Difference in bacterial motion between forward and backward swimming caused by the wall effect

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 μm 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.