Vibrio cholerae

Vibrio cholerae

Vibrio cholerae is the bacterium responsible for cholera, a disease caused by the ingestion of contaminated food or water and that, in the most severe cases, causes a profound dehydration, which may lead to death if left untreated.

Historical perspective

In 1854, the Italian anatomist Filippo Pacini observed for the first time this bacterium in stool samples from patients with cholera. However, due to the beliefs of that time, his work was ignored for many years. In 1883, Robert Koch, who was traveling to Egypt to study cholera, identified the bacterium that causes the disease, Vibrio cholerae.

Nomenclature and identification

Vibrio Cholerae (Electron Microscopy)

The Vibrio cholerae bacteria belongs to the Vibrionaceae family. It’s a curved-rod shape, facultative anaerobic, Gram-negative, and asporogenic bacillus which is able to have both a respiratory and a fermentative metabolism.  Vibrio cholerae is oxidase positive, it reduces nitrate and is moved by a single polar flagellum.

Vibrio cholerae is classified according to its somatic antigens (O antigens). The classification is organized in serogroups and about 200 serogroups are known. Until recently, it was thought that serogroup O1 included all the strains responsible for epidemics and endemics of cholera. Non-O1 Vibrio cholerae serogroups are associated mainly to sporadic cases of diarrhoea and extraintestinal infections. However, in 1992, a large cholera-like outbreak happened in Bangladesh and in India and turned out to be caused by a non-O1 Vibrio cholerae strain. That bacteria did not belong to any of the 138 known serogroups, it was then designated O139. Since then, Vibrio cholerae O139  persisted as the second  etiologic agent of cholera.

Serogroup O1 is organized into three serotypes: Ogawa, Inaba and Hikojima and, into two biotypes, classical and El Tor, based on chemical properties and susceptibility to bacteriophages. The growth of Vibrio cholerae O1 is inhibited by the compound O/129 (2,4-diamino-6,7-diisopropyl-pteridine), but, it is increasingly resistant to this compound, a fact usually associated with a resistance to  sulfamethoxazole-trimethoprim. Vibrio cholerae O139  is resistant to O/129.

Transmission

Vibrio cholerae is a bacteria with high mobility, which has  modest nutritional requirements and has  Human as its main reservoir. The disease results from the ingestion of contaminated food or water and it is related to an environmental mismanagement. Water scarcity or lack of clean water, lack of sanitation and an environment, in general, not very salubrious, are the main factors behind the spread of the disease. The risk areas are usually the suburbs where there are no basic infrastructures, as well as refugee camps and displaced persons, where the minimum conditions of purity of water and public health have not been met.

Pathogenicity

Infection due to Vibrio cholerae occurs when water or contaminated foods are ingested by the organism. After passing through the stomach acid barrier, the vibrio colonizes the small intestine epithelium through the toxin co-regulated pilus (TCP), among others. The choleric  enterotoxin (CT) (and possibly other toxins), produced by the attached vibrios, changes the ionic transport in the intestinal epithelium cells. The subsequent loss of water and electrolytes leads to a severe diarrhoea which is characteristic of cholera.

Virulence mechanisms

The pathogenicity of Vibrio cholerae is a complex process and involves several genes encoding virulence factors that enable it to reach the small intestine epithelium, to colonize this epithelium and to produce the CT. In Vibrio cholerae, most of the virulence genes, required for the pathogenicity, exists in groups. They include the genetic element CTX, which is the genome of the lysogenic bacteriophage CTXΦ (CTXphi) that carries the genes that code for CT, and pathogenicity island TCP that carries the genes of the colonization factor of pili, TCP.

Ecology

The Vibrio cholerae is part of the normal bacterial flora present in coastal and estuarine areas, however, the non-pathogenic strains are most often isolated from the environment than O1 and O139 strains. In addition, outside the areas of epidemics and away from the area that may have been contaminated, the strains isolated from the environment are mainly CT negative.

Several studies have illustrated the ability of Vibrio cholerae to be associated with a variety of aquatic organisms including crustaceans, zooplankton, phytoplankton and algae in order to prolong their survival in the aquatic environment.

References:

• Center for Disease Control and Prevention. (2014). Cholera – Vibrio cholerae infection. Available: http://www.cdc.gov/cholera/index.html. Last accessed 4th Jul 2016.
• Faruque, S.M. and G.B. Nair. 2002. Molecular ecology of toxigenic Vibrio cholera.
• Microbiol. Immunol., 46(2):59-66
• Lau, O.L., 2004. Vibrio cholera: Epidemiology, ecology, evolution and climate
• change. Reed College, Portland, OR.
• Peterson, K.M. 2002. Expression of V. cholera virulence genes in response to
• Environmental signals. Curr. Issues. Microbiol. 3:29 -38
• World Health Organization (2015). Cholera. Available: http://www.who.int/mediacentre/factsheets/fs107/en/. Last accessed 01st Jul 2016.