A minimal mechanism for bacterial pattern formation

Colonies of Escherichia coli or Salmonella typhimurium form geometrically c omplex patterns when exposed to, or feeding on, intermediates of the tricar boxylic acid (TCA) cycle. In response to the TCA cycle intermediate: the ba cteria secrete aspartate, a potent chemo-attractant. As a result, the cells form high-density aggregates arranged in striking regular patterns. The si mplest are temporary spots formed in a liquid medium by both E. coli and S. typhimurium. In semi-solid medium S. typhimurium forms concentric rings ar ising from a low-density bacterial lawn, which are either continuous or spo tted, whereas E. coli forms complex patterns arising from a dense swarm rin g, including interdigitated spots (also called sunflower spirals), radial s pots, radial stripes and chevrons. We present a mathematical model that cap tures all three of the pattern-forming processes experimentally observed in both E. coli and S. typhimurium, using a minimum of assumptions.