Streptococcus pyogenes (Group A Streptococcus)
Gram-positive, beta-haemolytic coccus forming chains; carries Lancefield Group A carbohydrate antigen (N-acetylglucosamine polymer); obligate human pathogen. M protein — the defining virulence factor (>250 M-types) — binds complement factor H, blocking C3b opsonisation, and shares epitopes with cardiac myosin and tropomyosin, driving acute rheumatic fever (ARF) via molecular mimicry. Pyrogenic exotoxins SpeA and SpeC are superantigens crosslinking MHC-II with TCR Vβ to activate up to 20% of T cells, producing Streptococcal TSS (STSS). Streptolysin O (SLO) provides the diagnostic ASO titre rise and kills neutrophils; streptodornase Sda1 dissolves neutrophil extracellular traps. Penicillin/amoxicillin remain universally active — no acquired penicillin resistance has ever been documented in GAS. ARF is preventable by treating pharyngitis within 9 days of symptom onset.
Classification & Structure
| Gram reaction | Gram-positive; thick peptidoglycan; Lancefield Group A carbohydrate (rhamnose-N-acetylglucosamine polymer); beta-haemolysis (complete, clear zone) on sheep blood agar; PYR positive; bacitracin sensitive; catalase negative |
| Morphology | Spherical cocci 2–>20 cells per chain; non-motile, non-spore-forming; obligate human pathogen; spread by respiratory droplets (pharyngitis) and direct contact (skin infections) |
| Cell wall / Capsule | Hyaluronic acid capsule: non-immunogenic (molecular mimicry of host HA) → no antibody recognition; anti-phagocytic physical barrier. Lancefield Group A carbohydrate antigen: basis of serological classification. M protein (surface fibrillar coiled-coil dimer): binds factor H → ↓C3b opsonisation; shares epitopes with cardiac myosin/tropomyosin/laminin → ARF molecular mimicry |
| Key virulence factors | M protein (>250 types: factor H binding, molecular mimicry, fibronectin binding); SpeA/SpeC superantigens (TSS); SpeB (cysteine protease, tissue invasion, NADase); SLO (streptolysin O: cholesterol-dependent cytolysin, ASO titre target, neutrophil lysis); SLS (streptolysin S: beta-haemolysis on plates, non-immunogenic); C5a peptidase (ScpA: degrades neutrophil chemoattractant); streptodornase Sda1 (DNase: dissolves NETs); streptokinase (fibrinolysis, systemic spread) |
Pathogenesis
1 · Pharyngeal colonisation and neutrophil evasion
M protein, Protein F1/SfbI, and pili mediate adhesion to pharyngeal epithelium via fibronectin binding. GAS then deploys multi-layer neutrophil evasion: M protein binds factor H → ↓C3b opsonisation; C5a peptidase (ScpA) cleaves the dominant neutrophil chemoattractant C5a → ↓neutrophil influx; hyaluronic acid capsule provides anti-phagocytic physical barrier; streptodornase Sda1 dissolves neutrophil extracellular traps (NETs); SLO lyses neutrophils that have engulfed bacteria.
2 · Spreading factors and tissue invasion
Hyaluronidase degrades HA in connective tissue; streptodornases (B, C, D) degrade extracellular DNA; streptokinase binds plasminogen → plasmin activity → fibrin dissolution → systemic spread through fascial planes. SpeB (cysteine protease) degrades immunoglobulins, complement proteins, extracellular matrix, and cytokines. NADase (Spn/NdaS) depletes intracellular NAD+ in invaded host cells → cytotoxicity. Collectively, these spreading factors convert localised pharyngitis or impetigo into necrotising fasciitis.
3 · Superantigen toxic shock (STSS)
Pyrogenic exotoxins SpeA and SpeC bind outside the MHC-II peptide groove (on α1 and β1 domains) and crosslink to TCR Vβ chains outside CDR3, activating ALL T cells bearing specific Vβ families (up to 20% of the entire T-cell repertoire vs. <0.01% in conventional antigen response). This triggers a tsunami of TNF-α, IL-1β, IL-6, IL-2 → fever, hypotension, multi-organ failure, DIC. STSS mortality is 20–70% even in modern ICUs. M1T1 strains carry SpeA and SpeC; M1 is the dominant global invasive lineage.
