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 seeligeri has been found in environmental sources (including the soil) and recovered from feces of sheep. It was the most commonly recovered Listeria in surveys of environmental water sources done in the USA, Canada, and Austria. It is not associated with disease in humans.