Introduction

 Most bacteria possess the ability to swim in order to seek suitable circum-stances for reproduction and to evade adversarial conditions. Bacterial cells are so small (on the order of 1 |im) that it might be assumed that the swim-ming motion of these cells is insignificant compared to the motion caused by the surrounding convective flow. Nevertheless, most bacteria possess organs for locomotion called flagella. Many other eukaryotic microorganisms also have locomotion organelles, e.g. cilia, flagella. This suggests that the ability to swim is essential, even for the microorganisms, because swimming ability was selected and developed over a long evolutionary process. Figure 1 illustrates a schematic bacterial cell, which consists of two no-ticeable elements. The cell body is a capsule that contains genetic information and the chemical systems for reproduction, adaptation, locomotion, sensing, etc. The cell body is the core of the small creature. The other element is the flagellar filament, which is a helical thin filament made of protein. Unlike the flagella of eukaryotic cells that contain microtubules and bend by themselves, the flagella of bacterial cells do not deform actively. Instead, each filament is driven by a rotary motor (flagellar motor) embedded in the cell body. The ro-tation of the helical flagellum produces a helical wave within the neighboring fluid that exerts a reaction force on the filament in the direction opposite to the wave motion. Thus, the cell is propelled by the reaction force due to the rota-tion of the flagellum. A flagellar motor can drive a flagellar filament in either a clockwise (CW) or counterclockwise (CCW) direction. Some bacterial cells such as Vibrio alginolyticus, which this paper specifically addresses, possess a single left-handed helix flagellum. When the motor rotates CCW, as seen from the distal end of the flagellum, the helical wave propagates from the proximal end to the distal end. As a result, the cell swims with the cell body preceding the flagel-lum, hereafter called "forward" motion. When the motor rotates CW, the cell swims in an orientation in which the flagellum pulls the cell body, which is hereafter called "backward" motion. Some bacterial cells like Salmonella ty-phimurium and Escherichia coli possess several flagella. Their motion differs from the singly flagellated bacterial cells and is usually referred to as "run" and "tumble." When the motors rotate CCW, the filaments form a helical-shaped bundle and the cell swims forward. However, these bacteria are inca-pable of backward motion. When the motors switch their rotational direction from CCW to CW, the inverted torque induces polymorphic transformations of the filaments, which result in the dissociation of the bundle. During the proc-ess, the cell changes its orientation due to fluid forces exerted on each fila-ment. When the motors rotate CCW again, the cell swims in a different direc-tion