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Virulence
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<h2 id="virulent-bacteria">Virulent bacteria</h2> <p>The ability of <a href="/facts/Bacteria/wC8xSCsU">bacteria</a> to 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 called <a href="/facts/Virulence_factor/jw099pff">virulence factors</a>. Many virulence factors are so-called <a href="/facts/Bacterial_effector_protein/2xo5F3fF">effector proteins</a> that are injected into the host cells by specialized secretion apparati, such as the <a href="/facts/Type_three_secretion_system/WL4VW48e">type three secretion system</a>. 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., <a href="/facts/Cytokine_storm/8fMp0yKC">cytokine storm</a>). </p><p>The virulence factors of bacteria are typically proteins or other molecules that are synthesized by <a href="/facts/Enzyme/DSI1Fh7y">enzymes</a>. These proteins are coded for by genes in <a href="/facts/Chromosome/FRGf11FC">chromosomal</a> DNA, <a href="/facts/Bacteriophage/L2Ll0T3q">bacteriophage</a> DNA or <a href="/facts/Plasmid/yf59JVlA">plasmids</a>. Certain bacteria employ <a href="/facts/Mobile_genetic_elements/cmVCkbA7">mobile genetic elements</a> and <a href="/facts/Horizontal_gene_transfer/FPyUIsf2">horizontal gene transfer</a>. Therefore, strategies to combat certain bacterial infections by targeting these specific virulence factors and mobile genetic elements have been proposed.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> Bacteria use <a href="/facts/Quorum_sensing/GDaHeJeY">quorum sensing</a> to synchronise release of the molecules. These are all proximate causes of morbidity in the host. </p> <h3>Methods by which bacteria cause disease</h3> 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 <a href="/facts/Cell_surface_receptor/0YGSjDdL">cell surface receptors</a> for bacteria are essential proteins for other functions. Due to the presence of <a href="/facts/Mucus/hnlBuHAy">mucus</a> 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. <i><a href="/facts/Helicobacter_pylori/9q4H68LP">Helicobacter pylori</a></i> is able to survive in the acidic environment of the human stomach by producing the enzyme <a href="/facts/Urease/qh6XKw4w">urease</a>. Colonization of the stomach lining by this bacterium can lead to <a href="/facts/Peptic_ulcer_disease/peMGnYCd">gastric ulcers</a> and <a href="/facts/Stomach_cancer/HJjYtkqS">cancer</a>. The virulence of various strains of <i>Helicobacter pylori</i> 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 <a href="/facts/Endocytosis/NJjChLNj">endocytosis</a> or <a href="/facts/Pinocytosis/99D49uSV">macropinocytosis</a> 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 <a href="/facts/Polysaccharide/lqXq2cJU">polysaccharide</a> <a href="/facts/Bacterial_capsule/FVSHDlXL">capsule</a> of <i><a href="/facts/Streptococcus_pneumoniae/B5XxK27j">Streptococcus pneumoniae</a></i> inhibits <a href="/facts/Phagocytosis/eBrv2XKV">phagocytosis</a> 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 <a href="/facts/Foodborne_illness/9lxqlJqg">food poisoning</a> <a href="/facts/Toxin/11oUNk7n">toxins</a> 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. <h2 id="virulent-viruses">Virulent viruses</h2> <p><a href="/facts/Virus/mf88Bcbl">Virus</a> virulence factors allow it to replicate, modify host defenses, and spread within the host, and they are toxic to the host.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p><p>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 <a href="/facts/Endocytosis/NJjChLNj">endocytosed</a> and the bound virus then enters the host cell. Virulent viruses such as <a href="/facts/HIV/ulY2rgzu">HIV</a>, which causes <a href="/facts/HIV/AIDS/RqLIEMuJ">AIDS</a>, have mechanisms for evading host defenses. HIV infects <a href="/facts/T-helper_cells/nhwAYkDj">T-helper cells</a>, 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-induced <a href="/facts/Fever/waVxR2Ac">fever</a>. Many viruses can exist inside a host for long periods during which little damage