Shigella bacterium. Coloured scanning electron micrograph (SEM) of a Shigella sp. bacterium (orange) being engulfed by a macrophage white blood cell. This process is called phagocytosis. Macrophages are cells of the body's immune system. They phagocytose and destroy pathogens, dead cells and cellular debris. Shigella sp. are Gram-negative rod-shaped bacteria. They infect the large intestine, causing dysentery, which can vary in severity from a mild attack of diarrhoea to an acute infection. Magnification: x 11,650 when printed at 10 centimetres wide.