4 · Molecular mimicry driving ARF and PSGN
ARF: Anti-M-protein antibodies (particularly against N-terminal hypervariable region of M5, M6, M19 types) cross-react with cardiac myosin, tropomyosin, laminin, and N-acetyl-β-D-glucosamine (valve carbohydrate) → myocarditis, valvulitis. The adaptive immune response attacks the heart, joints, and CNS without the bacteria being present. Jones criteria (2015): major = carditis, polyarthritis, chorea, erythema marginatum, subcutaneous nodules; minor = fever, ↑ESR/CRP, ↑PR interval. Penicillin within 9 days of symptom onset prevents ARF. PSGN: Streptococcal antigens (SPEB/zymogen, enolase, NaPl-1/plasmin receptor) deposit in glomeruli → in situ immune complexes → alternative complement activation (↓C3, normal C4) → acute nephritic syndrome (haematuria, proteinuria, hypertension, oedema).
Host Immune Response
Disease Spectrum
| Disease | Key features | Treatment / prevention |
|---|---|---|
| Pharyngitis / tonsillitis | Sore throat, fever, tonsillar exudate, cervical LAD, no cough; Centor/McIsaac score; RADT | Penicillin V 10 days or amoxicillin; prevents ARF if started within 9 days |
| Scarlet fever | Pharyngitis + sandpaper erythematous rash (SpeA-driven); strawberry tongue; circumoral pallor; Pastia lines in flexures | Same as pharyngitis; notifiable in many countries |
| Impetigo (GAS) | Honey-crusted vesicular/pustular superficial skin lesions; painless; associated with PSGN (not ARF) | Topical mupirocin (mild); oral amoxicillin-clavulanate (extensive); PSGN risk watch for 1–3 weeks |
| Erysipelas | Sharply demarcated, raised, fiery-red skin lesion; butterfly/facial or lower limb; fever; GAS (not S. aureus) | IV penicillin G or cefazolin; recurrence common |
| Necrotising fasciitis type 2 | Rapidly progressing deep fascial necrosis; severe pain out of proportion to skin findings; “dusky” skin → bullae → necrosis; STSS in ~50% | Immediate surgical debridement + IV penicillin G + clindamycin (protein synthesis blockade stops exotoxin production); mortality 20–30% even with surgery |
| Streptococcal TSS (STSS) | SpeA/SpeC superantigen → fever, hypotension, multi-organ failure, DIC; from any GAS focus | ICU; IV penicillin G + clindamycin; IVIG in severe cases (neutralises toxins); mortality 30–70% |
| Acute rheumatic fever (ARF) | Migratory polyarthritis, carditis (most serious), Sydenham’s chorea, erythema marginatum, subcutaneous nodules; follows pharyngitis ONLY (not skin GAS infection) | Penicillin V 10 days; anti-inflammatory (aspirin/prednisolone); long-term benzathine penicillin prophylaxis to prevent recurrence and RHD progression |
| Post-streptococcal GN (PSGN) | Haematuria (“cola/smoky urine”), proteinuria, hypertension, oliguria, oedema; ↓C3, normal C4; ↑ASO (pharyngeal) or ↑anti-DNase B (skin) | Supportive; antibiotics do NOT prevent PSGN (unlike ARF); monitor for CKD in adults |
Treatment & Prophylaxis
| First-line (all GAS) | Penicillin V 10 days (oral) or amoxicillin 10 days; NO acquired penicillin resistance in GAS has ever been documented. Single-dose benzathine penicillin G IM is equivalent for pharyngitis and preferred in resource-limited settings. |
| Invasive GAS (NF2, STSS) | IV penicillin G PLUS clindamycin (inhibits protein synthesis → blocks exotoxin SpeA/SpeC production; superior to penicillin alone in high bacterial burden). IVIG (1–2 g/kg) neutralises circulating superantigens in STSS; evidence of mortality benefit in severe STSS. |
| Penicillin allergy | Azithromycin (macrolide, 5 days) for pharyngitis; clindamycin for skin/invasive; resistance rates vary by region. |
| ARF prevention | Complete 10-day penicillin course within 9 days of pharyngitis symptom onset prevents ARF in virtually all cases, even though clinical improvement occurs earlier. Critical public health rationale for testing and treating strep throat in children. |
| Secondary ARF prophylaxis | Benzathine penicillin G IM every 3–4 weeks (or oral penicillin V daily); prevents recurrent GAS pharyngitis → limits progressive valvular RHD. Duration: 10+ years (or age 21) without carditis; lifelong in severe RHD or high-risk settings. |
Cross-Atlas Connections
References
- Bennett JE, Dolin R, Blaser MJ. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 9th ed. Elsevier; 2020.
- Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2021.
Contribute to the Pathogen Atlas
This entry covers GAS biology, M-protein molecular mimicry, and ARF/PSGN pathogenesis. Planned expansions: GAS vaccine pipeline (SpyCEP, M-protein-based), global RHD echocardiographic screening data, and STSS immunotherapy trials.