Helicobacter pylori bacterium Coloured trans mission electron micrograph TEM of a single Helicobacter pylori bacterium formerly known as Campylobacter pyloridis This gramnegative bacillus rodshaped bacterium is curved as here or sometimes spiral in shape Numerous hair like flagellae are visible at upper left Colonies of H pylori occur on the stomach mucous membrane in people who suffer from gastritis and this bacteria has been linked to stomach ulcer formation H pylori may also be a cofactor for gastric cancer as its presence increases the risk of stomach tumours Negatively stained Magnification x at xcm size