Abstract
Swarming colonies of the light sensitive bacteria Serratia marcescens grown on agar exhibit robust, fluctuating, collective flows that include vortices, jets and sinuous streamers spanning multiple bacterial lengths. Here, we study the effects of light, with a substantial ultra-violet component, on these collective flows. We expose regions of the swarm to light of different intensities and examine the accompanying changes in collective motility during exposure as well as immediately after cessation of exposure. For small exposure times and at low intensities, we find that collective mobility is negligibly affected. Increasing exposure times or intensity to higher values temporarily suppresses collective mobility. When the light is turned off, bacteria regain motility at the single cell level and eventually reestablish large scale flows. Thus with sub-optimal levels of light exposure, bacteria maintain a high chance of regaining collective motility. For long exposure times or high intensities, exposed bacteria are paralyzed and slow down, in the process forming jammed domains. The rate of formation of this jammed region and its initial dissolution rate upon ceasing exposure both strongly depend on duration of exposure. We hypothesize that this results from bacteria forming aligned domains as they slow down; erosion then involves dislodging bacteria from caged configurations. Our results complement studies on the effects of light on free-swimming bacteria and provide a foundation for studies on light-driven flow patterning in bacterial swarms.
Footnotes
↵‡ aepattes{at}syr.edu, agopinath{at}ucmerced.edu