Infectious Disease and Microbiology – Enterococcus Species

Overview

Enterococcus species are Gram-positive cocci that normally colonize the human gastrointestinal tract but are important causes of healthcare-associated and opportunistic infections. The most clinically important species are Enterococcus faecalis and Enterococcus faecium.

Major infections include urinary tract infection, bacteremia, endocarditis, intra-abdominal and pelvic infection, neonatal infection, meningitis, and skin and soft-tissue infection.

Important Species

Clinically recognized species include:

• Enterococcus faecalis

• Enterococcus faecium

• Enterococcus avium

• Enterococcus casseliflavus

• Enterococcus durans

• Enterococcus gallinarum

• Enterococcus hirae

• Other Enterococcus species

Among these, E. faecalis and E. faecium account for most clinically important infections.

Microbiologic Characteristics

Enterococcus species are:

• Gram-positive cocci

• Facultatively anaerobic organisms

• Normal inhabitants of the gastrointestinal tract

• Opportunistic pathogens

• Notable for substantial intrinsic and acquired antimicrobial resistance

They were historically classified among the group D streptococci.

Colonization and Incubation

A conventional incubation period is difficult to define.

Infection commonly develops after a prolonged period of intestinal colonization, particularly in hospitalized or medically complex patients.

The sequence is often:

Intestinal colonization → disruption of host barriers or medical intervention → invasion → clinical infection

Epidemiology

Enterococcus species have become increasingly important causes of healthcare-associated infection.

They are prominent causes of hospital-acquired bacteremia, urinary tract infection, and infections involving indwelling medical devices.

Risk Factors

Important risk factors for invasive enterococcal infection include:

• Prolonged hospitalization

• Broad-spectrum antibiotic exposure

• Urinary or vascular catheters

• Recent surgery

• Intra-abdominal disease

• Immunosuppression

• Severe underlying illness

Bacteremia

Enterococcus species are important causes of bloodstream infection.

Bacteremia may be either:

• Monomicrobial

• Polymicrobial, particularly when arising from gastrointestinal or intra-abdominal sources

Potential sources include urinary infection, intra-abdominal infection, vascular catheters, and endocarditis.

Urinary Tract Infection

Enterococci are important causes of urinary tract infection, especially in hospitalized patients.

Risk is increased by:

• Indwelling urinary catheters

• Urinary instrumentation

• Structural urinary tract disease

• Prolonged hospitalization

Clinical disease ranges from cystitis to complicated UTI and urosepsis.

Infective Endocarditis

Enterococci can cause acute or subacute infective endocarditis.

E. faecalis is particularly important in this setting.

Patients may develop:

• Persistent bacteremia

• Fever

• Cardiac murmur

• Valvular vegetations

• Embolic or immunologic complications

Enterococcal endocarditis can be difficult to eradicate and generally requires prolonged antimicrobial therapy.

Intra-Abdominal and Pelvic Infection

Because enterococci normally colonize the gastrointestinal tract, they may participate in:

• Intra-abdominal abscesses

• Peritonitis

• Biliary infection

• Pelvic infections

• Postoperative abdominal infections

These infections are frequently polymicrobial.

Neonatal Infection

Enterococci can occasionally cause serious infections in neonates, particularly in hospitalized or premature infants.

Manifestations may include:

• Sepsis

• Bacteremia

• Meningitis

Meningitis

Enterococcal meningitis is uncommon but may occur in:

• Neonates

• Neurosurgical patients

• Immunocompromised individuals

• Patients with invasive enterococcal infection

Pneumonia

Enterococcus may occasionally be isolated in patients with pulmonary disease.

However, true enterococcal pneumonia is uncommon, and isolation from respiratory specimens should be interpreted carefully because colonization can occur.

Skin and Soft-Tissue Infection

Enterococci may participate in skin, soft-tissue, and wound infections, particularly in hospitalized patients.

These infections are frequently polymicrobial and may occur in surgical wounds or chronic ulcers.

Diagnosis

Diagnosis is established by culture of the pathogen from the appropriate clinical specimen.

Examples include:

• Blood cultures

• Urine culture

• Wound or abscess cultures

• Cerebrospinal fluid culture

Because antimicrobial resistance is common, susceptibility testing is essential for clinically significant infection.

Enterococcus faecalis Treatment

The source lists the following agents for susceptible E. faecalis:

• Amoxicillin

• Ampicillin

• Penicillin G

Ampicillin is commonly active against susceptible E. faecalis isolates.

