Vibrio cholerae
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''Vibrio cholerae'' is a species of
Gram-negative Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...
, facultative anaerobe and comma-shaped
bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
. The bacteria naturally live in
brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...
or saltwater where they attach themselves easily to the
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...
-containing shells of crabs, shrimp, and other shellfish. Some strains of ''V. cholerae'' are pathogenic to humans and cause a deadly disease called
cholera Cholera () is an infection of the small intestine by some Strain (biology), strains of the Bacteria, bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea last ...
, 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 Heinrich Hermann Robert Koch ( ; ; 11 December 1843 – 27 May 1910) was a German physician and microbiologist. As the discoverer of the specific causative agents of deadly infectious diseases including tuberculosis, cholera and anthrax, he i ...
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 flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
(a tail like structure) at one pole and several pili throughout its cell surface. It undergoes respiratory and fermentative metabolism. Two serogroups called O1 and O139 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. Ringers 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 chromosomes produces the cholera toxin (CT), a
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
that causes profuse, watery
diarrhea Diarrhea (American English), also spelled diarrhoea or diarrhœa (British English), is the condition of having at least three loose, liquid, or watery bowel movements in a day. It often lasts for a few days and can result in dehydration d ...
(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 In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ran ...
bacteriophage A bacteriophage (), also known informally as a phage (), is a virus that infects and replicates within bacteria. The term is derived . Bacteriophages are composed of proteins that Capsid, encapsulate a DNA or RNA genome, and may have structu ...
(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 Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromat ...
, which further is converted to
serotonin Serotonin (), also known as 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter with a wide range of functions in both the central nervous system (CNS) and also peripheral tissues. It is involved in mood, cognition, reward, learning, ...
. As such, quorum sensing allows a
commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit f ...
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 The miasma theory (also called the miasmic theory) is an abandoned medical theory that held that diseases—such as cholera, chlamydia, or plague—were caused by a ''miasma'' (, Ancient Greek for 'pollution'), a noxious form of "bad air", a ...
, which posited that infections spread through contaminated air, was no longer a satisfactory explanation. The English physician
John Snow John Snow (15 March 1813 – 16 June 1858) was an English physician and a leader in the development of anaesthesia and medical hygiene. He is considered one of the founders of modern epidemiology and early germ theory, in part because of hi ...
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 The French Academy of Sciences (, ) is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French Scientific method, scientific research. It was at the forefron ...
on 23 April was recorded as: " ouchetcould verify that there existed in these holera patientsdejecta 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 The French Academy of Sciences (, ) is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French scientific research. It was at the forefront of scientific d ...
, Pouchet announced that the organisms were '' infusoria'', a name then used for microscopic
protists A protist ( ) or protoctist is any Eukaryote, eukaryotic organism that is not an animal, Embryophyte, land plant, or fungus. Protists do not form a Clade, natural group, or clade, but are a Paraphyly, paraphyletic grouping of all descendants o ...
, naming them as the Vibrio rugula'' of Mueller and Shrank', a species of
protozoa Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically ...
described by Danish naturalist
Otto Friedrich Müller Otto Friedrich Müller, also known as Otto Friedrich Mueller (2 November 1730 – 26 December 1784) was a Denmark, Danish natural history, naturalist and scientific illustrator. Biography Müller was born in Copenhagen. He was educated for the ch ...
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 Heinrich Hermann Robert Koch ( ; ; 11 December 1843 – 27 May 1910) was a German physician and microbiologist. As the discoverer of the specific causative agents of deadly infectious diseases including tuberculosis, cholera and anthrax, he i ...
. 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 Koch's postulates ( ) are four criteria designed to establish a causality, causal relationship between a microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884, based on earlier concepts described by ...
). 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 named 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 Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
means "to quiver". Except for ''V. cholerae'' and ''V. mimicus'', all other vibrio species are
halophilic A halophile (from the Greek word for 'salt-loving') is an extremophile that thrives in high salt concentrations. In chemical terms, halophile refers to a Lewis acidic species that has some ability to extract halides from other chemical species. ...
