Cholera bacteria. Coloured transmission electron micrograph (TEM) of Vibrio cholerae bacteria. These Gram-negative rod-shaped bacteria have a single polar flagellum (long, thin), which they use to propel themselves through water. They are the cause of cholera, an infection of the small intestine that is transmitted to humans via contaminated food or water. V. cholerae produces a toxin that causes diarrhoea and severe dehydration, which may be fatal if left untreated. Magnification: x12900 when printed 10 centimetres high.