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Intracellular pathogens such as Listeria monocytogenes and Rickettsiarickettsii move within a host cell by polymerizing a comet-tail of actin fibersthat ultimately pushes the cell forward. This dense network of cross-linkedactin polymers typically exhibits a striking curvature that causes bacteria tomove in gently looping paths. Theoretically, tail curvature has been linked todetails of motility by considering force and torque balances from a finitenumber of polymerizing filaments. Here we track beads coated with a prokaryoticactivator of actin polymerization in three dimensions to directly quantify thecurvature and torsion of bead motility paths. We find that bead paths are morelikely to have low rather than high curvature at any given time. Furthermore,path curvature changes very slowly in time, with an autocorrelation decay timeof 200 seconds. Paths with a small radius of curvature, therefore, remain sofor an extended period resulting in loops when confined to two dimensions. Whenallowed to explore a 3D space, path loops are less evident. Finally, wequantify the torsion in the bead paths and show that beads do not exhibit asignificant left- or right-handed bias to their motion in 3D. These resultssuggest that paths of actin-propelled objects may be attributed to slow changesin curvature rather than a fixed torque.

Curvature and torsion in growing actin networks