Cholera toxin. Computer artwork of the structure of cholera enterotoxin (intestinal toxin). The molecule consists of two subunits, A (top) and B (bottom). The B subunit, which is a pentamer, binds to intestinal epithelial cells, allowing the toxin to enter them. This toxin is produced by Vibrio cholerae bacteria the causative agent of cholera, an infection of the small intestine. It causes diarrhoea and severe dehydration, which can be fatal if left untreated.