INTRODUCTION — Cholera is a rapidly dehydrating diarrheal disease caused by a toxin-producing bacteria, Vibrio cholerae.
The etiologic agent and pathogenesis of infection with toxigenic V. cholerae is reviewed here. The clinical approach to patients with cholera is discussed separately. (See "Cholera: Clinical features, diagnosis, treatment, and prevention".)
Infections due to other strains of V. cholerae that do not cause epidemic cholera, are also discussed elsewhere. (See "Infections due to non-O1/O139 Vibrio cholerae".)
MICROBIOLOGY
Etiologic agent — V. cholerae is a distinctive, comma-shaped gram-negative rod. Organisms are highly motile and possess a single polar flagellum. V. cholerae is salt-tolerant, requiring NaCl for growth (halophilic) and exists naturally in aquatic environments. While in aquatic environments, V. cholerae may enter a viable but non-culturable form [1]. However, V. cholerae is readily grown from clinical specimens, including stool and rectal swabs, and can be identified in microbiology laboratories using selective media and biochemical tests. (See "Cholera: Clinical features, diagnosis, treatment, and prevention", section on 'Diagnosis'.)
Only cholera toxin-producing (toxigenic) strains of V. cholerae are associated with cholera. While some environmental V. cholerae are toxigenic and capable of causing cholera, most environmental V. cholerae isolates are not toxigenic. Toxigenic strains harbor a filamentous bacteriophage (CTXΦ) which encodes cholera toxin [2]. (See 'Genomic features' below.)
Classification — V. cholerae is a diverse species and includes pathogenic and non-pathogenic variants. The organism can be classified serologically based on differences in the structure of the O antigen of its lipopolysaccharide. More than 200 serogroups of V. cholerae have been reported. However, only serogroups O1 and O139 have been associated with large scale epidemics of cholera [3]. Other serogroups are generally grouped together as V. cholerae "non-O1, non-O139." Toxin-producing non-O1/O139 V. cholerae strains have been associated with isolated cases or small outbreaks of dehydrating diarrhea, whereas non-toxin-producing non-O1/O139 strains have been associated with sporadic gastroenteritis and sepsis. (See "Infections due to non-O1/O139 Vibrio cholerae".)
Serogroup O1 — V. cholerae O1 is the cause of the current global pandemic of cholera and is divided into two major serotypes, Inaba and Ogawa, which differ by the presence of a single methyl group on the Ogawa O-specific polysaccharide antigen. The change between serotypes Ogawa and Inaba arises from mutations in the WbeT methyltransferase [4].
V. cholerae O1 is also divided into two biotypes, El Tor and classical, which are differentiated by biochemical distinctions and susceptibility to specific bacteriophages. Previous V. cholerae pandemics were caused by the classical biotype, but this is now thought to be extinct. In conventional microbiologic tests, classical strains of V. cholerae O1 are sensitive to polymyxin B, have a negative Voges-Proskauer test (which detects the metabolic product acetoin), and do not agglutinate chicken red blood cells.
Serogroup O139 — V. cholerae O139 emerged in 1992 and was a major cause of epidemic cholera for a decade, but is no longer a major cause of cholera. The epidemic O139 strain arose via the acquisition of a single genomic island, the rfb region, which included the genes required for synthesis of the O139 antigen but the organism was otherwise identical to previously circulating V. cholerae O1 strains [5,6].
Genomic features — The genome of V. cholerae and other Vibrio species contains two chromosomes. This feature is distinct from many other bacteria. The first V. cholerae reference genome was derived from a strain, N16961, a V. cholerae O1 El Tor strain that was isolated from a cholera patient in Bangladesh in 1971 [7]. V. cholerae can evolve rapidly through acquisition of mobile genetic elements. Notable features of the V. cholerae genome include:
●CTXΦ – Toxin-producing strains of V. cholerae contain the bacteriophage CTXΦ. (A lysogenic bacteriophage is a virus that infects bacteria and inserts its genetic material into the host bacterium's genome.) This bacteriophage can be transmitted between bacteria and includes the genes that encode cholera toxin [2].
●VPI – The Vibrio Pathogenicity Island (VPI) [8] contains the genes that encode the toxin coregulated pilus (TCP), a specialized bacterial structure in pathogenic strains that allows colonization of the human intestine [9]. The production of TCP is coordinated with production of cholera toxin [10]. TCP is also the receptor for the bacteriophage CTXΦ [2].
