Vibrio cholerae is a species of Gram-negative, facultative anaerobe and comma-shaped bacteria. The bacteria naturally live in brackish or saltwater where they attach themselves easily to the chitin-containing shells of crabs, shrimp, and other shellfish. Some strains of V. cholerae are pathogenic to humans and cause a deadly disease called cholera, which can be derived from the consumption of undercooked or raw marine life species or drinking contaminated water.
V. cholerae was first described by Félix-Archimède Pouchet in 1849 as some kind of protozoa. Filippo Pacini correctly identified it as a bacterium and from him, the scientific name is adopted. The bacterium as the cause of cholera was discovered by Robert Koch in 1884. Sambhu Nath De isolated the cholera toxin and demonstrated the toxin as the cause of cholera in 1959.
The bacterium has a flagellum (a tail like structure) at one pole and several pili throughout its cell surface. It undergoes respiratory and fermentative metabolism. Two serogroups called O1NCBI: Vibrio cholerae O1 (serogroup) and O139NCBI: Vibrio cholerae O139 (serogroup) are responsible for cholera outbreaks. Infection is mainly through drinking contaminated water or ingestion of food contaminated with faecal matter from an infected person, therefore is linked to sanitation and hygiene. When ingested, it invades the intestinal mucosa which can cause diarrhea and vomiting in a host within several hours to 2–3 days of ingestion. Ringer's lactate and oral rehydration solution, combined with antibiotics such as fluoroquinolones and tetracyclines, are the common treatment methods in severe cases.
V. cholerae has two circular chromosomes. One chromosome produces the cholera toxin (CT), a protein that causes profuse, watery diarrhea (known as "rice-water stool"). But the DNA does not directly code for the toxin as the genes for cholera toxin are carried by CTXphi (CTXφ), a temperate bacteriophage (virus). The virus only produces the toxin when inserted into the bacterial DNA. Quorum sensing in V. cholerae is well studied and it activates host immune signaling and prolongs host survival, by limiting the bacterial intake of nutrients, such as tryptophan, which further is converted to serotonin. As such, quorum sensing allows a commensal interaction between host and pathogenic bacteria.
Discovery Initial observations During the third global pandemic of cholera (1846–1860), there was extensive scientific research to understand the etiology of the disease. The miasma theory, which posited that infections spread through contaminated air, was no longer a satisfactory explanation. The English physician John Snow was the first to give convincing evidence in London in 1854 that cholera was spread from drinking water – a contagion, not miasma. Yet he could not identify the pathogens, which made most people still believe in the miasma origin.
V. cholerae was first observed and recognized under microscope by the French zoologist Félix-Archimède Pouchet. In 1849, Pouchet examined the stool samples of four people having cholera. His presentation before the French Academy of Sciences on 23 April was recorded as: "[Pouchet] could verify that there existed in these [cholera patients] dejecta an immense quantity of microscopic infusoria." As summarised in the Gazette medicale de Paris (1849, p 327), in a letter read at the 23 April 1849 meeting of the Paris Academy of Sciences, Pouchet announced that the organisms were infusoria, a name then used for microscopic protists, naming them as the 'Vibrio rugula of Mueller and Shrank', a species of protozoa described by Danish naturalist Otto Friedrich Müller in 1786.
Identification of the bacterium An Italian physician, Filippo Pacini, while investigating cholera outbreak in Florence in the late 1854, identified the causative pathogen as a new type of bacterium. He performed autopsies of corpses and made meticulous microscopic examinations of the tissues and body fluids. From feces and intestinal mucosa, he identified many comma-shaped bacilli. Reporting his discovery before the Società Medico-Fisica Fiorentina (Medico-Physician Society of Florence) on 10 December, and published in the 12 December issue of the Gazzetta Medica Italiana (Medical Gazette of Italy), Pacini stated:Pacini thus introduced the name vibrioni (Latin vībro means "to move rapidly to and fro, to shake, to agitate"). A Catalan physician Joaquim Balcells i Pascual also reported such bacterium around the same time. The discovery of the new bacterium was not regarded as medically important as the bacterium was not directly attributed to cholera. Pacini also stated that there was no reason to say that the bacterium caused the disease since he failed to create a pure culture and perform experiments, which was necessary 'to attribute the quality of contagion to cholera'. The miasma theory was still not ruled out.
Rediscovery The medical importance and relationship between the bacterium and the cholera disease was discovered by German physician Robert Koch. In August 1883, Koch, with a team of German physicians, went to Alexandria, Egypt, to investigate the cholera epidemic there. Koch found that the intestinal mucosa of people who died of cholera always had the bacterium, yet he could not confirm if it was the causative agent. He moved to Calcutta (now Kolkata), India, where the epidemic was more severe. It was from here that he isolated the bacterium in a pure culture on 7 January 1884. He subsequently confirmed that the bacterium was a new species, and described it as "a little bent, like a comma." He reported his discovery to the German Secretary of State for the Interior on 2 February, and it was published in the Deutsche Medizinische Wochenschrift (German Medical Weekly).
