Tag: Tristan Ursell

Chemotaxis Expansion Waves in E. Coli

Presenter(s): Maria Dresser − Physics, Mathematics

Faculty Mentor(s): Tristan Ursell, Daniel Shoup

Poster 13

Research Area: Biophysics

Funding: Presidential Undergraduate Research Scholars (PURS) recipient

Communities of bacteria respond to environmental changes as a group with the combined behaviors of individual bacteria giving rise to unique collective behaviors that facilitate the growth and dispersal of bacteria. In particular, bacteria undergo a process called chemotaxis which utilizes a run and tumble method to move towards higher concentrations within a given chemical gradient. In liquid environments, collective consumption and chemotaxis towards nutrients results in a collective behavior known as an expansion wave which facilitates rapid range expansion. How environmental properties dictate the attributes of expansion waves is poorly understood yet critically important as expansion waves drive invasiveness, colonization, and may help bacteria define their interspecies boundaries in complex communities. Here we study the expansion of E. Coli in capillary tubes to replicate a one dimensional expansion environment. The use of various concentrations of galactose and three amino acids give rise to different expression profiles and observable behaviors. After inoculating cells into capillary tubes containing different nutrient media, we image the tubes using bright field microscopy and measure the wave speed and number of waves in each tube. Wave speed allows us to understand how quickly bacteria enter a new region and how this is affected by nutrient concentration. Because different waves may exhibit different phenotypic states such as consuming different nutrients and undergoing cell division at different rates, we are interested in understanding what nutrient concentrations give rise to multiple waves. We hypothesize that slower waves are undergoing cell division more rapidly, thus devoting more energy to division than to consumption. Results thus far show that expansion rate is constant until a threshold is met, and lower initial cell concentrations give rise to more waves.

Simulation of Bacterial Motion in Sterically Complex Environments

Presenter(s): Matthew Kafker—Physics, Mathematics

Faculty Mentor(s): Tristan Ursell

Session: Prerecorded Poster Presentation

Many species of bacteria navigate complex and heterogeneous environments to search for metabolic resources and avoid toxins . Common among such complexities is steric structure—solid objects whose surface curvature alters bacterial trajectories upon impact . In previous experiments, we characterized scattering of bacteria from vertical pillars of different radii, which provides the basis for understanding how impact with a solid, curved object alters bacterial motion . However, it remains poorly understood how multiple interactions affect bacterial trajectories and whether distinct object curvatures or length-scales of separation between steric objects have qualitatively distinct effects on bacterial motion . We address this question using agent-based computer simulations of cells moving within 2D environments . Each environment presents simulated cells with steric objects (i .e . circular pillars) of radius 8 .3μm and a controlled separation between pillars of L μm, where L is a parameter of the simulation . Cells then diffuse through this environment, scattering with pillars they encounter . By measuring the mean squared displacement (MSD) of the ensemble of trajectories in time for different values of L, we are able to quantify precisely how the length-scales of separation between steric structures affect bacterial trajectories . These MSD measurements will also allow us to compare our results with future experimental work . Ultimately, we hope that our results may contribute to a more realistic model of the behavior of motile cells in natural environments such as soils or a mammalian gut .