●SXT/R391 ICE – The SXT/R391 integrative and conjugative element (ICE) is an important part of the genome that allows V. cholerae to acquire certain types of foreign DNA and has allowed the bacteria to acquire antibiotic resistance phenotypes [11]. It was first identified in V. cholerae O139 [11]. Subsequently, this horizontally-acquired region was found in pandemic V. cholerae O1 El Tor.
Molecular epidemiology — The use of whole genome sequencing and single nucleotide polymorphisms (SNPs) as high resolution markers has allowed for a better understanding of the evolution of V. cholerae. Over the last two centuries, there have been seven pandemics of V. cholerae [3]. V. cholerae O1 El Tor is the cause of the current, seventh pandemic. A study of 154 representative isolates from the seventh pandemic El Tor (7PET) strains confirmed that this pandemic represents the expansion of a single bacterial lineage with a most recent common ancestral clone estimated to date to 1952 [5]. This analysis also showed that the current global cholera pandemic has actually been comprised of three independent waves of transmission, each coinciding with a major acquisition of new genetic material, which conferred increased environmental stability, drug resistance, or virulence to the previously circulating strains of V. cholerae [5]:
●The current cholera pandemic began with the emergence of V. cholerae O1 biotype El Tor, which replaced the previously circulating classical biotype, leading to the apparent extinction of the classical biotype [5].
●A second wave occurred when the original pandemic V. cholerae O1 El Tor strains were replaced by V. cholerae O1 isolates containing the SXT/R391 antibiotic resistance element. V. cholerae O139 strains arose from and are included in this second wave.
●The third wave, in which the most recent common ancestral clone is estimated to have originated in 1988, was associated with acquisition of a cholera toxin variant that shared characteristics of a cholera toxin sequence previously observed in the now extinct classical biotype.
Each wave of transmission originated in South Asia before spreading to distant locations throughout Asia, Africa, South America, and the Caribbean [5].
Higher resolution studies of the molecular epidemiology of V. cholerae have applied whole-genome analysis across different spatial and temporal scales to learn more about the generation of genetic diversity within hosts, transmission within households, the origin of individual epidemics, and the factors which shape the epidemiology of cholera within an individual region or continent. As an example, sampling of strains in Africa and South America have revealed detailed information on the transmission of pandemic cholera in these continents. In Africa, there have been at least 11 introductions of 7PET V. cholerae since 1970, including five distinct introductions of multidrug-resistant V. cholerae from Asia since 2000 [12]. In South America, two separate intercontinental transmissions of 7PET V. cholerae occurred when a first-wave 7PET strain was introduced into Peru in 1991 and when third-wave 7PET strains were introduced in Haiti in 2010 [13]. Genomic analyses rapidly demonstrated that the origin of 7PET V. cholerae in Haiti could be traced to the introduction of a single strain that had been most recently been circulating in South Asia, counter to the idea that the epidemic had originated from a local environmental strain [14]. Additional analyses have since confirmed that isolates from Haiti are virtually identical to those from Nepal [15], a finding consistent with epidemiologic evidence linking the first cases in Haiti to an area contaminated with raw sewage originating from Nepalese stabilization forces at a United Nations base [15].
PATHOGENESIS — V. cholerae may cause asymptomatic or mild infection. However, V. cholerae may also cause a uniquely severe secretory diarrhea that can be fatal in less than 12 hours [16]. The infectious dose of V. cholerae required to cause cholera is thought to be relatively high; an inoculum of 108 V. cholerae or higher generally results in severe infection in healthy North American volunteers [17,18].
In order for infection to occur, V. cholerae must survive the acidic environment of the stomach and then form colonies of bacteria on the surface of the small intestine. Organisms attached to the small intestine produce cholera toxin (CT), which is responsible for the profound secretory diarrhea that is the hallmark of cholera. As little as 5 micrograms of ingested CT may mimic a cholera-like illness [19]. (See 'Colonization' below and 'Cholera toxin and the pathogenesis of secretory diarrhea' below.)
Previous exposure and immunity, nutrition, genetic factors, and interaction with other microbes may influence human susceptibility to cholera. (See 'Host susceptibility' below and 'Interactions with other microbes' below.)
