Editing Virulence (section)

==Virulent bacteria==
The ability of [[bacteria]] 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 [[virulence factor]]s. Many virulence factors are so-called [[Bacterial effector protein|effector proteins]] that are injected into the host cells by specialized secretion apparati, such as the [[type 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]]).{{cn|date=January 2025}}

The virulence factors of bacteria are typically proteins or other molecules that are synthesized by [[enzyme]]s. These proteins are coded for by genes in [[chromosome|chromosomal]] DNA, [[bacteriophage]] DNA or [[plasmid]]s. 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.<ref>{{Cite journal | last1 = Keen | first1 = E. C. | title = Paradigms of pathogenesis: Targeting the mobile genetic elements of disease | doi = 10.3389/fcimb.2012.00161|journal=Frontiers in Cellular and Infection Microbiology|volume=2|page=161|year=2012|pmid = 23248780| pmc = 3522046| doi-access = free }}</ref> Bacteria use [[quorum sensing]] to synchronise release of the molecules. These are all proximate causes of morbidity in the host.{{cn|date=January 2025}}

===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 receptor]]s 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.{{cn|date=January 2025}}
; 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 [[Peptic ulcer disease|gastric ulcers]] and [[Stomach cancer|cancer]]. The virulence of various strains of ''Helicobacter pylori'' tends to correlate with the level of production of urease.{{cn|date=January 2025}}
; Invasion: Some virulent bacteria produce proteins that either disrupt host cell membranes or stimulate their own [[endocytosis]] or [[Pinocytosis#Macropinocytosis|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.{{cn|date=January 2025}}
; 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]] [[Bacterial capsule|capsule]] of ''[[Streptococcus pneumoniae]]'' inhibits [[phagocytosis]] of the bacterium by host immune cells.{{cn|date=January 2025}}
; Toxins: Many virulence factors are proteins made by bacteria that poison host cells and cause tissue damage. For example, there are many [[Foodborne illness|food poisoning]] [[toxin]]s 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.{{cn|date=January 2025}}