UV causes breaks in bacterial DNA, which in turn cause the bacterial SOS response, which stops many cellular activities and up-regulates DNA repair enzymes. If the UV damage is minimal the cells can recover when they enter more friendly environments, but if the damage is significant they will never reproduce again.
Ultraviolet germicidal irradiation (UVGI) is a sterilization method that uses ultraviolet (UV) light at sufficiently short wavelength to break down microorganisms. It is used in a variety of applications, such as food, air and water purification. UV has been a known mutagen at the cellular level for more than one-hundred years.