  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.
