Vibrio cholerae and Asiatic Cholera

Vibrio cholerae and Asiatic Cholera (page 1) 

(This chapter has 4 pages) 



Introduction

The genus Vibrio consists of Gram-negative straight or curved rods, motile by means of a single polar flagellum. Vibrios are capable of both respiratory and fermentative metabolism. O₂ is a universal electron acceptor; they do not denitrify. Most species are oxidase-positive. In most ways vibrios are related to enteric bacteria, but they share some properties with pseudomonads a well. The Family Vibrionaceae is found in the "Facultatively Anaerobic Gram-negative Rods" in Bergey's Manual (1986), on the level with the FamilyEnterobacteriaceae. In the revisionist taxonomy of 2001 (Bergey's Manual), based on phylogenetic analysis, Vibrionaceae, Pseudomonadaceae andEnterobacteriaceae are all landed in the  Gammaproteobacteria. Vibrios are distinguished from enterics by being oxidase-positive and motile by means of polar flagella. Vibrios are distinguished from pseudomonads by being fermentative as well as oxidative in their metabolism. Of the vibrios that are clinically significant to humans, Vibrio cholerae,the agent of cholera, is the most important.

Most vibrios have relatively simple growth factor requirements and will grow in synthetic media with glucose as a sole source of carbon and energy. However, since vibrios are typically marine organisms, most species require 2-3% NaCl or a sea water base for optimal growth. Vibrios vary in their nutritional versatility, but some species will grow on more than 150 different organic compounds as carbon and energy sources, occupying the same level of metabolic versatility asPseudomonas. In liquid media vibrios are motile by polar flagella that are enclosed in a sheath continuous with the outer membrane of the cell wall. On solid media they may synthesize numerous lateral flagella which are not sheathed.

Vibrios are one of the most common organisms in surface waters of the world. They occur in both marine and freshwater habitats and in associations with aquatic animals. Some species are bioluminescent and live in mutualistic associations with fish and other marine life. Other species are pathogenic for fish, eels, and frogs, as well as other vertebrates and invertebrates.

V. cholerae and V. parahaemolyticus are pathogens of humans. Both produce diarrhea, but in ways that are entirely different. V. parahaemolyticus is an invasive organism affecting primarily the colon; V. cholerae is noninvasive, affecting the small intestine through secretion of an enterotoxin. Vibrio vulnificusis an emerging pathogen of humans. This organism causes wound infections, gastroenteritis, or a syndrome known as "primary septicemia."

Campylobacter jejuni (formerly Vibrio fetus), is now moved to the classEpsilonproteobacteria in the the family Campylobacteraceae. Campylobacter jejuni has been associated with dysentery-like gastroenteritis, as well as with other types of infection, including bacteremic and central nervous system infections in humans. Another vibrio-like organism, Helicobacter pylori causes duodenal and gastric ulcers and gastric cancer. It is also reclassified into the class Epsilonproteobacteria family Helicobacteraceae.


Vibrio cholerae

Cholera

Cholera (frequently called Asiatic cholera or epidemic cholera) is a severe diarrheal disease caused by the bacterium Vibrio cholerae. Transmission to humans is by water or food. The natural reservoir of the organism is not known. It was long assumed to be humans, but some evidence suggests that it is the aquatic environment.

V. cholerae produces cholera toxin, the model for enterotoxins, whose action on the mucosal epithelium is responsible for the characteristic diarrhea of the disease cholera. In its extreme manifestation, cholera is one of the most rapidly fatal illnesses known. A healthy person may become hypotensive within an hour of the onset of symptoms and may die within 2-3 hours if no treatment is provided. More commonly, the disease progresses from the first liquid stool to shock in 4-12 hours, with death following in 18 hours to several days.

