Is Enterococcus faecalis sensitive to cephalexin if it is sensitive to ampicillin?

Enterococcus faecalis Sensitivity to Cephalexin When Sensitive to Ampicillin

No, Enterococcus faecalis that is sensitive to ampicillin will NOT be sensitive to cephalexin. 1

Mechanism of Resistance

Enterococci, including E. faecalis, have intrinsic resistance to cephalosporins regardless of their sensitivity to ampicillin. This is a fundamental microbiological characteristic of this genus that has important clinical implications:

• Cephalosporins and antistaphylococcal penicillins (oxacillin, nafcillin) have minimal or no in vitro activity against enterococci 1
• This intrinsic resistance exists even when the organism tests sensitive to ampicillin
• The resistance is due to low affinity of cephalosporins for enterococcal penicillin-binding proteins

Clinical Implications

This resistance pattern has several important clinical implications:

• Cephalexin should not be used for treating E. faecalis infections, even when the organism is ampicillin-sensitive
• Using cephalosporins for E. faecalis infections will likely result in treatment failure
• This can lead to progression of infection, increased morbidity, and potentially mortality if the infection is serious

Appropriate Treatment Options

For E. faecalis urinary tract infections that are sensitive to ampicillin:

1. First-line options:

   • Ampicillin or amoxicillin (with or without a beta-lactamase inhibitor)
   • Amoxicillin-clavulanic acid 500/125 mg orally three times daily for 7 days 2

2. Alternative options (if penicillin allergic):

   • Nitrofurantoin (for uncomplicated lower UTIs)
   • Fosfomycin (for uncomplicated lower UTIs)
   • Vancomycin (for serious infections in penicillin-allergic patients)

Special Considerations

For Complicated Infections

For more serious infections like endocarditis or bacteremia, combination therapy is often needed:

• Ampicillin plus gentamicin has been the traditional regimen 1
• Ampicillin plus ceftriaxone has emerged as an effective alternative, especially for high-level aminoglycoside-resistant strains 1
  • This combination works through saturation of different penicillin-binding proteins
  • Note that ceftriaxone alone has no activity against enterococci

Resistance Patterns

• E. faecalis generally remains more susceptible to ampicillin than E. faecium 3
• Resistance to ampicillin in E. faecalis is less common than in E. faecium 4
• The susceptibility patterns of enterococci should be monitored as they can change over time 5

Common Pitfalls to Avoid

1. Do not assume cross-sensitivity: The sensitivity of E. faecalis to ampicillin does not predict sensitivity to cephalosporins
2. Do not use cephalosporins empirically: When treating suspected enterococcal infections, avoid cephalosporins as empiric therapy
3. Do not misinterpret susceptibility testing: Laboratory reports showing ampicillin sensitivity should not be extrapolated to cephalosporins
4. Avoid monotherapy for serious infections: For invasive enterococcal infections, combination therapy is often necessary for bactericidal activity

In conclusion, when managing urinary tract infections caused by E. faecalis, it is essential to recognize that ampicillin sensitivity does not predict cephalexin sensitivity, and appropriate antibiotic selection should be guided by this understanding to ensure effective treatment.