Size-dependent swimming behaviors of bacteria and the role of multiflagellarity

Flagella are a crucial organelle of bacteria enabling their motility and often adapted in two arrangements, i.e., monotrichous bacteria with a single polar flagellum and peritrichous bacteria with multiple flagella forming a rotating bundle. Although the adaptation of bacterial cellular features are under strong evolutionary pressure, studies have shown that multiflagellarity confers no noticeable benefit to the swimming of bacteria in bulk fluids. The puzzling finding poses a long-standing question: why does multiflagellarity emerge given the high metabolic cost of flagellar synthesis? Contrary to the common view that its benefit lies beyond the basic function of flagella in steady swimming, here we show that multiflagellarity indeed provides a significant selective advantage to bacteria in their swimming ability, allowing bacteria of different body sizes to maintain a constant swimming speed. By synergizing experiments, modeling and simulations, we reveal how bacteria utilize the increasing number of flagella to regulate the flagellar motor load, which leads to faster flagellar rotation neutralizing the higher fluid drag on their larger bodies. Without such a balancing mechanism, the swimming speed of monotrichous bacteria decreases with increasing size. Taken together, our study sheds light on the origin of multiflagellarity. Our findings are important for understanding environmental influence on bacterial morphology and useful for designing artificial flagellated microswimmers.