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Streptococcus pyogenes

Beta-hemolytic, Lancefield group A Gram-positive coccus arranged in chains, responsible for pharyngitis, skin infections, rheumatic fever, and post-streptococcal glomerulonephritis.

Organism Card

DomainMust know
Identity
  • G+ve cocci in chains; facultative anaerobe [1]
  • Group A Streptococcus (GAS) = Streptococcus pyogenes
  • β-haemolytic (complete haemolysis) on blood agar [1][2]
  • Catalase −ve (vs Staphylococcus catalase +ve)
Lab discriminator
  • Bacitracin-sensitive (vs GBS = bacitracin-resistant) [2]
  • PYR (pyrrolidonyl arylamidase) +ve
  • Lancefield group A antigen
  • β-haemolytic + bacitracin-sensitive = GAS (key exam discriminator vs other β-haemolytic strep)
Reservoir / transmission
  • Reservoir: skin and mucous membranes of human host [3]
  • Transmission: airborne salivary droplets and nasal discharge [3]
  • Also direct contact (skin infections / fomites)
  • Hong Kong: scarlet fever remains notifiable; periodic outbreaks in children
Key virulence
  • M protein → anti-phagocytic; molecular mimicry with cardiac myosin → rheumatic fever [4]
  • Superantigen exotoxins (SpeA/B/C) → bind TCR shared by many T cells → massive T cell proliferation & cytokine storm → scarlet fever, streptococcal TSS [5]
  • Streptolysin O (oxygen-labile) → haemolysin; basis of ASO titre
  • Streptolysin S (oxygen-stable) → β-haemolysis on blood agar
  • Hyaluronidase, streptokinase → tissue spread
Clinical syndromes
  • Suppurative:
  • — Pharyngitis/tonsillitis (most common bacterial cause in children) [3]
  • — Scarlet fever: day-2 sandpaper rash, circumoral pallor, strawberry tongue, Pastia's lines, desquamation [5]
  • — Impetigo, cellulitis/erysipelas
  • — Necrotizing fasciitis (pain out of proportion, haemorrhagic bullae, dishwater discharge) [6]
  • — Streptococcal TSS: hypotension + ≥2 organ failures; GAS from sterile site = definite [3]
  • Toxin-mediated: scarlet fever, streptococcal TSS [5]
  • Non-suppurative / immune-mediated: [4]
  • — Acute rheumatic fever: 2–4 wk after pharyngitis ONLY (NOT skin infection); Jones criteria; molecular mimicry; pancarditis [4]
  • — PSGN: 1–3 wk post-pharyngitis OR 3–6 wk post-impetigo; ↓C3, ↑ASO; subepithelial humps on EM [7]
  • — Post-streptococcal reactive arthritis, PANDAS
Diagnosis
  • Throat swab for culture (gold standard for pharyngitis)
  • Rapid antigen detection test (RADT) — high specificity, moderate sensitivity
  • ↑ASO titre: confirms recent streptococcal infection (may be −ve in skin strains) [8]
  • Anti-DNase B: more reliable for skin infection–related PSGN
  • ↓C3 and CH50 with normal C4 → classic complement pattern in PSGN [7]
  • Blood C/ST for invasive disease (NF, TSS, bacteraemia)
Treatment
  • β-haemolytic streptococci are sensitive to penicillin [1]
  • Pharyngitis: Penicillin V (oral) — first-line [9]
  • Severe/invasive: IV benzylpenicillin (penicillin G)
  • Penicillin allergy: erythromycin or clindamycin [5]
  • NF: aggressive surgical debridement + IV broad-spectrum antibiotics [6]
  • NB: GAS has NO penicillin resistance reported (unlike pneumococcus)
  • IE due to GAS: penicillin G ± gentamicin [10]
Prevention
  • Scarlet fever is notifiable in Hong Kong
  • ARF prophylaxis: prolonged penicillin (secondary prophylaxis until age 21 or longer) [4]
  • Treat pharyngitis with antibiotics to prevent ARF (even though only hastens recovery by ~16 h) [5]
  • No vaccine currently available
  • Contact precautions for skin infections; droplet precautions for pharyngitis
Classic traps
  • ARF follows pharyngitis ONLY — NOT skin infection; PSGN follows BOTH pharyngitis and impetigo [4][7]
  • GAS = bacitracin-sensitive; GBS = bacitracin-resistant [2]
  • Scarlet fever vs Kawasaki: scarlet fever = day-2 rash + circumoral pallor + sandpaper texture; Kawasaki = ≥5 days fever + conjunctivitis + polymorphous rash + extremity changes
  • Streptococcal TSS vs Staphylococcal TSS: strep TSS requires GAS isolation & usually has NF; staph TSS = toxin-mediated, culture of S. aureus often from non-sterile site
  • PSGN complement pattern: ↓C3, normal C4 (vs lupus nephritis: ↓C3 AND ↓C4)

Exam Intelligence

  1. "Expert practice tip 3: β-haemolytic streptococci are sensitive to penicillin" — directly from GC 105 slides [1]. This is a recurring exam principle: GAS has no reported penicillin resistance. Penicillin V is first-line for pharyngitis; penicillin G for invasive disease.

  2. GC 105 lists GAS clinical spectrum as: pharyngitis, scarlet fever, impetigo, cellulitis, necrotizing fasciitis, TSS, and post-infectious syndromes (ARF, PSGN) [1]. Expect MCQs or SAQs listing "5 non-cardiac diseases caused by GAS" — pharyngitis, scarlet fever, TSS, cellulitis, PSGN is a classic five [10].

