Listeria are short, Gram-positive non-sporeforming bacilli that invade host cells to exist as intracellular parasites. The traditional species (Listeria sensu stricto) can grow at temperatures as low as 4 C and are motile when grown at 25 C but not at 37 C. Listeria are catalase positive and ferment acids but do not generate gas. They are among the most common causes of serious diseases in animals, and some species are of growing importance to human health due to infections through contaminated foods, including raw milk, dairy, and meats.
Listeria innocua is a cause of encephalitis in animals but is considered harmless for humans, thus allowing it to be used as a surrogate for studies aimed at preventing and detecting Listeria contamination in foods. It is common in the environment and was a significant portion of the Listeria spp. recovered in surveys of surface waters in the USA, Canada, and Austria.