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A Caulobacter crescentus swarmer cell is propelled by a helical flagellum, which is rotated by a motor at its base. The motor alternates between rotating in clockwise and counterclockwise directions and spends variable intervals of time in each state. We measure the distributions of these intervals for cells either free swimming or tethered to a glass slide. A peak time of around one second is observed in the distributions for both motor directions with counterclockwise intervals more sharply peaked and clockwise intervals displaying a larger tail at long times. We show that distributions of rotation intervals fit first passage time statistics for a biased random walker and the dynamic binding of CheY-P to FliM motor subunits accounts for this behavior. Our results also suggest that the presence of multiple CheY proteins in C. crescentus may be responsible for differences between its switching behavior and that of the extensively studied E. coli.

We measured the distribution of a forward swimming strain of Caulobacter crescentus near a surface using a three-dimensional tracking technique based on dark field microscopy and found that the swimming bacteria accumulate heavily within a micrometer from the surface. We attribute this accumulation to frequent collisions of the swimming cells with the surface, causing them to align parallel to the surface as they continually move forward. The extent of accumulation at the steady state is accounted for by balancing alignment caused by these collisions with the rotational Brownian motion of the micrometer-sized bacteria. We performed a simulation based on this model, which reproduced the measured results. Additional simulations demonstrate the dependence of accumulation on swimming speed and cell size, showing that longer and faster cells accumulate more near a surface than shorter and slower ones do.

Diffusion of the protein filaments F -actin confined in a thin layer between two walls is studied using the methods of single filament fluorescence imaging and particle tracking. The translational and rotational diffusion coefficients are measured for F -actin of lengths in the range of 1.5-5 microm. The length dependence of the measured diffusion coefficients is consistent with the predicted two-dimensional projection of the diffusion of a cylinder in an unbounded fluid. Fits based on the formulas for diffusion in the bulk fluid yield higher apparent viscosity values than that of the buffer solution by a factor of 2 for a layer thickness between 0.7 and 1.6 microm. We show that the measured results can be accounted for by correction based on the hydrodynamic theory of a long cylinder between confining walls.