Microscopic Fluid Mechanic Research for Propulsion of Bacteria with Flagella

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The swimming motion of Vibrio alginolyticus, a bacterium with a single flagellum was observed using a dark-field biological microscope with CCD camera and was record by a video recorder. The linear relation between the rotation rate of the cell body and the propulsive speed was obtained for each individual bacterium. Computations were carried out for propulsion of the bacteria with a single flagellum applying the boundary element method based on the Stokes flow. the comparison is well between the observations and the simulations.In the present investigation, the forward and backward swimming speeds were also measured for a lot of Vibrio alginolyticus. The speed of backward swimming is 1.5 times larger than that of forward swimming on average. The difference of the speed between the forward and the backward swimming might be caused by several factors such as asymmetry of the motor torque with respect to the rotational direction and deformation of the flagellum filament due to hydrodynamic forces.An analysis was conducted for the small deformation of flexible flagellum of swimming bacteria. The resistive force theory was applied to estimate the force exerted on flagellum. The torsional as well as the bending moments determine the curvature and the torsion of the deformed flagellum according to the Kirchhoff model for elastic rod. The speeds were simulated for the forward and the backward swimming taking account of deformation of flagellum. It is shown that the difference between the forward and the backward speeds is not generated by the deformation of the flagellum.