The actin-based motility of the bacterium, Listeria monocytogenes, is a model system for understanding motile cell functions involving actin polymerization. Although the biochemical and genetic aspects of Listeria motility have been intensely studied, biophysical data are sparse. Here we have used high-resolution laser tracking to follow the trailing ends of Listeria moving in the lamellae of COS7 cells. We found that pauses during motility occur frequently and that episodes of step-like motion often show pauses spaced at about 5.4 nm, which Corresponds to the spatial periodicity of F-actin. We occasionally observed smaller steps (<3nm), as well as periods of motion with no obvious pauses. Clearly, bacteria do not sense cytoplasmic viscoelasticity because they fluctuate 20 times less than adjacent lipid droplets. Instead, bacteria bind their own actin 'tails', and the anchoring proteins can 'step' along growing filaments within the actin tail. Because positional fluctuations are unusually small, the forces of association and propulsion must be very strong. Our data disprove the brownian ratchet model and limit alternative models, such as the 'elastic' brownian ratchet or the 'molecular' ratchet.
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