Above is a deconvolved image of two Listeria spreading from one cell into another. Green indicates GFP-actin, blue stains DNA (bacteria and host nuclei), and red is an antibody against E-cadherin, a structural protein that holds epithelial cells together like small patches of velcro. One can clearly see the E-cadherin outlining the two central protrusions; one can also see bacteria travelling freely through the cytoplasm nearby.

Using GFP-actin MDCK cells made by Angela Barth and Eugenio de Hostos, we can watch the spreading of L. monocytogenes from one cell into another for the first time. For a time lapse example of bacteria travelling in these cells (450k file), click here.

Listeria intercellular spread occurs when a bacterium propels itself into the plasma membrane of its host, distending that membrane into a protrusion. If a neighboring cell is very nearby, this protrusion may enter it, causing the neighbor to take up the protrusion in an as-yet not understood mechanism. This leaves the bacterium enclosed in a double membrane vacuole from which it must escape before it can begin its intracellular life cycle again.

Video observation shows that intercellular spread is accomplished by a series of reproducible, contingent steps with characteristic physical and temporal kinetics that give us clues about the relative roles of host and pathogen in this process.  The likelihood of protrusion formation decreases with increased monolayer age, hinting at structural maturation in MDCK cells which decreases lateral spread permissivity.  Kinetics of protrusion uptake suggest that it is a two-step process, with bacteria taking the initial active role and then waiting for engulfment by the recipient cell.  By differentially staining the membranes of infected cells, escape of Listeria into the cytoplasm of that cell can be documented (click here to see a movie (870k)).  In addition, low levels of intercellular membrane transfer are observed in the absence of infection, suggesting that the bacterium may co-opt an endogenous host process in order to spread from cell to cell.