Listeria monocytogenes exploits normal host cell processes to spread from cell to cell

The bacterial pathogen, Listeria monocytogenes, grows in the cytoplasm of h ost cells and spreads intercellularly using a form of actin-based motility mediated by the bacterial protein ActA. Tightly adherent monolayers of MDCK cells that constitutively express GFP-actin were infected with L. monocyto genes, and intercellular spread of bacteria was observed by video microscop y. The probability of formation of membrane-bound protrusions containing ba cteria decreased with host cell monolayer age and the establishment of exte nsive cell-cell contacts. After their extension into a recipient eel, inter cellular membrane-bound protrusions underwent a period of bacterium-depende nt fitful movement, followed by their collapse into a vacuole and rapid vac uolar lysis. Actin filaments in protrusions exhibited decreased turnover ra tes compared with bacterially associated cytoplasmic actin comet tails. Rec overy of motility in the recipient cell required 1-2 bacterial generations. This delay may be explained by acid-dependent cleavage of ActA by the bact erial metalloprotease, Mpl. Importantly, we have observed that low levels o f endocytosis of neighboring MDCK cell surface fragments occurs in the abse nce of bacteria, implying that intercellular spread of bacteria may exploit an endogenous process of paracytophagy.