Listeria sp. bacteria, coloured scanning electron micrograph (SEM). Listeria are Gram-positive rod-shaped bacteria. The species L. monocytogenes is a human pathogen, causing the disease listeriosis. Symptoms of listeriosis include fever, meningitis and encephalitis (swelling of the brain). It is caught from contaminated food and is most common in newborns, the elderly and immunocompromised patients.