Using calibrated numerical models as a noninvasive probe to study bacterial motility

The evolutionary pressures that cause morphological changes in bacteria are often difficult to determine because we lack precise information about fluid-structure interactions when bacteria are motile. Numerical modeling provides insights, but the models are often calibrated using biological measurements that have large uncertainties. Instead, our collaborative project (NSF POLS - 2210610) uses dynamically similar tabletop experiments to precisely calibrate the method of regularized Stokeslets (MRS) and the method of images for regularized Stokeslets (MIRS) to extract quantitatively accurate values of forces and torques on model bacteria moving near a boundary. The experiments also provide the first validation of theories for torque on cylinders (Jeffrey and Onishi 1981) and torque on spheres (O'Neil 1964) moving near a boundary. The simulations then serve as a noninvasive probe to measure swimming metrics such as the Purcell efficiency, energy per distance, and our new metabolic energy cost measure. We have shown that our method provides unique insights into how cylindrical body shape affects swimming performance measures near a boundary (Shindell et al., Fluids, 2021). This work also shows that the wavelength of a helical flagellum seems to be selected independently of body shape. See the related talk by Orrin Shindell for more details.