  3. Superantigen mechanism: toxin binds to part of TCR shared by many T cells → massive T cell proliferation and cytokine release [5]. This mechanism underlies both scarlet fever and streptococcal TSS and is a favourite MCQ discriminator.

  4. ARF follows streptococcal pharyngitis ONLY — impetigo/pyoderma does NOT cause ARF but CAN cause PSGN [4]. This is an extremely high-yield exam discriminator tested repeatedly.

  5. PSGN timing: 1–3 weeks after pharyngitis; 3–6 weeks after skin infection. Lab hallmark: ↓C3 with normal C4 + ↑ASO titre. Subepithelial humps ("starry sky" granular IgG/C3 on IF) [7][8].

  6. NF caused by S. pyogenes: pain out of proportion to clinical findings, haemorrhagic bullae, dishwater discharge; LRINEC score > 8 = high risk; management = aggressive debridement + IV antibiotics [6]. Exam favourite in surgery papers.

  • GAS vs GBS: GAS = bacitracin-sensitive, β-haemolytic, pharyngitis/skin/ARF/PSGN. GBS = bacitracin-resistant, β-haemolytic, neonatal sepsis/meningitis, peripartum infections, UTI.
  • Scarlet fever vs Kawasaki disease: Both present in children with fever and rash. Scarlet fever has sandpaper rash on day 2, circumoral pallor, strawberry tongue, Pastia's lines, and responds to penicillin. Kawasaki requires ≥5 days fever, bilateral non-exudative conjunctivitis, extremity changes, and needs IVIG + aspirin.
  • Streptococcal TSS vs Staphylococcal TSS: Strep TSS requires GAS isolation from normally sterile site + hypotension + ≥2 organ criteria. Staph TSS is toxin-mediated (TSST-1), associated with tampon use/wound packing, and does not require positive sterile-site culture.
  • PSGN complement pattern: ↓C3 with normal C4 (alternative pathway activation). If C3 AND C4 both low → think lupus nephritis or MPGN. If C3 remains low > 8 weeks → suspect lupus nephritis or MPGN, not PSGN [8].
  • ASO titre pitfall: ASO may be negative in skin-strain PSGN; anti-DNase B is more reliable for post-skin-infection PSGN [8].

2025 Fourth Summative MCQ — EMQ Section IV "Use of Antibiotics", Q20 [9]:

"Treatment of group A streptococcal pharyngitis." Options included: A. Amoxicillin-clavulanic acid, B. Cefazolin, C. Cefotaxime, D. Ceftaroline, E. Ceftazidime, F. Cloxacillin, G. Gentamicin, H. Nitrofurantoin, I. Penicillin V, J. Rifampicin.

Correct answer: I. Penicillin V. Rationale: GAS pharyngitis first-line is oral penicillin V. GAS remains universally penicillin-sensitive. Cloxacillin (anti-staphylococcal) and cephalosporins are not first-line. This directly tests the GC 105 "expert practice tip 3" principle [1].

2021 Fourth Summative MCQ — EMQ Section V "Investigating a Child with Fever and Rash", Q19 [11]:

"A 4-year-old girl presented with a low-grade fever, sore throat and a generalised, blanchable, erythematous, maculopapular rash. Her blood count revealed presence of atypical lymphocytes."

Correct answer: D. Monospot test. Rationale: Atypical lymphocytes + pharyngitis + maculopapular rash → EBV infectious mononucleosis, NOT scarlet fever. This is a discriminator trap: scarlet fever rash has sandpaper texture and typically circumoral pallor, and would NOT show atypical lymphocytes. The monospot differentiates EBV from GAS pharyngitis.

(Note: While Q19 above tests EBV, the EMQ stem set also includes options for ASO titre and throat swab for bacterial culture — examiners expected students to differentiate GAS-related investigations from EBV-related ones in this question set.)

No other past paper questions specifically featuring S. pyogenes as the primary organism were identified in the indexed past paper context. Questions on ARF/PSGN/NF may appear in SAQ or minicase format but the specific stems were not retrievable from the indexed text provided.

[1] Lecture slides: GC 105. Medically important microbes what every doctor should know.pdf — β-haemolytic streptococci slide, expert practice tip 3 [2] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — Gram +ve bacteria classification tree (p.13) [3] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf — GAS microbiology and tonsillopharyngitis (p.124) [4] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — Acute rheumatic fever / rheumatic heart disease (p.444) [5] Senior notes: Adrian Lui Pediatrics Notes.pdf — Scarlet fever and superantigen mechanism (p.474) [6] Senior notes: Maksim Surgery Notes.pdf — Necrotizing fasciitis (p.274) [7] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — PSGN morphology and diagnosis (p.999) [8] Senior notes: Ryan Ho Urogenital.pdf — PSGN clinical and laboratory features (p.66) [9] Past papers: 2025 Fourth Summative MCQ.pdf — EMQ Section IV Q20 (p.35) [10] Senior notes: Block A - Fever and a murmur_ Valvular heart diseases; Infective endocarditis.pdf — GAS in IE and non-cardiac diseases [11] Past papers: 2021 Fourth Summative Assessment MCQ.pdf — EMQ Section V Q19 (p.42)

Staphylococcus epidermidis

Coagulase-negative, Gram-positive coccus that is a normal skin commensal and a leading cause of biofilm-associated infections on indwelling medical devices and prosthetic implants.

Streptococcus pneumoniae

Encapsulated, alpha-hemolytic, Gram-positive lancet-shaped diplococcus and the leading cause of community-acquired pneumonia, bacterial meningitis, and otitis media.

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