Listeria monocytogenes is a intracellular bacterium that is the causative agent of Listeriosis. It is one of the most virulent foodborne pathogens. Listeria species are known to secrete a 60kD protein collectively termed p60, which is encoded by the "iap" (invasion-associated-protein) gene and secreted in large quantities into the growth media. p60 is a highly immunogenic murein hydrolase that is essential for cell division.