Effects of intermediate filaments on actin-based motility of Listeria monocytogenes

How does subcellular architecture influence the intracellular movements of large organelles and macromolecular assemblies? To investigate the effects of mechanical changes in cytoplasmic structure on intracellular motility, w e have characterized the actin-based motility of the intracellular bacteria l pathogen Listeria monocytogenes in normal mouse fibroblasts and in fibrob lasts lacking intermediate filaments. The apparent diffusion coefficient of L. monocytogenes was two-fold greater in vimentin-null fibroblasts than in wild-type fibroblasts, indicating that intermediate filaments significantl y restrict the Brownian motion of bacteria. However, the mean speed of L. m onocytogenes actin-based motility was statistically identical in vimentin-n ull and wild-type cells. Thus, environmental drag is not rate limiting for bacterial motility. Analysis of the temporal variations in speed measuremen ts indicated that bacteria in vimentin-null cells displayed larger fluctuat ions in speed than did trajectories in wild-type cells. Similarly, the pres ence of the vimentin meshwork influenced the turning behavior of the bacter ia; in the vimentin-null cells, bacteria made sharper turns than they did i n wild-type cells. Taken together, these results suggest that a network of intermediate filaments constrains bacterial movement and operates over dist ances of several microns to reduce fluctuations in motile behavior.