The goal of this research is to observe and track individual bacteria in order to elucidate the physical mechanisms of agellar motility. The central tool of the study is a Digital Tracking Microscope (DTM), which allows tracking of a single cell for arbitrarily long times. The first stage of the project is the refinement of the DTM, which already exists in prototype form. Once completed the DTM will be deployed to measure the velocity and orientation of single swimming cells over the course of several minutes. These measurements will yield detailed statistics for the swimming behavior of single bacterial cells such as C. crescentus, E. coli, and S. marascens. By correlating the swimming velocity and cell orientation with geometrical properties of the cell, such as cell shape, agellum length, and number of agella, and by using theory and computation to determine the fluid and elastic stresses acting on the cell, the PIs will elucidate how physical constraints determine swimming behavior. Stokes flow analysis will be coupled with elasticity theory in a combined manner, and with this synthesis, the PIs will explain why cells in 'pusher' mode, with the agellum behind the swimming cell, move faster than cells in 'puller' mode, with the agellum ahead of the swimming cell. The effects of the rheology of the suspending fluid on the swimming behavior will also be explored. Motivated by the fact that most infectious bacteria encounter mucus, a non-Newtonian fluid, when they enter the body, the PIs will track swimming bacteria in non-Newtonian fluids.
Intellectual Merit : The intellectual merit of this work is that the DTM allows a host of new measurements and observations. By observing single bacterial cells over time, it becomes possible to study the effects of individual variation across a population. Tracking of individual cells has already lead to new discoveries in C. crescentus, such as the difference in swimming speed between pushers and pullers. The high resolution of the DTM motivates the development of more sophisticated quantitative theories that integrate fluid mechanics with material properties and elasticity, and can be compared with our precise measurements. While recent years have seen active focus on idealized theoretical approaches and table-top models for swimming, the proposed work promises a new level of detail from live cell studies that have not previously been possible.
Broader Impacts : The PIs will participate in outreach and recruitment activities. The co-PIs will continue to participate in the Research Experiences for Teachers program, which invites K-12 teachers from local schools to the Brown campus to participate in laboratory research and course development projects during the summer, and in the Brown University Summer High School program, giving lectures and demonstrations on the physics of locomotion. The PIs will also contribute lectures and discussions to the Providence After School Alliance (PASA) - a local organization that provides expanded learning opportunities to high-school students through science-oriented programs. Lastly, the collaboration will also develop web-based educational media to illustrate the concepts of bacterial motility, agellar motion, microscopy, and PIV.
