Enterococcus faecalis vs faecium: Critical Antimicrobial Differences
E. faecalis is typically ampicillin-susceptible and responds well to β-lactam therapy, while E. faecium exhibits intrinsic penicillin resistance in most modern clinical isolates and requires alternative treatment strategies.
Key Antimicrobial Susceptibility Differences
E. faecalis
- Ampicillin/penicillin susceptible in the vast majority of cases 1
- Rarely produces β-lactamase (when it does, use ampicillin-sulbactam) 1
- Intrinsic penicillin resistance is uncommon 1
- Only ~3% of strains are multidrug resistant 1
- Vancomycin-resistant E. faecalis remains rare and many VRE E. faecalis strains remain penicillin-susceptible 1
E. faecium
- Intrinsic penicillin resistance is common in modern clinical isolates 1
- Most contemporary E. faecium strains are highly resistant to ampicillin 2
- Up to 95% of vancomycin-resistant enterococci (VRE) are E. faecium, with multidrug resistance to vancomycin, aminoglycosides, and penicillins 1
- Significantly more resistant than E. faecalis to penicillin, ampicillin, piperacillin, imipenem, and ciprofloxacin 3
Treatment Recommendations
For Ampicillin-Susceptible E. faecalis
Use ampicillin plus gentamicin or the ampicillin-ceftriaxone double β-lactam combination 4, 1
- The ampicillin-ceftriaxone combination is associated with similar cure rates and fewer adverse events compared to ampicillin-gentamicin 4
- This avoids aminoglycoside nephrotoxicity while maintaining efficacy
For E. faecium
Do NOT use the ampicillin-ceftriaxone combination for E. faecium, even if ampicillin-susceptible 4, 5
- Recent 2025 data demonstrates that despite in vitro synergy, the amoxicillin/ceftriaxone combination requires unachievable drug concentrations in vivo for E. faecium 5
- The minimal synergistic concentrations are consistently higher for E. faecium (median ceftriaxone 32 mg/L) compared to E. faecalis (ceftriaxone 2 mg/L) 5
- For ampicillin-susceptible E. faecium, ampicillin plus gentamicin may be used, though data quality is limited 4
- Consider limiting gentamicin to 2 weeks to mitigate toxicity while providing most benefit 4
For Multidrug-Resistant E. faecium (VRE)
Linezolid or high-dose daptomycin (10-12 mg/kg/day) are the only viable options 1
- All isolates in multiple studies remained susceptible to linezolid, vancomycin, and tigecycline 6, 7, 8
- Daptomycin monotherapy has variable success rates and emergence of resistance during treatment is documented 1
- Consider daptomycin combination therapy with ampicillin or ceftaroline, particularly for strains with daptomycin MICs in the higher susceptible range 1
- Quinupristin-dalfopristin is active only against E. faecium (not E. faecalis) but rarely used due to severe side effects including intractable muscle pain 1
Critical Clinical Pitfalls
Piperacillin-tazobactam exposure is an independent risk factor for E. faecium bacteremia 9. The proportion of E. faecium cases increased from 41% to 51% between 2015-2021, parallel to expanding pip/taz use 9. This antibiotic selection pressure favors E. faecium over E. faecalis.
Hospital-acquired infections and prior carbapenem exposure are also independent risk factors for E. faecium 9. These patients require heightened suspicion for E. faecium rather than E. faecalis.
Resistance Patterns to Monitor
- High-level aminoglycoside resistance (HLAR) occurs in ~38% of enterococcal isolates overall, eliminating synergy with β-lactams 3
- E. faecalis harbors broader virulence determinants compared to E. faecium, explaining its higher pathogenic potential despite better antimicrobial susceptibility 6, 7, 10
- Erythromycin resistance is nearly universal in both species 6
- Resistance to tetracycline, gentamicin, ciprofloxacin varies but is more prevalent in clinical versus environmental isolates 7
The species distinction is clinically critical: always obtain species-level identification and susceptibility testing before finalizing therapy, as empiric treatment appropriate for E. faecalis may fail catastrophically against E. faecium.