From the Guidelines
A minimum inhibitory concentration (MIC) of 1 mcg/ml for ampicillin is generally considered sufficient for treating enterococcal infections. Ampicillin remains the preferred therapy for susceptible enterococci, with standard dosing typically ranging from 2 grams IV every 4-6 hours for serious infections 1. For enterococci, susceptibility is defined as an MIC ≤8 mcg/ml, making an MIC of 1 mcg/ml well within the susceptible range. However, treatment success depends on several additional factors beyond just the MIC value.
Key Considerations
- The site of infection is important, as some sites like endocarditis or meningitis require combination therapy with an aminoglycoside (typically gentamicin) for synergistic killing 1.
- The patient's renal function may necessitate dose adjustments.
- It's also crucial to confirm the absence of beta-lactamase production, which would render ampicillin ineffective regardless of the MIC.
- The clinical response should be monitored closely, as laboratory susceptibility doesn't always translate to clinical efficacy, particularly in deep-seated or complicated infections where surgical intervention may also be necessary.
Treatment Guidelines
- For native valve endocarditis, 4-6 weeks of therapy is recommended, depending on the duration of symptoms before initiation of therapy 1.
- For prosthetic valve endocarditis, a minimum of 6 weeks of therapy is recommended 1.
- Combination therapy with ampicillin and gentamicin is recommended for enterococcal endocarditis caused by strains susceptible to both antibiotics 1.
From the Research
Susceptibility of Enterococcus to Ampicillin
- The minimum inhibitory concentration (MIC) of 1 mcg/ml for ampicillin against enterococci is considered low to intermediate resistance 2.
- However, the effectiveness of ampicillin in treating enterococcal infections depends on various factors, including the specific strain of enterococcus, the site of infection, and the presence of other resistance mechanisms 3, 4.
- Studies have shown that ampicillin can be effective against enterococci with low-level resistance, but its efficacy decreases against strains with high-level resistance or those producing beta-lactamases 3, 2.
Combination Therapy with Gentamicin
- Combining ampicillin with gentamicin can enhance its effectiveness against enterococci, particularly those with low-level resistance 3, 4.
- However, the synergy between ampicillin and gentamicin can be variable and may not always result in improved outcomes 4, 5.
- The use of combination therapy with gentamicin should be guided by susceptibility testing and clinical experience 2, 6.
Clinical Implications
- The treatment of enterococcal infections, particularly those caused by resistant strains, remains a challenge 2, 6.
- A MIC of 1 mcg/ml for ampicillin may not be sufficient to ensure effective treatment, especially in cases of serious infections such as endocarditis 3, 6.
- Further research is needed to determine the optimal treatment regimens for enterococcal infections and to improve patient outcomes 4, 6.