Cholera bacteria. Coloured environmental scanning electron micrograph (ESEM) of Vibrio cholerae bacteria, showing their flagella (tails). These Gram-negative rod-shaped (bacilli) bacteria 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.