Jude Taylor UCF Research: Unveiling the Environmental Roles of Vibrio Cholerae Virulence Factors
Vibrio cholerae, a facultative pathogen, naturally inhabits aquatic ecosystems. While some strains cause cholera, a severe diarrheal disease, most are nonpathogenic. This article explores the ecological roles of V. cholerae virulence factors, highlighting their connections between the human host and the bacteria's natural environment. It discusses how these factors, alongside host selective pressures, may have led to the emergence of pathogenic traits in V. cholerae.
Vibrio Cholerae: An Overview
Vibrio cholerae is a widely studied pathogenic species within the Vibrionaceae family of aquatic bacteria. While some species, including Vibrio vulnificus and Vibrio parahaemolyticus, can cause disease in humans, they are natural inhabitants of estuarine and brackish environments, and most strains are nonpathogenic. Cholera remains a significant health concern in areas with limited access to clean water and sanitation, with an estimated 3 to 5 million cases occurring globally each year.
Among the known serogroups of V. cholerae, only the O1 and O139 serogroups are associated with cholera symptoms. These serogroups belong to the pandemic genome (PG) group, which is phylogenetically confined. While only strains from this group have been found to cause cholera in humans, other strains of V. cholerae (non-O1/non-O139) can cause gastrointestinal infections.
Mobile Genetic Elements and Virulence Factors
Many virulence factors of V. cholerae are encoded within mobile genetic elements, acquired horizontally by pathogenic strains. Cholera toxin (CT), responsible for profuse watery diarrhea, is encoded within the CTXϕ lysogenic phage. Toxin-coregulated pilus (TCP), an essential colonization factor, is encoded within Vibrio pathogenicity island 1 (VPI-1).
In its natural environment, V. cholerae associates with various aquatic organisms, including copepods, crustaceans, arthropods, chironomid egg masses, cyanobacteria, shellfish, waterfowl, and fish. It also faces abiotic and biotic stressors such as nutrient limitations, pH changes, temperature and salinity fluctuations, grazing by protozoa, and phage predation. Mechanisms that allow the bacteria to colonize and persist in their natural environment provide preadaptations for virulence in human hosts.
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Convergence of Aquatic Environment and Human Host
Humans undeniably influence the emergence and evolution of pathogenic V. cholerae by selecting and amplifying virulent clones and their traits. However, recent studies have revealed that several virulence and colonization factors of V. cholerae also play roles in the bacteria's survival and persistence in its natural environment. This article will delve into the environmental roles of several V. cholerae virulence factors involved in various functions, including colonization, motility, adhesion, biofilm formation, quorum sensing (QS), and toxin secretion.
Type VI Secretion System (T6SS)
Some non-O1/non-O139 strains of V. cholerae can cause gastrointestinal infections. V. cholerae V52, a strain belonging to the O37 serogroup, encodes a nanosyringe-like system called the type VI secretion system (T6SS). This system induces inflammatory diarrhea, facilitates replication of V. cholerae within the rabbit intestine, and plays a role in competing against the gut microbiota. Since its discovery, T6SSs have been described in V. cholerae O1 strains and other bacterial species.
Inactivation of the T6SS attenuates V. cholerae pathogenesis in Drosophila melanogaster. Interestingly, the T6SS can be reactivated in the presence of commensal gut bacteria such as Acetobacter pasteurianus. The roles of the T6SS in intestinal colonization, virulence, and antagonistic interactions with gut microbes are governed by diverse regulatory mechanisms such as QS or carbon utilization and chitin-induced natural competency pathways. Recent findings show a direct regulatory relationship between the T6SS and QS; however, the possible contribution of the T6SS to the virulence regulatory cascade needs further elucidation.
Besides its critical role in the host, the T6SS plays a major role in the environmental survival of V. cholerae. In the environment, the T6SS confers protection against predators, aids in competition against antagonistic microorganisms, and facilitates gene acquisition and horizontal gene transfer. The T6SS secretes self-protecting proteins (TsiV1, TsiV2, and TsiV3) and toxic effector proteins (VasX, TseL, and VgrG-3), which provide a competitive advantage over other bacterial species in the natural environment and mediate cytotoxicity to both mammalian cell lines and the soil-living amoeba Dictyostelium discoideum.
Secretion of toxins and effectors by the T6SS provides a selective advantage during interspecies competition against numerous species, such as Escherichia coli and Salmonella enterica serovar Typhimurium. Besides serving as a predatory killing device, the T6SS is part of the competence regulon in V. cholerae. The T6SS-encoding gene cluster is under the positive control of the competence regulators TfoX and QstR and fosters horizontal gene transfer by making exogenous DNA accessible to V. cholerae cells. These findings highlight the critical roles of the T6SS both in the host and in the natural environment, allowing V. cholerae to prey on other microorganisms and also acquire novel genetic traits.
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Quorum Sensing (QS)
QS is a phenomenon by which bacteria monitor their cell population density through the extracellular accumulation of signaling molecules called autoinducers. Expression of hapR, a negative regulator of virulence, is repressed at low cell densities; however, during the late stages of colonization, when cell numbers are high, hapR becomes derepressed, thus negatively affecting virulence gene expression. The signaling molecules produced from QS at high cell densities also facilitate cellular processes that cause increased motility, repression of Vibrio polysaccharide (VPS) production, downregulation of TCP and CT, upregulation of the T6SS, and protease secretion.
