Quenching active swarms: effects of light exposure on collective motility in swarming Serratia marcescens

Abstract

Swarming colonies of the light-responsive bacteria Serratia marcescens grown on agar exhibit robust fluctuating large-scale flows that include arrayed vortices, jets and sinuous streamers. We study the immobilization and quenching of these collective flows when the moving swarm is exposed to intense wide-spectrum light with a substantial ultraviolet component. We map the emergent response of the swarm to light in terms of two parameters—light intensity and duration of exposure—and identify the conditions under which collective motility is impacted. For small exposure times and/or low intensities, we find collective motility to be negligibly affected. Increasing exposure times and/or intensity to higher values suppresses collective motility but only temporarily. Terminating exposure allows bacteria to recover and eventually reestablish collective flows similar to that seen in unexposed swarms. For long exposure times or at high intensities, exposed bacteria become paralysed and form aligned, jammed regions where macroscopic speeds reduce to zero. The effective size of the quenched region increases with time and saturates to approximately the extent of the illuminated region. Post-exposure, active bacteria dislodge immotile bacteria; initial dissolution rates are strongly dependent on duration of exposure. 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, inform models of patterning in gradated illumination and provide a starting point for the study of specific wavelengths on swarming bacteria.

Swarming bacterial fronts: Dynamics and morphology of active swarm interfaces propagating through passive frictional domains

“…In collectively-moving swarms, individual self-propelling cells are influenced by steric and hydrodynamic interactions with their neighbors 7,9,10,19,24 . These interactions result in complex structural and flow features including fluctuating regions of high vorticity and streamers as seen in Figure 1(a) (see also 24,25 ).The intensity and transient of these emergent flows is clearly evidenced upon seeding the swarm with small 2 µm colloidal spheres; these particles are strongly advected by the bacteria and follow tortuous trajectories as shown in Figure 1(b). The mean square displacement obtained from long time measurements provides an effective diffusivity that characterizes the activity of the suspension 24 .…”

“…Structural information such as size of coherently arrayed structures and length scales over which structural features are correlated may thus be obtained. Using these plugins we found that these domains exhibited a distribution of sizes, typically between 10-20 µm with continuously changing orientational fields due to emergent flows 25 . Figure 1(d) shows the result of such an image based analysis of the structural features demonstrating the presence of many intertwined aligned domains.…”

“…A prototypical example of these fronts are the interfaces that separate motile active domains from passive highly frictional regions comprised of dead or non-motile bacteria. In recent work 24,25 , we used the response to wide-spectrum light to create internal active-passive interfaces within dense active swarms of Serratia marcescens. The overall effect of light was found to depend on both exposure time and intensity; high intensities coupled with long exposure times caused a permanent loss of motility 25 .…”

“…In recent work 24,25 , we used the response to wide-spectrum light to create internal active-passive interfaces within dense active swarms of Serratia marcescens. The overall effect of light was found to depend on both exposure time and intensity; high intensities coupled with long exposure times caused a permanent loss of motility 25 . Combined with suitable screens, this provided us a controllable method to generate macroscopic domains with nearly circular or straight interfaces.…”

“…Combining experiments with simulations, we conclude that hydrodynamic and steric interactions enable different modes of surface dynamics, morphology and thus front invasion.ing depends strongly on the nutrient composition of the liquid in the substrate as well as viscosity of the ambient medium. Unlike biofilms, however, swarms are highly motile and feature long-range collective motions 7,9,[17][18][19][20][21][22][23][24][25] that involve structures with clear length scales and persistence times. From a biological standpoint, these emergent flow characteristics seem to confer benefits relevant to colonization and occupation of preferred niches 6,7,26,27 .…”