## 5 Coating Materials with Embedded Antibiotics and Other Antibofilm Agents
### Silver Nanoparticles and Silver Ions
#### 5.6.4. Denaturation of Bacterial Proteins and DNA Molecules  
Besides their capacity to form stable bonds with thiol group-containing proteins, silver ions might also catalyze the formation of disulfide bridges between cysteine residues, thereby altering proteins conformation and deactivating important enzymes [101]. Using Fourier transform infrared spectroscopy (FTIR), [PERSON] et al. [102] showed that silver ions form two types of complexes with DNA by binding to guanine N7 and adenine N7 (type I complex) and to guanine-cytosine and adenine-thymine base pairs (type II complex) at a higher cation/polynucleotide ratio. The morphological changes that occurred in _E. coli_ and _S. aureus_ bacterial cells following exposure to silver ions have been directly observed by [PERSON] et al. [103] through TEM imaging and X-ray microanalysis. The TEM micrographs of pristine cells revealed a remarkable electron-light region in the center of the cells, very rich in phosphorus as shown by energy-dispersive X-ray spectroscopy, which comprises the bacterial DNA. After treatment with silver ions, the electron-light region collapsed to form an electron-light dot indicating that DNA condensation took place. Similar results were obtained by [PERSON] et al. [104]. The authors assumed that this behavior was a self-defense mechanism; bacteria are conglomerating their DNA to protect it against attack from silver nanoparticles. However, this impeded DNA replication since DNA replication can be effectively conducted only when DNA molecules are in a relaxed state and not in a condensed one [105]. Despite the excellent antimicrobial properties of silver nanoparticles [106], care must be taken in their use since silver nanoparticles appear to have toxic effects in higher organisms, including humans as well. The more complex structural and functional organization of eukaryotic cells can be affected by silver nanoparticles through various mechanisms such as induction of programmed cell death via activation of the p53-mediated signaling pathway [107]. To end this discussion of the antibacterial mechanisms of silver, meaning both zero-valent silver nanoparticles and silver ions, we feel that the most appropriate conclusion is that one drawn by [PERSON] et al. [108]. Based on previous comparative studies like the one of [PERSON] et al. [109], the opinion of these authors is that once one accepts the presumption that silver nanoparticles act as silver ions sources, the suggestion that silver ions and silver nanoparticles have similar action mode appears as a reasonable one. However, because of scarce knowledge of mechanisms underlying the antibacterial activity of zero-valent silver nanoparticles and of how the particular synthetic method impacts these mechanisms, the possibility of major differences between silver nanoparticles and silver ions in the molecular basis of action and bacterial resistance cannot be ruled out. The synthetic routes to prepare silver nanoparticles can be classified as: (i) Physical methods, (ii) chemical methods, and (iii) biological methods [110; 111]. The most commonly used chemical method is based on chemical reduction and involves a metal precursor, a reducing agent, and a stabilizing/capping agent that prevent nanosilver from aggregation in solutions or on surfaces. [PERSON] et al. [112] choose an environment-friendly green synthesis [113] to prepare alginate stabilized silver nanoparticles (SA-AgNPs). The synthesis was carried out in the water as an environmentally benign solvent, using silver nitrate as a metal precursor, sodium borohydride as a reducing agent, and the biopolymer sodium alginate as a capping agent. Briefly, a solution of sodium alginate in deionized water was prepared, and solutions of the metal precursor and of the reducing agent were successively added to it under magnetic stirring. The reaction mixture was eventually dialyzed against water to remove any unreacted AgNO\({}_{3}\), NaBH\({}_{4}\), and salts formed during synthesis. The SA-AgNPs were freeze-dried [112].