Editing Cholera (section)

== Mechanism ==
[[File:Cholera role of biofilm in intestinal colonization.jpg|thumb|upright=1.4|The role of [[biofilm]] in the intestinal colonization of ''Vibrio cholerae'']]
When consumed, most bacteria do not survive the [[Gastric acid|acidic]] conditions of the [[stomach|human stomach]].<ref name="BliskaAlmagro-Moreno2015" /> The few surviving bacteria conserve their energy and stored [[nutrient]]s during the passage through the stomach by shutting down [[protein]] production. When the surviving bacteria exit the stomach and reach the [[small intestine]], they must propel themselves through the thick [[Mucus#Digestive system|mucus]] that lines the small intestine to reach the intestinal walls where they can attach and thrive.<ref name="BliskaAlmagro-Moreno2015">{{cite journal | vauthors = Almagro-Moreno S, Pruss K, Taylor RK | title = Intestinal Colonization Dynamics of Vibrio cholerae | journal = PLOS Pathogens | volume = 11 | issue = 5 | pages = e1004787 | date = May 2015 | pmid = 25996593 | pmc = 4440752 | doi = 10.1371/journal.ppat.1004787 | doi-access = free }}</ref>

Once the cholera bacteria reach the intestinal wall, they no longer need the [[flagellum|flagella]] to move. The bacteria stop producing the protein [[flagellin]] to conserve energy and nutrients by changing the mix of proteins that they express in response to the changed chemical surroundings. On reaching the intestinal wall, ''V.&nbsp;cholerae'' start producing the [[exotoxin|toxic proteins]] that give the infected person a watery diarrhea. This carries the multiplying new generations of ''V.&nbsp;cholerae'' bacteria out into the drinking water of the next host if proper sanitation measures are not in place.<ref>{{cite journal |last1=Wolfe |first1=Marlene |last2=Kaur |first2=Mehar |last3=Yates |first3=Travis |last4=Woodin |first4=Mark |last5=Lantagne |first5=Daniele |title=A Systematic Review and Meta-Analysis of the Association between Water, Sanitation, and Hygiene Exposures and Cholera in Case–Control Studies |journal=The American Journal of Tropical Medicine and Hygiene |date=2 August 2018 |volume=99 |issue=2 |pages=534–545 |doi=10.4269/ajtmh.17-0897 |pmid=29968551 |pmc=6090371 }}</ref>

The [[cholera toxin]] (CTX or CT) is an [[oligomer]]ic complex made up of six [[protein subunit]]s: a single copy of the A subunit (part A), and five copies of the B subunit (part B), connected by a [[disulfide bond]]. The five B subunits form a five-membered ring that binds to [[GM1]] [[ganglioside]]s on the surface of the intestinal epithelium cells. The A1 portion of the A subunit is an enzyme that [[ADP-ribosylation|ADP-ribosylates]] [[G protein]]s, while the A2 chain fits into the central pore of the B subunit ring. Upon binding, the complex is taken into the cell via receptor-mediated [[endocytosis]]. Once inside the cell, the disulfide bond is reduced, and the A1 subunit is freed to bind with a human partner protein called [[ADP-ribosylation factor 6]] (Arf6).<ref name=O>{{cite journal | vauthors = O'Neal CJ, Jobling MG, Holmes RK, Hol WG | title = Structural basis for the activation of cholera toxin by human ARF6-GTP | journal = Science | volume = 309 | issue = 5737 | pages = 1093–1096 | date = August 2005 | pmid = 16099990 | doi = 10.1126/science.1113398 | s2cid = 8669389 | bibcode = 2005Sci...309.1093O }}</ref> Binding exposes its active site, allowing it to permanently ribosylate the [[Gs alpha subunit]] of the [[heterotrimeric G protein]]. This results in constitutive [[Cyclic adenosine monophosphate|cAMP]] production, which in turn leads to the secretion of water, sodium, potassium, and bicarbonate into the lumen of the small intestine and rapid dehydration. The gene encoding the cholera toxin was introduced into ''V.&nbsp;cholerae'' by [[horizontal gene transfer]]. Virulent strains of ''V.&nbsp;cholerae'' carry a variant of a [[Temperateness (virology)|temperate]] [[bacteriophage]] called [[CTXφ Bacteriophage|CTXφ]].

