A 38-year-old woman develops a surgical site infection 5 days after open appendectomy. Gram stain of purulent wound fluid shows gram-positive cocci in clusters. Blood culture is pending. In the absence of MRSA risk factors or beta-lactam allergy, what is the drug of choice for empiric treatment?

Explanation

## Gram-Positive Cocci SSI: Empiric Treatment **Key Point:** When gram-positive cocci in clusters are identified on Gram stain (presumed Staphylococcus aureus) in a patient without MRSA risk factors or beta-lactam allergy, **antistaphylococcal penicillins** (nafcillin or oxacillin) are the drugs of choice because they provide superior bactericidal activity and tissue penetration compared to alternatives. ### Why Nafcillin/Oxacillin? 1. **Beta-lactamase-resistant penicillins** with excellent anti-staphylococcal activity 2. **Superior bactericidal kinetics** compared to vancomycin or clindamycin 3. **Excellent tissue penetration** into infected wounds and abscess cavities 4. **First-line for methicillin-susceptible S. aureus (MSSA)** in non-allergic patients 5. **Cost-effective** compared to glycopeptides or oxazolidinones **High-Yield:** Nafcillin and oxacillin are preferred over cephalosporins for confirmed or suspected S. aureus SSI because they are antistaphylococcal penicillins with superior activity against MSSA. Cephalosporins are acceptable alternatives but are not first-line. ### Comparison Table | Agent | MSSA Activity | MRSA Activity | Tissue Penetration | First-Line Status | |-------|---------------|---------------|-------------------|-------------------| | **Nafcillin/Oxacillin** | Excellent | No | Excellent | **YES (MSSA, no allergy)** | | Cefazolin | Good | No | Good | Alternative for MSSA | | Clindamycin | Variable (resistance ~30%) | No | Good | Not first-line; resistance concern | | Linezolid | Excellent | Excellent | Good | Reserved for MRSA or allergy | **Clinical Pearl:** Once culture confirms MSSA and susceptibilities are available, continue nafcillin/oxacillin. If MRSA is confirmed, switch to vancomycin or linezolid. If beta-lactam allergy is present, clindamycin is acceptable if susceptibility is confirmed, but linezolid is preferred for serious infections. **Mnemonic:** **NAPOX** = **NA**fcillin/**OX**acillin = **P**enicillins for **S. aureus** (MSSA). Remember: penicillins > cephalosporins > glycopeptides for MSSA in non-allergic patients. **Warning:** Do not confuse antistaphylococcal penicillins with ampicillin/amoxicillin—the latter are NOT beta-lactamase-resistant and are ineffective against S. aureus.