Staphylococci

Question 1

Distinction of staphylococci from other gram (+) cocci like streptococci

Answer 1

• Gram stain (+) in grape-like clusters
• Grows on blood agar, chocolate agar, but NOT on MacConkey agar
• On blood agar:
  
  • Staph are somewhat golden
  • Strep colonies look clear
• Catalase test:
  
  • Staph are positive (bubbles)
  • Strep are negative

Question 2

Staphylococcal virulence factors: cell wall

Answer 2

• Peptidoglycan
  
  • Stimulates cytokines –> aggessin, shock
• Teichoic acids
  
  • Adhesion to epithelium
• Slime (coagulase negative)
  
  • Extracellular matrix –> cements and protects
• Capsule
  
  • Extracellular matrix –> protects from phagocytosis
• Protein A
  
  • Binds IgG –> protects from phagocytosis
• Clumping factor
  
  • Binds fibrin –> large clumps, blocks phagocytosis
• Fibronectin-binding protein
  
  • Attaches to fibronectin –> attachment to cell
• PBP2a
  
  • Alters pencillin binding –> methicillin resistance

Question 3

Staphylococcal virulence factors: enzymes

Answer 3

• Catalase
  
  • Catalyzes H2O2 –> inhibits PMN killing
• Coagulase
  
  • Clots plasma –> abscess wall
• Panton-Valentine leukocidin
  
  • Destroys PMNs –> protects from phagocytosis
• Hemolysins
  
  • Cytotoxic –> tissue damage
• Lipase
  
  • Lipolysis –> skin infection
• Beta-lactamase
  
  • Penase –> penicillin G resistance

Question 4

Staphylococcal virulence factors: toxins

Answer 4

• Enterotoxin A-E
  
  • Vagal stimulator –> food poisoning
• Exfoliatin A-B
  
  • Granular cell layer cleavage –> scalded skin syndrome
• TSST-1
  
  • TNF, IL-1 stimulator –> Toxic shock syndrome

Question 5

Protein A interaction with antibodies

Answer 5

• S. aureus is protein A positive –> produces protein A
• Binds to Fc receptor of IgG and activates C’
• Protein A prevents antibody-mediated phagocytosis

Question 6

Coagulase & role in disease

Answer 6

• Coagulase = enzyme that can surround bacteria with fibrin coat, protection from phagocytosis
• Coagulase activates prothrombin, causing blood to clot
• Encapsulates infection, causes abscess formation
• Abscesses must be drained –> abx have difficulty penetrating capsule
  
  • Exceptions: lung and renal abscesses do not require surgical debridement
• Think staph A. clumps, or coagulates into clusters

Question 7

Identification of S. aureus infections in clinical laboratory

Answer 7

• SBA positive
• Chocolate agar positive
• MacConkey agar negative
• Hemolysis/pigment variable
  
  • Pigment likely golden
• Catalase positive
• Coagulase positive
• Clumping factor positive
• Protein A positive