Microbiologists have studied the [[gene expression|genetic mechanisms]] by which the ''V.&nbsp;cholerae'' bacteria turn off the production of some proteins and turn on the production of other proteins as they respond to the series of chemical environments they encounter, passing through the stomach, through the mucous layer of the small intestine, and on to the intestinal wall.<ref name="DiRita">{{cite journal | vauthors = DiRita VJ, Parsot C, Jander G, Mekalanos JJ | title = Regulatory cascade controls virulence in Vibrio cholerae | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 88 | issue = 12 | pages = 5403–5407 | date = June 1991 | pmid = 2052618 | pmc = 51881 | doi = 10.1073/pnas.88.12.5403 | bibcode = 1991PNAS...88.5403D | doi-access = free }}</ref> Of particular interest have been the genetic mechanisms by which cholera bacteria turn on the protein production of the toxins that interact with host cell mechanisms to pump [[chloride]] ions into the small intestine, creating an ionic pressure which prevents sodium ions from entering the cell. The chloride and sodium ions create a salt-water environment in the small intestines, which through osmosis can pull up to six liters of water per day through the intestinal cells, creating the massive amounts of diarrhea. The host can become rapidly dehydrated unless treated properly.<ref name="who.int">"Cholera Fact Sheet", World Health Organization. [https://www.who.int/mediacentre/factsheets/fs107/en/index.html who.int] {{webarchive|url=https://web.archive.org/web/20120505185900/http://www.who.int/mediacentre/factsheets/fs107/en/index.html |date=2012-05-05 }}. Retrieved November 5, 2013.</ref>

By inserting separate, successive sections of ''V.&nbsp;cholerae'' DNA into the DNA of other bacteria, such as ''[[Escherichia coli|E. coli]]'' that would not naturally produce the protein toxins, researchers have investigated the mechanisms by which ''V.&nbsp;cholerae'' responds to the changing chemical environments of the stomach, mucous layers, and intestinal wall. Researchers have discovered a complex cascade of regulatory proteins controls expression of ''V.&nbsp;cholerae'' [[virulence]] determinants.<ref>{{cite journal |last1=Peterson |first1=Kenneth M |last2=Gellings |first2=Patrick S |title=Multiple intraintestinal signals coordinate the regulation of Vibrio cholerae virulence determinants |journal=Pathogens and Disease |date=1 February 2018 |volume=76 |issue=1 |doi=10.1093/femspd/ftx126 |pmid=29315383 |doi-access=free }}</ref> In responding to the chemical environment at the intestinal wall, the ''V.&nbsp;cholerae'' bacteria produce the TcpP/TcpH proteins, which, together with the ToxR/ToxS proteins, activate the expression of the ToxT regulatory protein. ToxT then directly activates expression of [[virulence]] genes that produce the toxins, causing diarrhea in the infected person and allowing the bacteria to colonize the intestine.<ref name="DiRita" /> Current{{when|date=August 2017}} research aims at discovering "the signal that makes the cholera bacteria stop swimming and start to colonize (that is, adhere to the cells of) the small intestine."<ref name="DiRita" />

=== Genetic structure ===
{{Main|Vibrio cholerae}}
[[Amplified fragment length polymorphism]] [[DNA profiling|fingerprinting]] of the pandemic [[genetic isolate|isolates]] of ''V.&nbsp;cholerae'' has revealed variation in the genetic structure. Two [[gene cluster|clusters]] have been identified: Cluster I and Cluster II. For the most part, Cluster I consists of strains from the 1960s and 1970s, while Cluster II largely contains strains from the 1980s and 1990s, based on the change in the clone structure. This grouping of strains is best seen in the strains from the African continent.<ref>{{Unreliable medical source|date=December 2013}} {{cite journal | vauthors = Lan R, Reeves PR | title = Pandemic spread of cholera: genetic diversity and relationships within the seventh pandemic clone of Vibrio cholerae determined by amplified fragment length polymorphism | journal = Journal of Clinical Microbiology | volume = 40 | issue = 1 | pages = 172–181 | date = January 2002 | pmid = 11773113 | pmc = 120103 | doi = 10.1128/JCM.40.1.172-181.2002 }}</ref>

=== Antibiotic resistance ===
In many areas of the world, [[antibiotic resistance]] is increasing within cholera bacteria. In [[Bangladesh]], for example, most cases are resistant to [[tetracycline]], [[trimethoprim-sulfamethoxazole]], and [[erythromycin]].<ref name="NEJM2006" /> Rapid diagnostic [[assay]] methods are available for the identification of [[multiple drug resistance|multi-drug resistant]] cases.<ref name="Mackay">{{cite book|title=Real-Time PCR in microbiology: From diagnosis to characterization|publisher=Caister Academic Press|year=2007|isbn=978-1-904455-18-9|editor=Mackay IM}}{{page?|date=June 2024}}</ref> New generation antimicrobials have been discovered which are effective against cholera bacteria in ''in vitro'' studies.<ref name="Ramamurthy">{{cite book |title=Vibrio cholerae: Genomics and molecular biology |publisher=Caister Academic Press |year=2008 |isbn=978-1-904455-33-2 |chapter=Antibiotic resistance in ''Vibrio cholerae'' |author=Ramamurthy T |chapter-url=https://www.caister.com/hsp/abstracts/vib/12.html |page=195 }}</ref>