At high cell densities, quorum regulatory small RNAs become activated by HapR to activate T6SS genes, a phenomenon that is conserved across V. cholerae strains. The activity of the T6SS in V. cholerae is controlled by the combined actions of LuxO, a QS response regulator, and TsrA, a global regulator of V. cholerae. TsrA represses the production of the T6SS substrate Hcp. Disruption of LuxO and TsrA activates the T6SS, thus increasing intestinal colonization in the mouse model and inflammatory diarrhea in infant rabbits.
The influence of QS on the survivability and persistence of V. cholerae in aquatic habitats has been discussed previously. The production of HapR in the natural environment plays a role in preventing the bacteria from protozoal grazing through secretion of PrtV and, at high cell densities, regulates the transcription of hapA, which encodes a hemagglutinin protease (HAP) that cleaves biofilm proteins. PrtV plays a role in bacterial survivability against predators such as phages, protozoa, and bacteriovorous organisms such as Cafeteria roenbergensis and Tetrahymena pyriformis. In the human host, PrtV mediates degradation of the epithelial extracellular matrix and blood components and induces an inflammatory response.
HAP is a HapR-regulated metalloprotease that cleaves proteins in the biofilm matrix when the cell density increases, thus possibly facilitating bacterial cell dispersal in the late stages of colonization. In the aquatic environment, HAP digests the gelatinous matrix of chironomid egg masses, mediates associations with cyanobacteria, and aids in dissolving organic matter, thereby releasing nutrients for V. cholerae cells. Recently, a direct relationship between QS and the intestinal colonization of an arthropod host by V. cholerae has been reported.
Colonization Factors: GbpA and TCP
In its natural environment, V. cholerae can be typically found in association with the chitinaceous exoskeleton of crustaceans. GbpA is a chitin-binding protein that is highly conserved on the core genome of members of the family Vibrionaceae. GbpA promotes adherence, colonization, and interactions with various environmental biotic surfaces, such as crustacean shells, mussel hemocytes, and bivalves and their hepatopancreatic cells. Chitin is one of the most abundant carbon sources in the aquatic environment; therefore, binding to and degrading chitin provide a competitive advantage for V. cholerae outside the human host. Active interactions of GbpA during the intestinal colonization of soft-shelled turtles have been shown, prompting the proposal of the turtle gut as an alternative model system for V. cholerae colonization. In addition, GbpA has been shown to mediate attachment to human intestinal epithelial cells and is required for successful gut colonization, which provides a direct link between environmental and host colonization of V. cholerae.
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TCP, a type IV pilus, is an essential colonization factor that mediates microcolony formation in the intestine. Microcolonies are clusters of V. cholerae cells that confer numerous properties to the bacteria. For instance, TCP enhances attachment to intestinal epithelial cells, facilitates bacterium-bacterium interactions by tethering cells together, mediates secretion of the colonization factor TcpF, and provides protection against antimicrobial agents. The ability to form microcolonies correlates with the ability to colonize infant mice and humans. In addition, TCP also acts as the receptor of the CTXϕ phage. In aquatic environments, together with other pili such as mannose-sensitive hemagglutinin (MSHA) and chitin-regulated pilus (ChiRP), TCP mediates attachment to and colonization of the chitinaceous surface of copepods. Furthermore, it has been shown that mutant strains that do not secrete TCP are unable to form differentiated biofilms on those surfaces, which leads to increased sensitivity to stressors. Overall, it appears that the ability of V. cholerae to form biofilms is crucial for its survival in both the host and the environment.
Cholera Toxin (CT)
The production of CT in the intestine is directly responsible for the severity of the profuse diarrhea associated with cholera. CT constitutively activates adenylate cyclase by ADP-ribosylating a coupled G-protein, which leads to increased intracellular cAMP levels. This prompts the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel to be constitutively opened, Cl− to be effluxed with sodium, and water to follow passively. Although a direct environmental role of CT has yet to be reported, it has been shown that, due to the lysogenic nature of the CTXϕ phage, the insertion and deletion of this phage can enable gene recombination, which leads to diversity within the pandemic strains. This serves as an opportunity to increase the pathogenic potential of pandemic strains. Intriguingly, V. cholerae secretes CT while associated with the cyanobacterium Rhizoclonium fontanum; the biological reason behind this remains unknown. Furthermore, studies have shown that CT causes protein trafficking and death of D. melanogaster. CT also causes disruption of exocyst trafficking, which induces the breakdown of intestinal adherens junctions in both D. melanogaster and mammalian intestines in a manner dependent on Rab11, a conserved G protein. These unresolved associations indicate that CT plays a role in the environment; however, more research needs to be conducted in order to establish an evolutionary origin of the toxin. It was previously hypothesized that, given its inherent function, CT might act as an osmoregulator when produced in the gills of crustaceans, providing an advantage to the crustaceans as they move into environments of increasing salinity. It is tempting to speculate that V. cholerae might have coopted this mechanism to its own advantage.
ToxR
The transmembrane transcriptional activator ToxR is encoded in the core genome of every sequenced member of the family Vibrionaceae. It influences the expression of numerous genes (∼150 genes) involved in diverse cellular functions. In association with TcpP, ToxR is required for transcription of the gene encoding ToxT, which regulates the expression of the major pathogenicity factors of V. cholerae.
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