Microbiology Chapter 1: Staphylococci

Study Notes

Gram-positive spherical cocci, often in grape-like clusters.
Catalase positive, facilitating hydrogen peroxide breakdown.
Produces opaque, pigmented colonies on agar.
Divided into two groups based on coagulase production:
- Coagulase-positive:
  - Staphylococcus aureus: Most pathogenic, medically significant.
- Coagulase-negative:
  - Includes S. epidermidis and S. saprophyticus, generally less pathogenic.
S. aureus is a facultative anaerobe with a characteristic golden-yellow pigmentation.

Nutrient agar: Basic growth medium.
Blood agar: Exhibits complete β-hemolysis, indicating pathogenicity.
Mannitol salt agar: Selective for S. aureus, showing yellow colonies due to mannitol fermentation.

Coagulase: Converts plasma fibrinogen to fibrin, aiding infection localization and immune evasion.
Clumping factor (adhesin): Facilitates attachment to tissues and clots.
Protein A: Binds to IgG, inhibiting opsonization and immune response.
Hemolysins: Pore-forming toxins that lyse host cell membranes.
Invasins: Including leukocidin and staphylokinase, promote tissue spread.
Enterotoxins: Associated with food poisoning and toxic shock syndrome.

Pyogenic diseases:
- Common skin infections: Folliculitis, furuncles, carbuncles, and abscesses.
- Post-surgical infections and traumatic wounds.
- Potential for severe systemic infections leading to conditions like osteomyelitis and endocarditis.
Toxin-mediated diseases:
- Staphylococcal food poisoning: Fast onset (1-6 hours) often from contaminated food, particularly protein-rich sources.

Commonly associated with urinary tract infections, especially in young sexually active women.
Differentiated from S. epidermidis by being novobiocin resistant. Treatable with quinolone antibiotics.

Gram-positive ovoid cocci arranged in chains or pairs.
Catalase negative, distinguishing it from staphylococci.
Requires enriched media for growth, such as blood agar or serum.
Key species include Streptococcus pyogenes, Streptococcus agalactiae, and Streptococcus pneumoniae.

Categorized based on hemolysis on blood agar:
- Beta-hemolysis: Complete lysis (e.g., Streptococcus pyogenes).
- Alpha-hemolysis: Partial lysis (e.g., Streptococcus pneumoniae).

Antiphagocytic factors:
- M protein: Inhibits phagocytosis and promotes adherence.
- C5a peptidase: Cleaves C5a, disrupting immune signaling.
- Hyaluronic acid capsule: Mimics host tissue, aiding evasion.
Invasins:
- Streptokinase: Activates plasminogen, promoting tissue invasion.
- Streptolysins: Lyse host cells and aid in spread.
- Streptococcal pyrogenic exotoxins: Superantigens linked to severe conditions like toxic shock syndrome.