. Initial isolates are slightly curved, whereas they can appear as straight rods upon laboratory culturing. The bacterium has a
flagellum A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
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 acid Bile acids are steroid acids found predominantly in the bile of mammals and other vertebrates. Diverse bile acids are synthesized in the liver in peroxisomes. Bile acids are conjugated with taurine or glycine residues to give anions called bile ...
s and
antimicrobial peptides Antimicrobial peptides (AMPs), also called host defence peptides (HDPs) are part of the innate immune response found among all classes of life. Fundamental differences exist between Prokaryote, prokaryotic and eukaryota, eukaryotic cells that may ...
, ''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 Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
that causes profuse, watery
diarrhea Diarrhea (American English), also spelled diarrhoea or diarrhœa (British English), is the condition of having at least three loose, liquid, or watery bowel movements in a day. It often lasts for a few days and can result in dehydration d ...
(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. ''V. cholerae'' can cause syndromes ranging from asymptomatic to cholera gravis. In
endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...
areas, 75% of cases are asymptomatic, 20% are mild to moderate, and 2–5% are severe forms such as cholera gravis. Symptoms include abrupt onset of watery
diarrhea Diarrhea (American English), also spelled diarrhoea or diarrhœa (British English), is the condition of having at least three loose, liquid, or watery bowel movements in a day. It often lasts for a few days and can result in dehydration d ...
(a grey and cloudy liquid), occasional
vomiting Vomiting (also known as emesis, puking and throwing up) is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose. Vomiting can be the result of ailments like food poisoning, gastroenteritis, pre ...
, and abdominal cramps.
Dehydration In physiology, dehydration is a lack of total body water that disrupts metabolic processes. It occurs when free water loss exceeds intake, often resulting from excessive sweating, health conditions, or inadequate consumption of water. Mild deh ...
ensues, with symptoms and signs such as thirst, dry mucous membranes, decreased skin turgor, sunken eyes,
hypotension Hypotension, also known as low blood pressure, is a cardiovascular condition characterized by abnormally reduced blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood and is ...
, weak or absent
radial pulse In human anatomy, the radial artery is the main artery An artery () is a blood vessel in humans and most other animals that takes oxygenated blood away from the heart in the systemic circulation to one or more parts of the body. Exceptions ...
,
tachycardia Tachycardia, also called tachyarrhythmia, is a heart rate that exceeds the normal resting rate. In general, a resting heart rate over 100 beats per minute is accepted as tachycardia in adults. Heart rates above the resting rate may be normal ...
,
tachypnea Tachypnea, also spelt tachypnoea, is a respiratory rate greater than normal, resulting in abnormally rapid and shallow breathing. In adult humans at rest, any respiratory rate of 1220 per minute is considered clinically normal, with tachypnea b ...
, hoarse voice,
oliguria Oliguria or hypouresis is the low output of urine specifically more than 80 ml/day but less than 400ml/day. The decreased output of urine may be a sign of dehydration, kidney failure, hypovolemic shock, hyperosmolar Hyperglycemic Nonketotic Syndro ...
, cramps,
kidney failure Kidney failure, also known as renal failure or end-stage renal disease (ESRD), is a medical condition in which the kidneys can no longer adequately filter waste products from the blood, functioning at less than 15% of normal levels. Kidney fa ...
,
seizure A seizure is a sudden, brief disruption of brain activity caused by abnormal, excessive, or synchronous neuronal firing. Depending on the regions of the brain involved, seizures can lead to changes in movement, sensation, behavior, awareness, o ...
s,
somnolence Somnolence (alternatively sleepiness or drowsiness) is a state of strong desire for sleep, or sleeping for unusually long periods (compare hypersomnia). It has distinct meanings and causes. It can refer to the usual state preceding falling aslee ...
, coma, and death. Death due to dehydration can occur in a few hours to days in untreated children. The disease is also particularly dangerous for pregnant women and their fetuses during late pregnancy, as it may cause premature labor and fetal death. A study done by the Centers for Disease Control (CDC) in Haiti found that in pregnant women who contracted the disease, 16% of 900 women had fetal death. Risk factors for these deaths include: third trimester, younger maternal age, severe dehydration, and vomiting Dehydration poses the biggest health risk to pregnant women in countries that there are high rates of cholera. In cases of cholera gravis involving severe dehydration, up to 60% of patients can die; however, less than 1% of cases treated with rehydration therapy are fatal. The disease typically lasts 4–6 days. Worldwide, diarrhoeal disease, caused by cholera and many other pathogens, is the second-leading cause of death for children under the age of 5 and at least 120,000 deaths are estimated to be caused by cholera each year. In 2002, the WHO deemed that the case fatality ratio for cholera was about 3.95%.


