  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 \textit{Escherichia coli}, 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 new insights into the
functional benefit of multiflagellarity in bacteria.
