“We have witnessed in our days the birth of a new pestilence, which, in the short space of fourteen years, has desolated the fairest portions of the globe, and swept off at least fifty MILLIONS of our race. It has mastered every variety of climate, surmounted every natural barrier, conquered every people. It has not, like the simoon, blasted life and then passed away; the cholera, like the small-pox or plague, takes root in the soil which has once possessed.”
(The London and Paris Observer, 11/27/1831)
Introduction:
Cholera is a gastrointestinal disease caused by a toxin released by aquatic bacterium Vibrio cholerae. Symptoms of cholera are acute watery diarrhea and vomiting. In severe cases, patients may show signs of severe dehydration, such as sunken eyes, decreased skin turgor, muscle cramping, and decreased blood pressure. If left untreated, severe cases can result in death within a few hours. There are two serotypes of V. cholerae responsible for the global cholera pandemics: O1 and O139. O1 has two biotypes - classical and El Tor. Non-O1 and non-O139 V. cholerae infections can cause similar but with milder symptoms (1).
There have been seven recorded cholera pandemics beginning in 1816 and many more outbreaks in that time. Today, cholera infects approximately 3-5 million people every year and between 100,000-120,000 of these cases are fatal (1). Although cases of cholera are rare in developed countries, it continues to be a public health concern in regions with underdeveloped water treatment practices because of its relatively high death rate and persistence in the environment.
Mechanism of Infection:
Although most V. cholerae ingested dies in the acidic stomach, the survivors can quickly colonize the small intestine. The symptoms associated with V. cholerae infection are primarily caused by the cholera toxin, but there are several genes that contribute to the colonization and pathogenicity of V. cholerae. Many of the genes associated with the virulence of V. cholerae are located on a pathogenicity island believed to originate from phage (2). We chose several genes identified for their role in pathogenicity and colonization to target for our project.
Cholera Toxin:
The cholera toxin is an oligomeric protein made up 6 subunits: 1 copy of subunit A (ctxA) and 5 copies of subunit B (ctxB). The 5 ctxB subunits form a ring that directly binds to the GM1 ganglioside receptors on the surface of intestinal epithelial cells, causing the cholera toxin to be endocytosed. Once inside, the disfulfide bridges in the toxin are reduced, freeing the A subunit which then activates G proteins inside the cell, causing constitutive production of cAMP. This activates chloride ion channels, causing efflux of chloride ions, followed by water, Na+, and K+ due to osmotic and electrical gradients. This produces the characteristic watery diarrhea and rapid loss of water and electrolytes (3).
Transcriptional regulator toxT:
ToxT is a transcriptional regulator that indirectly activates the cholera toxin operon and the toxin coregulated pilus operon (4). ToxT is activated when ToxR recruits TcpP to the upstream promoter.
Toxin Coregulated Pilus:
The toxin coregulated pili are bundles of fimbriae extending from the poles of V. cholerae that are coregulated with the cholera toxin. They have been shown to play an important role in V. cholerae colonization of the gut (5). Their exact cell surface interactions are unknown.
Accessory Colonization Factor:
The exact mechanism of the accessory colonization factor proteins are unknown, but they have been shown to enhance colonization of V. cholerae in the gut in infant mice (6).
[Image of gene distribution on Vibrio pathogenicity island]
Cholera and the beginnings of phage therapy:
In the late 1800s, English chemist Ernest Hanbury Hankin traveled to India to study the cholera outbreak. He observed bactericidal activity in the water taken from the Ganges river when applied to cultures of V. cholerae. In his notes, he described the agent responsible as being able to pass through a fine porcelain filter but became inactive when boiled. He also noted that people living in regions that obtained their water from the Ganges river were not experiencing a cholera outbreak (7).
Sources:
1. World Health Organization. (2014). Cholera (Fact sheet No. 107). Retrieved from http://www.who.int/mediacentre/factsheets/fs107/en/
2. Rajanna, C., Wang, J., Zhang, D., Xu, Z., Ali, A., Hou, Y.-M., and Karaolis, D. K. R. The Vibrio Pathogenicity Island of Epidemic Vibrio cholerae Forms Precise Extrachromosomal Circular Excision Products. J. Bacteriol. December 2003 185:23 6893-6901
3. Holmgren J. Actions of cholera toxin and the prevention and treatment of cholera. Nature. 1981 Jul 30;292(5822):413–417.
4. Krukonis, E.S., Yu, R.R., Dirita, V.J. The Vibrio cholerae ToxR/TcpP/ToxT virulence cascade: distinct roles for two membrane-localized transcriptional activators on a single promoter. Mol Microbiol. 2000 Oct;38(1):67-84
5. Herrington, D.A., Hall, R.H., Losonsky, G., Mekalanos, J.J., Taylor, R.K., and Levine, M.M., Toxin, toxin-coregulated pili, and the toxR regulon are essential for Vibrio cholerae pathogenesis in humans. J Exp Med Oct 1 1988 168:1487-1492
6. Peterson, K.M., Mekalanos, J.J., Characterization of the Vibrio cholerae ToxR regulon: identification of novel genes involved in intestinal colonization. Infect Immun. 1988 Nov;56(11):2822-9.
7. Abedon, S.T., Thomas-Abedon, C., Thomas, A., Mazure, H. Bacteriophage prehistory: Is or is not Hankin, 1896, a phage reference? Bacteriophage. 2011 May-Jun; 1(3): 174–178
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