| Target site |
Mechanisms of antimicrobial action |
| Reaction of Silver nanoparticles with peptidoglycan cell wall |
Silver ions being highly reactive, interact, and bind to the negatively charged bacterial cell wall, alter its permeability, and cause cell damage. Gram-negative bacteria constitute a thin layer of peptidoglycan which is covered by an outer cell envelope of lipoprotein, phospholipid, and lipopolysaccharide. The role of the outer lipid layer is to allow selective permeability to certain products. The bacterial membrane is known to contain many sulfur-containing proteins. Since silver has a higher propensity to react with sulfur and phosphorus compounds, biologically active silver ions bind to the outer membrane creating changes in membrane morphology and leading to an increase in membrane permeability. Some studies demonstrate mutual electrostatic attraction between the negatively charged bacterial cell and positively charged silver ions [8]. In gram-positive bacteria, the thick cellular wall may decrease the permeation of silver nanoparticles. The bactericidal effect of silver nanoparticles is dependent on their size and dose. Silver nanoparticles with sizes 1-10 nm show maximum interaction with the bacterial surface [6]. |
| Action on Plasma membrane |
Apart from the ability to release silver ions, nanoparticles themselves can lead to cell lysis. Nanoparticles of smaller than 10 nm size are highly reactive owing to the greater surface area of interaction with bacterial cells. [6] These nanosized particles can permeate through the cell membrane, causing structural changes within the bacterial casing, and inhibiting the uptake of phosphate compound, which is one of the essential components for several biological processes within the bacteria such as energy metabolism, membrane integrity, inheritance of genetic materials, and intracellular signaling [9]. Ionic silver interacts with thiol group compounds of respiratory enzymes present within the bacterial cell, thereby affecting cellular respiration and producing reactive oxygen species [6]. Transmission electron microscopy (TEM) revealed that E. Coli bacteria treated with silver showed enlargement of periplasmic space suggestive of shrinkage of the inner membrane and its separation from the cell wall. However, Staphylococcus aureus, gram-positive bacteria, underwent comparable morphological changes as E. coli, howbeit to a smaller extent, having a thicker cell envelope suggesting greater resistance to silver ions [10,11]. |
| Action on cytoplasmic DNA |
Ag+ ions are strong nucleic acid binders having an affinity to interact and form complexes with bases present in DNA molecules. The transmission electron microscopic image showed a normal, random distribution of electron-light region assigned as normal DNA. However, following treatment with Ag+ ion, an electron light region appeared in the center of the E. coli showing a tightly condensed substance twisted together. [10] Such damage to DNA blocks its replication ability. When DNA molecules are relaxed and distributed in all parts of the cell, cellular replication can effectively take place. But when DNA is twisted in the center of the cell, it is no longer able to continue replication; hence, leading to cell death. [12] |
