An investigation on a peculiar fluid dynamic phenomenon in the motion of bacteria swimming close to a rigid boundary.

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The objective of this study is to clarify the interesting phenomenon in swimming motion of bacteria that has been observed only when the bacteria swim close to a rigid boundary. The trajectory close to a wall is noticeably different from the trajectory in free space. In addition to the trajectory, the swimming speed is affected by the presence of a wall. This is important because most observations of motion of microorganisms or even micro-machines have been done with suspensions of them sealed between two glasses. The motion as well as bacterial motion can be affected significantly by the glasses.The relationships between swimming speeds and rotational rates of the cell bodies of bacteria were experimentally investigated. They were measured in both motions of forward and backward when the bacteria moves straightly. The speed and the rotational rate was proportional for every cells. The proportional constant of forward motion and that of backward motion are the same. This means that the deformation of the cell body or the flagellum does not occur as much as it affects on the swimming motion.Assuming that the peculiar motion is caused by fluid dynamic interaction due to the presence of a rigid boundary, we perfumed a boundary element analysis into account the non-slip condition on a wall. The result shows that as for pitching, forward motion is stable and backward unstable. According to the results, an image of bacterial motion close to a boundary can be deduced and the image qualitatively agrees with the observed phenomenon.We also tried to develop an microscope that automatically tracks a bacterium. This enables us to obtain successive data such as swimming speed or positions without losing the image of the bacterium out of sight. An motor-driven stage equipped to an dark-filed microscope is controlled by the signal sent by an image processing software. The response time is not sufficient to track a bacterium smoothly so that improvement is needed.