Analysis of the actin polymerization-driven movement of Listeria monocytogenes

The pathogenic bacterium Listeria monocytogenes is propelled at a rate of several lengths per minute in the cytoplasm of an infected host cell, leaving a 'comet tail' of polymerized filaments made of the host cell protein actin in its wake. From the point of view of L. monocytogenes, this process is surprisingly simple---all that is required for its movement is the presentation of the surface protein ActA, which then triggers a complex biochemical cascade resulting in the polymerization of the host cell actin. Several host cell cytoskeletal proteins are implicated in the reaction.; To more precisely characterize the relationship between protein localization near the bacterial surface and movement, specialized image processing techniques were used to track the movement of individual bacteria while simultaneously measuring the localization of fluorescently labeled proteins to the bacterial surface over time. Tracking and fluorescence data for many individual bacteria were stored in a large database.; Analysis of bacterial velocity fluctuations across a large (n > 1000) population of Listeria monocytogenes in Xenopus egg cytoplasmic extract revealed widespread, significant low frequency periodic fluctuations in individual bacterial velocity over time. The average longitudinal spatial actin tail profile of individual bacteria in the population was found to be well described by a four parameter 'blurred exponential' function, the shape of which varied systematically with average bacterial speed. Average and instantaneous bacterial velocities in the population were positively correlated with actin density in a region directly behind the bacterium, and negatively correlated with density along the front of the bacterial surface. The results are consistent with a model in which the actin tail generates both propulsive and retarding forces. How both forces may be mediated by specific molecular interactions at the surface of the bacterium is discussed.