MRCEM Notes

Staphylococcus aureus

Microbiology
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Background

Staphylococcus aureus is a Gram-positive, catalase-positive, coagulase-positive coccus that typically appears in grape-like clusters on microscopy. It is a facultative anaerobe and produces a variety of exotoxins. It is commonly found as a commensal organism in the anterior nares, skin, perineum, and other mucosal sites.

Transmission

Up to 40% of individuals carry S. aureus asymptomatically, particularly in the nose, axilla, and perineum. Transmission occurs via:
  • Direct contact with colonised individuals
  • Indirect contact via contaminated fomites (e.g. clothing, medical equipment)
  • Respiratory droplets, particularly in crowded environments
Endogenous infection can occur when the skin barrier is compromised or the immune system is suppressed.

Virulence

A number of factors determine how virulent strains of Staphylococcus aureus are. One key virulence factor is the expression of Protein A which binds the Fc region of IgG, interfering with opsonization, phagocytosis and complement activation.

Clinical Features

Localised infections:
  • Skin and soft tissue infections: impetigo, abscesses, cellulitis, infected eczema
  • Wound infections, particularly post-operative
  • Device-associated infections: intravenous lines, prosthetic joints
Invasive infections:
  • Pneumonia (especially post-influenza; often cavitating)
  • Infective endocarditis: rapid, destructive, and common in intravenous drug users
  • Osteomyelitis
  • Septic arthritis
  • Bacteraemia/sepsis
Toxin-mediated syndromes:
  • Toxic shock syndrome (TSS)
  • Staphylococcal scalded skin syndrome (SSSS)
  • Food poisoning: rapid onset due to preformed enterotoxins

MRSA (Methicillin-Resistant Staphylococcus aureus)
MRSA strains are resistant to beta-lactam antibiotics and are common in healthcare settings. Risk factors include:
  • Prior antibiotic use
  • Long hospital stays
  • ICU admission
  • Surgical wounds
  • Dialysis
  • Indwelling devices

MRSA causes the same range of diseases as methicillin-sensitive strains.

Treatment
Methicillin-sensitive S. aureus (MSSA):
  • First-line: Flucloxacillin

MRSA:
  • First-line: Vancomycin or Teicoplanin
  • Alternatives: Linezolid, Daptomycin (depending on site and severity)

Prevention
Infection control: isolate MRSA-positive patients, use barrier nursing

Decolonisation:
  • Mupirocin nasal ointment
  • Chlorhexidine body washes

Hand hygiene and environmental cleaning in healthcare settings are key to reducing spread
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