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

Treatment of Methicillin-Sensitive Staphylococcus aureus (MSSA) Infections

Beta-lactam antibiotics, specifically oxacillin, nafcillin, or cefazolin, are the recommended first-line treatments for MSSA infections due to their superior efficacy compared to vancomycin. 1

First-Line Treatment Options

• Antistaphylococcal penicillins are the preferred treatment for MSSA infections:

  • Oxacillin: 250-500 mg IV every 4-6 hours for mild to moderate infections; 1 gram IV every 4-6 hours for severe infections 2
  • Nafcillin: 2 g IV every 6 hours 3
  • Dicloxacillin: 125-250 mg PO every 6 hours (mild to moderate infections) 4

• Cefazolin is an effective alternative to antistaphylococcal penicillins:

  • 0.5-1 g IV every 8 hours 3
  • Particularly useful for patients with non-anaphylactic penicillin allergy 5

Treatment by Infection Type

Skin and Soft Tissue Infections

• For simple abscesses or boils, incision and drainage may be adequate 3
• For purulent cellulitis:
  • Clindamycin: 300-450 mg PO TID 3
  • TMP-SMX: 1-2 DS tablets PO BID 3
  • Doxycycline: 100 mg PO BID 3

Bacteremia and Endocarditis

• For uncomplicated MSSA bacteremia: minimum 2 weeks of therapy 1
• For complicated bacteremia: 4-6 weeks of therapy 1
• For MSSA endocarditis: 6 weeks of nafcillin or oxacillin 3
• Blood cultures should be obtained 2-4 days after initial positive cultures to document clearance of bacteremia 1

Pyomyositis

• Cefazolin or antistaphylococcal penicillin (nafcillin or oxacillin) is recommended 3
• Early drainage of purulent material is essential 3
• Initial IV antibiotics, followed by oral antibiotics once clinically improved 3
• Total duration of 2-3 weeks is recommended 3

Special Considerations

Penicillin Allergy

• Patients with reported penicillin allergy should have their allergy evaluated 5
• For non-anaphylactic penicillin allergy, cefazolin is preferred over vancomycin 5
• For severe penicillin allergy, vancomycin is recommended 3

Pediatric Dosing

• Oxacillin: 50-100 mg/kg/day IV in equally divided doses every 4-6 hours 2
• Dicloxacillin: 12.5-25 mg/kg/day PO in equally divided doses every 6 hours 4
• Clindamycin: 10-13 mg/kg/dose PO every 6-8 hours, not to exceed 40 mg/kg/day 3

Duration of Therapy

• For severe staphylococcal infections: minimum 14 days 2
• For endocarditis and osteomyelitis: 6 weeks or longer 3, 2
• Continue therapy for at least 48 hours after patient becomes afebrile, asymptomatic, and cultures are negative 2

Common Pitfalls to Avoid

• Using vancomycin for MSSA infections when beta-lactams can be used - beta-lactams have superior efficacy 1, 5
• Inadequate duration of therapy, especially for complicated infections 2
• Failure to obtain follow-up blood cultures to document clearance of bacteremia 1
• Inadequate source control (e.g., drainage of abscesses, removal of infected devices) 1
• Using once-daily antibiotics like ceftriaxone as primary therapy for MSSA bacteremia - evidence is insufficient 6

Monitoring

• Assess clinical response within 48-72 hours of initiating therapy 7
• Adjust antibiotics based on culture and susceptibility results 7, 1
• For bacteremia, repeat blood cultures to document clearance 1

Beta-lactam antibiotics remain the cornerstone of MSSA treatment due to their superior efficacy and safety profile compared to alternatives like vancomycin 1, 8, 5.