Treatment of Methicillin-Sensitive Staphylococcus aureus (MSSA) Bacteremia
Antistaphylococcal beta-lactams such as nafcillin, oxacillin, or cefazolin are the first-line treatments for MSSA bacteremia due to their superior efficacy compared to vancomycin. 1
First-Line Treatment Options
- Nafcillin or oxacillin 2g IV every 6 hours is the preferred treatment for MSSA bacteremia 1
- Cefazolin 1-2g IV every 8 hours is an acceptable alternative with similar clinical outcomes compared to nafcillin/oxacillin 1, 2
- Beta-lactams demonstrate significantly better outcomes than vancomycin for MSSA infections 1, 3
- Comparative studies have shown no significant difference in mortality between nafcillin/oxacillin and cefazolin for MSSA bacteremia 2, 4
Treatment Duration
- For uncomplicated MSSA bacteremia: minimum 2 weeks of therapy 5
- For left-sided native valve endocarditis: 6 weeks of nafcillin or equivalent antistaphylococcal penicillin 6
- For complicated infections (endocarditis, metastatic foci): at least 6 weeks of therapy 6
Special Considerations
Beta-Lactam Allergies
- For patients with confirmed severe beta-lactam allergy, vancomycin 15-20 mg/kg IV every 8-12 hours is recommended 1, 6
- Vancomycin should be adjusted based on trough levels (15-20 μg/mL for serious infections) 6
Central Nervous System Involvement
- Nafcillin should be used instead of cefazolin for brain abscess resulting from MSSA infection due to better blood-brain barrier penetration 6
Hemodialysis Patients
- Cefazolin is preferred over vancomycin even in hemodialysis patients, as studies show higher treatment failure rates with vancomycin (31.2% vs. 13%) 3
Monitoring and Follow-up
- Obtain follow-up blood cultures 2-4 days after initial positive cultures to document clearance of bacteremia 1
- Echocardiography is recommended for all adult patients with S. aureus bacteremia to rule out endocarditis 1, 5
- Monitor for hepatic dysfunction when using nafcillin, as the liver/biliary tract is the primary route of nafcillin clearance 7
- Perform periodic assessment of organ system function including renal, hepatic, and hematopoietic function during prolonged therapy 7
Important Clinical Pearls
- The addition of gentamicin is not recommended for treatment of native valve endocarditis or bacteremia due to increased nephrotoxicity without improved outcomes 6, 1
- Rifampin is not recommended as adjunctive therapy for uncomplicated bacteremia or native valve endocarditis 1
- Piperacillin/tazobactam is associated with higher mortality compared to nafcillin/oxacillin/cefazolin and should not be used as definitive monotherapy for MSSA bacteremia 2
- Early transition from empiric vancomycin to beta-lactam therapy is crucial once MSSA is identified, ideally within 3 days 8
- Source control (removal of infected devices, drainage of abscesses) is a critical component of treating S. aureus bacteremia 5
Drug Interactions and Precautions
- Nafcillin may decrease the effects of warfarin; monitor prothrombin time closely when used concomitantly 7
- Nafcillin may result in subtherapeutic cyclosporine levels in organ transplant patients 7
- Monitor for penicillin allergic reactions when using beta-lactams, especially in patients with histories of significant allergies 7