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 agent for MSSA bacteremia, particularly for complicated infections or those involving the central nervous system 2, 1
- Cefazolin 1-2g IV every 8 hours is an equally effective alternative for most cases of MSSA bacteremia 1, 3
- Beta-lactams demonstrate significantly better outcomes than vancomycin for MSSA bacteremia, with lower treatment failure rates 1, 4
Special Considerations
Alternative Agents for Beta-Lactam Allergies
- For patients with confirmed severe beta-lactam allergy, vancomycin 15-20 mg/kg IV every 8-12 hours can be used 2, 1
- Daptomycin is FDA-approved for S. aureus bacteremia and may be considered in patients who cannot tolerate first-line therapy 5
- Clinical trials show daptomycin has comparable efficacy to standard therapy for MSSA bacteremia (success rates of 45% for daptomycin vs. 49% for comparator in MSSA infections) 5
Specific Clinical Scenarios
- For brain abscess resulting from MSSA infection, nafcillin should be used instead of cefazolin due to better blood-brain barrier penetration 2, 1
- In hemodialysis-dependent patients, cefazolin is superior to vancomycin for MSSA bacteremia (treatment failure rates: 13% vs. 31.2%) 4
- The median time to clearance of MSSA bacteremia is approximately 4 days 5
Duration of Therapy
- For uncomplicated MSSA bacteremia, 2 weeks of therapy is generally sufficient 6
- For complicated bacteremia or infective endocarditis, 4-6 weeks of therapy is recommended 2
- The American Heart Association recommends 6 weeks of nafcillin for uncomplicated left-sided native valve endocarditis caused by MSSA 2
Important Clinical Pearls
- Gentamicin should not be used for treatment of native valve endocarditis caused by MSSA due to increased nephrotoxicity without improved outcomes 2, 1
- Empirical combination therapy with vancomycin plus an antistaphylococcal β-lactam antibiotic while awaiting susceptibility results has uncertain benefit 2
- Follow-up blood cultures should be obtained 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, 6
- Source control is critical for successful treatment, including removal of infected intravascular devices and drainage of abscesses 6
- Once-daily treatment options (ceftriaxone, ertapenem) are not recommended as first-line therapy for MSSA bacteremia despite their convenience 7
Treatment Algorithm
- Initial empiric therapy: Start with vancomycin or daptomycin if MRSA is a concern 6
- Once MSSA is confirmed: De-escalate to nafcillin, oxacillin, or cefazolin 1, 8
- For CNS involvement: Prefer nafcillin over cefazolin 2, 1
- For beta-lactam allergy: Use vancomycin or daptomycin 1, 5
- Evaluate for metastatic foci of infection and ensure appropriate source control 6
- Continue therapy for appropriate duration based on complexity of infection 2, 6