A mechanistic study of the antibacterial effect of silver ions on Escherichia coli and Staphylococcus aureus

To investigate the mechanism of inhibition of silver ions on microorganisms , two strains of bacteria, namely Gram-negative Escherichia coli (E. coli a nd Gram-positive Staphylococcus aureus (S. aureus), were treated with AgNO3 and studied using combined electron microscopy and X-ray microanalysis. Si milar morphological changes occurred in both E.coli and S. aureus cells aft er Ag+ treatment. The cytoplasm membrane detached from the cell wall. A rem arkable electron-light region appeared in the center of the cells, which co ntained condensed deoxyribonucleic acid (DNA) molecules. There are many sma ll electron-dense granules either surrounding the cell wall or depositing i nside the cells. The existence of elements of silver and sulfur in the elec tron-dense granules and cytoplasm detected by X-ray microanalysis suggested the antibacterial mechanism of silver: DNA lost its replication ability an d the protein became inactivated after Ag+ treatment. The slighter morpholo gical changes of S. aureus compared with E. coli recommended a defense syst em of S. aureus against the inhibitory effects of Ag+ ions. (C) 2000 John W iley & Sons, Inc.