Cholera illness and symptoms

''V. cholerae'' infects the intestine and causes diarrhea, the hallmark symptom of cholera. Infection can be spread by eating contaminated food or drinking contaminated water. It also can spread through skin contact with contaminated human feces. Not all infection indicate symptoms, only about 1 in 10 people develop diarrhea. The major symptoms include: watery diarrhea, vomiting, rapid heart rate, loss of skin elasticity, low blood pressure, thirst, and muscle cramps. This illness can get serious as it can progress to kidney failure and possible coma. If diagnosed, it can be treated using medications.


Disease occurrence

''V. cholerae'' has an endemic or epidemic occurrence. In countries where the disease has been for the past three years and the cases confirmed are local (within the confines of the country) transmission is considered to be "endemic." Alternatively, an outbreak is declared when the occurrence of disease exceeds the normal occurrence for any given time or location."World Health Organization, Disease Outbreaks." World Health Organization, World Health Organization, 8 Mar. 2016, www.searo.who.int/topics/disease_outbreaks/en/. Epidemics can last several days or over a span of years. Additionally, countries that have an occurrence of an epidemic can also be endemic. The longest standing ''V. cholerae'' epidemic was recorded in Yemen. Yemen had two outbreaks, the first occurred between September 2016 and April 2017, and the second began later in April 2017 and recently was considered to be resolved in 2019."Mystery of Yemen Cholera Epidemic Solved." ScienceDaily, ScienceDaily, 2 Jan. 2019, www.sciencedaily.com/releases/2019/01/190102140745.htm. The epidemic in Yemen took over 2,500 lives and impacted over 1 million people of Yemen. More outbreaks have occurred in Africa, the Americas, and Haiti.


Preventive measures

When visiting areas with epidemic cholera, the following precautions should be observed: drink and use bottled water; frequently wash hands with soap and safe water; use chemical toilets or bury feces if no restroom is available; do not defecate in any body of water and cook food thoroughly. Supplying proper, safe water is important. A precaution to take is to properly sanitize. Hand hygiene is an essential in areas where soap and water is not available. When there is no sanitation available for hand washing, scrub hands with ash or sand and rinse with clean water. A single dose
vaccine A vaccine is a biological Dosage form, preparation that provides active acquired immunity to a particular infectious disease, infectious or cancer, malignant disease. The safety and effectiveness of vaccines has been widely studied and verifi ...
is available for those traveling to an area where cholera is common. There is a ''V. cholerae'' vaccine available to prevent disease spread. The vaccine is known as the, "oral cholera vaccine" (OCV). There are three types of OCV available for prevention: Dukoral®, Shanchol™, and Euvichol-Plus®. All three OCVs require two doses to be fully effective. Countries who are endemic or have an epidemic status are eligible to receive the vaccine based on several criteria: Risk of cholera, Severity of cholera, WASH conditions and capacity to improve, Healthcare conditions and capacity to improve, Capacity to implement OCV campaigns, Capacity to conduct M&E activities, Commitment at national and local level"Oral Cholera Vaccines." World Health Organization, World Health Organization, 17 May 2018, www.who.int/cholera/vaccines/en/. Since May the start of the OCV program to May 2018 over 25 million vaccines have been distributed to countries who meet the above criteria.


Treatment

The basic, overall treatment for Cholera is re-hydration, to replace the fluids that have been lost. Those with mild dehydration can be treated orally with an oral rehydration solution (ORS). When patients are severely dehydrated and unable to take in the proper amount of ORS, IV fluid treatment is generally pursued. Antibiotics are used in some cases, typically fluoroquinolones and tetracyclines.


