The inhibitory effect of silver is probably the sum of distinct mechanisms of action. A number of studies suggest that silver ions react with SH groups of proteins20,21 and play an essential role in bacterial inactivation.7 Micromolar levels of silver ions have been reported to uncouple respiratory electron transport from oxidative phosphorylation, which inhibits respiratory chain enzymes or interferes with membrane permeability to protons and phosphate.21 Studies conducted by Feng et al.21 and by Jung et al.22 have shown the activity of silver ions on Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive), respectively. Feng et al.21 treated these bacteria with AgNO3 and studied the effects on cell morphology using combined electron microscopy (TEM and SEM) and X-ray microanalyses. E. coli and S. aureus underwent similar morphological changes after silver ion treatment characterized by a cytoplasm membrane detachment from cell walls and the appearance of an electron-light region in the center of the cells, which contained condensed deoxyribonucleic acid (DNA) molecules probably formed to protect DNA from injuries mediated by the silver ions. Small electron-dense granules either surrounding the cell wall or deposited inside the cells were also present.21 Recently, Jung et al.22 reported results corroborating the morphological changes described by Feng et al.21 and also suggested that in the presence of silver ions, bacterial cells reach an active but non-culturable state and eventually die. Jung et al.22 also suggested that the thickness of the peptidoglycan layer of gram-positive bacteria may prevent to some extent, the action of the silver ions, since they found a higher inhibitory activity of silver ion solution against E. coli than against S. aureus which also corroborates the conclusion of Feng et al.21
The presence of silver ions and sulfur in the electron-dense granules observed after silver ions treatment in the cytoplasm of bacterial cells suggests an interaction with nucleic acids that probably results in impairment of DNA replication.21