Colonization — After ingestion of V. cholerae and passage through the upper gastrointestinal tract, the organisms colonize the small intestine. An important factor for colonization is the toxin-coregulated pilus (TCP), which is a rope-like structure produced by toxigenic V. cholerae [9]. TCP also leads to bacterial aggregation and likely allows the bacteria to resist killing by bile [20]. Bacterial motility is another important factor for colonization, and a hallmark of V. cholerae is the rapid propulsion of the bacterial by a single flagellum which enables it to swim through mucous and arrive at the mucosal surface [21].
Cholera toxin and the pathogenesis of secretory diarrhea — Cholera toxin (CT) is the main virulence factor of pathogenic strains of V. cholerae, and is the primary cause of the characteristic profuse watery diarrhea of cholera. CT is an AB5 protein toxin that consists of a single A subunit (CtxA) non-covalently attached to a donut-shaped ring of five B subunits (CtxB). Once it colonizes the epithelial surface, V. cholerae secretes CT. Post-translational modification by a protease nicks the CtxA subunit, yielding two CtxA portions that remain connected by a single disulfide bond [22]. The CtxB pentamer attaches with high affinity to the apical surface of small intestinal epithelial cells via the monosialoganglioside GM1 receptor [23]. The entire CT then undergoes endocytosis via a retrograde transport pathway and travels to the endoplasmic reticulum, where CtxA is cleaved into two polypeptide chains by reduction of a disulfide bond to yield CtxA1 and CtxA2 peptides [24]. The enzymatically active CtxA1 is translocated to the cytosol, where it activates the G-protein regulated adenylyl cyclase [25]. This increases concentrations of intracellular cyclic AMP, resulting in secretion of chloride via apical cystic fibrosis transmembrane conductance regulator (CFTR) channels and inhibition of sodium chloride absorption [26]. The ultimate result is massive fluid secretion into the small intestine, resulting in large amounts of fluid losses with high concentrations of sodium, chloride, bicarbonate, and potassium [27].
While CT is the proximate bacterial cause of the secretory diarrhea and rapid fluid loss associated with cholera, other bacterial factors also contribute to the pathogenesis of V. cholerae. A notable virulence factor is the V. cholerae sialidase, NanH (or VcN), which remodels intestinal gangliosides and enhances the availability of the GM1 receptor, augmenting the effects of CT [28]. Along with lipopolysaccharide and CT, NanH is a third immunodominant target of the human immune response to cholera [29], and it is hypothesized that V. cholerae is a human-restricted infection because of the evolutionary loss in humans of the N-glycolylneuraminic acid, which is resistant to the effects of the V. cholerae sialidase [30].
Regulation of virulence — Toxigenic V. cholerae coordinate the expression of virulence genes through a regulatory network known as the ToxR regulon [31]. This network responds to and integrates complex environmental signals, which can include amino acid levels, bacterial cell density (through quorum sensing pathways), and other mechanisms. Downstream components of the ToxR regulon are controlled by ToxT, a transcriptional activator that directly activates transcription of genes encoding CT, TCP, and other virulence factors of toxigenic V. cholerae [31].
Hyperinfectivity — V. cholerae shed by humans with cholera is more than a hundred times more infective in mice compared with organisms grown in vitro [32,33]. This hyperinfectivity is transient and is lost in less than 24 hours after the bacteria is shed into the environment [34]. However, epidemiologic models suggest that person-to-person spread by hyperinfective V. cholerae contributes to the explosive [35] nature of cholera epidemics. Studies of V. cholerae shed in human stool suggest that the downregulation of chemotaxis genes may contribute to this hyperinfective phenotype [36].
Host susceptibility — Most risk factors for cholera reflect the underlying mode of transmission via contaminated food and water. However, a number of specific host factors associated with a higher risk of V. cholerae infection and cholera have been identified and reflect the biological interaction between the host and pathogen.
A major determinant of host susceptibility to cholera is immunity acquired through previous infection or cholera vaccination (see "Cholera: Clinical features, diagnosis, treatment, and prevention", section on 'Vaccines'). Both human challenge and longitudinal studies in endemic areas demonstrate that a single episode of cholera protects against subsequent infection [37,38]. However, this protection is serogroup specific, and the level and duration of protection following V. cholerae O1 infection varies depending on the biotype and serotype of the organism [39].
