A bacterial cell that has a single polar flagellum alternately repeats forward swimming, in which the flagellum pushes the cell body, and backward swimming, in which the flagellum pulls the cell body. We have reported that the backward swimming speeds of Vibrio alginolyticus are on average greater than the forward swimming speeds. In this study, we quantitatively measured the shape of the trajectory as well as the swimming speed. The trajectory shape in the forward mode was almost straight, whereas that in the backward mode was curved. The same parameters were measured at different distances from a surface. The difference in the motion characteristics between swimming modes was significant when a cell swam near a surface. In contrast, the difference was indistinguishable when a cell swam >60 microm away from any surfaces. In addition, a cell in backward mode tended to stay near the surface longer than a cell in forward mode. This wall effect on the bacterial motion was independent of chemical modification of the glass surface. The macroscopic behavior is numerically simulated on the basis of experimental results and the significance of the phenomenon reported here is discussed.

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The difference between forward and backward swimming speeds has been observed for Vibrio alginolyticus by use of high-intensity dark-field microscopy. The reason that causes the difference is not clear at present. There are some possible reasons we may take into account ; the deformation of a flagellum which is caused by the fluid force exerting on the flagellum, the hydrodynamic interaction between a cell and a slide and the changes in rotational speed of a flagellar motor . In order to investigate the reason for the difference, we measure cell body rotation rates simultaneously with swimming speeds of a cell swimming forwards and backwards . The results show that the difference is mainly caused by the variation in the rotation rates of the cell body at least when the cell moves straightly.

Bacteria swimming close to a rigid boundary o 丘 毛 cn trace circular trajectoI ・ ies． An E ． Coli cell ， which utilizes a bundle of severa 田 agella as a screw ， swims clockwise above a boundary when it is observed Erom above ． In colltrast ， a Vibrio cell ， whicll possesses a single flagellunll swims coullterclockwise ill a way that the flagcllum pulls the cell body ． Also ， the swimmillg speed of them are affected by the boundary ． In this study ， the velocities and anglllar velocities of bacterium models swimminig dose to a rigid boundary are calculated by use of the boundary elcment analysis ， They are compared to those of models swimming illEree space ， The degree of fiuid dynamic illtcractions ， between the cell body and the boundary ， and between the flagellurn and the boundary is crucial to explain the variety of motions f bacteria swimlnillg close to aboundary ．