GAS
Summary
GAS most commonly = Group A Streptococcus (Streptococcus pyogenes), a gram-positive, beta-hemolytic, bacitracin-sensitive, PYR-positive coccus in chains. It causes pharyngitis, impetigo, erysipelas, cellulitis, necrotizing fasciitis, scarlet fever, and TSS, plus post-infectious sequelae (acute rheumatic fever, post-streptococcal glomerulonephritis). Other uses: General Adaptation Syndrome (Selye's stress response) and Generalized Anxiety/other contexts.
Detail
**1. Group A Streptococcus (S. pyogenes) – the high-yield meaning**
*Microbiology:* Gram-positive cocci in chains; catalase-negative; beta-hemolytic (complete hemolysis on blood agar); bacitracin-sensitive; PYR-positive; Lancefield group A carbohydrate (C-carbohydrate). Distinguish from Group B (S. agalactiae): bacitracin-resistant, hippurate-positive, CAMP-positive.
*Virulence factors:* - **M protein** – antiphagocytic; antigenic variation; basis of molecular mimicry in rheumatic fever (cross-reacts with cardiac myosin/tissue). - **Hyaluronic acid capsule** – weakly immunogenic, antiphagocytic. - **Streptolysin O** – oxygen-labile hemolysin; immunogenic → ASO titer (rises after pharyngitis; not after skin infection). - **Streptolysin S** – oxygen-stable, non-immunogenic. - **Exotoxins SpeA/SpeC (pyrogenic/erythrogenic toxin)** – superantigens (bind MHC II and TCR Vβ → massive cytokine release); cause scarlet fever and streptococcal toxic shock syndrome. Encoded by a lysogenic bacteriophage. - **Streptokinase** (fibrinolysis), **DNase** (anti-DNase B titer rises after skin infection), **hyaluronidase** (spreading), **C5a peptidase**.
*Suppurative (direct infection) diseases:* - **Pharyngitis** – fever, tonsillar exudates, tender anterior cervical lymphadenopathy, NO cough (Centor/McIsaac criteria). Dx: rapid antigen test/throat culture. Tx: penicillin V or amoxicillin (macrolide/cephalosporin if allergic). Resistance to penicillin has never been established. - **Scarlet fever** – pharyngitis + sandpaper-like erythematous rash (sparing palms/soles), strawberry tongue, circumoral pallor, Pastia lines, later desquamation. - **Skin:** impetigo (honey-colored crusts), erysipelas (raised, sharply demarcated, superficial dermis/lymphatics), cellulitis (deeper), **necrotizing fasciitis (type II)** – pain out of proportion, rapid spread, crepitus less common; needs urgent surgical debridement + broad-spectrum antibiotics incl. clindamycin (inhibits toxin synthesis). - **Streptococcal toxic shock syndrome** – shock, multiorgan failure, often with soft-tissue infection; positive blood cultures are common (unlike staph TSS).
*Non-suppurative (immune-mediated) sequelae:* - **Acute rheumatic fever** (2–4 weeks after PHARYNGITIS only): type II hypersensitivity/molecular mimicry. Jones criteria – **J**oints (migratory polyarthritis), **O**(heart: carditis, mitral valve most common), **N**odules (subcutaneous), **E**rythema marginatum, **S**ydenham chorea; plus fever, elevated ESR/CRP, prolonged PR. Histology: Aschoff bodies with Anitschkow cells (caterpillar cells). Prevent recurrence with penicillin prophylaxis (benzathine penicillin G). Treating pharyngitis prevents ARF. - **Post-streptococcal glomerulonephritis** (1–3 weeks after pharyngitis OR skin infection): type III hypersensitivity (immune complex); nephritic syndrome, cola-colored urine, periorbital edema, hypertension, low C3; LM: hypercellular glomeruli; IF: “lumpy-bumpy” granular IgG/C3; EM: subepithelial humps. Treatment is supportive; antibiotics do not prevent PSGN. Children usually recover fully. - Also: reactive arthritis-like syndromes, PANDAS (controversial), guttate psoriasis trigger, erythema nodosum.
**2. Other meanings of “GAS” on exams** - **General Adaptation Syndrome** (Hans Selye): alarm → resistance → exhaustion stages of the stress response (behavioral science/physiology; cortisol and catecholamines). - **Gas** in other contexts (e.g., GAS = growth arrest–specific gene, GAS = gamma-activated sequence in JAK-STAT signaling; “gas gangrene” = Clostridium perfringens – alpha toxin/lecithinase, crepitus, double zone of hemolysis). - Do not confuse with **Group B Strep (GBS)** – neonatal sepsis/meningitis, prevented by intrapartum penicillin prophylaxis.
**Board pearls:** Bacitracin-sensitive + beta-hemolytic = GAS. ASO ↑ → pharyngitis-related; anti-DNase B ↑ → skin-related. Rheumatic fever follows only pharyngitis; PSGN follows pharyngitis or skin infection. Antibiotics prevent rheumatic fever but not PSGN.
Sources
- First Aid for the USMLE Step 1 – Microbiology: Gram-positive organisms (Streptococci)
- Robbins & Cotran Pathologic Basis of Disease – Rheumatic heart disease; Glomerular diseases
- Sketchy Medical / Pathoma – Streptococcus pyogenes and post-strep sequelae
- Murray's Medical Microbiology – Streptococcus pyogenes
- IDSA Clinical Practice Guideline for Group A Streptococcal Pharyngitis
- AHA/Revised Jones Criteria for Acute Rheumatic Fever
Reviewed by Medusa editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.