Virulence factors and streptococcal infections
Virulence factors of streptococcus:
- Capsular hyaluronic acid
- Group specific polysaccharide antigen
- Type specific antigen
- Protein F and G
- Hemolysin: Streptolysin
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:
- 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.
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.
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
- Sore throat: acute tonsillitis and pharyngitis
- Scarlet fever: it occurs as a complication of sore throat. And characterized by generalized erythematous rashes.
- Impetigo: superficial discrete crushed spot especially in children. It lasts for 1-2 weeks and heal spontaneously without scar.
- Erysipelas: it is an acute spreading lesion of skin. Infected areas of skin shows massive edema with erythema.
- Cellulitis: it is sub-cutaneous infection
- Otitis media
- Streptococcus shock syndrome
- Rheumatic fever