The clinical description of cholera begins with sudden onset of massive diarrhea. The patient may lose gallons of protein-free fluid and associated electrolytes, bicarbonates and ions within a day or two. This results from the activity of the cholera enterotoxin which activates the adenylate cyclase enzyme in the intestinal cells, converting them into pumps which extract water and electrolytes from blood and tissues and pump it into the lumen of the intestine. This loss of fluid leads to dehydration, anuria, acidosis and shock. The watery diarrhea is speckled with flakes of mucus and epithelial cells ("rice-water stool") and contains enormous numbers of vibrios. The loss of potassium ions may result in cardiac complications and circulatory failure. Untreated cholera frequently results in high (50-60%) mortality rates.

Treatment of cholera involves the rapid intravenous replacement of the lost fluid and ions. Following this replacement, administration of isotonic maintenance solution should continue until the diarrhea ceases. If glucose is added to the maintenance solution it may be administered orally, thereby eliminating the need for sterility and iv. administration. By this simple treatment regimen, patients on the brink of death seem to be miraculously cured and the mortality rate of cholera can be reduced more than ten-fold. Most antibiotics and chemotherapeutic agents have no value in cholera therapy, although a few (e.g. tetracyclines) may shorten the duration of diarrhea and reduce fluid loss.

Cholera has smoldered in an endemic fashion on the Indian subcontinent for centuries. There are references to deaths due to dehydrating diarrhea dating back to Hippocrates and Sanskrit writings. Epidemic cholera was described in 1563 by Garcia del Huerto, a Portuguese physician at Goa, India. The mode of transmission of cholera by water was proven in 1849 by John Snow, a London physician. In 1883, Robert Koch successfully isolated the cholera vibrio from the intestinal discharges of cholera patients and proved conclusively that it was the agent of the disease.

The first long-distance spread of cholera to Europe and the Americas began in 1817, such that by the early 20th century, six waves of cholera had spread across the world in devastating epidemic fashion. Since then, until the 1960s, the disease contracted, remaining present only in southern Asia. In 1961, the "El Tor" biotype (distinguished from classic biotypes by the production of hemolysins) reemerged and produced a major epidemic in the Philippines to initiate a seventh global pandemic (See map below). Since then, this biotype has spread across Asia, the Middle East, Africa, and parts of Europe.

There are several characteristics of the El Tor strain that confer upon it a high degree of "epidemic virulence" allowing it to spread across the world as previous strains have done. First, the ratio of cases to carriers is much less than in cholera due to classic biotypes (1: 30-100 for El Tor vs. 1: 2 - 4 for "classic" biotypes). Second, the duration of carriage after infection is longer for the El Tor strain than the classic strains. Third, the El Tor strain survives for longer periods in the extraintestinal environment. Between 1969 and 1974, El Tor replaced the classic strains in the heartland of endemic cholera, the Ganges River Delta of India.

The global spread of cholera during the seventh pandemic, 1961-1971. (CDC)

El Tor broke out explosively in Peru in 1991 (after an absence of cholera there for 100 years), and spread rapidly in Central and South America, with recurrent epidemics in 1992 and 1993. From the onset of the epidemic in January 1991 through September 1, 1994, a total of 1,041,422 cases and 9,642 deaths (overall case-fatality rate: 0.9%) were reported from countries in the Western Hemisphere to the Pan American Health Organization. In 1993, the numbers of reported cases and deaths were 204,543 and 2362, respectively.

In 1982, in Bangladesh, a classic biotype resurfaced with a new capacity to produce more severe illness, and it rapidly replaced the El Tor strain which was thought to be well-entrenched. This classic strain has not yet produced a major outbreak in any other country.

In December, 1992, a large epidemic of cholera began in Bangladesh, and large numbers of people have been involved. The organism has been characterized asV. cholerae O139 "Bengal". It is derived genetically from the El Tor pandemic strain but it has changed its antigenic structure such that there is no existing immunity and all ages, even in endemic areas, are susceptible. The epidemic has continued to spread. and V. choleraeO139 has affected at least 11 countries in southern Asia. Specific totals for numbers of V. cholerae O139 cases are unknown because affected countries do not report infections caused by O1 and O139 separately.

