Active Matter Phases and Phase Transitions in a Model Social Bacterium

Princeton University, Princeton NJ

Investigators

Abstract

The ability of groups of individual organisms to form complex and dynamic spatial patterns is a key aspect of biological phenomena ranging from bird flocking and fish schooling to multicellularity and embryonic development. In bacterial populations, this often involves complicated signaling mechanisms between cells. However, some species have evolved to take advantage of a special kind of physics called active matter to perform specific biological functions without the need for chemical signaling between cells. In the social bacterium Myxococcus xanthus, the Shaevitz group have shown that cells can form multicellular groups that resemble bird flocks, ant trails, and liquid droplets. These cells can change the way they move in response to environmental cues to change the behavior of the population. This project lays out a series of experiments and theoretical analyses to understand these processes from a physicist’s perspective, using tools to measure the spatial orientations, flows, and forces in these populations. Results from this work will answer fundamental questions about the mechanism and function of M. xanthus collective behavior and inform our understanding of how groups of organisms act collectively. In the future, this may lead to new methods for developing micro- and nanotechnologies that self-assemble using the principles of active physics. This project focuses on three questions: how three-dimensional droplets form during starvation, how a surface-wave instability aids predatory behavior, and how these behaviors manifest in naturalistic soil-like environments. To understand how 3D fruiting bodies emerge from 2D wetted layers, the investigators will track nematic order, cell motion, and mechanical stress throughout the population during development. To understand the instability that governs rippling behavior during predation, the investigators will measure the height of the population and track cell motion while perturbing motility, interactions between cells, and the environment. To probe M. xanthus phase transitions on more realistic substrates, the investigators will measure motility, fruiting body formation, and rippling on curved substrates and artificial colloidal models of hydrated soils. 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.