- Group A streptococci
- Gram-positive cocci
- Arranged in chains.
- Senstivity to bacitracin. (Maxted’s observation).
- Catalase negative (S aureus is Catalase Positive)
- CAMP Negative (Group B Streptococci are CAMP Positive)
- Not soluble in 10% bile.
- Hydrolyse PYR (Used for Presumptive Diagnosis of S pyogenes)
- Ferment trehalose but not ribose.
- Virulent strains produce ‘matt’ (finely granular) colony
- Avirulent strains produce ‘glossy’ colonies.
- Capsulated strains produce mucoid colonies, corresponding in virulence to the matt type.
METHOD OF TRANSMISSION:
- Respiratory droplets
- Hand contact with nasal discharge and skin contact with impetigo lesions
- Pathogen can also be found in its carrier state (anus, vagina, skin, pharynx)
- Can spread from cattle to humans through raw milk and contaminated foods (salads, milk, eggs)
- Polysacchaide capsule:
- Composed of hyaluronic acid.
- Protect from ingestion and killing by phagocytosis.
- Also helps in colonization in the pharynx by binding to CD44
- Inner layer made of peptidoglycan.
- Middle layer made of carbohydrate (basis of Lancefielil classification).
- Outer layer made of protein and lipoteochoic acid. e.g. M Protein T, R.
- M protein is basis of Griffith Typing. Inhibit phagocytosis.
- Antibody to M is protective.
- Hair-like pilli (fimbria): Important for attachment to epithelial cells.
- Antigen of streptococci are similar to normal human cells
- Streptococcal infection is associated with autoimmune disease like rheumatic fever.
- Capsular hyaluronic acid —>Synovial fluid
- Cell wall protein —>Myocardium
- Group A carbohydrates —> Cardiac valves
- Cytoplasmic membrane —>Vascular intima
- Peptidoglycan (mucoprotein) —->Skin antigen
Toxins and Virulence Factors
- Oxygen labile
- Activity only on pour plate
- Antigenic specific.
- Oxygen stable and serum soluble]
- Non antigenic
- Hemolysis on surface
- Pyrogenic Exotoxin = Erythrogenic = Dick = Scadatinal Toxin:
- This is superantigen causing TSS.
- Identify children susceptible to scarlet fever by intradermal injection (Dicktest) and Schultz Charlton Reaction.
- .Three types : Types A (MC) and Type C are coded by bacteriophage while type B is chromosomal.
- Streptokinase (Fibrinolysin): Facilitates spread of infection.
- Spy Lep: A serine protease that cleaves and inactivate IL-8, thereby inhibiting neutrophil recruitment to the site of infections.
- Deoxyribo nuclease (Streptodornase): Responsible for thin serous character of strep Exudates. Also called as DNAase.
- Nicotinamide Adenine Dinucleotidase (NAD-ase).
- Hyaluronidase: Favor spread of infection.
- Serum opacity factor: Lipoproteinase.
- Anti Streptolysin O titre used in retrospective diagnosis; > 200 units is significant .
Clinical Manifestations of Streptococcus pyogenes
Infections typically begin in the throat or skin. The most striking sign is a strawberry-like rash
- Pharyngitis (strep throat):
- Sore Throat is the M/C Streptococcal Infection
- Localized skin infection (impetigo)
- Erysipelas(Superficial + S/C Tissue) and cellulitis (superficial form of cellulitis)
- Necrotizing fasciitis
- Scarlet fever:Streptococcal Pharyngitis + Rash with Minute Papules (Sand Paper Skin),
- Associated with Circumoral palor + Strawberry Tounge
- Streptococcal toxic shock syndrome
- Autoimmune-mediated complications( rheumatic fever and acute postinfectious glomerulonephritis
- Genital Infections
- Anaerobic Streptococci are most important cause of puerperal sepsis
- Bacteremia , Pneumonia and Toxic Shock Syndrome
- Non Suppurative Complications
|Acute Rheumatic Fever||Acute Glomerulonephritis|
|Post Throat Infection (Any Serotype)||Skin / Throat Serotypes 49, 53-55, 59-64, 1 & 12|
|Repeated Attacks Common||Not seen|
|Penicillin Prophylaxis Indicated||Not indicated|
|Course – Progressive / Static||Self limiting|
|ASO Titre Raised||May or May not (Skin Infection) Raised|
|Marked Immune Response No Change in Complement||Moderate Immune Response with ↓ Complement Level|
Lab Diagnosis of Streptococcus pyogenes
- Acute Pharyngitis – Swab Culture (Gold Standard)
- Transport Media – Pike’s Media
- Sheep Blood Agar recommended (As it is inhibitory to H. hemolyticus)
- ARF and Ac GN retroscpective with ↑ ASO Titres ( ASO > 200)
- In Ac GN & Pyoderma Anti DNAse and Antihyaluronidase used for retrospective diagnosis
- Streptozyme Test :- Passive Haemagglutination Test (Specific and Sensitive for all Streptococcal Infections)
Management of Streptococcus pyogenes
- Penicillin :- Pharyngitis / Impetigo / Erypsipelas / Cellulitis
- Penicillin + Empyema Drainage:-Empyema or Pnemonia
- Penicillin + Clindamycin + Surgical Debridement :-Necrotizing Fascitis / Myositis
- Penicillin + Clindamycin + i/v Ig :- Streptococcal TSS
For PDF Version of the Notes on
Streptococcus pyogenes Click Below to Download
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