Virulence Factors

What is pathogenicity?

The ability of a pathogen to produce an infectious disease in an organism. Virulence is defined as the degree of pathogenicity.

Factors

Virulence factors are molecules expressed and secreted by pathogens (bacteria, viruses, fungi and protozoa) that enable them to achieve the following:

• Colonization of a niche in the host (this includes adhesion to cells)
  
• Evasion of the host's immune response
  
• Entry into and exit out of cells (if the pathogen is an intracellular one)
  
• Obtain nutrition from the host

They are very often responsible for causing disease in the host as they inhibit certain host functions. There are basically two types of virulence factors, bacterial virulence and viral virulence.

Bacterial Virulence

• Adhesion - The tendency of certain dissimilar molecules to cling together due to attractive forces.
  
• Extracellular Enzymes - An enzyme, such as a digestive enzyme, that functions outside the cell from which it originates. Examples of it are the hyaluronidase, which are a family of enzymes that degrade hyaluronic acid and coagulase, which is an enzyme produced by Staphylococcus aureus that converts fibrinogen to fibrin.
  
• Anti-phagocytic factors.
  
• Toxins – a poisonous substance that produced by living things or organisms.

Viral Virulence

• Adhesion - The tendency of certain dissimilar molecules to cling together due to attractive forces.
  
• Host evasion.
  
• Latency – hides or incorporate into genome.
  
• High mutability – ability to change the protein coat.

Relative Virulence of Virus

More virulent -------------------------------------------------> Less virulent

Influenza virus > Hepatitis virus > HIV > Ebola virus