Chemomodulation of cellular movement, collective formation of vortices by swarming bacteria, and colonial development

Bacterial colonies have developed sophisticated modes of cooperative behavior which enable them to respond to adverse growth conditions. It has been shown that such behavior can be manifested in formation of complex colonial patterns. Certain Bacillus species exhibit collective migration, ''turbulent like'' flow and emergence of whirlpools during colonial development. Here we present experimental observations of collective behavior and a generic model to explain such behavior. The model incorporates self-propelled and interacting ''particles'' (swarmers). We show that velocity interaction between the particles can lead to a synchronized movement. To explain vortices formation, we propose a plausible mechanism involving a special chemotactic response (rotational chemotaxis) which is based on speed modulations according to the concentration of a chemoattractant. This mechanism differs from that exhibited by swimming bacteria. We show that the chemomodulation of swarmers' speed together with the velocity interactions impose a torque on the collective motion and can lead to formation of vortices. The inclusion of both attractive and repulsive rotational chemotaxis in the model captures the salient features of the observed growth patterns.