Is Enterococcus faecalis generally considered a contaminant in clinical cultures?

Is Enterococcus faecalis Typically a Contaminant?

No, Enterococcus faecalis is generally NOT considered a contaminant and should be treated as a true pathogen in most clinical scenarios. Unlike typical skin flora such as coagulase-negative staphylococci or Corynebacterium species, E. faecalis isolated from blood cultures usually represents genuine infection requiring treatment.

Key Clinical Distinctions

When E. faecalis Represents True Infection

E. faecalis accounts for approximately 90% of enterococcal infections 1, 2 and is a well-established cause of:

• Infective endocarditis - requiring 4-6 weeks of combination antimicrobial therapy 2
• Catheter-related bloodstream infections - with relatively low risk of endocarditis (only 1.5% in one large series) 3
• Nosocomial bacteremia - representing 10% of all hospital-acquired bloodstream infections 3

Rare Contamination Scenarios

Contamination with E. faecalis should only be suspected when:

• Mixed with typical skin organisms (coagulase-negative staphylococci, Corynebacterium, Bacillus, Micrococcus) in a single blood culture 4
• Vancomycin-resistant strains isolated from single blood cultures 5
• Ampicillin-resistant strains in single cultures 5
• Laboratory cross-contamination with quality control strains (rare but documented) 6

Approximately 17% of positive blood cultures with enterococci mixed with skin organisms are probable contaminants 4. However, this is the exception rather than the rule.

Clinical Approach Algorithm

Single positive blood culture for E. faecalis:

1. Assume true bacteremia unless mixed with skin flora
2. Evaluate for source (urinary tract, intra-abdominal, catheter-related)
3. Assess for endocarditis risk factors (prosthetic valve, persistent bacteremia >72 hours, new murmur) 3
4. Obtain susceptibility testing to penicillin, vancomycin, and high-level aminoglycoside resistance 2

Multiple positive blood cultures:

• Definitively represents true infection 7
• Clinical outcomes similar whether single or multiple cultures positive 7
• Proceed with appropriate antimicrobial therapy based on susceptibilities

Treatment Implications

The fact that E. faecalis requires specific, prolonged combination therapy underscores its pathogenic nature:

• Ampicillin-susceptible strains: Ampicillin plus gentamicin for 4-6 weeks 2, 3
• Catheter-related infections: 7-14 days if catheter removed, with antibiotic lock therapy if retained 3
• Endocarditis: Minimum 4-6 weeks combination therapy 2

Critical Pitfalls to Avoid

Do not dismiss E. faecalis as a contaminant based solely on:

• Single positive blood culture (unless mixed with skin flora)
• Community-onset versus hospital-onset acquisition 5
• Number of blood cultures drawn per day 5

Always obtain infectious disease consultation for enterococcal bacteremia, particularly when 8:

• Endocarditis is suspected
• Multidrug resistance is present
• Persistent bacteremia occurs despite appropriate therapy

The standard definition of blood culture contamination explicitly excludes enterococci from the list of typical contaminants (which includes coagulase-negative staphylococci, Corynebacterium, Bacillus, Micrococcus, and Cutibacterium acnes) 9. This fundamental distinction reflects the clinical reality that enterococci are established pathogens requiring definitive antimicrobial therapy.