Integrating bacterial polarity with host-cytoskeletal dynamics: Initiation of Listeria monocytogenes actin-based motility

The bacterial pathogen, Listeria monocytogenes, is able to integrate its own spatial dynamic organization with that of its host cell to allow it to move through the host-cellular cytoplasm and spread from cell to cell while evading the immune system. L. monocytogenes achieve motility through the polar expression of a single bacterial protein, ActA, to recruit the necessary host-cytoskeletal factors asymmetrically to the surface. ActA promotes the nucleation of actin filaments, which eventually leads to the formation of a self-renewing, polymerizing actin comet tail that propels the bacterium forward. Previous work has focused on the steady-state motility of L. monocytogenes. Here we investigate how this bacterium is able to initiate its motility.; Upon entering the host-cell cytoplasm, L. monocytogenes begin expression and polarization of the ActA protein de novo. By using a fully functional ActA-RFP (red fluorescent protein) fusion protein, we directly visualized the process of ActA polarization. We found that the polar ActA surface distribution may be inherent to the differential cell wall growth rates along the bacterium. Protein is initially secreted at distinct sites, slowly spread over the cylindrical cell body through helical cell wall growth, and eventually incorporated into the poles. We further found that bacteria modify the surface localization of ActA based on environmental conditions, possibly owing to changes in the properties of the bacterial cell wall.; We directly examined the process by which L. monocytogenes initiate motility both in vitro and in vivo. This process was found to be distinct from motility initiation of spherical, symmetrically coated cargo and dependent on bacterial rod-shaped geometry, likely contributing to the bacteria's ability to move within the host-cell. We also directly determined the effect of the polarized ActA surface distribution on initiation and maintenance of bacterial motility. Surprisingly, we found that robust motility developed through a time-dependent maturation from erratic, more sensitive movement to steady-state. Finally, we showed that various motility parameters, including the timing of tail formation, speed variability, path curvature, and sensitivity to the environment can occur through subtle changes in the biochemical interactions at the bacterial surface between ActA and the host-actin cytoskeleton.