In April 1997, a cholera outbreak occurred among 90,000 Rwandan refugees residing in temporary camps in the Democratic Republic of Congo. During the first 22 days of the outbreak, 1521 deaths were recorded, most of which occurred outside of health-care facilities.

In the United States, cholera was prevalent in the 1800s but has been virtually eliminated by modern sewage and water treatment systems. However, as a result of improved transportation, more persons from the United States travel to parts of Latin America, Africa, or Asia where epidemic cholera is occurring. U.S. travelers to areas with epidemic cholera may be exposed to the bacterium. In addition, travelers may bring contaminated seafood back to the United States. A few foodborne outbreaks have been caused by contaminated seafood brought into this country by travelers. Greater than 90 percent of the cases of cholera in the U.S. have been associated with foreign travel.

V. choleraemay also live in the environment in brackish rivers and coastal waters. Shellfish eaten raw have been a source of cholera, and a few persons in the United States have contracted cholera after eating raw or undercooked shellfish from the Gulf of Mexico.

Antigenic Variation and LPS Structure in Vibrio cholerae

Antigenic variation plays an important role in the epidemiology and virulence of cholera. The emergence of the Bengal strain, mentioned above, is an example. The flagellar antigens of V. cholerae are shared with many water vibrios and therefore are of no use in distinguishing strains causing epidemic cholera. O antigens, however, do distinguish strains of V. cholerae into 139 known serotypes. Almost all of these strains of V. cholerae are nonvirulent. Until the emergence of the Bengal strain (which is "non-O1") a single serotype, designated O1, has been responsible for epidemic cholera. However, there are three distinct O1 biotypes, named Ogawa, Inaba and Hikojima, and each biotype may display the "classical" or El Tor phenotype. The Bengal strain (O139) is a new serological strain with a unique O-antigen which partly explains the lack of residual immunity.

Antigenic Determinants of Vibrio cholerae

Serotype	O Antigens
Ogawa	A, B
Inaba	A, C
Hikojima	A, B, C

Endotoxin is present in Vibrio cholerae as in other Gram-negative bacteria. Fewer details of the chemical structure of Vibrio cholerae LPS are known than in the case of E. coli and Salmonella, but some unique properties have been described. Most importantly, variations in LPS occur in vivo and in vitro, which may be correlated with reversion in nature of nonepidemic strains to classic epidemic strains and vice versa.

Cholera Toxin

Cholera toxin activates the adenylate cyclase enzyme in cells of the intestinal mucosa leading to increased levels of intracellular cAMP, and the secretion of H₂0, Na⁺, K⁺, Cl^-, and HCO₃^- into the lumen of the small intestine. The effect is dependent on a specific receptor, monosialosyl ganglioside (GM1 ganglioside) present on the surface of intestinal mucosal cells. The bacterium produces an invasin, neuraminidase, during the colonization stage which has the interesting property of degrading gangliosides to the monosialosyl form, which is the specific receptor for the toxin.

The toxin has been characterized and contains 5 binding (B) subunits of 11,500 daltons, an active (A1) subunit of 23,500 daltons, and a bridging piece (A2) of 5,500 daltons that links A1 to the 5B subunits. Once it has entered the cell, the A1 subunit enzymatically transfers ADP ribose from NAD to a protein (called Gs or Ns), that regulates the adenylate cyclase system which is located on the inside of the plasma membrane of mammalian cells.

Enzymatically, fragment A1 catalyzes the transfer of the ADP-ribosyl moiety of NAD to a component of the adenylate cyclase system. The process is complex. Adenylate cyclase (AC) is activated normally by a regulatory protein (GS) and GTP; however activation is normally brief because another regulatory protein (Gi), hydrolyzes GTP. The normal situation is described as follows.