Serious Infection and Endocarditis

The source describes treatment of serious infection or endocarditis using a cell-wall-active β-lactam combined with gentamicin, provided the organism does not demonstrate high-level aminoglycoside resistance.

The purpose of combination therapy is to achieve synergistic bactericidal activity.

Ampicillin + Gentamicin Synergy

The traditional principle is:

Ampicillin or penicillin damages the bacterial cell wall

• ●

Gentamicin enters the organism more effectively

→ Synergistic killing

However, high-level aminoglycoside resistance eliminates this synergistic effect.

Endocarditis Duration

Enterococcal endocarditis generally requires prolonged therapy.

The source describes:

Approximately 6 weeks of antimicrobial treatment

The exact regimen and duration depend on the valve involved, susceptibility pattern, prior therapy, and clinical circumstances.

Enterococcus faecium

E. faecium is particularly important because it is generally more antimicrobial-resistant than E. faecalis.

The source describes treatment with:

Vancomycin or teicoplanin, potentially combined with gentamicin when appropriate susceptibility is demonstrated.

However, resistant E. faecium strains are a major modern clinical problem.

Vancomycin-Resistant Enterococcus

A major high-yield concept is vancomycin-resistant Enterococcus (VRE).

VRE occurs particularly among E. faecium strains and is an important cause of healthcare-associated infection.

Resistance can substantially limit therapeutic options.

Important Intrinsic Vancomycin Resistance

E. gallinarum and E. casseliflavus have characteristic intrinsic low-level vancomycin resistance, associated with the VanC phenotype.

This distinguishes them from the acquired high-level vancomycin resistance encountered in many clinically important E. faecium isolates.

Additional Treatment Options

The source also lists:

• Imipenem

• Vancomycin

• Teicoplanin

• Amoxicillin–clavulanate

• Ampicillin–sulbactam

• Piperacillin–tazobactam

Actual therapy should be selected according to species identification, infection site, severity, and susceptibility testing.

Cystitis

For uncomplicated lower urinary infection, the source lists agents such as:

• Nitrofurantoin

• Ciprofloxacin

• Trimethoprim–sulfamethoxazole

However, susceptibility varies substantially, so urinary isolates should be interpreted according to the individual organism and susceptibility profile.

Important Resistance Characteristics

Enterococci are notable for resistance to many commonly used antimicrobial agents.

Important concepts include:

Intrinsic resistance to cephalosporins

Reduced susceptibility to many β-lactams

Possible high-level aminoglycoside resistance

Vancomycin resistance, especially in E. faecium

This combination of resistance mechanisms makes enterococci particularly important hospital pathogens.

E. faecalis vs. E. faecium

Enterococcus faecalis

→ More common in many clinical infections

→ Often more susceptible to ampicillin

→ Important cause of endocarditis and UTI

Enterococcus faecium

→ Generally more drug resistant

→ Frequently ampicillin resistant

→ Strongly associated with VRE

→ Particularly important in healthcare-associated infection

High-Yield Clinical Pattern

Hospitalized patient

• ●

Prolonged antibiotic exposure or indwelling catheter

• ●

UTI, bacteremia, or endocarditis

• ●

Gram-positive cocci with substantial antimicrobial resistance

→ Think Enterococcus

Endocarditis High-Yield Pattern

Persistent enterococcal bacteremia

• ●

Valvular vegetation/endocarditis

• ●

Need for prolonged therapy and bactericidal combination strategy when appropriate

→ Think Enterococcus faecalis

Exam Essentials

Genus: Enterococcus

Type: Gram-positive cocci

Normal habitat: Gastrointestinal tract

Major species: E. faecalis and E. faecium

Major infections: UTI, bacteremia, endocarditis, intra-abdominal and pelvic infection

Healthcare association: Strong

Diagnosis: Culture + susceptibility testing

E. faecalis: Often ampicillin susceptible

E. faecium: Generally more resistant

Important resistant phenotype: VRE, especially E. faecium

Aminoglycoside issue: High-level resistance eliminates synergistic killing

Endocarditis: Usually requires prolonged treatment

E. gallinarum/E. casseliflavus: Intrinsic VanC-mediated low-level vancomycin resistance

Cephalosporins: Enterococci are intrinsically resistant

Key clinical pearl: The major distinction is E. faecalis = often ampicillin susceptible and a classic cause of endocarditis, whereas E. faecium = substantially more drug resistant and strongly associated with VRE; serious enterococcal infections require susceptibility-guided therapy because intrinsic and acquired resistance are central features of this genus.