The Fluid Mechanics of Bacterial Swimming in Yield Stress Fluids

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Biological fluids such as mucous can behave like elastic solids or complex viscous liquid depending on the applied stress. The goal of this project is to understand the fluid mechanics of bacterial swimming in model complex fluids that resemble biological fluids. This comprehensive theoretical, computational, and experimental study will fill an important knowledge gap in understanding the relationship between the properties of biological fluids and the mechanics of bacterial motility. Swimming capabilities significantly contribute to the virulence of pathogenic bacteria by allowing them to reach the plasma membrane of susceptible cells and cause systemic infections. Therefore, data acquired in the project will be useful to researchers who want to mitigate or treat bacterial diseases. It will also be crucial for bioengineers aiming to deliver drugs via bacteria or synthetic swimmers. In addition, the project will provide outreach that inspires interest in engineering among K-12 students by illustrating the societal impact of engineering science. Bacterial motility in biological fluids such as mucous is complicated by the presence of a yield stress, a minimal stress necessary to deform the fluid, the coupled viscous and elastic responses of the fluid, and the non-continuum nature arising because the bacterium’s flagella have a thickness comparable with the spacing between mucus strands. A novel computational approach accounting for this complex rheology will capture the bacterium cell using an immersed boundary method and the bundle of flagella using slender-body theory. The solvent and polymer will be modeled as two interpenetrating fluids with the thin flagella exerting their stresses directly only on the solvent. In complementary experiments, nanoparticle organic hybrid bio-compatible materials will be designed with rheology and structure that mimic the biological fluids. Dark-field and phase-contrast microscopy of suspensions of E. coli will provide statistically accurate measurements of the swimming speed and cell rotation rate distribution. In microscopy tracking individual cells, separate fluorescent dyes will enable the determination of the different rotation rates of the cell and flagella bundle. Observations in Newtonian fluids will be used to characterize the rotary motors that drive the flagella. The motility measurements will be performed in collaboration with Professor Wilson Poon at the University of Edinburgh. The experiments will determine the criterion for cells to swim, the speed of cells that do swim, and the distinct resistances the non-continuum fluid exert on the cell and flagella, thereby testing three crucial predictions of the computation and theory.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.