BACTERIA · Gram-positive cocci
Streptococcus pyogenes (Group A Strep)
Streptococcus pyogenes — Gram stain (chains)
Stain: Gram stainMorphology: Gram-positive cocci in chainsYield: HIGHDifficulty: EASY
Image: Wikimedia Commons · File:Gram_stain_of_Streptococcus_pyogenes.jpg · Gary E. Kaiser PhD (CC BY 3.0)
Key facts
**Pathogenesis**: M protein inhibits phagocytosis and shares epitopes with cardiac myosin (molecular mimicry → rheumatic fever). Streptolysin O is antigenic (ASO titers). **Diagnostic clue**: Beta-hemolytic, catalase-negative, bacitracin-sensitive (vs Group B which is resistant), PYR-positive. **Virulence**: Erythrogenic exotoxin A (scarlet fever, strep TSS — superantigen), streptokinase, hyaluronidase, DNase B.
Boards buzzwords
- chains of cocci
- beta-hemolytic
- bacitracin-sensitive
- PYR-positive
- M protein
- strawberry tongue
Associated diseases
- Pharyngitis (strep throat)
- Scarlet fever
- Impetigo / erysipelas / cellulitis
- Necrotizing fasciitis
- Acute rheumatic fever
- Post-streptococcal glomerulonephritis
- Strep toxic shock syndrome
Treatment
Penicillin V or amoxicillin first-line; cephalexin or clindamycin if penicillin-allergic; clindamycin added to penicillin for necrotizing fasciitis/STSS (suppresses toxin synthesis)
Related organisms
Listeria monocytogenes
Gram-positive rod (facultative intracellular)
Mycobacterium tuberculosis
Acid-fast bacillus (mycolic acid cell wall)
Nocardia spp.
Gram-positive filamentous, partially acid-fast
Treponema pallidum
Spirochete
Mycobacterium leprae
Acid-fast bacillus (obligate intracellular)
Neisseria meningitidis
Gram-negative diplococci