The other key part of the pathogenicity of L. monocytogenes is the gene LLO, which produces the protein Listeriolysin O.
This protein let's the bacterium escape death after being engulfed by macrophages through the process of phagocytosis. Listeriolysin O breaks up the phagosome after the bacterium is internalized, allowing its escape into the cytoplasm.
Bacteria and viruses are sneaky bastards.