(221s) Interplay between Swimming Speeds, Motor Torques and Filament Numbers in Multi-Flagellated Cells

Flagellar motors propel bacterial bodies by rotating helical flagellar filaments. The magnitude of the flagellar thrust determines the swimming speed. The swimming behavior of a monoflagellated cell can be modeled well as a sphere coupled to a helical filament. However, modeling the behavior of multi-flagellated cells is challenging owing to a complex dependence of flagellar thrust on filament numbers, filament forms, motor torque, as well as cell geometry among other things. Here, we experimentally investigated these relationships by comparing swimming behavior in two bacterial species, Bacillus subtilis and Escherichia coli. These motile species were observed to develop differential motor torques as a function of viscous loads, and carried markedly different number of filaments. Significantly, the cell lengths in the former were almost twice that in the latter. Yet, the swimming speeds in the two species were similar. A considerable difference in the predictions from the resistive force theory and experimental measurements was found. We will discuss how more rigorous models mitigate this discrepancy.