Staphylococcus Aureus

Aka: Staphylococcus Aureus, MSSA, Methicillin-Sensitive Staphylococcus Aureus

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II. Pathophysiology

  1. Characteristics
    1. Gram Positive Cocci in clusters (as with all Staphylococcus)
  2. Structural Defenses
    1. Microcapsule (outer)
    2. Peptidoglycan Cell Wall
      1. Depends on Transpeptidase (Penicillin Binding Protein) for formation
      2. Transpeptidase is a target of Penicillins, but many Staphylococcus have developed resistance
        1. See Penicillin Binding Protein 2A below
    3. Cell Membrane (inner)
  3. Immune System Defenses
    1. Protein A
      1. Binds IgG Fc, blocking Opsonization (and Phagocytosis)
    2. Coagulase
      1. Promotes clotting (via plasmin activation, and in turn, Fibrin formation)
    3. Hemolysin (a, b, g, d)
      1. Like alpha and beta hemolytic Streptococcus, lyse Red Blood Cells
      2. May also lyse PMNs, Macrophages and Platelets
      3. Staphylococcus Aureus is named for its gold coloration on BAL
    4. Leukocidin
      1. Leukocidins kill WBCs
    5. Penicillinase
      1. Beta-Lactamase that inactivates Penicillins
      2. Penicillinase-resistant Antibiotics (e.g. Methicillin, Nafcillin) are effective against MSSA
    6. Penicillin Binding Protein 2A (MRSA strains)
      1. Encoded by a segment of acquired chromosomal DNA (mecA)
      2. Replaces transpeptidase (Penicillin Binding Protein) in Peptidoglycan cell wall assembly
        1. Transpeptidase is the typical Penicillin target
        2. MRSA strains express Penicillin Binding Protein 2A, which confers resistance to most Penicillins
  4. Tissue Invasion Mediators
    1. Hyaluronidase (spreading factor)
      1. Lyses connective tissue proteoglycans
    2. Staphylokinase
      1. Lyses Fibrin clots (similar to Streptokinase)
    3. Lipase
      1. Breaks down skin surface oils
    4. Protease
      1. Lyses tissue Proteins
  5. Exotoxins
    1. Enterotoxin
      1. See Food Poisoning
      2. See Staphylococcal Enterotoxin B
      3. Heat-stable, preformed toxin resulting in Gastroenteritis soon after ingestion (e.g. unrefrigerated cold salads, meats)
    2. Exfoliatin
      1. See Staphylococcal Scalded Skin Syndrome
      2. Strains that carry exfoliative toxins A and B (only 5% of S. aureus strains)
      3. Breaks down desmoglein-1 resulting in epidermolysis or Acantholysis (breakage of cell to cell adhesions)
        1. Results in diffuse skin slouging
    3. Panton Valentine Leukocidin (PVL) Toxin
      1. Promotes abscess formation
      2. More common with community acquired MRSA Infections
    4. Toxic Shock Syndrome Toxin (TSST-1)
      1. Similar to pyrogenic toxin from Streptococcus Pyogenes
      2. Superantigens bind Macrophages and trigger severe Cytokine release

III. Infections

  1. Skin Infections
    1. Cellulitis (esp. with abscess)
    2. Furuncle or Carbuncle
    3. Surgical Wound Infection (onset within 48 hours)
    4. Impetigo
    5. Superficial Folliculitis
    6. Toxic Shock Syndrome
    7. Staphylococcal Scalded Skin Syndrome
    8. Intravenous Catheter associated infection
    9. Hydradenitis suppurativa
    10. Mastitis
  2. Cardiopulmonary infections
    1. Staphylococcal Pneumonia (Empyema risk)
    2. Infective Endocarditis
  3. Musculoskeletal infections
    1. Osteomyelitis
    2. Septic Arthritis
    3. Septic Bursitis
  4. CNS Infections
    1. Meningitis
    2. Brain Abscess
    3. Epidural Abscess
  5. Gastrointestinal
    1. Staphylococcal Enterotoxin BFood Poisoning
      1. Unrefrigerated cold salads, meats, cream pastries
      2. Onset in 1 to 6 hours (duration to 24 to 48 hours)

IV. Management: Methicillin Resistant

  1. See Methicillin Resistant Staphylococcus Aureus

V. Management: Methicillin-Sensitive

  1. First line Antibiotics (non-Penicillin allergic)
    1. IV: Nafcillin or Oxacillin
    2. PO: Dicloxacillin
  2. Alternative agents
    1. IV: Cefazolin (Ancef)
    2. PO: Cephalexin (Keflex)

VI. References

  1. Gladwin, Trattler and Mahan (2014) Clinical Microbiology, Medmaster, Fl, p. 41-5

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