Gastroenteritis. Artwork of bacteria shown below the human digestive system in which they cause gastroenteritis, the inflammation of the stomach and intestines. The stomach is at upper centre above the small intestine (at centre). The large intestine (colon), which ends at the rectum (at bottom centre), surrounds the small intestine. Various bacteria can be seen at bottom, including bacilli (rod-shaped) and cocci (circular) forms. The few bacteria that cause gastroenteritis include Escherichia coli, Listeria sp. and Salmonella sp. Food poisoning, due to contaminated food or water, is the most common form of gastroenteritis. Treatment is rarely necessary.