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