Streptococcus Pyogens: Morphology,Transmission, Antigenicity and Lysogeny

MORPHOLOGY

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

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.

Biochemical reactions

• 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.

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

• 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

• 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.

• 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.

1. Capsular hyaluronic acid → Synovial fluid
2. Cell wall protein → Myocardium
3. Group A carbohydrates → Cardiac valves
4. Cytoplasmic membrane → Vascular intima
5. Peptidoglycan (mucoprotein) → Skin antigen

LYSOGENY:

Toxins and Virulence Factors

Hemolysin

1. Oxygen labile
2. Activity only on pour plate
3. Antigenic specific.
4. Cardiotoxic

Streptolysin

1. Oxygen stable and serum soluble]
2. Non antigenic
3. Hemolysis on surface

Pyrogenic Exotoxin = Erythrogenic = Dick = Scadatinal Toxin:

1. This is superantigen causing TSS.
2. Identify children susceptible to scarlet fever by intradermal injection (Dicktest) and Schultz Charlton Reaction.
3. .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 .

Exam Question

Biochemical reactions

• Senstivity to bacitracin(Maxted’s observation).
• Catalase negative
• Not soluble in 10% bile.
• Hydrolyse PYR
• Ferment trehalose but not ribose.

Antigenic Structure

• Polysacchaide capsule:
• CellWall
• Hair-like pilli (fimbria): Important for attachment to epithelial cells.
• LYSOGENY:

Toxins and Virulence Factors

Produce numerous exotoxins and exoenzymes

• Hemolysin
• Streptolysin
• Pyrogenic Exotoxin = Erythrogenic = Dick = Scadatinal Toxin
• Streptokinase (Fibrinolysin): Facilitates spread of infection.
• Spy Lep: A serine protease that cleaves and inactivate IL-8
• 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.