Radioation is almost at the bottom of the list for causing mutation in bacteria that are not in a artificial setting. It appear that the system (SOS system) that repairs the damage cause by UV plays a role a in something called "adaptive" mutation. It is also referred to stress-induce mutation. It appears that bacteria are subject to various stress such as the lack of proper nutrient are capable of activating the SOS system and therefore increase the mutation rate.
Radiation would probably have a bad effect for the most bacteria as most mutation are deleterious and bacteria at some point would be come resistance and the radiation would have not significant effect.