Here we report on the triggering of antibacterial activity by a new type of silver nanoparticle coated with porous silica, Ag@silica, irradiated at their surface plasmon resonant frequency. The nanoparticles are able to bind readily to the surface of bacterial cells, although this does not affect bacterial growth since the silica shell largely attenuates the intrinsic toxicity of silver. However, upon simultaneous exposure to light corresponding to the absorption band Of the nanoparticles, bacterial death is enhanced selectively on the irradiated zone. Because of the low power density used, for the treatments, we discard thermal effects as the cause of cell killing. 1, Instead, We propose that the increase in toxicity is due to the enhanced electromagnetic field. in the proximity of the nanoparticles, which indirectly, most likely through induced photochemical reactions, is able to cause cell death.