MRSA

Overview

  • Staphylococcus aureus has innovative mechanisms to evade host defenses.
    • Produces molecules that inhibit neutrophils and phagocytosis.

Methicillin Resistance

  • History: Resistance was present by the 1950s.
    • Methicillin is an old beta-lactam antibiotic.
    • The term now applies to strains of Staphylococcus aureus resistant to all beta-lactams.

Acquisition of Resistance

  1. Intrinsic Resistance:
    • Innate resistance of the cell wall to certain antibiotics.
  2. Acquired Resistance:
    • Acquisition of the mecA gene:
      • Encodes for penicillin-binding protein (PBP).
      • Binds to beta-lactam antibiotics, reducing their effect on the cell wall.
    • Formation of glycocalyx and biofilm when adherent to metalware:
      • Increases resistance by over 100 times.

Community-Acquired MRSA

  • More virulent and resistant than hospital-acquired strains.
  • Produce cytotoxins causing:
    • Skin necrosis.
    • Leucocyte destruction.
  • Possess the mecA gene and penicillin-binding protein.

MRSA Carriage

  • Nasal Carriage:
    • Strains match those cultured from surgical site infections (SSI) in 85%.
    • May spread to the bloodstream during intubation.
    • Represents skin colonization.
  • High-Risk Colonization Areas:
    • Skin, groin, and axilla.
  • Risks of Nasal Colonization with Staphylococcus aureus:
    • Greatest independent risk factor for SSI.
    • 9 times higher risk than uncolonized individuals.
    • Fourfold increase in infection risk.

Screening

  • Literature supports institution-wide screening, leading to a significant reduction in post-operative SSIs.
    • Screening reduces transmission to elective patients undergoing arthroplasty.
  • Routine Pre-Operative Screening:
    • Swabs from nasal, groin, and axilla regions for Staphylococcus aureus.
    • 20% of patients are Staphylococcus carriers.
    • 5% are MRSA carriers.

Treatment

  • Standard Treatment:
    • Mupirocin nasal ointment and chlorhexidine washes for 5 days.
    • Re-swab and repeat if still positive.
    • Require 3 clear swabs at different times post-treatment prior to surgery.
  • Success Rate:
    • 90% eradication of nasal carriage.

Antibiotic Prophylaxis in Arthroplasty

  • For MRSA-Treated Patients:
    • Use of vancomycin is recommended.
  • Routine Vancomycin Use:
    • Not advised due to the risk of VRSA strain development.
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