Staphylococcus aureus and MRSA Study Notes

Staphylococcus aureus Overview

  • Classification: Gram-positive bacterium, first described in 1880.

  • Environment: Produces catalase, survives anaerobically, colonizes skin and respiratory system.

  • Diseases: Causes infections like scalded skin syndrome and endocarditis.

Virulence Factors

  • Stress Resistance: Highly cross-linked peptidoglycan, lipoteichoic acids, and staphyloxanthin provide protection against desiccation and reactive oxygen species.

  • Immune Manipulation: Inhibits complement activation, chemotaxis, and lyses leukocytes. Corrupts immune response.

Key Virulence Mechanisms

  • Adhesins: Bind to host tissues and inhibit immune response.

  • Coagulases: Form staphylothrombin, aiding in evasion of phagocytosis.

  • Toxins: Alpha-toxin, beta-toxin, and gamma-toxin damage host cells.

  • Superantigens: Activate T-cell proliferation, linked to severe immune responses.

Scalded Skin Syndrome

  • Characteristics: Primarily affects children; symptoms include skin peeling, blisters, fever, and Nikolsky sign.

Sepsis and Septicemia

  • Septicemia: Blood infection with high mortality (30-50%); can lead to organ failure.

Diagnostics

  • Culture Technique: Mannitol-salt agar (MSA) differentiates S. aureus by mannitol fermentation.

  • Tests: Catalase test (bubbles in hydrogen peroxide) and coagulase test (agglutination).

Treatment Options

  • Antibiotics: Oxacillin, cephalosporins, clindamycin, linezolid, vancomycin, mupirocin for skin infections.

Methicillin-Resistant Staphylococcus aureus (MRSA)

  • Types: Hospital-acquired (HA-MRSA) and community-acquired (CA-MRSA); classified based on mecA gene encoding PBP2a.

  • Reservoirs: Can carry MRSA without symptoms; colonization versus infection.

Trends in Bacteraemia Rates (2025)

  • MSSA: 24.3 per 100,000 (community onset).

  • MRSA: 1.3 per 100,000 (community onset).

Prophylaxis and Infection Control

  • Practices: Hand hygiene, use of protective equipment, screening for infections, isolating infected individuals.

Other Notes

  • S. lugdunensis: Coagulase-negative staphylococci that inhibit S. aureus colonization through production of lugdunin.