The complete taxonomic hierarchy of Listeria monocytogenes is: Bacteria (Domain), Eubacteria (Kingdom), Firmicutes (Phylum), Bacilli (Class), Bacillales (Order), Listeriaceae (Family), Listeria (Genus), and Listeria monocytogenes (Species).
L. monocytogenes is a gram-positive, rod-shaped bacteria. The rods of L. monocytogenes are often arranged into short chains, and in direct smears can appear coccoid and be mistaken for steptococci. Longer cells of L. monocytogenes can ressemble corynebacteria. L. monocytogene's ressemblance to other bacteria sometimes cause it to be disregarded as harmless, even though it is a deadly foodborne pathogen.
L. monocytogenes has pertricous flagella, which allow it to have tumbling motility. One unique property of L. monocytogenes is that it is only motile via flagella in a limited temperature range around room temperature (from between 20 and 25 C). The motility of L. monocytogenes flagella is greatly reduced at body temperature, which is significant because L. monocytogenes is a facultative intracellular pathogen.
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monocytogenes has developed a unique form of actin-based motility in order to move from within one human host cell to another. The protein ActA is anchored to the surface of L. monocytogenes, and this protein triggers the polymerization and cross-linking of actin filaments. This propels the bacterium forward, and leaves behind a comet tail made of short cross-linked actin filaments. This unique form of motility may be the reason for why L. monocytogenes can cross the tight endotheial cell junctions of the placenta and blood brain barrier, which most bacteria cannot