Cholera

Cholera is caused by a bacteria called Vibrio cholerae, which secretes a toxin, or poison, that binds to the cells of the small intestine. One of the functions of the small intestine in humans is to regulate the amount of fluid that is absorbed by cells. Normally, small intestine cells absorb most of the fluid that is ingested; only a small amount of fluid is excreted in the feces. Under abnormal conditions, such as in response to a pathogen, cells do not absorb fluid, and as a result a lot of fluid enters the small intestine and is excreted in the feces. These frequent, watery stools are called diarrhea. Diarrhea can actually be helpful, as the rapid movement of fluid flushes the gastrointestinal tract of harmful bacteria and other pathogens. But if diarrhea is severe or long lasting, such as occurs in cholera, too much fluid is lost and the body becomes dehydrated. If fluids are not replaced, death can result.

Along with causing fluid loss, the binding of cholera toxin to small intestine cells also results in loss of electrolytes. Electrolytes are chemicals that the body needs to function properly, such as potassium chloride, sodium chloride (salt), and bicarbonate. Electrolytes are crucial in the control of blood pressure, excretion of metabolic wastes, and maintenance of blood sugar levels. If the amount of electrolytes in the body deviates even slightly, these crucial body functions are imperiled. Cholera toxin prompts the small intestine cells to secrete large amounts of electrolytes into the small intestine. These electrolytes are then excreted in the watery diarrhea.

The cholera toxin consists of two subunits, the A subunit and the B subunit. The B subunit is a ring, and the A subunit is suspended within it. By itself, the B subunit is nontoxic; the A subunit is the poisonous part of the toxin. The B subunit binds to the small intestine cell and creates a channel within the cell membrane through which the A subunit enters. Once inside the small intestine cell, the A subunit disrupts the cascade of reactions that regulates the cell's fluid and electrolyte balance. Fluid and electrolytes leave the cell and enter the small intestine. The resultant diarrhea may cause a fluid loss that exceeds 1 qt (1 liter) per hour.

V. cholerae lives in aquatic environments, and especially favors salty or brackish waters. V. cholerae frequently colonize shellfish; in fact, cholera cases in the United States are almost always traced to eating raw or undercooked shellfish. Interestingly, V. cholerae can also cause skin and other soft tissue infections. Cases of such infection with these bacteria have been found in persons who have sustained injuries in marine environments; apparently, V. cholerae in water can penetrate broken skin and cause infection.