The A1 fragment catalyzes the attachment of ADP-Ribose (ADPR) to the regulatory protein forming Gs-ADPR from which GTP cannot be hydrolyzed. Since GTP hydrolysis is the event that inactivates the adenylate cyclase, the enzyme remains continually activated. This situation can be illustrated

Thus, the net effect of the toxin is to cause cAMP to be produced at an abnormally high rate which stimulates mucosal cells to pump large amounts of Cl^- into the intestinal contents. H₂O, Na⁺ and other electrolytes follow due to the osmotic and electrical gradients caused by the loss of Cl^-. The lost H₂O and electrolytes in mucosal cells are replaced from the blood. Thus, the toxin-damaged cells become pumps for water and electrolytes causing the diarrhea, loss of electrolytes, and dehydration that are characteristic of cholera. 

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Mechanism of action of cholera enterotoxin according to Finkelstein inBaron, Chapter 24. Cholera toxin approaches target cell surface. B subunits bind to oligosaccharide of GM1 ganglioside. Conformational alteration of holotoxin occurs, allowing the presentation of the A subunit to cell surface. The A subunit enters the cell. The disulfide bond of the A subunit is reduced by intracellular glutathione, freeing A1 and A2. NAD is hydrolyzed by A1, yielding ADP-ribose and nicotinamide. One of the G proteins of adenylate cyclase is ADP-ribosylated, inhibiting the action of GTPase and locking adenylate cyclase in the "on" mode.

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Colonization of the Small Intestine

There are several characteristics of pathogenic V. cholerae that are importantdeterminants of the colonization process. These include adhesins,neuraminidase, motility, chemotaxis and toxin production. If the bacteria are able to survive the gastric secretions and low pH of the stomach, they are well adapted to survival in the small intestine. V. cholerae is resistant to bile salts and can penetrate the mucus layer of the small intestine, possibly aided by secretion of neuraminidase and proteases (mucinases). They withstand propulsive gut motility by their own swimming ability and chemotaxis directed against the gut mucosa.

Specific adherence of V. cholerae to the intestinal mucosa is probably mediated by long filamentous fimbriae that form bundles at the poles of the cells. These fimbriae have been termed Tcp pili (for toxin coregulated pili), because expression of these pili genes is coregulated with expression of the cholera toxin genes. Not much is known about the interaction of Tcp pili with host cells, and the host cell receptor for these fimbriae has not been identified. Tcp pili share amino acid sequence similarity with N-methylphenylalanine pili of Pseudomonasand Neisseria.

Two other possible adhesins in V. cholerae are a surface protein that agglutinates red blood cells (hemagglutinin) and a group of outer membrane proteins which are products of the acf (accessory colonization factor) genes. acf mutants have been shown to have reduced ability to colonize the intestinal tract. It has been suggested that V. cholerae might use these nonfimbrial adhesins to mediate a tighter binding to host cells than is attainable with fimbriae alone.

V. cholerae produces a protease originally called mucinase that degrades different types of protein including fibronectin, lactoferrin and cholera toxin itself. Its role in virulence is not known but it probably is not involved in colonization since mutations in the mucinase gene (designated hap for hemagglutinin protease) do not exhibit reduced virulence. It has been suggested that the mucinase might contribute to detachment rather than attachment. Possibly the vibrios would need to detach from cells that are being sloughed off of the mucosa in order to reattach to newly formed mucosal cells.

Genetic Organization and Regulation of Virulence Factors in Vibrio cholerae

In Vibrio cholerae, the production of virulence factors is regulated at several levels. Regulation of genes at the transcriptional level, especially the genes for toxin production and fimbrial synthesis, has been studied in the greatest detail.

