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.