Clinical manifestation and Pathogenesis of Streptococcus pyogenes

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

1. Streptokinase

• 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.

2. Deoxyribonucleases

• Streptococcal deoxyribonucleases A, B, C, and D degrades DNA (DNases) and similar to streptokinase facilitate the spread of streptococci in tissue by liquefying pus.

3. Hyaluronidase

• Hyaluronidase splits hyaluronic acid, an important component of the ground substance of connective tissue. Thus, hyaluronidase aids in spreading infecting microorganisms (spreading factor).

4. 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.

5. Hemolysins

• 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

1. Erysipelas

• Portal of entry is skin
• Raised lesion and red
• Brawny edema

2. Cellulitis

• Acute
• 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
• Septicemia

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

Streptococcal sore throat

• Sub acute nasopharyngitis
• Thin serous discharge
• Fever
• Infection extend to middle ear or mastoid
• Enlarged cervical lymph nodes
• Tonsillitis
• 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

Streptococcal toxic shock syndrome and scarlet fever

• Shock
• Bacteremia
• Respiratory failure
• Multiorgan failure
• Necrotizing fasciitis
• Myositis
• Fever
• Erythema and desquamation

Poststreptococcal disease

Acute glomerulonephritis

• Blood and protein in urine
• Edema
• 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
• Fever
• Malaise
• A migratory non suppurative polyarthritis
• Inflammation of all parts of heart ( endocardium, myocardium and pericardium)
• Thickened and deformed valves