Virulence factors are molecules expressed and secreted by that enable them to colonize the host, evade or inhibit the immune responses of the host, enter into or out of a host cell, and/or obtain nutrition from the host. The following are virulence factors that promote bacterial colonization of the host .
1. The ability to use motility and other means to contact host cells and disseminate within a host.
2. The ability to adhere to host cells and resist physical removal.
3. The ability to invade host cells.
4. The ability to compete for iron and other nutrients.
5. The ability to resist innate immune defenses such as phagocytosis and complement.
6. The ability to evade adaptive immune defenses.
- 5.0: Prelude to Virulence Factors that Promote Bacterial Colonization
- Virulence factors are molecules expressed on or secreted by microorganisms that enable them to colonize the host, evade or inhibit the immune responses of the host, enter into or out of a host cell, and/or obtain nutrition from the host. To cause infectious disease, a bacterium must produce virulence factors that promote bacterial colonization of the host, as well as virulence factors that impair or damage the host.
- 5.1: The Ability to Use Motility and Other Means to Contact Host Cells
- Bacteria have to make physical contact with host cells before they can adhere to those cells and resist being flushed out of the body. Motile bacteria can use their flagella and chemotaxis to swim through mucus towards mucosal epithelial cells. Because of their thinness, their internal flagella (axial filaments), their corkscrew shape, and their motility, certain spirochetes are more readily able enter lymph vessels and blood vessels and spread to other body sites.
- 5.2: The Ability to Adhere to Host Cells and Resist Physical Removal
- One of the body's innate immune defenses is the ability to physically remove bacteria from the body. Bacteria may resist physical removal by producing pili, cell wall adhesin proteins, and/or biofilm-producing capsules that enable bacteria to adhere to host cells. At the end of the shaft of a bacterial pilus is an adhesive tip structure having a shape corresponding to that of specific receptor on a host cell for initial attachment. Bacteria can typically make a variety of different adhesive tips
- 5.3: The Ability to Invade Host Cells
- Some bacteria produce molecules called invasins that activate the host cell's cytoskeletal machinery enabling bacterial entry into the cell by phagocytosis. Entering a non-defense host cell can provide the bacterium with a ready supply of nutrients, as well as protect the bacterium from complement, antibodies, and other body defense molecules. Some bacteria invade phagocytic cells, neutralize their killing ability, and turn them into a safe haven for bacterial replication.
- 5.4: The Ability to Compete for Nutrients
- The ability to be pathogenic is directly related to the bacterium's ability to compete successfully with host tissue and normal flora for limited nutrients. They compete for nutrients by synthesizing specific transport systems or cell wall components capable of binding limiting substrates and transporting them into the cell. Iron is an essential nutrient for both bacterial growth and human cell growth. Both bacteria and their host synthesize compounds capable of binding iron for their use.
- 5.5: The Ability to Resist Innate Immune Defenses
- Some bacteria are able to resist innate immune defenses such as phagocytosis and the body's complement pathways. We will break this down into two categories: (1) The ability to resist phagocytic engulfment (attachment and ingestion) and (2) the ability to resist phagocytic destruction and complement serum lysis.
- 5.6: The Ability to Evade Adaptive Immune Defenses
- There are various ways that the antibodies the body makes during adaptive immunity protect the body against bacteria. Some antibodies such as IgG and IgE function as opsonins and stick bacteria to phagocytes (opsonization or enhanced attachment). Antibodies, such as IgG, IgA, and IgM, can bind to bacterial adhesins, pili, and capsules and in this way block their attachment to host cells.
- 5.E: Virulence Factors that Promote Colonization (Exercises)
- These are homework exercises to accompany Kaiser's "Microbiology" TextMap. Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell (unicellular), cell clusters or no cell at all (acellular). This includes eukaryotes, such as fungi and protists, and prokaryotes. Viruses and prions, though not strictly classed as living organisms, are also studied.
Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)