Study on the bacterial locomotion mechanism

许多细菌具有长约1.5μm的细胞体和细长的螺旋鞭毛，长度为5-10μm，直径为30 nm。细菌通过旋转其鞭毛细丝来游泳，鞭毛细丝充当螺旋推进器。细菌的游泳速度有时被认为是随着粘度的增加而增加的。这种现象是细菌运动所特有的。Berg和Turner(1979)指出，这种现象是由于加入聚合物分子以增加粘度而形成的松散的准刚性网络造成的。我们通过引入两个表观粘度从数学上发展了它们的概念，并得到了与以前报道的实验数据类似的结果。聚合物的加入提高了推进效率，超过了鞭毛转速的下降，游泳速度随着粘度的增加而增加。鞭毛细丝由称为鞭毛蛋白的蛋白质组成。如果鞭毛的形状发生较大变化，则推进效率将发生显著变化。据推测，鞭毛具有像硬弹簧一样的弹性，尽管它们在旋转过程中可能会膨胀和收缩。我们发展了一种用激光暗场显微镜观察游泳过程中鞭毛形状的实验方法。溶藻弧菌的鞭毛节距在细胞向前游动时减小，而在细胞向后游动时增大。当细胞以100μm/S的速度向前或向后游动时，变化约为3%。相反，在实验误差范围内没有观察到半径的显著变化。观察到的鞭毛螺旋的变形被认为只会对推进效率产生轻微的影响，因为与推进效率相对应的游泳速度与鞭毛转速之比(v-f比)的变化估计只有几个百分点的变化。

Many bacteria have cell bodies about 1.5 μm long and thin helical flagellar filaments that are 5 - 10 μm in length and 30 nm in diameter. Bacteria swim by rotating their flagellar filaments, which act as screw propellers.Bacterial swimming speed is sometimes known to increase with viscosity. This phenomenon is peculiar to bacterial motion. Berg and Turner (1979) indicated that the phenomenon was caused by a loose, quasi-rigid network formed by polymer molecules that were added to increase viscosity. We mathematically developed their concept by introducing two apparent viscosities and obtained results similar to the experimental data reported before. Addition of polymer improved the propulsion efficiency, which surpasses the decline in flagellar rotation rate, and the swimming speed increased with viscosity.Flagellar filaments consist of protein called flagellin. If flagella change their shapes largely, the efficiency of propulsion will change significantly. It is presumed that flagella have elasticity like hard spring, though they possibly expand and contract during rotation. We developed an experimental method to observe the flagellar shape during swimming by using laser dark-field microscopy. The flagellar pitch of Vibrio alginolyticus was observed to decrease when a cell swam forward, while it was observed to increase when a cell swam backward. The change was about 3 % when a cell swam forward or backward at the speed of 100 μm/s, respectively. On the contrary, the significant change in radius was not observed within the experimental error. The deformation in flagellar helix observed was considered to only slightly influence the propulsion efficiency because the change in the ratio of swimming speed to flagellar rotation rate (v-f ratio) that corresponds to the propulsion efficiency was estimated to change by only a few percent.