Although Koch was convinced that the bacterium was the cholera pathogen, he could not entirely procure critical evidence that the bacterium produced the symptoms in healthy subjects (an important element in what was later known as Koch's postulates). His experiment on animals using his pure bacteria culture did not lead to the appearance of the disease in any of the subjects, and he correctly deduced that animals are immune to the human pathogen. The bacterium was by then known as "the comma bacillus." It was only in 1959, in Calcutta, that Indian physician Sambhu Nath De isolated the cholera toxin and showed that it caused cholera in healthy subjects, hence fully proving the bacterium-cholera relationship.
Taxonomy Pacini had used the name "vibrio cholera", without proper binomial rendering, for the name of the bacterium. Following Koch's description, a scientific name Bacillus comma was popularised. But an Italian bacteriologist Vittore Trevisan published in 1884 that Koch's bacterium was the same as that of Pacini's and introduced the name Bacillus cholerae. A German physician Richard Friedrich Johannes Pfeiffer renamed it as Vibrio cholerae in 1896. The name was adopted by the Committee of the Society of American Bacteriologists on Characterization and Classification of Bacterial Types in 1920. In 1964, Rudolph Hugh of the George Washington University School of Medicine proposed to use the genus Vibrio with the type species V. cholerae (Pacini 1854) as a permanent name of the bacterium, regardless of the same name for protozoa. It was accepted by the Judicial Commission of the International Committee on Bacteriological Nomenclature in 1965, and the International Association of Microbiological Societies in 1966.
Characteristics V. cholerae is a highly motile, comma shaped, gram-negative rod. The active movement of V. cholerae inspired the genus name because "vibrio" in Latin means "to quiver". Except for V. cholerae and V. mimicus, all other vibrio species are halophilic. Initial isolates are slightly curved, whereas they can appear as straight rods upon laboratory culturing. The bacterium has a flagellum at one cell pole as well as pili. It tolerates alkaline media that kill most intestinal commensals, but they are sensitive to acid. It is an aerobe while all other Vibrios are facultative anaerobes, and can undergo respiratory and fermentative metabolism. It measures 0.3 μm in diameter and 1.3 μm in length with average swimming velocity of around 75.4 μm/sec.
Pathogenicity
V. cholerae pathogenicity genes code for proteins directly or indirectly involved in the virulence of the bacteria. To adapt the host intestinal environment and to avoid being attacked by bile acids and antimicrobial peptides, V. cholera uses its outer membrane vesicles (OMVs). Upon entry, the bacteria sheds its OMVs, containing all the membrane modifications that make it vulnerable for the host attack.
During infection, V. cholerae secretes cholera toxin (CT), a protein that causes profuse, watery diarrhea (known as "rice-water stool"). This cholera toxin contains 5 B subunits that plays a role in attaching to the intestinal epithelial cells and 1 A subunit that plays a role in toxin activity. Colonization of the small intestine also requires the toxin coregulated pilus (TCP), a thin, flexible, filamentous appendage on the surface of bacterial cells. Expression of both CT and TCP is mediated by two component systems (TCS), which typically consist of a membrane-bound histidine kinase and an intracellular response element. TCS enable bacteria to respond to changing environments. In V. cholerae several TCS have been identified to be important in colonization, biofilm production and virulence. Quorum regulatory small RNAs (Qrr RNA) have been identified as targets of V. cholerae TCS. Here, the small RNA (sRNA) molecules bind to mRNA to block translation or induce degradation of inhibitors of expression of virulence or colonization genes. In V. cholerae the TCS EnvZ/OmpR alters gene expression via the sRNA coaR in response to changes in osmolarity and pH. An important target of coaR is tcpI, which negatively regulates expression of the major subunit of the TCP encoding gene (tcpA). When tcpI is bound by coaR it is no longer able to repress expression tcpA, leading to an increased colonization ability. Expression of coaR is upregulated by EnvZ/OmpR at a pH of 6,5, which is the normal pH of the intestinal lumen, but is low at higher pH values. V. cholerae in the intestinal lumen utilizes the TCP to attach to the intestinal mucosa, not invading the mucosa. After doing so it secretes cholerae toxin causing its symptoms. This then increases cyclic AMP or cAMP by binding (cholerae toxin) to adenylyl cyclase activating the GS pathway which leads to efflux of water and sodium into the intestinal lumen causing watery stools or rice watery stools.