Microbiology

Question 1

Streptococci

Answer 1

• Gram-positive
• Seen in chains or pairs (Streps form straight lines)
• Classification criteria are:
  
  • Biochemical (Species)
  • Serological (Lancefield Groups - polysaccharide antigens)
  • Haemolytic (Lysis seen on Sheep Blood Agar)
    
    • Beta (clear)
    • Alpha (green)
    • Gamma (nil)
• Investigations include gram stain, culture, and serology for serum antibodies, e.g. antistreptolysin O.
• Disease is caused by invasion, local and distant spread, effect of toxins (as in Scarlett Fever), and via immune responses (as in Rheumatic Fever)

NB: Streptolysin O is a toxin created by streptococci which lyses cells (e.g. red cells and white cells).

Strep. pyogenes is:

• Found in throat and nose
• Lancefield Group A
• Beta haemolytic
• Scarlett Fever and Rheumatic Fever

Strep. agalactiae is:

• Found in vagina and gut
• Lancefield Group B
• Beta haemolytic

Strep. pneumoniae is:

• Found in nasopharynx
• Not groupable
• Alpha haemolytic

Strep. viridans is:

• Found in mouth
• Not groupable
• Alpha haemolytic

Notes:

• VRE = Vancomycin Resistant Enterococci
• Pneumococcal vaccine is used in those with splenectomy or immunocompromise.

Reference: Clinical Microbiology and Infectious Diseases (An Illustrated Colour Text) [2nd Edition], p.36-37.

Question 2

Staphylococci

Answer 2

• Gram-positive
• Either coagulase-positive (S. aureus) or coagulase-negative (S. epidermidis or S. saprophyticus)
• Coagulase is an enzyme which protects the organism by formation of fibrin.
• Toxins released by Staphylococci are what result in conditions such as scalded skin syndrome.

S. epidermidis

Important in prosthetic infections and catheter infections

S. saprophyticus

Cause of urinary infections

Reference: Clinical Microbiology and Infectious Diseases (An Illustrated Colour Text) [2nd Edition], p.36-37.