Helicobacter pylori bacterium. Coloured trans- mission electron micrograph (TEM) of a single Helicobacter pylori bacterium (formerly known as Campylobacter pyloridis). This gram- negative bacillus (rod-shaped) 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 co-factor for gastric cancer as its presence increases the risk of stomach tumours. Negatively stained. Magnification: x16, 650 at 6x7cm size.