BACTERIA · Gram-positive cocci

Streptococcus pyogenes (Group A Strep)

Streptococcus pyogenes — Gram stain (chains)

Stain: Gram stainMorphology: Gram-positive cocci in chainsYield: HIGHDifficulty: EASY

Streptococcus pyogenes (Group A Strep) microscopic image — Gram-positive cocci, Gram stain

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