Virulence
Virulenceis apathogen's ormicroorganism'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 itshost.[1]Thepathogenicityof an organism—its ability to causedisease—is determined by itsvirulence factors.[2][3]In the specific context ofgene for genesystems, often in plants, virulence refers to a pathogen's ability to infect a resistant host.[4]
Thenounvirulence(Latinnounvirulentia) derives from theadjectivevirulent,meaning disease severity.[5]The wordvirulentderives from the Latin wordvirulentus,meaning "a poisoned wound" or "full of poison."[5][6]The termvirulencedoes not only apply to viruses.
From anecologicalstandpoint, virulence is the loss offitnessinduced by a parasite upon its host. Virulence can be understood in terms ofproximate causes—those specific traits of the pathogen that help make the host ill—andultimate causes—the evolutionary pressures that lead to virulent traits occurring in a pathogen strain.[7]
Virulent bacteria
[edit]The ability ofbacteriato cause disease is described in terms of the number of infecting bacteria, the route of entry into the body, the effects of host defense mechanisms, and intrinsic characteristics of the bacteria calledvirulence factors.Many virulence factors are so-calledeffector proteinsthat are injected into the host cells by specialized secretion apparati, such as thetype three secretion system.Host-mediated pathogenesis is often important because the host can respond aggressively to infection with the result that host defense mechanisms do damage to host tissues while the infection is being countered (e.g.,cytokine storm).
The virulence factors of bacteria are typically proteins or other molecules that are synthesized byenzymes.These proteins are coded for by genes inchromosomalDNA,bacteriophageDNA orplasmids.Certain bacteria employmobile genetic elementsandhorizontal gene transfer.Therefore, strategies to combat certain bacterial infections by targeting these specific virulence factors and mobile genetic elements have been proposed.[8]Bacteria usequorum sensingto synchronise release of the molecules. These are all proximate causes of morbidity in the host.
Methods by which bacteria cause disease
[edit]- 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 hostcell surface receptorsfor bacteria are essential proteins for other functions. Due to the presence ofmucuslining 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 pyloriis able to survive in the acidic environment of the human stomach by producing the enzymeurease.Colonization of the stomach lining by this bacterium can lead togastric ulcersandcancer.The virulence of various strains ofHelicobacter pyloritends to correlate with the level of production of urease.
- Invasion
- Some virulent bacteria produce proteins that either disrupt host cell membranes or stimulate their ownendocytosisormacropinocytosisinto 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. ThepolysaccharidecapsuleofStreptococcus pneumoniaeinhibitsphagocytosisof 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 manyfood poisoningtoxinsproduced 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
[edit]Virusvirulence 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 areendocytosedand the bound virus then enters the host cell. Virulent viruses such asHIV,which causesAIDS,have mechanisms for evading host defenses. HIV infectsT-helper cells,which leads to a reduction of the adaptive immune response of the host and eventually leads to an immunocompromised state. Death results from opportunistic infections secondary to disruption of the immune system caused by AIDS. Some viral virulence factors confer ability to replicate during the defensive inflammation responses of the host such as during virus-inducedfever.Many viruses can exist inside a host for long periods during which little damage is done. Extremely virulent strains can eventuallyevolveby mutation andnatural selectionwithin the virus population inside a host. The term "neurovirulent"is used for viruses such asrabiesandherpes simplexwhich can invade thenervous systemand cause disease there.
Extensively studiedmodel organismsof virulent viruses includevirus T4and otherT-even bacteriophageswhich infectEscherichia coliand a number of relatedbacteria.
Thelytic life cycleof virulent bacteriophages is contrasted by thetemperate lifecycleof temperate bacteriophages.[10][11]
See also
[edit]- 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
[edit]
- ^Pirofski LA, Casadevall A (2012)."Q and A: What is a pathogen? A question that begs the point".BMC Biology.10:6.doi:10.1186/1741-7007-10-6.PMC3269390.PMID22293325.
- ^"MeSH - Medical Subject Headings, Karolinska Institute, 13 April 2010".Archived fromthe originalon 3 March 2016.Retrieved13 April2010.
- ^Biology Online (7 October 2019)."Virulence".Biology Online.
- ^Thrall, Peter H.; Burdon, Jeremy J. (2003). "Evolution of Virulence in a Plant Host-Pathogen Metapopulation".Science.299(5613): 1735–7.Bibcode:2003Sci...299.1735T.doi:10.1126/science.1080070.ISSN0036-8075.PMID12637745.S2CID6894315.
- ^ab"virulent".Oxford English Dictionary(Online ed.).Oxford University Press.(Subscription orparticipating institution membershiprequired.)
- ^Lewis, Charlton T.; Short, Charles."vīrŭlentus".A Latin Dictionary.Retrieved2023-01-02.
- ^"Plant disease | Importance, Types, Transmission, & Control | Britannica".britannica.Retrieved2023-01-02.
- ^Keen, E. C. (2012)."Paradigms of pathogenesis: Targeting the mobile genetic elements of disease".Frontiers in Cellular and Infection Microbiology.2:161.doi:10.3389/fcimb.2012.00161.PMC3522046.PMID23248780.
- ^Flint, S.Jane; Enquist, Lynn W.; Racaniello, Vincent R.; Skalka, Anna Marie (2009).Principles of Virology. Vol. II Pathogenesis and Control(3rd ed.). Washington, D.C.: ASM. pp. 42–7.ISBN978-1-55581-480-9.
- ^Madigan, M. T.; Martinko, J. M. (2006).Brock Biology of Microorganisms(11th ed.). Prentice Hall.ISBN978-0-13-144329-7.
- ^"lytic phage | virus | Britannica".britannica.Retrieved2023-01-02.