What is the recommended dosage of oral ampicillin for a patient with normal renal function and no known allergy to penicillins (PCN) with an Enterococcus faecalis wound infection?

Oral Ampicillin Dosing for Enterococcus faecalis Wound Infection

For an uncomplicated Enterococcus faecalis wound infection in a patient with normal renal function and no penicillin allergy, oral amoxicillin 500 mg three times daily (total 1500 mg/day) for 7-14 days is the recommended treatment, as ampicillin has poor oral bioavailability and amoxicillin is the preferred oral beta-lactam formulation. 1

Critical Context: Oral vs. IV Therapy

• Oral ampicillin is not recommended due to poor and erratic oral bioavailability (approximately 40%), making it unsuitable for reliable treatment of enterococcal infections 1
• Amoxicillin is the oral beta-lactam of choice for E. faecalis infections, as it achieves superior oral absorption compared to ampicillin while maintaining equivalent antimicrobial activity 1
• All guideline recommendations for "ampicillin" in enterococcal infections refer to intravenous administration at doses of 2 g every 4-6 hours (12 g/day total) 2, 3

Recommended Oral Regimen

For uncomplicated wound infections:

• Amoxicillin 500 mg orally three times daily (every 8 hours) 1
• Duration: 7-14 days depending on clinical response and source control 3, 1
• The full 1500 mg total daily dose is necessary for adequate tissue penetration 1

When IV Therapy is Required Instead

You must use IV ampicillin (not oral) if:

• The wound infection shows signs of systemic involvement (bacteremia, sepsis) 3
• There is deep tissue involvement, osteomyelitis, or prosthetic material present 4
• The patient has endocarditis (requires IV ampicillin 2 g every 4 hours plus gentamicin or ceftriaxone) 2, 3
• Blood cultures are positive for E. faecalis 3

IV ampicillin dosing when required:

• Ampicillin 2 g IV every 6 hours (8 g/day) for uncomplicated bacteremia with source control 3
• Ampicillin 2 g IV every 4 hours (12 g/day) for more serious infections or when combination therapy is needed 2

Essential Pre-Treatment Steps

• Confirm susceptibility testing before relying on oral therapy, as E. faecium (not E. faecalis) may be ampicillin-resistant 1
• Ensure adequate source control of the wound infection, as failure to achieve this will result in treatment failure regardless of antibiotic choice 3
• Rule out bacteremia with blood cultures if the patient has fever, systemic symptoms, or the wound infection is not responding to initial therapy 3

Common Pitfalls to Avoid

• Do not use oral ampicillin - it has poor bioavailability and is not recommended for enterococcal infections 1
• Do not underdose amoxicillin - the full 500 mg three times daily is required, not 250 mg doses 1
• Do not assume all enterococci are E. faecalis - E. faecium has different resistance patterns and may not respond to amoxicillin 1
• Do not use fluoroquinolones (such as levofloxacin) as they lack reliable activity against enterococci 3
• Do not use amoxicillin for vancomycin-resistant strains - these require linezolid 600 mg PO every 12 hours instead 1

Step-Down Therapy Option

• Oral amoxicillin can serve as step-down therapy after initial IV treatment once clinical improvement is documented 1
• This approach is appropriate for patients who initially required IV therapy but have achieved source control and clinical stability 1

Monitoring and Follow-Up

• Reassess at 48-72 hours to ensure clinical improvement 3
• Obtain follow-up cultures if bacteremia was present to document clearance 3
• Consider transesophageal echocardiography if bacteremia persists >72 hours despite appropriate therapy to rule out endocarditis 3