Session W43: Invited Session: Physical Mechanisms of Collective Microbial Dynamics

Chiral patterning in Paenibacillus colonies under stress

One of the most striking examples of bacterial colony patterning occurs in the C-morphotype of Paenibacillus strains. Here, macroscopic chirality results from the interaction of local liquid-crystal ordering of the long bacterial cells with the self-propelled motility driven by the non-reflection-symmetric flagella. This talk will review some of the original experimental data from the Ben-Jacob lab as well as recent insight obtained via genomics. I will then discuss attempts to model and simulate the chiral patterns via solving reaction-diffusion equations on random lattices. At the end, I will introduce the challenges still to be faced in understanding transitions between these patterns and more common branching structures   

The effects of self-induced noise on the onset of collective behavior in suspensions of swimming bacteria

Collective dynamics of self-locomoting micro-organisms, such as bacteria and algae have attracted enormous attention, with a large number of experimental and theoretical works published in the last few years. A plethora of nontrivial properties have been predicted and consequently studied, including dynamic instabilities, anomalous density fluctuations, nontrivial stress-strain relations, rectification of chaotic motion, and viscosity reduction. Here we investigate the viscosity of a suspension of swimming bacteria analytically and numerically. We propose a simple model that takes into account excluded volume constraints and allows for efficient computation for a large number of bacteria. Our calculations show that long-range hydrodynamic interactions, intrinsic to self-locomoting objects in a viscous fluid, result in a dramatic reduction of the effective viscosity. In agreement with experiments on suspensions of \textit{Bacillus subtilis}, we show that the viscosity reduction is related to the onset of large-scale collective motion due to interactions between the swimmers. The simulations reveal that the viscosity reduction occurs only for relatively low concentrations of swimmers: further increases of the concentration yield an increase of the viscosity. We derive an explicit asymptotic formula for the effective viscosity in terms of known physical parameters and show that hydrodynamic interactions are manifested as self-induced noise in the absence of any explicit stochasticity in the system. We also explain the increase in the viscosity for pullers by analysis of the deviations from the mean field approximation   

Synchronization of flagella

Motivated by the observed coordination of nearby beating flagella, we use highly controlled simple model experiments with rotating paddles to study how hydrodynamic interactions can lead to phase-locking. The agreement between our numerical models and experimental results confirms that hydrodynamic interactions can lead to synchronization or phase-locking if the system has sufficient flexibility. We also present a simple theory, valid for weakly interacting paddles, for both viscous and viscoelastic fluids.   

Formation and propagation of high density waves during swarming of \textit{P. aeruginosa}

We will describe in this talk how the bacterium \textit{P. aeruginosa }alters its local physical environment by propagating high density waves of cells into branched tendrils during surface motility described as swarming. Biologically justified model simulations will be used to suggest a mechanism of wave propagation and branched tendril formation that depend upon competition between the changing viscosity of the bacterial liquid suspension and the liquid film boundary expansion caused by Marangoni forces. Thus, \textit{P. aeruginosa} controls physical forces responsible for liquid film expansion to efficiently colonize surfaces.   

Alignment vs noise in self-propelled particles: minimal models for collective motion and their continuous descriptions

Two important 1995 papers have marked the birth of collective motion studies in physics: Vicsek et al introduced what could now be described as the ``Ising model'' of this new subfield. This prompted Toner and Tu to propose a continuum theory of flocks which they showed to give rise to long-range orientational order even in two space dimensions. In this setting, the complexity of most natural instances of collective motion is reduced to the competition between local alignment and noise in interacting self-propelled particles. As I will show, this nevertheless gives rise to important and new physics. In this talk, I will give an update of our current knowledge about the Vicsek model, the Toner-Tu theory, and their relationship. I will also present the emerging picture of universality classes brought about by recent progress in the study of Vicsek-like models together with their continuous descriptions.