Patterns that Grow
  Chemotactic bacteria have been observed to congregate into highly regular patterns. When the bacteria are placed in the center of a dish, a wave of bacteria can travel outward, leaving a regular pattern of spots or stripes in its wake. Although chemotaxis and excretion of an attractant can readily cause a pattern forming instability from a uniform state, they are not capable of generating patterns starting from a single spot. These patterns are apparently formed with the help of bacterial growth and depletion of nutrients in the growth medium. 
William J. Bruno