is done. Extremely virulent strains can eventually <a href="/facts/Evolution/Hj0qAaBk">evolve</a> by mutation and <a href="/facts/Natural_selection/JXTte6en">natural selection</a> within the virus population inside a host. The term "<a href="/facts/Neurotropic_virus/HC4v3sGj">neurovirulent</a>" is used for viruses such as <a href="/facts/Rabies/rPDyNHwL">rabies</a> and <a href="/facts/Herpes_simplex/8AQktlTV">herpes simplex</a> which can invade the <a href="/facts/Nervous_system/CDxRKU2U">nervous system</a> and cause disease there. </p><p>Extensively studied <a href="/facts/Model_organism/vNv6q4yb">model organisms</a> of virulent viruses include <a href="/facts/T4_phage/Af4UyJxy">virus T4</a> and other <a href="/facts/T-even_bacteriophages/Af4UyJxy">T-even bacteriophages</a> which infect <a href="/facts/Escherichia_coli/nR4Gvl56">Escherichia coli</a> and a number of related <a href="/facts/Bacteria/wC8xSCsU">bacteria</a>. </p><p>The <a href="/facts/Lytic_cycle/lBHxkeAj">lytic life cycle</a> of virulent bacteriophages is contrasted by the <a href="/facts/Lysogenic_cycle/rrWxs5KG">temperate lifecycle</a> of temperate bacteriophages.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a><a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Host%E2%80%93pathogen_interaction/seV2tLTk">Host–pathogen interaction</a></li> <li><a href="/facts/Membrane_vesicle_trafficking/mX8USWA0">Membrane vesicle trafficking</a></li> <li><a href="/facts/Bacterial_effector_protein/2xo5F3fF">Bacterial effector protein</a></li> <li><a href="/facts/Infectious_disease/182XrncP">Infectious disease</a></li> <li><a href="/facts/Law_of_declining_virulence/xe8euNvV">Law of declining virulence</a> – Disproved hypothesis of epidemiologist Theobald Smith</li> <li><a href="/facts/Optimal_virulence/f8FXNBMq">Optimal virulence</a></li> <li><a href="/facts/Super-spreader/sL9HSwLg">Super-spreader</a></li> <li><a href="/facts/Theory_of_virulence/PkD9PlG0">Theory of virulence</a> – Theory by biologist Paul W. Ewald</li> <li><a href="/facts/Verotoxin-producing_Escherichia_coli/xhtWPSCd">Verotoxin-producing Escherichia coli</a></li> <li><a href="/facts/Virulence_factor/jw099pff">Virulence factor</a></li> <li><a href="/facts/Antivirulence/RR91CxBK">Antivirulence</a></li></ul> Wikimedia Commons has media related to Virulence. <h2 id="references">References</h2> <ol> <li id="fn:1"><p>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. PMC 3269390. PMID 22293325. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3269390" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3269390</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"MeSH - Medical Subject Headings, Karolinska Institute, 13 April 2010". Archived from the original on 3 March 2016. Retrieved 13 April 2010. <a href="https://web.archive.org/web/20160303190453/http://mesh.kib.ki.se/swemesh/show.swemeshtree.cfm?Mesh_No=G06.930&tool=karolinska" target="_blank">https://web.archive.org/web/20160303190453/http://mesh.kib.ki.se/swemesh/show.swemeshtree.cfm?Mesh_No=G06.930&tool=karolinska</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Biology Online (7 October 2019). "Virulence". Biology Online. <a href="https://www.biologyonline.com/dictionary/virulence" target="_blank">https://www.biologyonline.com/dictionary/virulence</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>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. ISSN 0036-8075. PMID 12637745. S2CID 6894315. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"virulent". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.) <a href="https://www.oed.com/view/Entry/223857" target="_blank">https://www.oed.com/view/Entry/223857</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"virulent". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.) <a href="https://www.oed.com/view/Entry/223857" target="_blank">https://www.oed.com/view/Entry/223857</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Lewis, Charlton T.; Short, Charles. "vīrŭlentus". A Latin Dictionary. 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