We study the collective dynamics of bacterial cultures under various environmental conditions. Initially, we found that when bacteria are subjected to a temperature gradient, they first move and accumulate near their natural temperature due to thermotaxis. The maximum of the bacterial density profile then drifts to lower temperature with a velocity proportional to the initial concentration of bacteria. However, above a critical concentration of, a new mode develops from the initial accumulation in the form of a sharp pulse moving at a much faster velocity as depicted in the movie to the right. This mode we found to be a result of a positive feedback mechanism provided by interbacterial communication through secretion of an attractant. For more details see Salman et al 2006.

 

 

In addition, since the motility of bacteria is affected by the oxygen concentration in their surroundings, we found that oxygen penetrating into an anaerobic sample induces the coexistence of two domains of motile and nonmotile bacteria. This coexistence generates a bacterial accumulation at the border that propagates slowly with a constant velocity (see the image and movie to the right). For more details see Douarche et al 2009.