  We report a novel morphological transition in a Bacillus subtilis colony
initially growing under ambient conditions, after ultraviolet radiation
exposure. The bacteria in the central regions of the colonies are observed to
migrate towards the colony edge forming a ring during uniform spatial exposure.
When the radiation is switched off, the colonies were observed to grow both
inward into the evacuated regions as well as outward indicating that the
pattern is not formed due to depletion of nutrients at the center of the
colony. We also propose a reaction-diffusion model in which waste-limited
chemotaxis initiated by the UV radiation leads to the observed phenomenology.
