contributes to cell lysis and the release of intracellular DNA, which is an important component of enterococcal biofilm and might contribute to the release of bacteria from vegetations.

    Enterococcal adhesins and surface proteins are thought to influence virulence by facilitating the attachment of bacteria to components of the extracellular matrix (i.e., different types of collagen, laminin, fibronectin, among others), cells and platelets. This property appears critical in the pathogenesis of enterococcal endocarditis and UTIs.

    The enterococcal stress proteins play an important role in the pathogenesis of endocarditis in E. faecalis and peritonitis in E. faecium by contributing to bacterial survival in the GI tract.

    The large megaplasmids of E. faecium (approximately 150-300kb) increase the colonization of E. faecium in the GI tract of mice and also enhance the pathogenic properties in experimental peritonitis, endocarditis, and UTIs. The exact mechanism is unknown, but within these plasmids there are several genes that may offer a competitive advantage for survival in the GI tract and other tissues.

    Collagen binding is thought to be of paramount importance in the pathogenesis of endocarditis of S. gallolyticus subsp. gallolyticus. Of interest, premalignant colonic lesions appear to have increased expression of collagen type IV, which could give S. gallolyticus a competitive binding advantage and could explain, at least in part, the association with this disease.

WHAT’S THE EVIDENCE for specific management and treatment recommendations?

The following ratings are based on the Canadian Task Force recommendations:

  • Strength of recommendation

    A: Good evidence to support a recommendation for or against use

    B: Moderate evidence to support a recommendation for or against use

    C: Poor evidence to support a recommendation

  • Quality of evidence

    I: Evidence from more than one properly randomized, controlled trial

    II: Evidence from more than one well-designed clinical trial without randomization; from cohort or case-controlled analytic studies (preferably from more than one center); from multiple time-series; or from dramatic results from uncontrolled experiments

    III: Evidence from opinions of respected authorities, based on clinical experiences, descriptive studies, or reports of expert committees

Penicillin/ampicillin plus gentamicin or streptomycin has been the regimen of choice for many decades for the treatment of ampicillin-susceptible enterococcal endocarditis with isolates not exhibiting high-level resistance HLR to aminoglycosides. (Strength of recommendation A-II.)

Ampicillin plus ceftriaxone was initially recommended for the treatment of E. faecalis endocarditis with isolates that exhibit high-level resistance to aminoglycosides (HLR) (strength of recommendation A-II), but a recent study indicated that it is equivalent to the combination of ampicillin plus aminoglycosides for the treatment of E. faecalis endocarditis and some consider this the regimen of choice due to lower toxicity.

Linezolid is not the first choice for the treatment of ampicillin and vancomycin enterococcal endocarditis. (Strength of recommendation B-III.) Linezolid may be appropriate for the treatment of catheter-associated bacteremia. (Strength of recommendation C-III.)

Quinupristin-dalfopristin (Q/D) monotherapy is an alternative for bacteremia (Strength of recommendation B-II). Q/D combinations are favored over Q/D monotherapy for the treatment of E. faecium endocarditis. (Strength of recommendation C-III.)

High-dose daptomycin monotherapy is an alternative in the treatment of bacteremia without concomitant endocarditis caused by vancomycin-resistant enterococci. (Strength of recommendation B-III.) In endocarditis, daptomycin should be used with caution because of the potential emergence of resistance during therapy. (Strength of recommendation C-III.) Combination of daptomycin with other agents (i.e., ampicillin, gentamicin, tigecycline) may improve the therapeutic efficacy in endocarditis. (Strength of recommendation C-III.)

Tran. Mbio. vol. 4. 2013. pp. e00281

Clin Infect Dis.. vol. 56. 2013 May. pp. 1261-8.