There are many different forms of Vibrio cholerae, a bacterium that is found naturally in aquatic environments. The bacterium is motile (can move) and uses a cell structure known as a flagellum to allow it to move through water. Fig. 4.1 is a drawing of four cells of one form of V. cholerae. [Figure 4.1] Two main forms of V. cholerae, O1 and O139, are able to colonise the small intestine and cause cholera. These two forms are able to produce a toxin, choleragen, which causes the symptoms of diarrhoeal disease. Mutant V. cholerae that lack flagella are less able to cause disease.