What is the first-line oral antibiotic for a foot infection caused by Enterococcus?

First-Line Oral Antibiotic for Enterococcus Foot Infection

Amoxicillin is the first-line oral antibiotic for enterococcal foot infections when the organism is susceptible to penicillin/ampicillin. 1, 2

Primary Treatment Algorithm

Step 1: Obtain Susceptibility Data

• Culture and susceptibility testing should be obtained before initiating therapy when possible 2
• Test for ampicillin/penicillin susceptibility (MIC ≤8 mg/L defines susceptibility) 3
• If immediate treatment is required before susceptibilities are available, proceed to empirical therapy 2

Step 2: Choose Antibiotic Based on Susceptibility

For Penicillin-Susceptible Enterococcus:

• Amoxicillin (oral formulation of ampicillin) is the preferred agent 1, 2
• This recommendation comes from IDSA guidelines for prosthetic joint infections and diabetic foot infections, which consistently list amoxicillin/ampicillin as first-line for enterococcal infections 1
• Amoxicillin has poor oral bioavailability compared to other agents, but remains the standard of care when susceptibility is confirmed 1

For Penicillin-Resistant or Unknown Susceptibility:

• Linezolid 600 mg orally twice daily is the alternative first-line agent 1, 2, 4
• Linezolid maintains 97-99% susceptibility against enterococci, including multidrug-resistant strains 2, 5, 6, 7
• Linezolid has excellent oral bioavailability (equivalent to IV formulation) 1
• FDA-approved for complicated skin and soft tissue infections including diabetic foot infections 1, 5

Step 3: Treatment Duration

• 7-14 days for uncomplicated foot infections without systemic involvement 2, 4
• Assess clinical response within 72 hours; lack of improvement warrants reassessment 2

Important Clinical Considerations

Advantages of Amoxicillin (When Susceptible)

• More cost-effective than linezolid 2
• Narrower spectrum reduces collateral damage to normal flora 1
• Established track record for enterococcal infections 1

Advantages of Linezolid

• Can be started empirically before susceptibilities are known 2, 4
• Active against vancomycin-resistant enterococci (VRE) 1, 2, 5, 6, 7
• Excellent tissue penetration in diabetic foot infections 1, 8
• Bacteriostatic against enterococci but clinically effective 4, 7

Critical Pitfalls to Avoid

• Never use cephalosporins as monotherapy - enterococci are intrinsically resistant 3, 4, 9
• Do not use clindamycin - while listed in some guidelines for streptococcal infections, it lacks reliable enterococcal coverage 1
• Avoid fluoroquinolones (levofloxacin, ciprofloxacin) - these are not recommended for enterococcal foot infections despite being listed for other pathogens 1

Monitoring for Linezolid

• Watch for bone marrow suppression (thrombocytopenia occurs in ~2% of patients) 4, 5, 6
• Monitor for peripheral neuropathy with prolonged use (>14 days) 2, 4
• Be aware of drug interactions, particularly with serotonergic agents 2, 4

Special Situations

Moderate to Severe Infections

• For moderate infections requiring initial parenteral therapy, start with IV ampicillin or vancomycin, then transition to oral amoxicillin or linezolid based on susceptibilities 1
• The 2012 IDSA diabetic foot infection guidelines recommend ampicillin-sulbactam or ertapenem for moderate infections with broader coverage, then narrowing to amoxicillin once enterococcus is identified 1

Diabetic Foot Infections

• Enterococci are common in diabetic foot infections, particularly in chronic wounds 1
• Linezolid has been specifically studied in diabetic foot infections with documented efficacy 1, 5, 8
• Consider broader initial coverage if polymicrobial infection is suspected, then narrow to enterococcal-specific therapy 1

Vancomycin-Resistant Enterococcus (VRE)

• Linezolid is the oral agent of choice for VRE 1, 2, 4, 5, 6, 7
• Alternative is daptomycin, but this requires IV administration 1, 4

Practical Treatment Approach

If susceptibilities are known: Use amoxicillin for susceptible strains 1, 2

If susceptibilities are unknown and empirical treatment is needed: Start linezolid 600 mg orally twice daily, then switch to amoxicillin if organism proves ampicillin-susceptible to optimize cost and spectrum 2, 4

If patient has penicillin allergy: Linezolid is the preferred alternative 1, 2