Study on the bacterial locomotion mechanism

许多细菌的细胞体长约1.5 μm，有细长的螺旋鞭毛丝，长5 ~ 10 μm，直径30 nm。细菌通过旋转鞭毛丝游泳，鞭毛丝就像螺旋桨一样。细菌的游动速度有时会随着黏度的增加而增加。这种现象是细菌运动所特有的。Berg和Turner（1979）指出，这种现象是由加入聚合物分子以增加粘度而形成的松散的准刚性网络引起的。我们通过引入两种表观粘度，从数学上发展了它们的概念，得到了与之前报道的实验数据相似的结果。聚合物的加入提高了推进效率，超过了鞭毛旋转速率的下降，并且游动速度随粘度的增加而增加。鞭毛丝由一种叫做鞭毛蛋白的蛋白质组成。如果鞭毛的形状发生很大的变化，推进的效率就会发生很大的变化。据推测，鞭毛具有像硬弹簧一样的弹性，但也可能在旋转过程中膨胀和收缩。建立了一种用激光暗场显微镜观察游泳过程中鞭毛形态的实验方法。藻酸解弧菌鞭毛间距在细胞向前游动时减小，在细胞向后游动时增大。当细胞以100 μm/s的速度向前或向后游动时，其变化约为3%。相反，在实验误差范围内没有观察到明显的半径变化。观察到的鞭毛螺旋变形被认为对推进效率的影响很小，因为与推进效率相对应的游动速度与鞭毛旋转速率之比（v-f比）的变化估计只有几个百分点。

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.