Walsh & Hoyt: Listeria Monocytogenes

The genus Listeria includes seven species of bacteria, but only one, Listeria monocytogenes, named for its association with monocytosis in the peripheral blood of infected rabbits and guinea pigs, causes disease in humans. L. monocytogenes is a gram-positive, nonspore-forming, facultatively anaerobic rod that is motile at room temperature. It replicates entirely intracellularly. Organisms with which it can be confused in the laboratory include Corynebacterium species, E. rhusiopathiae, and, because it can occur in pairs, various gram-positive cocci, particularly S. pneumoniae.