Multiflagellarity allows bacteria to maintain constant motility across cell size*

We measure the swimming speed of E. coli, a model strain of multiflagellar bacteria, as a function of their body length. We find that the population-averaged swimming speed of bacteria is constant over three fold increase in their body length. We show how bacteria utilize the increasing number of flagella to regulate flagellar motor load, which results in higher rotational speeds as well as a constant swimming speed for large cell sizes. We subsequently perform simulations that reveal the role of interflagellar interactions in controlling the increase of rotational speeds. Our mechanism predicts that the swimming speed of uniflagellar species decreases with increasing cell size, which we verify directly through experiments on several strains of uniflagellar bacteria. Thus, the stark difference between the uniflagellar and multiflagellar swimming demonstrated in our study provides an insight into the crucial role of multiflagellarity in maintaining optimum motility for navigation and survival of bacteria in their native habitats.