Genome

''V. cholerae'' (and Vibrionaceae in general) has two circular
chromosomes A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most importa ...
, together totalling 4 million
base pairs A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...
of
DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
sequence and 3,885 predicted
genes In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
. The genes for cholera toxin are carried by CTXphi (CTXφ), a
temperate In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ran ...
bacteriophage A bacteriophage (), also known informally as a phage (), is a virus that infects and replicates within bacteria. The term is derived . Bacteriophages are composed of proteins that Capsid, encapsulate a DNA or RNA genome, and may have structu ...
inserted into the ''V. cholerae'' genome. CTXφ can transmit cholera toxin genes from one ''V. cholerae'' strain to another, one form of
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
. The genes for toxin coregulated pilus are coded by the Vibrio pathogenicity island (VPI), which is separate from the
prophage A prophage is a bacteriophage (often shortened to "phage") genome that is integrated into the circular bacterial chromosome or exists as an extrachromosomal plasmid within the bacterial cell (biology), cell. Integration of prophages into the bacte ...
. The larger first chromosome is 3 Mbp long with 2,770
open reading frame In molecular biology, reading frames are defined as spans of DNA sequence between the start and stop codons. Usually, this is considered within a studied region of a prokaryotic DNA sequence, where only one of the six possible reading frames ...
s (ORFs). It contains the crucial genes for toxicity, regulation of toxicity, and important cellular functions, such as transcription and
translation Translation is the communication of the semantics, meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The English la ...
. The second chromosome is 1 Mbp long with 1115 ORFs. It is determined to be different from a
plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria and ...
or megaplasmid due to the inclusion of housekeeping and other essential genes in the genome, including essential genes for metabolism, heat-shock proteins, and
16S rRNA 16S ribosomal RNA (or 16Svedberg, S rRNA) is the RNA component of the 30S subunit of a prokaryotic ribosome (SSU rRNA). It binds to the Shine-Dalgarno sequence and provides most of the SSU structure. The genes coding for it are referred to as ...
genes, which are ribosomal subunit genes used to track evolutionary relationships between bacteria. Also relevant in determining if the replicon is a chromosome is whether it represents a significant percentage of the genome, and chromosome 2 is 40% by size of the entire genome. And, unlike plasmids, chromosomes are not self-transmissible. However, the second chromosome may have once been a megaplasmid because it contains some genes usually found on plasmids, including a P1 plastid-like
origin of replication The origin of replication (also called the replication origin) is a particular sequence in a genome at which replication is initiated. Propagation of the genetic material between generations requires timely and accurate duplication of DNA by semi ...
.


Bacteriophage CTXφ

CTXφ (also called CTXphi) is a filamentous phage that contains the genes for cholera toxin. Infectious CTXφ particles are produced when ''V. cholerae'' infects humans. Phage particles are secreted from bacterial cells without
lysis Lysis ( ; from Greek 'loosening') is the breaking down of the membrane of a cell, often by viral, enzymic, or osmotic (that is, "lytic" ) mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a ...
. When CTXφ infects ''V. cholerae'' cells, it integrates into specific sites on either chromosome. These sites often contain tandem arrays of integrated CTXφ
prophage A prophage is a bacteriophage (often shortened to "phage") genome that is integrated into the circular bacterial chromosome or exists as an extrachromosomal plasmid within the bacterial cell (biology), cell. Integration of prophages into the bacte ...
. In addition to the ''ctxA'' and ''ctxB'' genes encoding cholera toxin, CTXφ contains eight genes involved in phage reproduction, packaging, secretion, integration, and regulation. The CTXφ genome is 6.9 kb long.


Ecology and epidemiology

The main reservoirs of ''V. cholerae'' are aquatic sources such as
river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...
s,
brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuary ...
s, and
estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...
, often in association with copepods or other
zooplankton Zooplankton are the heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...
,
shellfish Shellfish, in colloquial and fisheries usage, are exoskeleton-bearing Aquatic animal, aquatic invertebrates used as Human food, food, including various species of Mollusca, molluscs, crustaceans, and echinoderms. Although most kinds of shellfish ...
, and aquatic plants. Cholera infections are most commonly acquired from drinking water in which ''V. cholerae'' is found naturally or into which it has been introduced from the feces of an infected person. Cholera is most likely to be found and spread in places with inadequate water treatment, poor sanitation, and inadequate hygiene. Other common vehicles include raw or undercooked fish and shellfish. Transmission from person to person is very unlikely, and casual contact with an infected person is not a risk for becoming ill.''V. cholerae'' thrives in an
aquatic environment An aquatic ecosystem is an ecosystem found in and around a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms—aquatic life—that are dependent on each other and on their environm ...
, particularly in surface water. The primary connection between humans and pathogenic strains is through water, particularly in economically reduced areas that do not have good water purification systems. Nonpathogenic strains are also present in water ecologies. The wide variety of pathogenic and nonpathogenic strains that co-exist in aquatic environments are thought to allow for so many genetic varieties. Gene transfer is fairly common amongst bacteria, and recombination of different ''V. cholerae'' genes can lead to new virulent strains. A symbiotic relationship between ''V. cholerae'' and ''Ruminococcus obeum'' has been determined. ''R. obeum'' autoinducer represses the expression of several ''V. cholerae'' virulence factors. This inhibitory mechanism is likely to be present in other gut microbiota species which opens the way to mine the gut microbiota of members in specific communities which may utilize autoinducers or other mechanisms in order to restrict colonization by ''V. cholerae'' or other enteropathogens. Autoinducers, specifically with ''V. cholerae,'' can develop biofilms and control virulence in response to extracellular quorum-sensing molecules. Outbreaks of Cholera cause an estimated 120,000 deaths annually worldwide. There has been roughly seven pandemics since 1817, the first. These pandemics first arose in the Indian subcontinent and spread.


