Staph aureus: Overview

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Bacteria - Gram Positive

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