V. cholerae enterotoxin is a product of ctx genes. ctxA encodes the A subunit of the toxin, and ctxB encodes the B subunit. The genes are part of the same operon. The transcript (mRNA) of the ctx operon has two ribosome binding sites (rbs), one upstream of the A coding region and another upstream of the B coding region. The rbs upstream of the B coding region is at least seven-times stronger than the rbs of the A coding region. In this way the organism is able to translate more B proteins than A proteins, which is required to assemble the toxin in the appropriate 1A: 5B proportion. The components are assembled in the periplasm after translation. Any extra B subunits can be excreted by the cell, but A must be attached to 5B in order to exit the cell. Intact A subunit is not enzymatically active, but must be nicked to produce fragments A1 and A2 which are linked by a disulfide bond. Once the cholera toxin has bound to the GM1 receptor on host cells, the A1 subunit is released from the toxin by reduction of the disulfide bond that links it to A2, and enters the cell by an unknown translocation mechanism. One hypothesis is that the 5 B subunits form a pore in the host cell membrane through which the A1 unit passes.

Transcription of the ctxAB operon is regulated by a number of environmental signals, including temperature, pH, osmolarity, and certain amino acids. Several other V. cholerae genes are coregulated in the same manner including the tcp operon, which is concerned with fimbrial synthesis and assembly. Thus the ctx operon and the tcp operon are part of a regulon, the expression of which is controlled by the same environmental signals.

The proteins involved in control of this regulon expression have been identified as ToxR, ToxS and ToxT. ToxR is a transmembranous protein with about two-thirds of its amino terminal part exposed to the cytoplasm. ToxR dimers, but not ToxR monomers, will bind to the operator region of ctxAB operon and activate its transcription. ToxS is a periplasmic protein. It is thought that ToxS can respond to environmental signals, change conformation, and somehow influence dimerization of ToxR which activities transcription of the operon.  ToxR and ToxS appear to form a standard two-component regulatory system with ToxS functioning as a sensor protein that phosphorylates and thus converts ToxR to its active DNA binding form. ToxT is  a cytoplasmic protein that is a transcriptional activator of the tcp operon. Expression of ToxT is activated by ToxR, while ToxT, in turn, activates transcription of tcp genes for synthesis of tcp pili.

Thus, the ToxR protein is a regulatory protein which functions as an inducer in a system of positive control. Tox R is thought to interact with ToxS in order to sense some change in the environment and transmit a molecular signal to the chromosome which induces the transcription of genes for attachment (pili formation) and toxin production. It is reasonable to expect that the environmental conditions that exist in the GI tract (i.e., 37^o temperature, low pH, high osmolarity, etc.), as opposed to conditions in the extraintestinal (aquatic) environment of the vibrios, are those that are necessary to induce formation of the virulence factors necessary to infect. However, there is conflicting experimental evidence in this regard, which leads to speculation of the ecological function of the toxin during human infection.

Immunity to Cholera

Infection with V. cholerae results in a spectrum of responses ranging from life-threatening secretory diarrhea to mild or unapparent infections of no manifestation except a serologic response. The reasons for these differences are not known. One idea is that individuals differ in the availability of intestinal receptors for cholera vibrios or for their toxin, but this has not been proven. Prior immunologic experience is certainly a major factor. For example, in heavily endemic regions such as Bangladesh, the attack rate is relatively low among adults in comparison with children.

After natural infection by V. cholerae, circulating antibodies can be detected against several cholera antigens including the toxin, somatic (O) antigens, and flagellar (H) antigens. These antibodies are also raised by parenteral injection of antigens as vaccine components. Antibodies directed against Vibrio O antigens are considered "vibriocidal" antibodies because they will lyse V. cholerae cells in the presence of complement and serum components. Vibriocidal antibodies reach a peak 8-10 days after the onset of clinical illness, and then decrease, returning to the baseline 2 - 7 months later. Their presence correlates with resistance to infection, but they may not be the mediators of this protection, and the role of circulating antibodies in natural infection is unclear.

After natural infection, people also develop toxin-neutralizing antibodies butthere is no correlation between antitoxic antibody levels and the incidence of disease in cholera zones.

