Streptococcus pyogenes
One-Sentence Definition
Streptococcus pyogenes (Group A Streptococcus) is a Gram-positive coccus in chains causing pharyngitis, SSTI, necrotizing fasciitis, toxin-mediated shock, and post-infectious sequelae (RF, PSGN).
Taxonomy & Morphology
- β-hemolytic; Lancefield group A
- Catalase-negative; bacitracin-sensitive (classic teaching)
- Gram Stain: chains / pairs
Virulence Highlights
- M protein (antiphagocytic; emm typing)
- Streptolysins, hyaluronidase, streptokinase
- Superantigens (scarlet fever / STSS)
- Capsule (hyaluronic acid) in some strains
Clinical Syndromes
- Pharyngitis, impetigo, erysipelas, cellulitis
- Necrotizing fasciitis, streptococcal toxic shock
- Rheumatic fever; post-strep glomerulonephritis
Diagnosis
- Throat culture / RADT for pharyngitis
- Wound/blood culture for invasive disease
- ASO/anti-DNase B for post-strep syndromes (serology)
Treatment Notes (conceptual)
- Penicillin remains drug of choice when susceptible (almost universal for GAS)
- Add clindamycin in toxin-severe invasive disease (toxin suppression adjunct — clinical practice point)
- Surgical source control for necrotizing infection
AMR
- Penicillin resistance not a practical problem in GAS; macrolide resistance varies by region
Related Notes
- Diseases: Streptococcal Pharyngitis · Cellulitis and Skin Abscess · Necrotizing Soft Tissue Infection
- MOC - Bacteriology · MOC - Diseases by System · MOC - Immunology (post-infectious)
- Contrast: Streptococcus pneumoniae · Staphylococcus aureus
Active Recall
- Name two post-infectious complications of GAS.
- Why add clindamycin in severe toxin disease?
- Lancefield group of S. pyogenes?