All Listeria species are small, Gram-positive rods, which are sometimes arranged in short chains. In direct smears they may be coccoid, so they can be mistaken for streptococci. Longer cells can be suggestive of corynebacteria. Flagella are produced at room temperature rather than at 37 C. Hemolysin production is an important marker for L monocytogenes, although it is not definitive, as L ivanovii and L seeligeri are likewise hemolytic on blood agar. Further biochemical characterization is necessary to distinguish between the different Listeria species.
Bacteria therefore can accumulate in contaminated food stored in the refrigerator.
An invasion factor secreted by the pathogenic bacteria enables them to penetrate host cells of the epithelial lining. Since this microorganism is widely distributed, this event may occur rather often. Normally, the immune system eliminates the infection before it spreads. Indeed, most adults who have no history of listeriosis have T lymphocytes primed specifically by Listeria antigens. If the immune system is compromised, however, systemic disease may develop.