Staph aureus: Overview

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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