Individuals with blood group O are more likely to develop severe cholera than individuals with other blood groups [40,41]. The mechanism for this is uncertain. In one cohort study from Bangladesh, blood group O was actually associated with a lower risk for V. cholerae infection overall despite the risk for higher severity of illness once infected, suggesting a susceptibility to the effects of cholera toxin rather than an increased susceptibility to colonization [41,42].
Other genetic variations are also likely to affect the risk of developing cholera. As an example, a candidate gene study demonstrated that variants in the innate immunity protein BPIFB1 (also known as LPLUNC1) are associated with susceptibility to cholera [43]. A genome-wide study of a population from a cholera-endemic region in Bangladesh found a number of genetic regions, including genes related to innate immunity and encoding potassium channels, to be associated with cholera [44].
Other host factors also modulate susceptibility to infection. Ingested bacteria must survive the gastric acid barrier of the stomach in order to reach the small intestine. V. cholerae are sensitive to gastric acid, and hypochlorhydria lowers the infectious dose needed to cause infection [45]. Among nutritional factors, retinol deficiency is associated with an increased risk of cholera, and breastfeeding has been consistently shown to be protective against cholera [46,47].
Interactions with other microbes
Gut microbiota — Metagenomic analysis during and after cholera demonstrates that the intestinal microbiota is drastically altered by cholera, and then undergoes a reproducible pattern of successive repopulation [48,49]. This repopulation may mirror the accumulation of bacterial taxa of the gut microbiota in childhood [49]. The pattern of microbial repopulation after cholera may be affected by changing enteric oxygen and carbohydrate levels after diarrheal illness [48].
Prior to exposure to V. cholerae, the gut microbiota may also predict susceptibility to cholera. In one study of household contacts of patients with cholera, certain taxa in the gut microbiome (such as Paracoccus animovorans, which promoted the growth of V. cholerae) were associated with an increased risk of infection, while others were associated with a decreased risk of infection [50]. Furthermore, a candidate live-attenuated V. cholerae vaccine strain was able to protect against V. cholerae infection in an infant rabbit model of cholera through a probiotic-like effect (following colonization with the vaccine strain but prior to the induction of immunity) [51]. In gnotobiotic mice, transplantation of human fecal microbiomes with key bacterial species that produce bile salt hydrolases, such as Blautia obeum, reduced V. cholerae colonization [52]. These findings suggest that V. cholerae gut microbiota interactions may ultimately be harnessed to effectively provide protection against human cholera.
Lytic bacteriophages — Lytic bacteriophages are viruses that kill bacteria. A number of lytic phages that attack toxigenic V. cholerae have been identified [53]. Evidence from environmental studies suggests that increasing amounts of lytic phages in the environment may coincide with the termination of cholera epidemics [54,55]. Furthermore, the simultaneous ingestion of toxigenic V. cholerae and lytic phage has been shown to exert a strong selective pressure for the bacteria to develop mutations that may make it less virulent in order to resist killing by the bacteriophage [56].
SUMMARY
●Vibrio cholerae is a facultative gram-negative rod found in aquatic environments. Only cholera toxin-producing (toxigenic) strains of V. cholerae are capable of causing cholera, a rapidly dehydrating diarrheal disease. (See 'Etiologic agent' above.)
●V. cholerae is classified serologically, with more than 200 reported serogroups. Only serogroups O1 and O139 have been associated with large scale epidemics of cholera. V. cholerae O1 can be further classified by serotype or biotype. V. cholerae O1 biotype El tor is the cause of the current global pandemic of cholera. (See 'Classification' above and 'Molecular epidemiology' above.)
●Horizontal gene transfer is the primary mechanism responsible for the evolution of pathogenic V. cholerae. (See 'Genomic features' above and 'Molecular epidemiology' above.)
●In order for infection to occur, V. cholerae must survive the acidic environment of the stomach, colonize the small intestine, and produce cholera toxin, an AB5 bacterial toxin that is the primary cause of the characteristic profuse watery diarrhea of cholera. (See 'Pathogenesis' above.)
●Previous exposure and immunity, nutrition, genetic factors, and interaction with other microbes may influence human susceptibility to cholera. (See 'Host susceptibility' above and 'Interactions with other microbes' above.)
ACKNOWLEDGMENT — The editorial staff at UpToDate acknowledge Joan R Butterton, MD, who contributed to an earlier version of this topic review.
