USMLE

Strep pyogenes: Overview

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Bacteria - Gram Positive
  1. Staph aureus: Overview
  2. Staph aureus: Presentation
  3. Methicillin-Resistant Staph aureus (MRSA)
  4. Staph saprophyticus
  5. Strep pneumoniae: Overview
  6. Strep pneumoniae: Presentation
  7. Strep viridans
  8. Strep pyogenes: Overview
  9. Strep pyogenes: Presentation
  10. Strep agalactiae
  11. Strep bovis
  12. Enterococcus
  13. Bacillus anthracis
  14. Bacillus cereus
  15. Clostridium tetani
  16. Clostridium perfringens
  17. Clostridium botulinum
  18. Clostridium difficile
  19. Corynebacterium diphtheriae
  20. Listeria monocytogenes
  21. Nocardia
  22. Actinomyces

Streptococcus pyogenes overview 

  • Part of Group A streptococci (GAS)
  • Characteristics
    • Gram + cocci in chains
    • Catalase -
      • First step in gram positive cocci; determines staph (catalase positive) vs. strep 
    • Beta-hemolytic
      • Second step, narrows type of strep
    • Pyrrolidonyl arylamidase (PYR) +
      • Because the bacitracin test is not very specific for S pyogenes, it has been replaced in many laboratories by the pyrrolidonyl arylamidase (PYR) test
    • Bacitracin sensitive
      • Has been widely replaced with the PYR test
  • Virulence Factors
    • Hyaluronic acid capsule
    • Protein M
      • Main virulence factor for strep. pyogenes
      • Inhibits phagocytosis and activation of complement
      • Also cytotoxic to neutrophils and mediator of bacterial attachment
    • Streptolysin (Hemolysin) O
      • Lyses RBCs (causing beta-hemolysis) and polymorphonuclear leukocytes.
      • Anti-streptolysin O (ASO) titers used as marker of recent infection
    • Pyrogenic exotoxin
      • Pyrogenic = causes fever; cause of Scarlet fever
      • Superantigen that results in tissue injury and septic shock; e.g. Toxic shock syndrome
    • Streptokinase
      • Fibrinolytic (clot-buster) that is now used as a medication
      • Enables more spread from initial site of infection
    • DNAse
      • DNase is an extracellular enzyme that depolymerizes viscous DNA in pus and disintegrates polymorphonuclear leukocytes, allowing the streptococcus to move more freely in the tissue.
      • Anti-DNAse is also a titer measured as a marker of recent infection