A genus of rod-shaped aerobic motile Gram-positive bacteria. Only one species, L. monocytogenes, causes disease (listeriosis). It is resistant to physical and chemical treatments and can occur as a contaminant in certain foods, in faeces, etc. Listeriosis can take various forms, depending on the site of infection: localization in the central nervous system causes meningoencephalitis, while uterine infection can result in abortion or congenital handicap in the fetus.