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Staphylococcus aureus

Up Coag. -ve staph. Listeria Staphylococcus aureus Streptococci


Factors related to pathogenicity

  • S. aureus cells express surface proteins that promote attachment to host proteins that form part of the extracellular matrix on epithelial and endothelial cell surfaces as well as being a component of blood clots.
  • These binding proteins are probably also involved in the binding of S. aureus to biomaterials such as prosthetic valves and intravascular catheters
  • Interbacterial adherence results in biofilm formation on biomedical devices.
  • S. aureus can adhere to the surface of endothelial cells and may be internalised by a process similar to phagocystosis
  • Some S. aureus have an outer capsule which interferes with opsonization and therefore phagocytosis
  • Toxin production

Toxins

  • a , b and d toxins damage host cell membranes
  • g toxin and Panton-Valentine leukocidin act synergistically to damage neutrophils, monocytes and macrophages
  • Enterotoxins and toxic shock syndrome toxin-1 cause toxic shock syndrome when released systemically
  • Epidermolytic toxins cause widespread blistering of the skin and exfoliation.

MRSA

  • increasing problem in many parts of the world
  • infection with MRSA appears to be associated with higher mortality after controlling for confounding factors, despite no increase in virulence. May be related to poor tissue penetration of glycopeptides
  • no apparent advantage in giving quinupristin/dalfopristin compared to vancomycin but retrospective subgroup analysis suggests that linezolid may result in better outcomes than vancomycin in nosocomial pneumonia, especially ventilator associated pneumonia

Deep infections

Wide variety of deep tissue infections including:

  • osteomyelitis, arthritis

  • endocarditis

  • cerebral, pulmonary and renal abscesses

  • breast abscesses in lactating women

  • pneumonia (almost always secondary to viral infection)

Infections often metastatic (may occur in 25% of bacteraemic patients)

Predisposing factors:

  • Alcoholism

  • Malignancy

  • old age

  • corticosteroids

  • cytotoxics

Wound infection

Risk of developing wound infection depends on interaction between:

  • Host factors

  • Surgical factors (eg disruption of tissue perfusion, use of foreign bodies)

  • Staphylococcal factors

  • Use of antimicrobial prophylaxis

Staphylococcal scalded skin syndrome

  • Group of primarily cutaneous diseases including generalized scalded skin syndrome (toxic epidermal necrolysis), staphylococcal scalded skin syndrome and bullous impetigo
  • Caused by epidermolytic toxins

Generalized scalded skin syndrome

  • Results from systemic absorption of epidermolytic toxin produced in a staphylococcal lesion
  • Intraepidermal desquamation occurs at sites which may be remote from the lesion and from which S. aureus cannot be isolated
  • Most common in neonates and children <8 years
  • Face, axilla and groin are most often affected first but generalized involvement of the skin may follow
 

©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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