Untitled Quiz

Pathogenesis and Virulence Factors

• Staphylococcal toxins are encoded on chromosomal elements called pathogenicity islands.
• Toxic shock syndrome toxin is produced by strains that cause toxic shock syndrome.
  • This toxin is a superantigen.
  • It causes fever, a rash, shock, and multisystem involvement.
• Enterotoxins are produced by 50% of Staphylococcus aureus strains.
  • These toxins are heat-stable and resistant to gut enzymes.
  • They cause diarrhea and vomiting associated with staphylococcal food poisoning.
  • The toxins are also superantigens.
  • There are multiple types of enterotoxins (A-E, G-J, K-R, and U,V).
• Panton-Valentine Leukocidin is produced mainly by community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA).
  • It is encoded on a mobile phage.
  • It lyses leukocytes, releasing inflammatory mediators that lead to necrosis and severe inflammation.

Cell Wall Components

• Protein A binds to the Fc portion of IgG at the complement binding site, preventing complement activation.
  • This reduces opsonization and phagocytosis of the organism.
• Teichoic Acid mediates adherence to mucosal surfaces.
  • It also has endotoxin-like properties.
• Peptidoglycan forms a microcapsule that is anti-phagocytic.
• Polysaccharide is also anti-phagocytic.

Enzymes and Toxins

• Coagulase is a surface protein that causes coagulation of plasma.
  • This results in fibrin deposition around lesions.
  • This rendering the organism more resistant to phagocytosis.
• Clumping Factor is an adhesin that allows for adherence to fibrinogen and fibrin in plasma.
  • It promotes aggregation of the organism and promotes attachment to blood clots and traumatized tissues.
  • It increases the invasive pathogenic potential of the bacteria.
• Catalase limits the ability of phagocytic cells to kill bacteria by degrading hydrogen peroxide (H2O2).
• Exfoliative Toxins consist of two distinct proteins (A and B).
  • They are epidermolytic and cause desquamation seen in the staphylococcal "scalded skin syndrome" in young children.
  • These toxins are superantigens.
• Hemolysins and other enzymes are produced by Staphylococcus aureus.

Diseases Caused by Staphylococcus aureus

• Pyogenic (Pus-Forming) Staphylococcal Diseases
  • Focal Suppuration
    • Abscess formation is common in Staphylococcal infections.
    • The presence of coagulase contributes to abscess formation by depositing fibrin around the lesions.
    • Examples include folliculitis, carbuncles, boils, and abscesses.
  • Invasive Conditions
    • Organisms can spread via lymphatics or blood (bacteremia) to other parts of the body.
    • This can cause deep-seated lesions such as osteomyelitis, necrotizing pneumonia, empyema, endocarditis, meningitis, multiple abscesses in tissues, and septicemia.
  • Outbreaks
    • Hospital-acquired post-operative wound infections are common, particularly with antibiotic-resistant strains.
    • Foreign bodies, such as sutures and intravenous catheters, predispose to these infections.
• Toxigenic Staphylococcal Diseases
  • Food Poisoning
    • This is caused by ingestion of preformed enterotoxin in contaminated food that is improperly cooked and kept unrefrigerated.
    • The source of contamination is often the hands or nose of a cook or food handler.
    • Carbohydrate-rich foods such as cakes, pastry, and milk products are often the source of food poisoning.