Treatment of Methicillin-Sensitive Staphylococcus Aureus (MSSA) Bacteremia
Antistaphylococcal beta-lactams (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 equally effective alternative to nafcillin/oxacillin for MSSA bacteremia 1, 2
- For uncomplicated left-sided native valve endocarditis caused by MSSA, a 6-week course of nafcillin (or equivalent antistaphylococcal penicillin) is recommended 3
- For complicated left-sided endocarditis (e.g., perivalvular abscess formation, septic metastatic complications), at least 6 weeks of nafcillin is recommended 3
Clinical Considerations for Beta-Lactam Selection
- Cefazolin has shown similar clinical efficacy to antistaphylococcal penicillins (ASPs) in recent studies with better tolerability and lower rates of discontinuation due to adverse drug reactions 4
- There is no significant difference in mortality between cefazolin and cloxacillin (an ASP) for the treatment of MSSA bacteremia 2
- Oxacillin MIC (≥1 versus <1 μg/mL) does not appear to affect acute treatment outcomes regardless of whether ASP or cefazolin is used as definitive therapy 5
- In cases of brain abscess resulting from MSSA IE, nafcillin should be used instead of cefazolin due to better blood-brain barrier penetration 3
Alternative Treatment Options
- For patients with confirmed severe beta-lactam allergy, vancomycin 15-20 mg/kg IV every 8-12 hours can be used 1
- Daptomycin is FDA-approved for the treatment of S. aureus bacteremia, including MSSA, and may be a reasonable alternative when beta-lactams cannot be used 6
- For patients with a well-defined history of non-anaphylactoid reactions to penicillins (e.g., simple skin rash), cefazolin is a reasonable alternative 3
Monitoring and Follow-up
- 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
- The median time to clearance of MSSA bacteremia is approximately 4 days when treated with appropriate antibiotics 6
Important Clinical Pearls
- Gentamicin should not be used for treatment of native valve endocarditis caused by MSSA 3
- Beta-lactams (nafcillin, oxacillin, cefazolin) are superior to vancomycin for MSSA bacteremia, with significantly lower treatment failure rates 1
- For persistent MSSA bacteremia, some recent data suggests that adding a carbapenem (ertapenem or meropenem) to standard therapy may result in faster blood culture sterilization, though this approach requires further study 7
- Ampicillin/sulbactam may be considered as an alternative for beta-lactamase-producing MSSA bacteremia when first-line agents are unavailable, though liver dysfunction may be more common compared to cefazolin 8