Abstract
The presence of a nearby boundary is likely to be important in the life cycle and evolution of motile flagellate bacteria. This has led many authors to employ numerical simulations to model near-surface bacterial motion and compute hydrodynamic boundary effects. A common choice has been the method of images for regularized Stokeslets (MIRS); however, the method requires discretization sizes and regularization parameters that are not specified by any theory. To determine appropriate regularization parameters for given discretization choices in MIRS, we conducted dynamically similar macroscopic experiments and fit the simulations to the data. In the experiments, we measured the torque on cylinders and helices of different wavelengths as they rotated in a viscous fluid at various distances to a boundary. We found that differences between experiments and optimized simulations were less than 5% when using surface discretizations for cylinders and centerline discretizations for helices. Having determined optimal regularization parameters, we used MIRS to simulate an idealized free-swimming bacterium constructed of a cylindrical cell body and a helical flagellum moving near a boundary. We assessed the swimming performance of many bacterial morphologies by computing swimming speed, motor rotation rate, Purcell’s propulsive efficiency, energy cost per swimming distance, and a new metabolic energy cost defined to be the energy cost per body mass per swimming distance. All five measures predicted that the optimal flagellar wavelength is eight times the helical radius independently of body size and surface proximity. Although the measures disagreed on the optimal body size, they all predicted that body size is an important factor in the energy cost of bacterial motility near and far from a surface.
Highlights
Living organisms emerge, evolve, and reside within habitats, and the physical interactions among organisms and their environments impose selective forces on their evolution.In their low Reynolds number surroundings, bacteria such as Escherichia coli and Pseudomonas aeruginosa have evolved a mechanical motility system to propel themselves through fluids
Flagellar motor rotation is driven by an ion flow through the motor, causing the flagellum and the bacterial cell body to rotate in opposite directions [1]
The other three performance measures we studied are based on the mechanical energy cost to achieve motility: the Purcell inefficiency, the inverse of distance traveled per energy input, and the metabolic energy cost, which
Summary
Evolve, and reside within habitats, and the physical interactions among organisms and their environments impose selective forces on their evolution. In their low Reynolds number surroundings, bacteria such as Escherichia coli and Pseudomonas aeruginosa have evolved a mechanical motility system to propel themselves through fluids. This system consists of one or more helical flagella, and these flagellar organelles are attached to the cell body by rotary nanomotors. The interaction between the bacterium and the fluid generates a flow that results in the net motion of the bacterium
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