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Virulence factors of Staphylococcus aureus
A. Cell wall components
- Polysaccharide Capsule: inhibits phagocytosis
- Peptidoglycan: activates complement, IL-1, chemotactic to PMNs
- Teichoic acid: species specific, mediate binding to fibronectin
- Protein A: It binds to the Fc region of IgG and complement ,exerting an anti-opsonin effect.
- Fibronectin binding protein(FnBP): promote binding to ,mucosal cells and tissue matrices.
- Clumping factor: FnBP enhances clumping of organism in presence of plasma.
- Catalase enzyme: conversion of hydrogen peroxide into water and oxygen
- Coagulase enzyme and clumping factor: an enzyme like protein that clots oxalated or citrated plasma.
- Other enzymes: hyaluronidase(spreading factor), staphylokinase(fibrinolysis), protienases, lipases, β- lactamases
- Exotoxins: comprises of four toxins α,β,γ,δ: also called hemolysin,
– α exotoxins- heterogenous protein acts on broad spectrum of eukaryotic cell membranes
– β exotoxins- degardes sphingomyelin
– δ exotoxins- disrupts biological membrane
– γ exotoxins- interact with two proteins to form six potential two component toxins. All six toxins lyse WBC by pore formation in the cellular membranes that increase cation permeability
- Panton Valentine leukocidin: composed of two components S and F whichact synergistically to kill white blood cells
- Exfoliative toxins: composed of type A- located on phage and headt stable and type B- plasmid mediated and heat labile: yield generalized desquamation scalded skin syndrome
- Toxic shock syndrome toxins (TSST-1): prototypical superantigen which bind with MHC-II yielding T-cell stimulation. Toxic is associated with fever, shock and multisystem involvement.
- Enterotoxins: altogether 15 enterotoxins(A-E,G-P), heat stable, resistant to gut enzymes
Clinical manifestation of Staphylococcus aureus
A. Localized skin infections
- Infections are small superficial abscesses involving hair follicle, sweat or sebaceous glands.
- Subcutaneous abscesses called furuncles (boils) often formed around foreign bodies
- Carbuncles are larger, deeper, multiloculated skin infections that can lead to bacteremia
- Impetigo is usually localized, superficial, spreading crusty skin lesions
B. Deep localized infections
- Metastatic from superficial infections or skin carriage or may result from trauma.
- Acute infection of joint space in children.
C. Acute endocarditis
- Generally associated with intravenous drug users.
- Bacteria can be introduced into soft tissue and the bloodstream.
- Generalized infection with sepsis or bacteremia associated with a known focus or not
F. Nosocomial infections
- hospital associated infections often of wounds or bacteremia associated with catheter.
G. Toxin mediated infections
Toxic shock syndrome
- High Fever
- Multiorgan involvement
Scalded skin syndrome
- Appearance of superficial bullae
- Marked epithelial desquamation