Biology subjects

Gopinath, A.

Publications and source records attributed to Gopinath, A..

3 recordsLinked to original sources

bioRxiv2018

Filament extensibility and shear stiffening control persistence of strain and loss of coherence in cross-linked motor-filament assemblies

Cross-linked flexible filaments deformed by active molecular motors occur in many natural and synthetic settings including eukaryotic flagella, the cytoskeleton and in vitro motor assays. In these systems, an important quantity that controls spatial coordination and emergent collective behavior is the length scale over which elastic strains persist. We estimate this quantity in the context of ordered composites comprised of cross-linked elastic filaments sheared by active motors. Combining a mean-field theory valid for negligibly noisy systems with discrete simulations for noisy systems, we show that the effect of localized strains - be they steady or oscillatory - persist over distances determined by motor kinetics, motor elasticity and filament extensibility. The cut-off length that emerges from these effects controls the transmission of mechanical information and determines the criterion for spatially separated motor groups to stay synchronized. Our results generalize the notion of persistence in passive, Brownian filaments to active, cross-linked filaments.

biophysics↗

bioRxiv2018

Quenching a swarm: Effect of light exposure on suppression of collective motility in swarming Serratia marcescens

Swarming colonies of the light responsive bacteria Serratia marcescens grown on agar exhibit robust fluctuating large-scale collective flows that include arrayed vortices, jets, and sinuous streamers. We study the immobilization and quenching of these large-scale flows when the moving swarm is exposed to light with a substantial ultra-violet component. We map the response to light in terms of two independent parameters - the light intensity and duration of exposure and identify the conditions under which mobility is affected significantly. For small exposure times and/or low intensities, we find collective mobility to be negligibly affected. Increasing exposure times and/or intensity to higher values temporarily suppresses collective mobility. Terminating exposure allows bacteria regain motility and eventually reestablish large scale flows. For long exposure times or at high intensities, exposed bacteria become paralyzed, with macroscopic speeds eventually reducing to zero. In this process, they form highly aligned, jammed domains. Individual domains eventually coalesce into a large macroscopic domain with mean radial extent growing as the square root of exposure time. Post exposure, active bacteria dislodge exposed bacteria from these jammed configurations; initial dissolution rates are found to be strongly dependent on duration of exposure suggesting that caging effects are substantial at higher exposure times. Based on our experimental observations, we propose a minimal Brownian dynamics model to examine the escape of exposed bacteria from the region of exposure. Our results complement studies on planktonic bacteria and inform models for pattern formation in gradated illumination.

biophysics↗

bioRxiv2018

The propagation of active-passive interfaces in bacterial swarms

Propagating interfaces are ubiquitous in nature, underlying instabilities and pattern formation in biology and material science. Physical principles governing interface growth are well understood in passive settings; however, our understanding of interfaces in active systems is still in its infancy. Here, we study the evolution of an active-passive interface using a model active matter system, bacterial swarms. We use ultra-violet light exposure to create compact domains of passive bacteria within Serratia marcescens swarms, thereby creating interfaces separating motile and immotile cells. Post-exposure, the boundary re-shapes and erodes due to self-emergent collective flows. We demonstrate that the active-passive boundary acts as a diffuse interface with mechanical properties set by the flow. Intriguingly, interfacial velocity couples to local swarm speed and interface curvature, suggesting that an active analogue to classic Gibbs-Thomson-Stefan conditions controls boundary propagation. Our results generalize interface theories to mixing and segregation in active systems with collective flows.

biophysics↗