Steps and fluctuations of Listeria monocytogenes during actin-based motility

The actin-based motility of the bacterium, Listeria monocytogenes, is a mod el system for understanding motile cell functions involving actin polymeriz ation(1). Although the biochemical and genetic aspects of Listeria motility have been intensely studied(2-5), biophysical data are sparse(6). Here we have used high-resolution laser tracking to follow the trailing ends of Lis teria moving in the lamellae of COS7 cells. We found that pauses during mot ility occur frequently and that episodes of step-like motion often show pau ses spaced at about 5.4 nm, which corresponds to the spatial periodicity of F-actin(7). We occasionally observed smaller steps (<3 nm), as well as per iods of motion with no obvious pauses. Clearly, bacteria do not sense cytop lasmic viscoelasticity because they fluctuate 20 times less than adjacent l ipid droplets. Instead, bacteria bind their own actin 'tails', and the anch oring proteins can 'step' along growing filaments within the actin tail. Be cause positional fluctuations are unusually small, the forces of associatio n and propulsion must be very strong. Our data disprove the brownian ratche t model(8) and limit alternative models, such as the 'elastic' brownian rat chet(9) or the 'molecular' ratchet(4,10).