MRSA (Methicillin-resistant Staphylococcus aureus) Infection

Definition | Aetiology | Pathophysiology | Risk Factors | Signs and Symptoms | Investigations | Management

Definition

MRSA (Methicillin-resistant Staphylococcus aureus) is a strain of Staphylococcus aureus that is resistant to beta-lactam antibiotics, leading to infections ranging from mild skin and soft tissue infections to severe, life-threatening systemic infections.

Aetiology

  • Caused by Staphylococcus aureus carrying the mecA gene, which encodes penicillin-binding protein 2a (PBP2a), conferring resistance to methicillin and other beta-lactams.
  • Spread via direct contact with colonised or infected individuals or contaminated surfaces.
  • Commonly acquired in healthcare settings (hospital-acquired MRSA, HA-MRSA) or in the community (community-acquired MRSA, CA-MRSA).

Pathophysiology

  • MRSA colonises the skin and nasal passages, acting as a reservoir for infection.
  • Skin barrier breaches allow bacterial entry, leading to localised or systemic infections.
  • Produces virulence factors such as toxins and biofilms, enhancing pathogenicity and antibiotic resistance.

Risk Factors

  • Hospitalisation or residence in long-term care facilities.
  • Indwelling medical devices (catheters, central lines).
  • Recent antibiotic use, particularly broad spectrum antibiotics.
  • Immunosuppression (HIV, chemotherapy, diabetes).
  • Close contact sports or crowded living conditions.

Signs and Symptoms

  • Skin and soft tissue infections (SSTIs):
    • Boils, abscesses, cellulitis.
    • Painful, erythematous nodules, often with purulent drainage.
  • Severe infections:
    • Pneumonia (necrotising, hospital-acquired).
    • Bacteraemia and sepsis.
    • Endocarditis (particularly in IV drug users).
    • Osteomyelitis and septic arthritis.

Investigations

  • Wound swab or pus culture: confirms MRSA and determines antibiotic sensitivity.
  • Blood cultures: indicated in suspected bacteraemia or sepsis.
  • PCR for mecA gene: rapid detection of MRSA.
  • Chest X-ray: if pneumonia is suspected.
  • Echocardiography: if endocarditis is suspected.

Management

1. Skin and Soft Tissue Infections:

  • Incision and drainage of abscesses.
  • Topical antiseptics (e.g., chlorhexidine) for decolonisation.
  • Oral antibiotics: doxycycline, trimethoprim, or clindamycin.

2. Severe MRSA Infections:

  • IV vancomycin (first line for systemic infections).
  • Source control (removal of infected catheters, debridement of necrotic tissue).

3. Prevention:

  • Hand hygiene and contact precautions in healthcare settings.
  • Decolonisation therapy (nasal mupirocin, chlorhexidine body washes) in high risk patients.
  • Screening of high risk patients prior to hospital admission.

4. Referral:

  • Infectious diseases: for severe or recurrent MRSA infections.
  • Microbiology: for antibiotic resistance testing and treatment guidance.
  • Orthopaedics: if osteomyelitis or septic arthritis is suspected.
Infectious DiseasemypanotesMRSA (Methicillin-resistant Staphylococcus aureus) Infection