Virulence factors and streptococcal infections

Virulence factors and streptococcal infections

Virulence factors of streptococcus:

  1. Capsular hyaluronic acid
  2. Group specific polysaccharide antigen
  3. Type specific antigen
  4. Protein F and G
  5. Exotoxins
  6. Hemolysin: Streptolysin
  7. Enzymes

1. Capsular Hyaluronic acid:

  • Hyaluronic acid capsule are present in group A and group C streptococci.
  • The capsule is not immunogenic
  • It protects the bacteria against phagocytosis.

2. Group specific polysaccharide antigen:

  • Group specific polysaccharide antigen plays a pathogenic role in streptococcus infection.
  • These antigens are chemically similar in structure and composition to human tissues particularly to those of heart, kidney and joints.
  • As a result the immune response toward streptococci become auto-reactive and causes endocarditis, glomerulonephritis or arthritis.

3. Type specific antigens:

i. M-protein:

  • It is associated with fimbriae on cell wall.
  • It is heat and acid resistant but susceptible to tryptic digestion and soluble in alcohol.
  • M-protein promotes adherence and enhance virulence.
  • It also protect phagocytosis.
  • Streptococci without M-protein is non-virulent.

ii. T and R protein:

  • T and R proteins have no relation to virulence
  • They are heat labile protein.
  • Some pathogenic S. pyogens possess R protein

4. Protein F and G:

  • Protein –F promotes binding to pharyngeal epithelial cell. It is fibronectin binding protein.
  • Protein-G bind to Fc receptor of IgG and prevents antibody mediated phagocytosis.

5. Exotoxins:

i. Erythrogenic toxins:

  • It is immunogenic.
  • It causes characteristic rashes and scarlet fever.

ii. Exotoxin A:

  • It is associated with toxic shock syndrome (TSS) caused by S.aureus.
  • It is a super antigen.

iii. Exo-toxin B:

  • It is a cysteine protease and responsible for tissue destruction.

iv. Cardio-hepatic toxin:

  • It is responsible for heart and liver dysfunction.

6. Hemolysin: Streptoysin

  • Most group A and some group C and G streptococci produces hemolysin.
  • It is also known as streptolysin.
a) Streptolysin O:
  • It is an Oxygen labile hemolysin.
  • Streptolysin O lyses RBCs by binding to cholesterol containing cell membrane/
  • It causes complete hemolysis around colony in Blood agar
  • Weight 60,000 Da
  • Anti-streptolysin O appears on serum of human following streptococcus infection.
b) Streptolysin S:
  • It is an oxygen stable hemolysin.
  • It causes surface colony hemolysis
  • It is lyses RBCs and it has also leucocidal action.
  • It is non-antigenic.

7. Enzymes:

i. Streptokinase (fibrinolysin):

  • Streptokinase is a fibrinolysin and promotes lysis of blood clot.
  • Two types; streptokinase A and B.
  • Streptokinase promotes spread of streptococci infection by preventing formation of clot barriers.

ii. Streptococcal deoxyribonuclease (DNase):

  • Streptococcal DNase degrade host cell DNA.
  • There are 4-types of DNase enzymes-type A, B, C and D.
  • DNase liquefies viscus DNA in pus.
  • Nicotinamide adenine denucleotidase (NADase):
  • NADase acts as co-enzyme.
  • It is antigenic and has leuco-toxic effects.

iii. Hyaluronidase: It is antigenic

iv. Lipoproteinase: It is antigenic

v. Neuraminidase

vi. Lipase

vii. Phosphatase

viii. Esterase

ix. NAGase

x. Amylase


Streptococcal infections

  1. Sore throat: acute tonsillitis and pharyngitis
  2. Scarlet fever: it occurs as a complication of sore throat. And characterized by generalized erythematous rashes.
  3. Impetigo: superficial discrete crushed spot especially in children. It lasts for 1-2 weeks and heal spontaneously without scar.
  4. Erysipelas: it is an acute spreading lesion of skin. Infected areas of skin shows massive edema with erythema.
  5. Cellulitis: it is sub-cutaneous infection
  6. Sepsis
  7. Otitis media
  8. Bacteremia
  9. Pneumonia
  10. Streptococcus shock syndrome
  11. Rheumatic fever

Virulence factors and streptococcal infections