Three-dimensional (3D) computer-generated image of a grouping of Listeria monocytogenes bacteria. The artistic recreation was based upon scanning electron microscopic (SEM) imagery. Listeria monocytogenes is the species of pathogenic bacteria that causes the infection listeriosis. Listeriosis, a serious infection usually caused by eating food contaminated with the bacterium Listeria monocytogenes, is an important public health problem in the United States. The disease primarily affects older adults, pregnant women, newborns, and adults with weakened immune systems. However, rarely, persons without these risk factors can also be affected. The risk may be reduced by following recommendations for safe food preparation, consumption, and storage. It is a facultative anaerobic bacterium, capable of surviving in the presence or absence of oxygen. Listeria monocytogenes is a Gram-positive bacterium, in the division Firmicutes, named after Joseph Lister.