Streptococcus pyogenes

Streptococcus Pyogenes

 

Morphology

  • Group A streptococci
  • Gram-positive cocci
  • Arranged in chains.

Biochemical Characterstics

  • 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.

Cultural Characteristics

  • 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)

Antigenic Structure

  1. Polysacchaide capsule:
    1. Composed of hyaluronic acid.
    2. Protect from ingestion and killing by phagocytosis.
    3. Also helps in colonization in the pharynx by binding to CD44
  2. CellWall:
    1. Inner layer made of peptidoglycan.
    2. Middle layer made of carbohydrate (basis of Lancefielil classification).
    3. Outer layer made of protein and lipoteochoic acid. e.g. M Protein T, R.
    4. M protein is basis of Griffith Typing. Inhibit phagocytosis.
    5. Antibody to M is protective.
  3.  Hair-like pilli (fimbria): Important for attachment to epithelial cells.

Antigenic Similarity

  • 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

  •  Hemolysin
    • Oxygen labile
    • Activity only on pour plate
    • Antigenic specific.
    • Cardiotoxic
  • Streptolysin
    • 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

  1. Pharyngitis (strep throat):
    1. Sore Throat is the M/C Streptococcal Infection
  2. Localized skin infection (impetigo)
  3. Erysipelas(Superficial + S/C Tissue) and cellulitis (superficial form of cellulitis)
  4. Necrotizing fasciitis
  5. Scarlet fever:Streptococcal Pharyngitis + Rash with Minute Papules (Sand Paper Skin),
    1.  Associated with Circumoral palor + Strawberry Tounge
  6. Streptococcal toxic shock syndrome
  7. Autoimmune-mediated complications(  rheumatic fever and acute postinfectious glomerulonephritis
  8. Genital Infections
    1. Anaerobic Streptococci are most important cause of puerperal sepsis
  9. Bacteremia
    1. Bacteremia , Pneumonia and Toxic Shock Syndrome
  10. 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

[wpViralQuiz id=1703 columns=1]

[wpViralQuiz resultsonly=1703]

Leave a Reply

%d bloggers like this:
Malcare WordPress Security