Virulence
Virulence is a pathogen's or microorganism's ability to cause damage to a host.
In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host.[1] The pathogenicity of an organism—its ability to cause disease—is determined by its virulence factors.[2][3] In the specific context of gene for gene systems, often in plants, virulence refers to a pathogen's ability to infect a resistant host.[4]
The noun virulence (Latin noun virulentia) derives from the adjective virulent, meaning disease severity.[5] The word virulent derives from the Latin word virulentus, meaning "a poisoned wound" or "full of poison."[5][6] The term virulence does not only apply to viruses.
From an ecological standpoint, virulence is the loss of fitness induced by a parasite upon its host. Virulence can be understood in terms of proximate causes—those specific traits of the pathogen that help make the host ill—and ultimate causes—the evolutionary pressures that lead to virulent traits occurring in a pathogen strain.[7]
Virulent bacteria
The ability of
The virulence factors of bacteria are typically proteins or other molecules that are synthesized by enzymes. These proteins are coded for by genes in chromosomal DNA, bacteriophage DNA or plasmids. Certain bacteria employ mobile genetic elements and horizontal gene transfer. Therefore, strategies to combat certain bacterial infections by targeting these specific virulence factors and mobile genetic elements have been proposed.[8] Bacteria use quorum sensing to synchronise release of the molecules. These are all proximate causes of morbidity in the host.
Methods by which bacteria cause disease
- Adhesion
- Many bacteria must first bind to host cell surfaces. Many bacterial and host molecules that are involved in the adhesion of bacteria to host cells have been identified. Often, the host cell surface receptors for bacteria are essential proteins for other functions. Due to the presence of mucus lining and of anti-microbial substances around some host cells, it is difficult for certain pathogens to establish direct contact-adhesion.
- Colonization
- Some virulent bacteria produce special proteins that allow them to colonize parts of the host body. Helicobacter pylori is able to survive in the acidic environment of the human stomach by producing the enzyme urease. Colonization of the stomach lining by this bacterium can lead to gastric ulcers and cancer. The virulence of various strains of Helicobacter pylori tends to correlate with the level of production of urease.
- Invasion
- Some virulent bacteria produce proteins that either disrupt host cell membranes or stimulate their own endocytosis or macropinocytosis into host cells. These virulence factors allow the bacteria to enter host cells and facilitate entry into the body across epithelial tissue layers at the body surface.
- Immune response inhibitors
- Many bacteria produce virulence factors that inhibit the host's immune system defenses. For example, a common bacterial strategy is to produce proteins that bind host antibodies. The polysaccharide capsule of Streptococcus pneumoniae inhibits phagocytosis of the bacterium by host immune cells.
- Toxins
- Many virulence factors are proteins made by bacteria that poison host cells and cause tissue damage. For example, there are many food poisoning toxins produced by bacteria that can contaminate human foods. Some of these can remain in "spoiled" food even after cooking and cause illness when the contaminated food is consumed. Other bacterial toxins are chemically altered and inactivated by the heat of cooking.
Virulent viruses
Virus virulence factors allow it to replicate, modify host defenses, and spread within the host, and they are toxic to the host.[9]
They determine whether infection occurs and how severe the resulting viral disease symptoms are. Viruses often require receptor proteins on host cells to which they specifically bind. Typically, these host cell proteins are
Extensively studied
The lytic life cycle of virulent bacteriophages is contrasted by the temperate lifecycle of temperate bacteriophages.[10][11]
See also
- Host–pathogen interaction
- Membrane vesicle trafficking
- Bacterial effector protein
- Infectious disease
- Law of declining virulence– Disproved hypothesis of epidemiologist Theobald Smith
- Optimal virulence
- Super-spreader
- Theory of virulence– Theory by biologist Paul W. Ewald
- Verotoxin-producing Escherichia coli
- Virulence factor
- Antivirulence
References
- PMID 22293325.
- ^ "MeSH - Medical Subject Headings, Karolinska Institute, 13 April 2010". Archived from the original on 3 March 2016. Retrieved 13 April 2010.
- ^ Biology Online (7 October 2019). "Virulence". Biology Online.
- S2CID 6894315.
- ^ a b "virulent". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
- ^ Lewis, Charlton T.; Short, Charles. "vīrŭlentus". A Latin Dictionary. Retrieved 2023-01-02.
- ^ "Plant disease | Importance, Types, Transmission, & Control | Britannica". www.britannica.com. Retrieved 2023-01-02.
- PMID 23248780.
- ISBN 978-1-55581-480-9.
- ISBN 978-0-13-144329-7.
- ^ "lytic phage | virus | Britannica". www.britannica.com. Retrieved 2023-01-02.