Helicobacter pylori. Coloured Scanning Electron Micrograph (SEM) of Helicobacter pylori bacteria on the surface of the human stomach. Bacteria are coloured yellow, seen on epithelial cells of the stomach mucosa. The epithelial cells (blue) have surface microvilli or tiny bumps. These bacteria are rod-shaped Gram-negative bacilli. Colonies of Helicobacter pylori occur in the stomach in people suffering gastritis, and there is evidence that these bacteria are an important cause of stomach ulcers. Helicobacter pylori may also increase the risk of stomach tumours and of gastric cancer. Magnification: x5,100 at 5x7cm size. Magnification: x17,875 at 8x10 inch size.