shown. During the first and last circular motions, the flagellum precedes the cell body; hence, the cell moves backward. In straight motion, on the other hand, the cell swims forward. Figures 3(c) and (d) are the swimming traces of a YM42 cell, and Figs. 3(e) and (f) are for a NMB102 cell. The differences in trajectory between forward and backward motions are plainly demonstrated: a forward swimming cell moves straight and a backward swimming cell moves circularly. As mentioned above, the thickness of the liquid layer in this experiment was about 7 |im. Because the size of the cell was around 1 |im, which is com-parable to its thickness, the cells were likely to be affected by the glass sur-faces. The influence of a surface on cell motion was investigated by compar-ing the trajectories of cells swimming near a glass slide with the trajectories of cells swimming at a distance from a surface. In order to do so, a simple chamber was made. Spacers were inserted between the glass slide and the cover slip so as to make the distance between them about 170 |im. High-intensity dark-field microscopy cannot be applied to this thick sample, so in-verted phase-contrast microscopy was used for observation.

Typical examples of swimming traces of V. alginolyticus cells observed with an inverted phase-contrast microscope, (a)-(c) Wild type cells, (d)-(f) Smooth-swimming mutant cells, (g)-(i) Inverse smooth-swimming mutant cells, (a), (d) and (g) Cells swimming near slide glasses, (b), (e) and (h) Cells swimming several tens of micrometers from glass surfaces, (c), (f) and (i) Cells swimming near cover glasses. The arrows indicate the swimming direction. From Kudo et al. (2005) with permis-sion.

 Typical examples of the trajectories for YM4 cells are shown in Figs. 4(a)-(c). The trajectory contains circular motion and straight lines when the cells swim close to a top or bottom boundary, as shown in Figs. 4(a) and (c), while the trajectory is a series of straight lines when the cell swims at a distance from both of the glass surfaces, as shown in Fig. 4(b). Figures 4(d)-(f) indi-cate that YM42 cells always move in straight lines regardless of their distance from the surfaces. The NMB102 cells draw circular traces when the cells swim close to a surface, as depicted in Figs. 4(g) and (i). When there is not surface influence, the cells move in straight lines (Fig. 4(h)). These results suggest that asymmetric trajectory appears only when the cell swims close to a surface. A forward swimming cell moves in straight lines, while a backward swimming cell curves. Circular motions have also been reported on bacterial cells with several flagella. Berg and Turner (1990) described the circular motion ofE. coli cells. The three-dimensional tracking of the bacterial cells showed that circular mo-tion appears when the cells swim very close to a surface (Frymier et al. 1995). DiLuzi et al. (2005) demonstrated sorting of cells using branched narrow pas-sages. The motion of bacterial cells with several flagella is referred to as "run" and "tumble", or in other words, "forward" and "change directions." The asymmetrical characteristics between forward and backward motions are unique to bacterial cells with a single flagellum.