Which oral antibiotic is recommended for left lower leg cellulitis with a wound culture showing susceptibility to ampicillin, benzylpenicillin, streptomycin, and vancomycin, and resistance to gentamicin?

Oral Ampicillin is the Recommended Treatment

Based on your culture susceptibilities showing benzylpenicillin and ampicillin susceptibility, oral ampicillin 500 mg four times daily is the appropriate choice for this cellulitis. 1

Rationale for Ampicillin

Your wound culture demonstrates clear susceptibility to penicillins (benzylpenicillin and ampicillin), which directly targets the causative organism. Since the organism is susceptible to ampicillin and you have an oral formulation available, this is the most targeted and appropriate therapy.

• Ampicillin provides excellent coverage for beta-hemolytic streptococci and methicillin-sensitive Staphylococcus aureus, the most common causes of non-purulent cellulitis 2
• The organism's susceptibility pattern (sensitive to penicillins, resistant to gentamicin) is consistent with streptococcal species, which are the predominant pathogens in typical cellulitis
• Oral therapy is as effective as parenteral therapy for uncomplicated cellulitis 3

Dosing and Administration

• Ampicillin 500 mg orally four times daily for 5-10 days
• Must be taken on an empty stomach (30 minutes before or 2 hours after meals) with a full 8 oz glass of water 1
• For streptococcal infections, ensure at least 10 days of therapy to prevent sequelae 1

Why Not Other Options?

Vancomycin is only available intravenously for systemic infections 4 and is reserved for MRSA or severe penicillin-allergic patients—neither applies here.

Gentamicin resistance is irrelevant since your organism is penicillin-susceptible, and aminoglycosides are not used as monotherapy for cellulitis anyway.

Key Clinical Pearls

• The susceptibility to streptomycin but resistance to gentamicin further supports a streptococcal etiology (streptococci are naturally resistant to aminoglycosides) 5
• Adherence to antibiotic guidelines using penicillin-based therapy is associated with favorable outcomes in cellulitis 6
• Monitor for treatment response within 48-72 hours; failure to improve warrants reassessment
• Ensure the patient completes the full course even if symptoms improve early 1

Common Pitfalls to Avoid

• Don't empirically cover MRSA when culture shows penicillin susceptibility—this leads to unnecessary broad-spectrum therapy 2
• Don't use oral vancomycin, which is only effective for C. difficile colitis, not systemic infections 4
• Don't skip doses or take with food, as this significantly reduces ampicillin absorption 1