Vibrio cholerae is a comma-shaped, Gram-negative bacterium that causes cholera, a severe diarrheal disease. This bacterium thrives in brackish or saltwater environments, often attaching to shellfish. Cholera continues to be a major public health concern, particularly in regions with inadequate sanitation and limited access to clean water.
The Cholera Toxin
The cholera toxin (CT) is the primary factor responsible for the profuse, watery diarrhea characteristic of cholera. This toxin is an AB5-type hexameric protein, meaning it has one enzymatically active A subunit and five identical receptor-binding B subunits. The genes encoding these subunits, ctxA and ctxB, are located on a filamentous bacteriophage called CTXɸ.
The B subunit pentamer binds specifically to GM1 ganglioside receptors on the surface of intestinal epithelial cells. After binding, the entire toxin is taken into the cell through endocytosis and transported to the endoplasmic reticulum. In the endoplasmic reticulum, the A subunit separates from the B subunits and is then cleaved into CTA1 and CTA2, which remain connected by a disulfide bond.
The active CTA1 domain moves into the cell’s cytoplasm where it performs its damaging function. It acts as an enzyme, attaching an ADP molecule to a specific G protein (Gsα subunit) involved in cell signaling. This modification permanently activates the G protein, which in turn continuously stimulates adenylate cyclase, an enzyme that converts ATP into cyclic AMP (cAMP).
The dramatic increase in intracellular cAMP levels activates protein kinase A (PKA). PKA then phosphorylates and activates cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels in the cell membrane. This leads to a massive efflux of chloride ions, bicarbonate, sodium ions, and water into the intestinal lumen, while also inhibiting the uptake of sodium and water by the intestinal cells. The resulting osmotic imbalance causes the characteristic watery diarrhea, often described as “rice-water stool,” which can lead to severe dehydration and electrolyte imbalances.
Colonization and Adherence Factors
For Vibrio cholerae to cause disease, it must successfully colonize the host’s small intestine. A major factor in this process is the Toxin-Co-regulated Pilus (TCP), a type IV pilus. TCP are hair-like filaments on the bacterial surface that mediate bacterium-bacterium interactions, allowing V. cholerae to aggregate and form microcolonies.
These microcolonies are important for establishing a high bacterial load in the intestine and may offer protection from host immune responses. TCP also facilitates adherence to intestinal epithelial cells, which is a prerequisite for the delivery of cholera toxin. The genes for TCP biogenesis are found within a cluster known as the Vibrio pathogenicity island (VPI).
Beyond adherence, motility and chemotaxis play a part in V. cholerae’s ability to colonize the intestine. The bacterium possesses a single polar, sheathed flagellum that enables it to move through the viscous mucus layer of the small intestine. Chemotaxis, the ability to sense and move toward or away from chemical stimuli, helps the bacterium navigate to its preferred intestinal niches. While motility is known to be necessary for infection establishment, the precise role of chemotaxis in V. cholerae pathogenesis is still under investigation.
Accessory Virulence Elements
Beyond the cholera toxin and TCP, Vibrio cholerae possesses other factors that enhance its ability to cause disease. The Zonula Occludens Toxin (Zot) is encoded by the CTXɸ bacteriophage, the same phage that carries the cholera toxin genes. Zot increases the permeability of the small intestine by affecting the tight junctions between intestinal epithelial cells.
Zot’s mechanism involves a rearrangement of the epithelial cell cytoskeleton, specifically through protein kinase Cα-dependent F-actin polymerization. This action allows for the passage of macromolecules through the paracellular route, potentially contributing to fluid loss.
The Accessory Cholera Enterotoxin (Ace) is another factor found within the “virulence cassette” alongside CT and Zot. Ace contributes to fluid secretion in the intestine, though its effect is generally milder than that of the cholera toxin. It increases transcellular ion transport and causes fluid accumulation, acting as a classic enterotoxin.
The Type VI Secretion System (T6SS) is a syringe-like apparatus used by V. cholerae to deliver toxins into adjacent target cells. This system contributes to interbacterial competition, allowing V. cholerae to kill other bacteria and manipulate host cells. It can facilitate V. cholerae’s survival in aquatic environments and within the host, influencing competition with resident gut microbes and contributing to intestinal inflammation.
The O-antigen component of lipopolysaccharide (LPS) on the outer membrane of V. cholerae also contributes to its survival and immune evasion. The O-antigen can undergo serotype conversion, which may be involved in immune evasion. This surface component is a major target for both bacteriophages and the host immune system.
How Virulence Factors Cause Cholera
Cholera infection begins with the ingestion of Vibrio cholerae from contaminated sources. The bacteria survive the stomach’s acidic environment and use flagellar motility to reach the small intestine’s epithelial surface. There, the Toxin-Co-regulated Pilus (TCP) enables them to multiply, form microcolonies, and adhere to intestinal cells, a critical step for establishing infection.
Once colonized, V. cholerae secretes the cholera toxin (CT), which causes the characteristic massive fluid and ion efflux into the intestinal lumen, leading to severe watery diarrhea. Accessory toxins like Zonula Occludens Toxin (Zot) and Accessory Cholera Enterotoxin (Ace) amplify this fluid secretion. The Type VI Secretion System (T6SS) further supports colonization and persistence. The combined effect of these virulence factors results in severe dehydration and electrolyte imbalance, which can be fatal if untreated.