Escherichia coli Chemotaxis
Title: Modeling Escherichia coli aspartate chemotaxis in a Stokes flow
Students: Rob Bierman and Jordan Bush
Advisors: Dr. Frank Healy and Dr. Hoa Nguyen
Abstract:
The bacterium Escherichia coli uses long extracellular filamentous appendages known as flagella (singular flagellum) to swim through its environment. The bidirectional (clockwise or counterclockwise) propeller-like rotary motion of flagellar filaments results in the net movement of the cell through gradients of chemoattractant nutrient molecules such as glucose or the amino acid aspartate toward areas of high concentrations of these attractants. Clockwise flagellar rotation occurs more frequently in environments characterized by low attractant concentrations and results in a random "tumbling" behavior of the cell. Conversely, counterclockwise rotation, which occurs more frequently in environments with high attractant concentrations, causes cells to "run" in a single direction. Directional switching of the flagellar motor is governed by a phosphorelay circuit that transfers phosphoryl groups from donor to acceptor proteins; protein phosphorylation state is controlled by binding of chemoattractants to specific receptors. The ultimate goal of this project is to model the movement of a cell propelled by a flexible flagellum within a three dimensional aspartate concentration gradient. To achieve this end, it was necessary to combine two separate models. The first describes physical flagellar motion that results from either clockwise or counterclockwise rotation while the second determines the state of the internal phosphorelay system at different aspartate concentrations.
The flagellar motion is modeled using the method of regularized Stokeslets where a single regularized rotlet is applied to the base of the flagellum. The rotlet then affects the direction of flagellar rotation which causes the cell to either run or tumble. To describe the behavior of the phosphorelay system, a dynamical system is used to model a simplified phosphorylation cascade. The results will be incorporated into the flagellar motion model to determine the bacterium’s motility in a Stokes flow. Our initial study shows that the dynamical system behaves as we would expect, with bacteria successfully finding and remaining in high aspartate environments. We are refining the flagellar motion model before coupling both models to achieve our ultimate goal.