The modes of toxic actions caused by nano-sized particles have been analyzed by using ten different bioluminescent bacteria responsive to different toxic modes of actions. In this study, for nano-sized both silver and gold particles, the modes of toxic actions have been characterized and compared by using ten different recombinant bioluminescent bacteria in which each is known to be specifically responsive to a certain type of stresses such as DNA damage, oxidative damage or membrane damage. The results show that one of the strong toxic actions caused by nano-sized silver particles is oxidative damages, mainly caused by the formation of superoxide radicals while gold nanoparticles caused very weak membrane and heat shock damaging stresses to E. coli. The results have been evidenced by bioluminescent bacteria responsive only to superoxide radical- specific and confirmed by using enzyme-based experimental design with real time PCR technique. Superoxide radicals generated by nano-sized silver nanoparticles were not found to be completely removed by using superoxide dismutase (SOD) enzyme in vitro, which indicated that silver nanoparticle generated superoxide radicals, a source of oxidative damage to E. coli, both in vivo and in vitro, and that superoxide radical in vitro could be harmful to E. coli.