Helicobacter pylori bacterium. Coloured trans- mission electron micrograph (TEM) of a section through a Helicobacter pylori bacterium. This bacterium was formerly known as Campylobacter pyloridis. This Gram-negative rod-shaped (bacillus) bacterium has numerous hair-like flagellae (green). 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. Magnification unknown.