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Streptococcus pyogenes

Streptococcus pyogenes

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Streptococci Overview
  • Streptococci grow in pairs and chains.
  • They are facultative anaerobes, and are catalase-negative.
Streptococci classification:
  • Hemolysis of blood agar:
– Alpha-hemolytic species incompletely lyse red blood cells; this casts a greenish hue around the bacteria, which include Viridans group and S. pneumoniae. – Beta-hemolytic streptococci completely lyse red blood cells, which produces a clear outline around the bacteria; this group includes S. pyogenes and S. agalactiae. – Gamma-hemolytic streptococci do not lyse red blood cells; thus, there is no outline around the bacteria. This group includes the Enterococci.
  • Lancefield Serologic groupings categorize the beta-hemolytic streptococci based upon the C-carbohydrates in their cell walls.
Streptococcus Pyogenes
Streptococcus pyogenes is beta-hemolytic; it is also known as the Group A streptococci (GAS).
Virulence factors
M-protein is one of the most important virulent factors of S. pyogenes.* – It extends from the inner cytoplasmic membrane to block complement activation and phagocytosis. Strains with different M proteins are associated with different kinds of infections.
  • Outer capsule comprises hyaluronic acid, which also prevents phagocytic clearance (in some strains).
  • Lipoteichoic acid and F proteins bind fibronectin, thus adhering the bacteria to the host's extracellular matrix.
  • M and F proteins also facilitate epithelial cell invasion, which promotes persistent infection and migration to deeper tissues.
  • M-like surface proteins bind fibronectin in the ECM; they also bind antibodies to block phagocytosis.
  • Surface C5a peptidase inactivates C5a, which would otherwise attract neutrophils and other phagocytes.
  • Toxins & Enzymes:
– Streptococcal pyrogenic exotoxins (formerly called erythrogenic toxins) are superantigens that stimulate T cells and macrophages to release IL-1, IL-2, and TNF. – Streptolysins S and O lyse blood cells. – Streptokinases A and B are enzymes that break up blood clots to promote bacterial spread. – DNases decrease pus viscosity, which also promotes bacterial spread, and aids in the evasion of Neutrophil Extracellular Traps (NET's).
Diseases
  • Streptococcal pharyngitis ("strep throat") is characterized by throat and tonsil inflammation with swollen cervical lymph nodes, in addition to fever and headache. In the image, we can see whitish tonsillar exudate.
– Antibiotic treatment, comprising penicillin and amoxicillin, is important to prevent later complications.
  • Scarlet fever is a complication of streptococcal pharyngitis; within days of the initial infection, affected patients experience a rash that first appears on the chest, then the limbs.
– Initially the tongue may be covered in a yellowish coating; this sheds, leaving behind a bright red "Strawberry tongue" surface.
  • Pyoderma, aka, impetigo, is characterized by pustules that crust upon rupture; it is most common among children.
– Pyoderma is also associated with other bacterial strains, including Staphylococcus aureus.
  • Erysipelas (erythros = red; pella = skin) is characterized by raised red lesions, typically on the face but sometimes on the legs, and is accompanied by pain, fever, and chills.
– It is most common in young children or older adults.
  • Cellulitis involves the skin and underlying tissues; affected areas are red, swollen, and warm to the touch. It is usually painful and can spread within the deep tissues.
– Be aware that Staphylococcus aureus is also associated with cellulitis; determination of the causative agent is an important part of effective treatment.
  • Necrotizing fasciitis, aka, streptococcal gangrene. In our image, the skin is significantly damaged; in more severe cases, infection spreads to deeper tissues, where it can cause multi-organ failure and death. Diabetes is thought to be a risk factor for necrotizing fasciitis.
– This disease lends S. pyogenes its nickname: "the flesh-eating bacteria."
  • Toxic Shock Syndrome via production of specific pyogenic exotoxins. Patients experience fever, chills, vomiting and diarrhea, and, as with Staphylococcus aureus-induced toxic shock syndrome, the disease can affect multiple organ systems.
– Additionally, streptococcal toxic shock syndrome is associated with bacteremia and necrotizing fasciitis. – Treatment includes administration of intravenous penicillin and clindamycin; intravenous fluids and/or vasopressors can correct hypotension, and surgical removal of necrotic tissue may be required.
Immune-mediated diseases that can occur after Streptococcal pyogenes* infection: – Acute glomerulonephritis, which is characterized by deposition of antigen-antibody complexes in the glomerular basement membrane and infiltration of neutrophils. Inflammation leads to edema, hypertension, hematuria, and proteinuria. – Rheumatic fever and heart disease can occur after streptococcal pharyngitis; inflammation cardiac tissues can damage the cardiac valves and impede blood flow. Other manifestations include inflammation of the joints and blood vessels. Patients with pharyngitis should be given antibiotics within the first 10 days of infection to prevent later development of rheumatic fever.

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