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Streptococcus pyogenes

Group A streptococcal infection

Organism: Streptococcus pyogenes

The group A streptococcus bacterium is responsible for most cases of streptococcal illness. Other types (B, C, D, and G) may also cause infection. Group B streptococci cause most streptococcal infections in newborns and maternal post-labor/delivery infections.

Morphology

Gram positive cocci arranged in chains. Some strains are capsulated.

Cultural characteristics

Streptococcus pyogenes grows on blood agar producing beta hemolysis.

Pathogenesis

Protein antigens

Streptococcus pyogenes is divided into types based on their content of protein M. It is the most important virulence factor as it is antiphagocytic and promotes adherence to host epithelial cells. Protein M has an important role in the pathogenesis of rheumatic fever.

Toxins and enzymes

Streptokinase

(Fibrinolysin) activates plasmin in blood which can dissolve fibrin clots, thrombi and emboli. Streptokinase has been given intravenously for the treatment of pulmonary emboli and coronary artery and venous thrombosis.

Streptodornase

(Deoxyribonulcease) digests DNA protein which is abundant in purulent exudates and is responsible for their viscosity. Both streptokinase and streptodornase are responsible for the spreading nature of streptococcal infections.

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Hyaluronidase

Hyaluronidase destroys hyaluronic acid which is the cement substance of connective tissue. This helps the spread of infection.

Erythrogenic toxin

Causes vasodilatation of capillaries leading to scarlet fever.

Streptolysins

• Streptolysin O is antigenic and antibodies to it develop in the body.
• Streptolysin S is not antigenic but is responsible for beta hemolysis.

Pyrogenic exotoxin A

Is a superantigen similar to staphylococcal TSS toxin

Exotoxin B

Is a protease responsible for the pathogenesis of necrotizing fasciitis.

Some of the major syndromes associated with group A strep infection are:

• Streptococcal pharyngitis or "strep throat"
• Scarlet fever, most often preceded by a sore throat (Scarlatina).
• Skin infections (impetigo, cellulitis/erysipelas)
• Focal infections, limited to a particular body site, e.g., pneumonia, septic arthritis
  bacteremia, sepsis, streptococcal toxic shock syndrome
•  Necrotizing fasciitis

Complications of streptococcal infections

• Acute rheumatic fever
• Poststreptococcal glomerulonephritis

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