What is the recommended treatment for Methicillin-Sensitive Staphylococcus aureus (MSSA) bacteremia?

Treatment of Methicillin-Sensitive Staphylococcus aureus (MSSA) Bacteremia

For MSSA bacteremia, antistaphylococcal beta-lactams such as nafcillin, oxacillin, or cefazolin are the recommended first-line treatments due to their superior efficacy compared to vancomycin. 1, 2, 3

First-Line Treatment Options

• Preferred agents for MSSA bacteremia:

  • Nafcillin or oxacillin 2g IV every 6 hours 1
  • Cefazolin 1-2g IV every 8 hours 1, 3

• Duration of therapy:

  • Uncomplicated bacteremia: Minimum 2 weeks 1, 2
  • Complicated bacteremia (with metastatic foci): 4-6 weeks 1, 2
  • Infective endocarditis: 6 weeks 1

Special Considerations

Penicillin Allergy

• For patients with reported penicillin allergy, allergy evaluation is strongly recommended as most patients reporting penicillin allergy are not truly allergic 4
• If history excludes anaphylactic features, cefazolin can be safely used 4
• For patients with confirmed severe beta-lactam allergy:
  • Vancomycin 15-20 mg/kg IV every 8-12 hours (adjusted to achieve trough levels of 15-20 mg/L) 1
  • Daptomycin 6 mg/kg IV once daily (FDA-approved for S. aureus bacteremia) 5

Complicated Infections

• For brain abscess complicating MSSA IE, nafcillin is preferred over cefazolin due to better blood-brain barrier penetration 1
• For persistent bacteremia or treatment failure, consider:
  • Source control (removal of infected devices, drainage of abscesses) 1, 2
  • Infectious disease consultation for consideration of alternative agents 1

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
• For persistent bacteremia, evaluate for:
  • Undrained foci of infection 1
  • Metastatic complications 2
  • Endocarditis 1

Important Clinical Pearls

• Beta-lactams (nafcillin, oxacillin, cefazolin) are superior to vancomycin for MSSA bacteremia, with significantly lower treatment failure rates 6, 3
• Vancomycin should not be continued beyond empiric therapy once MSSA is identified, as it is associated with higher treatment failure rates (31.2% vs 13% for cefazolin) 6
• Addition of gentamicin to treatment regimens is not recommended for native valve endocarditis or bacteremia due to increased nephrotoxicity without improved outcomes 1
• Addition of rifampin is not recommended for uncomplicated bacteremia or native valve endocarditis 1

Treatment Algorithm

1. Initial empiric therapy (before susceptibility results):

   • Vancomycin or daptomycin (if high risk for MRSA)
   • Consider adding an antistaphylococcal beta-lactam empirically while awaiting susceptibilities 1

2. Once MSSA is confirmed:

   • Switch to nafcillin, oxacillin, or cefazolin 1, 3
   • Discontinue vancomycin to avoid unnecessary toxicity 6

3. Evaluate for complications:

   • Perform echocardiography 1
   • Assess for metastatic foci of infection 2
   • Implement source control measures 1, 2

4. Determine duration based on clinical scenario:

   • Uncomplicated: 2 weeks 1
   • Complicated: 4-6 weeks 1
   • Endocarditis: 6 weeks 1

By following this evidence-based approach, optimal outcomes can be achieved in the management of MSSA bacteremia.