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 monocytogenes is commonly found as a normal inhabitant of the gastrointestinal tract of numerous mammals and vertebrates. It causes infections of numerous organ systems in many domestic and wild animals, most notably meningoencephalitis and abortion in ruminants. It is the most common species of Listeria confirmed to be a human pathogen. Infections often lead to bacteremia, transplacental fetal sepsis, and meningitis/encephalitis resulting in fetal death (up to 50% of all infectious deaths), and they pose a severe threat to infants, pregnant women, the elderly, and the immunosuppressed. It is the third leading cause of foodborne deaths due to microbes in the USA.