Pathogenicity is the capacity to cause disease. Some bacteria are naturally pathogenic, while others can be so when a person is stressed or there is trauma allowing the microbe to enter unnatural areas of the body. Such microbes are called opportunistic pathogens - they become pathogenic when given a favorable opportunity.
Virulence is a measure of pathogenicity. It is typically measured in relationship to the ID50, infectious dose (number of bacteria) required to produce an infection in 50% of a test animal population. In cases where the pathogen can produce a deadly infection, the number used is the LD50, or lethal dose (number of bacteria) required to kill 50% of the population of test animals. Obviously, virulence is inversely proportional to LD50: the fewer microbes it takes to kill half the test population, the more virulent the microbe must be. So remember: low LD50/ID50 means high virulence, and high LD50/ID50 means low virulence.
What makes a highly virulent and dangerous microbe that way? Virulence factors are structures and products of bacterial cells that allow them to overcome the natural defenses of the body to cause disease. They come in several forms categorized in these two ways:
- Invasive factors. Structures and products of bacteria that enable them to eliminate control over their movement due to barriers in the body, avoid flushing mechanisms, survive phagocytosis. Among these substances are adhesins (allow attachment to cells), capsules (attachment or antiphagocytotic), and exoenzymes (cytolytic, among other). The innate (nonspecific) immunity defenses are made less effective against bacteria possessing invasive factors.
- Toxins. Many bacteria possess toxins that can kill immune cells or otherwise enable more severe consequences to their presence, resulting in illness. These fall into two major categories: exotoxins and endotoxins. Exotoxins are typically enzymes excreted from bacterial cells that have one of three effects: cause diarrhea, kill cells, or disrupt nerve signals. Endotoxin is the Gram negative cell wall layer called lipopolysaccharide, which causes fever, headache, and the other symptoms of food poisoning.
For a deeper look into pathogenicity and virulence factors, go to Micro Digital Media Exercise 22.
Your instructor will advise you on what documentation to submit upon completion of MDM Exercise 22: Virtual Lab Report, MDM Exercise 22, or other.
When you have finished this work, you may be asked to take a quiz over the material.