# Review on Silver Nanoparticles as a Novel Class of Antibacterial Solutions
## 4 Silver Nanoparticles and Their Antibacterial Role
### Antibacterial Mechanisms Activated by Silver Nanoparticles  
Reactive oxygen species (ROS) is created as a result of oxidative stress, a toxic effect of silver nano particles and ions. Such ROS are: superoxide anions, hydrogen peroxide, hydroxyl radical, singlet oxygen, hypochlorous acid [45]. Authors [51] studied the effect of Ag nanoparticles on intracellular ROS. The authors have discovered that the production of ROS is dose and surface- dependent. The action of Ag nanoparticles on bacterial cells begins with the appearance of oxidative stress, then membrane damage and ending with cell death. Cell death occurs due to the interaction between the active oxygen species and DNA, respiratory enzymes and cell membrane (Figure 2). Potential mechanisms in nanosilver activity are (1) attachment to cell membrane, to protein component of the membrane and DNA structure, resulting in function alteration (blue interrupted line marks the mechanism); (2) silver ions release, impacting the membrane, nucleic acids( DNA) and protein content of the cell (red interrupted line marks the described mechanism); (3) oxidative stress mounting as a result of reactive oxygen species release with effects on cell membrane, nuclei acids (DNA) and protein content of the membrane (the black interrupted line marks the mechanism) [45]. Silver ions present in the medium can interact with the negatively charged bacterial cell and may promote the appearance of pores in the cell membrane. These pores result in a loss of proteins and nucleic acids, affect the basic metabolic functions and thus lead to cell death. The level of cell membrane disruption is correlated with the dosage of Ag  
Figure 2: Interaction between silver nanoparticles and cells. nanoparticles [51]. A different cause of cell death is the reaction between colloidal silver and sulfhydryl groups present on the cell wall generating disulfide bridges which blocks cell respiration. Silver ions and nanoparticles can generate too many free radicals which result in hyper oxidation of: proteins, lipids, DNA, cell membrane [45]. Silver nanoparticles antimicrobial action is exerted at different cell compartments and follows multiple mechanisms. (1) Interaction with the cell membrane, using physical interaction, lipid bilayer disturbance and/or cationic ion release. (2) Contact and reaction of cationic silver with bacterial DNA, resulting in structural changes. (3) Interaction with cytoplasmic structures, including protein synthesis ribosomal machinery, causing block of translation and transcription with reduced protein synthesis, (4) ROS formation, resulting in oxidative stress-based bactericidal action (see Figure 3) [52]. Gram positive and Gram-negative bacteria react differently to Ag nanoparticles. Gram positive bacteria (_Staphylococcus aureus, Lysteria monocytogenes, Bacillus cereus_) have a greater resistance to Ag nanoparticles than Gram negative bacteria (_E. coli, Salmonella Typhimurium, Pseudomonas aeruginosa) due to their ticker cell walls [30; 52]. The thicker cell envelope in the Gram-positive bacteria is due to a thick layer of peptidoglycan (see Figure 3). Peptidesolcyan is a polymer consisting of sugars and amino acids which is present around the plasma membrane acting as a cell wall. This polymer might prevent the silver ions from entering the vital cellular structure and cytoplasm. Its negative charge may also be responsible for trapping positively charged silver ions [43]. [PERSON] et al. studied the potential of silver nanoparticles as antibacterial agents upon several bacteria. According to the collected data by authors, some of the most important mechanisms underlying the antibacterial effect of nanoparticles are alteration of cell and cytoplasm, alteration of membrane permeability and respiration, inhibition of bacterial DNA replication, bacterial cytoplasm membranes damage, and irreversible damage on bacterial cells. Electron transport interference, with alteration of intracellular ATP levels has also been cited as being a significant mechanism of action [52].