Listeria monocytogenes bacteria. Coloured scanning electron micrograph of Listeria monocytogenes bacteria, the causative agent of the human disease listeriosis. These Gram-positive rod-shaped bacteria have a thick cell wall covered by a fibrous capsular material. Listeriosis is contracted through contaminated food. Those vulnerable include pregnant women, in which the baby may be infected; the elderly; cancer patients and those on immunosuppressant drugs. These bacteria may also be a cause of encephalitis and purulent meningitis. Magnification: x16, 000 at 6x4.5cm size.