Silver and Copper Acute Effects on Membrane Proteins and Impact on Photosynthetic and Respiratory Complexes in Bacteria

Silver (Ag(+)) and copper (Cu(+)) ions have been used for centuries in industry, as well as antimicrobial agents in agriculture and health care. Nowadays, Ag(+) is also widely used in the field of nanotechnology. Yet, the underlying mechanisms driving toxicity of Ag(+) ions in vivo are poorly characterized. It is well known that exposure to excess metal impairs the photosynthetic apparatus of plants and algae. Here, we show that the light-harvesting complex II (LH2) is the primary target of Ag(+) and Cu(+) exposure in the purple bacterium Rubrivivax gelatinosus. Ag(+) and Cu(+) specifically inactivate the 800-nm absorbing bacteriochlorophyll a (B800), while Ni(2+) or Cd(2+) treatment had no effect. This was further supported by analyses of CuSO(4)- or AgNO(3)-treated membrane proteins. Indeed, this treatment induced changes in the LH2 absorption spectrum related to the disruption of the interaction of B800 molecules with the LH2 protein. This caused the release of B800 molecules and subsequently impacted the spectral properties of the carotenoids within the 850-nm absorbing LH2. Moreover, previous studies have suggested that Ag(+) can affect the respiratory chain in mitochondria and bacteria. Our data demonstrated that exposure to Ag(+), both in vivo and in vitro, caused a decrease of cytochrome c oxidase and succinate dehydrogenase activities. Ag(+) inhibition of these respiratory complexes was also observed in Escherichia coli, but not in Bacillus subtilis.