Virulence factors of Streptococcus pyogenes
A. Antigenic structure
- M protein: rod like coiled structure with two major structural classes; Class I and Class II ;major virulence factor;resist phagocytosis and intracellular killing by polymorphonuclear leukocytes in the absence of antibodies.
B. Toxins and enzymes
- It is also called as fibrinolysin.
- It transforms the plasminogen of human plasma into plasmin, an active proteolytic enzyme that digests fibrin and other proteins, allowing the bacteria to escape from blood clots.
- Streptococcal deoxyribonucleases A, B, C, and D degrades DNA (DNases) and similar to streptokinase facilitate the spread of streptococci in tissue by liquefying pus.
- Hyaluronidase splits hyaluronic acid, an important component of the ground substance of connective tissue. Thus, hyaluronidase aids in spreading infecting microorganisms (spreading factor).
- Pyrogenic exotoxins (Erythrogenic toxins)
- It act as superantigens, which stimulate T cells by binding to class II MHC complex and the activated T cell release cytokines that mediate shock and tissue injury.
- It is associated with Streptococcal toxic shock syndrome and scarlet fever.
- Two hemolysins are produced.
- Streptolysin O which is oxygen labile and immunogenic in nature. It induces the production of Anti-Streptolysin O(ASO) after the infection with streptococci.
- Streptolysin S which is oxygen stable and not immunogenic in nature. It s an agent responsible for the hemolytic zone around the streptococcal colonies on surface of blood agar.
Clinical manifestation of Streptococcus pyogenes
Disease attributable to invasion by S pyogenes
- Portal of entry is skin
- Raised lesion and red
- Brawny edema
- Spreading infection of the skin and subcutaneous tissue
- Pain, tenderness, swelling and erythema
3. Necrotizing fasciitis
- Rapidly spreading necrosis of skin tissue and fascia
4. Puerperal fever
- If the organism enter uterus after delivery, puerperal fever develops
5. Bacteremia and sepsis
- Infection of traumatic or surgical wounds with streptococci results in bacteremia, which can rapidly be fatal
Disease attributable to local infection with S pyogenes and their byproduct
1. Streptococcal sore throat
- Sub acute nasopharyngitis
- Thin serous discharge
- Infection extend to middle ear or mastoid
- Enlarged cervical lymph nodes
- Intense redness and edema of mucous membranes
- Purulent exudates
2. Streptococcal pyoderma
- Infection of superficial layer of skin : impetigo
- Superficial vesicles
- Denuded surface covered with pus and later encrusted
Invasive Group A Streptococcal infection
1. Streptococcal toxic shock syndrome and scarlet fever
- Respiratory failure
- Multiorgan failure
- Necrotizing fasciitis
- Erythema and desquamation
1. Acute glomerulonephritis
- Blood and protein in urine
- High blood pressure
- Urea nitrogen retention
- Low serum complement levels
- Chronic form leads to kidney failure
2. Rheumatic fever
- Most serious sequela
- Damage to heart muscles and valves
- A migratory non suppurative polyarthritis
- Inflammation of all parts of heart ( endocardium, myocardium and pericardium)
- Thickened and deformed valves