Virulence factors of Bacteria


Virulence factors of Bacteria

Pathogenicity is referred to as the ability of microbial species to produce disease and virulence is referred to the ability of microbial strains to produce diseases.

Why some of the pathogens are more virulent than others?

This is because of the virulence factors they produce which decide the severity of disease they may cause. The pathogen’s virulence factor is determined by Koch postulates.

Koch postulates states as-

  1. The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.
  2. The microorganism must be isolated from a diseased organism and grown in pure culture.
  3. The cultured microorganism should cause disease when introduced into a healthy organism.
  4. The microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

A) Pathogenesis Steps

  • The first step of pathogenesis involves exposure and adhesion. Adhesins are cell surface appendage in bacteria which allows adherence to other cells mostly the host cells for attachment
  • The second step involves the invasion which is followed by pathogens entering into the host body through blood stream
  • Less invasive organisms cause localized lesion, e.g. staphylococcal abscess. Highly invasive organisms cause generalized infection, e.g. streptococcal septicemia.

B) Toxigenecity

Bacteria produce two types of toxins—
(1) Exotoxin

It has following characters.
1. Heat labile proteins.
2. Diffuse readily into the surrounding medium.
3. Highly potent
4. They are generally formed by Gram positive organism and also by Gram negative organisms like shigella, Vibrio cholerae and Escherichia coli.
5. Exotoxin are specifically neutralized by antitoxin.
6. Can be separated from culture by filtration.
7. Action is enzymatic.
8. It has specific tissue affinity.
9. It is highly antigenic.
10. Specific pharmacological effects for each exotoxin.
11. Can be toxoided.
12. Cannot cause pyrexia in a host.

(2) Endotoxin

Endotoxin (Lipid a portion of lipopolysaccharide) has biological activities causing fever, muscle proteolysis, uncontrolled intravascular coagulation and shock. These may be mediated by production from mononuclear cells of IL-I, TNFX α 2 probably IL-6. It has following characters.

  1. Proteins polysaccharide lipid complex heat stable.
  2. Forms part of cell wall and will not diffuse into the medium.
  3. Obtained only by cell lysis.
  4. They have no enzymatic action.
  5. Effect is nonspecific action.
  6. No specific tissue affinity
  7. Active only in large doses 5 to 25 mg.
  8. Weakly antigenic.
  9. Neutralization by antibody ineffective.
  10. Cannot be toxoided.
  11. Produce in Gram negative bacteria.
  12. Can cause pyrexia in a host.


This is ability of parasite to spread from one host to another. It determines the survival and distribution of organism in a community. Highly virulent organism may not exhibit a high degree of communicability due to rapid lethal effect on hosts. Infections in which pathogen is shed in secretions as in respiratory and intestinal diseases are highly communicable.

D)Other bacterial products

1. Coagulase (Staphylococcus aureus) which prevents phagocytosis by forming fibrin barrier around bacteria.
2. Fibrinolysin promotes the spread of infection by breaking down the fibrin barrier in tissues.
3. Hyaluronidase split hyaluronic acid (component of connective tissue) thus facilitating spread of infection along tissue spaces.
4. Leucocidins damage polymorphonuclear leucocytes.
5. Hemolysin is produced by some organisms capable of destroying erythrocytes.
6. Ig A1 proteases: Gonococci, meningococci, Hemophilus influenzae pneumococci, may produce IgA1 protease which splits IgA and inactivates its antibody activity.

E) Bacterial appendages

Capsulated bacteria like pneumococcus, Klebsiella pneumonia and Hemophilus influenzae will stand phagocytosis. Surface antigen, e.g. Vi antigen of Salmonella typhi and K antigen of Escherichia coli resist phagocytosis and lytic activity of complement.

F)Infecting dose

The minimum infection dose (MID) or minimum lethal dose (MLD) is the minimum number of organisms required to produce clinical evidence of infection or death of susceptible animal.

G) Route of infection

Vibrio cholerae is effective orally. No effect when it is introduced subcutaneously. Streptococci
can initiate infection whatever be the mode of entry. They also differ in ability to produce damage to different organs in different species, e.g. tubercle bacilli injected into rabbit cause lesion mainly in
kidney and infrequently in liver and spleen. In guinea pig, main lesion is in liver and spleen whereas kidney is spared.