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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
, 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
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