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