Diversity and evolution

Two serogroups of ''V. cholerae'', O1 and O139, cause outbreaks of cholera. O1 causes the majority of outbreaks, while O139 – first identified in
Bangladesh Bangladesh, officially the People's Republic of Bangladesh, is a country in South Asia. It is the List of countries and dependencies by population, eighth-most populous country in the world and among the List of countries and dependencies by ...
in 1992 – is confined to Southeast Asia. Many other serogroups of ''V. cholerae'', with or without the cholera toxin gene (including the nontoxigenic strains of the O1 and O139 serogroups), can cause a cholera-like illness. Only toxigenic strains of serogroups O1 and O139 have caused widespread epidemics. ''V. cholerae'' O1 has two biotypes, classical and El Tor, and each biotype has two distinct serotypes, Inaba and Ogawa. The symptoms of infection are indistinguishable, although more people infected with the El Tor biotype remain asymptomatic or have only a mild illness. In recent years, infections with the classical biotype of ''V. cholerae'' O1 have become rare and are limited to parts of Bangladesh and
India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...
. Recently, new variant strains have been detected in several parts of Asia and Africa. Observations suggest these strains cause more severe cholera with higher case fatality rates.


Natural genetic transformation

''V. cholerae'' can be induced to become competent for natural genetic transformation when grown on
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...
, a biopolymer that is abundant in aquatic habitats (e.g. from crustacean exoskeletons). Natural genetic transformation is a sexual process involving DNA transfer from one bacterial cell to another through the intervening medium, and the integration of the donor sequence into the recipient genome by
homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...
. Transformation competence in ''V. cholerae'' is stimulated by increasing cell density accompanied by nutrient limitation, a decline in growth rate, or stress. The ''V. cholerae'' uptake machinery involves a
competence Broad concept article: *Competence (polyseme), capacity or ability to perform effectively Competence or competency may also refer to: *Competence (human resources), ability of a person to do a job properly **Competence-based management, performa ...
-induced pilus, and a conserved DNA binding protein that acts as a ratchet to reel DNA into the cytoplasm. There are two models of genetic transformation, sex hypothesis and competent bacteria.


Gallery

File:Vibrio diagram.png, Diagram of the bacterium, ''V. cholerae'' File:Vibrio cholerae on TCBS agar.jpg, Yellow colored (sucrose-fermenting) colonies of ''Vibrio cholerae'' on TCBS agar. File:Vibrio cholerae.jpg, Transmission electron microscope image of ''Vibrio cholerae'' that has been negatively stained.


See also

*
Drinking water Drinking water or potable water is water that is safe for ingestion, either when drunk directly in liquid form or consumed indirectly through food preparation. It is often (but not always) supplied through taps, in which case it is also calle ...
* Haiti cholera outbreak *
Cholera vaccine A cholera vaccine is a vaccine that is effective at reducing the risk of contracting cholera. The recommended cholera vaccines are administered orally to elicit local immune responses in the gut, where the intestinal cells produce antibodies ...


References


External links


Copepods and cholera in untreated water

Vibrio cholerae El Tor N16961 Genome Page

Type strain of ''Vibrio cholerae'' at Bac''Dive'' – the Bacterial Diversity Metadatabase
{{Authority control Vibrionales Food microbiology Gram-negative bacteria Cholera Bacteria described in 1854 Pathogenic bacteria