Since cholera is essentially a topical disease of the small intestine, it would seem that topical defense might be a main determinant of protection against infection by V. cholerae. Recurrent infections of cholera are in fact, rare, and this is probably due to local immune defense mediated by antibodies secreted onto the surfaces of the intestinal mucosa. Moreover, in children who are nursing cholera is less likely to occur, presumably due to protection afforded by secretory antibody in mother's milk.

Secretory IgA, as well as IgG and IgM in serum exudate, can be detected in the intestinal mucosa of immune individuals. Although these antibodies presumably have to function in the absence of complement they still bring about protective immunity. Motility is important in pathogenesis, and antibodies against flagella could immobilize the vibrios. Antibodies against flagella or somatic O antigens could cause clumping and arrested motion of cells. Antitoxic antibodies could react with toxin at the epithelial cell surface and block binding or activity of the the toxin.  Since the process by which the vibrios attach to the intestinal epithelium is highly specific, antibodies against Vibrio fimbriae or other surface components (LPS?) could block attachment.

The observation that natural infection confers effective and long-lasting immunity against cholera has led to efforts to develop a vaccine which will elicit protective immunity. The first attempts at a vaccine in 1960s were directed at whole cell preparations injected parenterally. At best, 90% protection was achieved and this immunity waned rapidly to the baseline within one year. Purified LPS fractions from different biotypes have also been given as vaccines with variable success. The cholera toxin can be converted to toxoid in the presence of formalin and glutaraldehyde. The toxoid is a poor antigen, however, and it elicits a very low level of protection.

At the present time, the manufacture and sale of the only licensed cholera vaccine in the United States has been discontinued. Two recently developed oral vaccines for cholera are licensed and available in other countries (Dukoral®, Biotec AB and Mutacol®, Berna). Both vaccines appear to provide  somewhat better immunity and fewer side-effects than the previously available vaccine. However, neither of these two vaccines is recommended for travelers nor are they available in the United States.  Nor are the vaccines recommended for inhabitants of regions where cholera is entrenched, since their use may render complacency with regard to individual susceptibility to disease. One of the vaccines also advertises protection against enterotoxigenic E. coli (ETEC) which produces a toxin (LT) identical to cholera toxin, and which is an important cause of traveller's diarrhea.

The oral vaccines are made from a live attenuated strains of V. cholerae.The ideal properties of such a "vaccine strain" of the bacterium would be to possess all the pathogenicity factors required for colonization of the small intestine (e.g. motility, fimbriae, neuraminidase, etc.) but not to produce a complete toxin molecule. Ideally it should produce only the B subunit of the toxin which would stimulate formation of antibodies that could neutralize the binding of the native toxin molecule to epithelial cells.

A new vaccine has been developed to combat the Vibrio cholerae Bengal strain that has started spreading in epidemic fashion in the Indian subcontinent and Southeast Asia. The Bengal strain differs from previously isolated epidemic strains in that it is serogroup 0139 rather than 01, and it expresses a distinct polysaccharide capsule. Since previous exposure to 01 Vibrio cholerae does not provide protective immunity against 0139, there is no residual immunity in the indigenous population to the Bengal form of cholera.

The noncellular vaccine is relatively nontoxic and contains little or no LPS and other impurities. The vaccine will be used for active immunization against Vibrio cholerae O139 and other bacterial species expressing similar surface polysaccharides. In addition, human or other antibodies induced by this vaccine could be used to identify Vibrio cholerae Bengal for the diagnosis of the infection and for environmental monitoring of the bacterium.

Cholera References and Links

Baron Medical Microbiology Textbook
Cholera, Vibrio cholerae O1 and O139, and Other Pathogenic Vibrios by Richard A. Finkelstein

World Health Organization
Cholera

CDC
Cholera
Travelers' Health Information on Cholera

FDA Bad Bug Book
Vibrio cholerae Serogroup O1

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