Virulence and Pathogenicity

Pathogenicity: the capacity of microbes to cause disease

Virulence: the degree of pathogenicity of a specific microbe

Based on:

• Invasive qualities
• Toxic qualities
• Presence of pile or fimbriae for adhesion
• Ability to avoid host defenses (mutate)

Virulence Factors and Toxins

• Enzymatic Virulence Factors Examples: Coagulase (Staphylococcus aureus), Streptokinase (Streptococcus pyogenes), Hyaluronidase (Many pathogens), Collagenase (Many pathogens), Leukocidin (Many pathogens), Hemolysin  (Many pathogens)
• Adhesion Factors Examples: Protein A (Staphylococcus aureus), Protein M (Streptococcus pyogenes)
• Virulence factors help bacteria to, invade the host,  cause disease, and evade host defenses. 
• The following are types of virulence factors: 
• Adherence Factors: Many pathogenic bacteria colonize mucosal sites by using pili (fimbriae) to adhere to cells.
• Invasion Factors: Surface components that allow the bacterium to invade host cells can be encoded on plasmids, but more often are on the chromosome.
• Capsules: Many bacteria are surrounded by capsules that protect them from opsonization and phagocytosis
• Exotoxins - A type of bacterial toxin with the following properties: May be produced by either gram-positive or gram-negative bacteria, The action of the exotoxin does not necessarily require the presence of the bacteria in the host and Most exotoxins are peptide or protein 
• Most exotoxins are heat sensitive (exception: enterotoxin of Staphylococcus aureus)
• Exotoxins include several types of protein toxins and enzymes produced and/or secreted from pathogenic bacteria. Major categories include cytotoxins, neurotoxins, and enterotoxins.

Classes of exotoxins: Neurotoxic, cytotoxic, or enterotoxic exotoxins

• Neurotoxins: Interfere with proper synaptic transmissions in neurons
• Cytotoxins: Inhibit specific cellular activities, such as protein synthesis 
• Enterotoxins: Interfere with water reabsorption in the large intestine; irritate the lining of the gastrointestinal tract

Endotoxins

• A type of bacterial toxin having the following properties:
• Produced only by gram-negative bacteria 
• Endotoxins are a component of the gram-negative cell wall 
• The action of endotoxin requires the presence of the bacteria in the host. The endotoxin may be released from the cell wall as the cells die and disintegrate
• Endotoxin is composed of Lipid A: Part of the lipopolysaccharide layer 
• Mode of action: Irritation/inflammation of epithelium, GI irritation, capillary/blood vessel inflammation, hemorrhaging

• Endotoxins: The lipopolysaccharide endotoxins on Gram-negative bacteria cause fever, changes in blood pressure, inflammation, lethal shock, and many other toxic events.