Trail following in bacteria*

Bacteria form coordinated structures out of millions of cells while individuals only sense and interact with their local environment, thus they are an excellent model system for studying active matter physics. One tool that bacteria use is stigmergy, the same mechanism ants use to form pheromone trails. When individual cells move into unexplored regions on a substrate they modify their environment, signaling other bacteria to follow in their paths. This trail following behavior plays an important role in the life cycle of several different types of bacteria including colony expansion in P. aeruginosa, and the formation of streams in M. xanthus. We study the motion of individual M. xanthus cells as they create and follow along trails. Through the use of a 3D confocal profilometer, we map out the height of a surface as cells move across it and find that single cells are capable of forming physical furrows in the substrate which other cells then move along. The patterns that arise from stigmergy are widespread in biology and much can be learned from this microscopic view of individuals creating and interacting with measurable trails.