What is Methicillin-Sensitive Staphylococcus aureus (MSSA) bacteremia?

Methicillin-Sensitive Staphylococcus aureus (MSSA) Bacteremia

Methicillin-Sensitive Staphylococcus aureus (MSSA) bacteremia is a serious bloodstream infection caused by Staphylococcus aureus bacteria that remain susceptible to methicillin and other beta-lactam antibiotics, requiring prompt identification and targeted antibiotic therapy to reduce mortality and complications. 1, 2

Definition and Diagnosis

• MSSA bacteremia is defined as the presence of methicillin-susceptible Staphylococcus aureus in the bloodstream, confirmed through positive blood cultures 1
• Rapid multiplex polymerase chain reaction (rmPCR) testing can accurately identify S. aureus and determine methicillin susceptibility directly from positive blood culture bottles, allowing for faster diagnosis and appropriate treatment 1
• Implementation of rapid PCR testing has been shown to decrease time to optimal MSSA antibiotic therapy by approximately 20 hours and reduce duration of bacteremia 3

Clinical Presentation and Evaluation

• Patients with MSSA bacteremia require a thorough evaluation including:

  • Repeat blood cultures to document clearance of bacteremia 1, 2
  • Echocardiography (transthoracic echocardiogram) to evaluate for infective endocarditis 1, 2
  • Clinical assessment to identify the source and extent of infection 1

• Risk stratification is recommended to guide further workup:

  • Low-risk: hospital-acquired, blood cultures clear in <48 hours, no persistent fever, timely antibiotic start 1
  • High-risk: risk factors for endocarditis, implanted prostheses, history of injection drug use, blood cultures positive >48 hours 1

Treatment

First-Line Therapy

• Antistaphylococcal beta-lactams are the preferred first-line treatment for MSSA bacteremia 4, 2:
  • Nafcillin or oxacillin 2g IV every 6 hours 2
  • Cefazolin 1-2g IV every 8 hours as an effective alternative 4, 2
  • These agents are superior to vancomycin for MSSA bacteremia with significantly lower treatment failure rates 2

Alternative Therapies

• For patients with severe beta-lactam allergies:
  • Vancomycin 15-20 mg/kg IV every 8-12 hours 2
  • Daptomycin 6 mg/kg IV once daily (FDA-approved for S. aureus bacteremia) 5

Duration of Therapy

• For uncomplicated bacteremia: minimum of 2 weeks of therapy 1, 2
• For complicated bacteremia: 4-6 weeks of therapy 1, 2
• For infective endocarditis: 6 weeks of therapy 1, 2

Monitoring and Follow-up

• Obtain follow-up blood cultures 2-4 days after initial positive cultures to document clearance of bacteremia 1, 2
• Persistent bacteremia (>48-72 hours) is a poor prognostic sign and should prompt additional investigation for metastatic foci of infection 1
• Some patients may have intermittently negative blood cultures prior to complete clearance (the "skip phenomenon"), which has implications for timing of reimplantation of cardiac devices or intravascular catheters 1

Complications and Special Considerations

• MSSA bacteremia can lead to metastatic infections including:

  • Infective endocarditis
  • Osteomyelitis
  • Septic arthritis
  • Deep tissue abscesses

• Source control is critical for successful treatment:

  • Surgical debridement and drainage of associated soft-tissue abscesses 1
  • Removal of infected intravascular devices 1

Common Pitfalls to Avoid

• Failure to transition from empiric broad-spectrum coverage to targeted beta-lactam therapy once MSSA is confirmed 4, 3
• Inadequate duration of therapy, especially in complicated cases 2
• Failure to obtain follow-up blood cultures to document clearance of bacteremia 1
• Inadequate source control, such as delayed drainage of abscesses or removal of infected devices 1
• Using vancomycin for MSSA when beta-lactams are available and not contraindicated 2

Prognosis

• MSSA bacteremia carries significant morbidity and mortality, though outcomes are generally better than with MRSA bacteremia 6
• Early appropriate antibiotic therapy and source control are the most important factors in improving outcomes 2, 3