Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455.
National Institute for Mathematical Sciences, Daejeon 34047, Republic of Korea.
Proc Natl Acad Sci U S A. 2023 Nov 28;120(48):e2310952120. doi: 10.1073/pnas.2310952120. Epub 2023 Nov 22.
To swim through a viscous fluid, a flagellated bacterium must overcome the fluid drag on its body by rotating a flagellum or a bundle of multiple flagella. Because the drag increases with the size of bacteria, it is expected theoretically that the swimming speed of a bacterium inversely correlates with its body length. Nevertheless, despite extensive research, the fundamental size-speed relation of flagellated bacteria remains unclear with different experiments reporting conflicting results. Here, by critically reviewing the existing evidence and synergizing our own experiments of large sample sizes, hydrodynamic modeling, and simulations, we demonstrate that the average swimming speed of , a premier model of peritrichous bacteria, is independent of their body length. Our quantitative analysis shows that such a counterintuitive relation is the consequence of the collective flagellar dynamics dictated by the linear correlation between the body length and the number of flagella of bacteria. Notably, our study reveals how bacteria utilize the increasing number of flagella to regulate the flagellar motor load. The collective load sharing among multiple flagella results in a lower load on each flagellar motor and therefore faster flagellar rotation, which compensates for the higher fluid drag on the longer bodies of bacteria. Without this balancing mechanism, the swimming speed of monotrichous bacteria generically decreases with increasing body length, a feature limiting the size variation of the bacteria. Altogether, our study resolves a long-standing controversy over the size-speed relation of flagellated bacteria and provides insights into the functional benefit of multiflagellarity in bacteria.
为了在粘性流体中游泳，鞭毛菌必须通过旋转鞭毛或一束多根鞭毛来克服其身体的流体阻力。由于阻力随细菌尺寸的增加而增加，因此理论上预期细菌的游动速度与其体长成反比。然而，尽管进行了广泛的研究，但鞭毛菌的基本大小-速度关系仍不清楚，不同的实验报告结果相互矛盾。在这里，通过批判性地回顾现有证据，并协同我们自己的大量样本实验、流体动力学建模和模拟，我们证明了, 一种周生鞭毛菌的主要模型，其平均游动速度与其体长无关。我们的定量分析表明，这种违反直觉的关系是细菌体长与鞭毛数之间线性相关所决定的集体鞭毛动力学的结果。值得注意的是，我们的研究揭示了细菌如何利用增加的鞭毛数量来调节鞭毛马达的负载。多个鞭毛之间的集体负载分担导致每个鞭毛马达的负载降低，从而使鞭毛旋转更快，这弥补了细菌较长身体上更高的流体阻力。如果没有这种平衡机制，单生鞭毛菌的游动速度通常会随着体长的增加而降低，这一特征限制了细菌的大小变化。总的来说，我们的研究解决了鞭毛菌大小-速度关系的长期争议，并为细菌多鞭毛性的功能益处提供了新的见解。
