  We report the spatio-temporal response of {\it Bacillus subtilis} growing on
a nutrient-rich layer of agar to ultra-violet (UV) radiation. Below a crossover
temperature, the bacteria are confined to regions that are shielded from UV
radiation. A forced convection of the population is effected by rotating a UV
radiation shield relative to the petri dish. The extinction speed at which the
bacterial colony lags behind the shield is found to be qualitatively similar to
the front velocity of the colony growing in the absence of the hostile
environment as predicted by the model of Dahmen, Nelson and Shnerb. A
quantitative comparison is not possible without considering the slow dynamics
and the time-dependent interaction of the population with the hostile
environment.
