Medicine & USMLE

Staph aureus: Overview

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

Staphylococcus Aureus Overview

  • Characteristics
    • Gram + cocci in clusters
    • Catalase +
      • First test to determine staph vs. strep (catalase negative)
    • Coagulase +
      • Second test to differentiate staph aureus from coagulase negative staph (e.g. epidermidis, saprophyticus, etc)
      • Enzyme produced by Staphylococcus aureus that activates prothrombin, resulting in the conversion of fibrinogen to fibrin. This process leads to fibrin-coating of the organism and resistance to phagocytosis.
    • golden/yellow colony color
      • due to yellow pigment (staphyloxanthin) produced by S. aureus
      • Aureus = “gold” in Latin
    • beta-hemolytic
      • Staph aureus produces hemolysin, an enzyme that damages red blood cells, platelets, neutrophils, and macrophages
      • This is less used in the laboratory algorithm, since catalase and coagulase can differentiate
    • ferments mannitol
      • Mannitol fermentation is a property of S. aureus. The other Staphylococci are not able to ferment mannitol.
    • PYR-negative
      • This test is more often used for identifying beta-hemolytic strep and enterococci
    • Facultative anaerobe
  • Virulence factors
    • Protein A
      • Binds Fc region of IgG to inhibit complement activation, opsonization, and phagocytosis.
    • Pencillianse
      • secreted form of β-lactamase, which makes S. aureus resistant to some penicillins
      • Treatment of S. aureus requires penicillinase-resistant penicillins
    • Hemolysins 
      • damages red blood cells, platelets, neutrophils, and macrophages
    • Leukocidins 
      • damages white blood cells
  • Treatment
    • Penicillianse-resistant penicillins
      • Methicillin
      • Nafcillin
      • Oxacillin
      • Dicloxacillin