**2. Silver nanoparticles for pathogen control**

Silver nanoparticles (AgNPs) offer oligodynamic action which is also of low toxicity and broad spectrum [17–19]. Moreover, compared with synthetic biocides, there is also only a low chance that microbial resistance might develop. These AgNPs have been exploited against Gram-negative bacteria, such as *Acinetobacter, Escherichia, Pseudomonas, Salmonella* and *Vibrio,* and against Gram-positive bacteria including *Bacillus, Clostridium, Enterococcus, Listeria, Staphylococcus* and *Streptococcus* [20]. A number of research reports have demonstrated that their antimicrobial nature depends on the surface-capping agent and the size and shape of the nanoparticle [21, 22].

The effectiveness of AgNPs also depends on bacterial dose [23]. Silver nanoparticles affect the growth of bacteria in a dose-dependent manner. In a study conducted by Agnihotri et al. [23], concentrations of 10 and 20 μg/ml Ag (10 nm) caused reductions of ~18 and ~53% in *E. coli*, respectively. Meanwhile, AgNP concentrations at 30 and 40 μg/ml eliminated all bacterial growth.

Silver nanoparticles smaller than 100 nm, and containing between 10,000 and 15,000 silver atoms, are effective as antibacterial agents [20]. The AgNPs' antibacterial potential increases as size decreases. This effect is more pronounced for AgNPs of size <10 nm, because contact with the bacterial cell is direct [24].

Research into the antimicrobial activity of AgNPs against Gram-positive and Gram-negative bacteria shows Gram-negative bacteria are more sensitive to AgNPs than Gram-positive ones [23, 25], although their relative sensitivity cannot be explained based only on a difference in the composition of the cell membrane.

In studies using discs impregnated with AgNP in culture media with bacteria, the formation of a clear zone of inhibition around the impregnated discs is an indicator of bactericidal potential of AgNP > 15 nm [21]. Bacteria are unable to survive in this area, possibly because of the release of silver in the form of nanoparticles or of silver ions.

In addition, nanoparticle silver can be released by the mobility of small size AgNPs through the semisolid agar, whereby a zone of inhibition is observed.

**205**

[27] (**Figure 1**).

*Silver Nanoparticles Offer Effective Control of Pathogenic Bacteria in a Wide Range of Food…*

the chitosan-silver colloid had a broad spectrum of antimicrobial activity.

**3. Some mechanisms of bactericidal action of silver nanoparticles** 

A way to transport active silver cations to the bacteria can occur on the cell membrane or within the cell. When combined with protonated chitosan, the positively charged AgNPs bind well to the negatively charged bacterial membrane

The first bacterial contact with AgNP can trigger an antibacterial mechanism by facilitating the entry of AgNPs into the bacterial cells. This is followed by an explosive release of silver ions inside the bacterial cells causing the bactericidal effect. The nature of the AgNP, bacteria interaction and its antibacterial effect have been analyzed by a number of methods. Bacteria exposed to AgNPs show high protein leakage and morphological changes [26]. As an example, *E. coli* treated with AgNPs (∼10 nm) appeared to shrink and develop an irregular shape. Micrographs show AgNPs on the cell membrane attached to the lipopolysaccharide layer of the cell wall, and a proportion of AgNPs were found inside the bacterial cell [23].

Biao et al. [21] noticed that bacterial strains have intact membranes and smooth surfaces in the absence of silver colloid, whereas after exposure to chitosan-silver colloid, the cell membrane and surface become shriveled, invaginated and disrupted. This cell membrane damage indicates the mode of action of chitosan-silver colloid. Its bactericidal effect is attributed to the release of silver cation from AgNPs and to alteration of the bacterial cell wall structure and associated physicochemical changes. Osmoregulation of the bacterial cell can also be affected causing extrusion of intracellular material and hence cell death. The deformed or wrinkled cell wall can

In addition, AgNPs can penetrate bacterial membranes, facilitating internalization. The rupture of perforation of the cell wall is an evidence of internalization of AgNP and of uncontrolled transport through the cytoplasm resulting in cell death

Multiple pathways of AgNP can occur after internalization. Silver atoms in nanoparticles are characterized by a high affinity with sulfur and phosphoruscontaining compounds such as DNA. In this way, they readily combine with cell

Silver ions can also inhibit bacterial replication by binding and denaturing bacterial DNA. Silver ions react with the thiol groups of enzymes, followed by DNA

Blocking of respiration is also a result of the interaction with cell membranes [30].

In a previous study conducted by Biao et al. [21], chitosan was combined with silver nanoparticles to form composites. They found that chitosan-silver colloid has a high inhibition ratio against the prokaryotes *E. coli* and *Staphylococcus aureus* (*S. aureus*) and the eukaryote *Candida albicans* (*C. albicans*). They concluded that

*DOI: http://dx.doi.org/10.5772/intechopen.89403*

**(AgNPs)**

**3.1 Electrostatic attraction**

proteins through electrostatic attraction [23].

**3.2 Alterations in the bacterial membrane**

also cause leakage of cytoplasmic contents.

constituents and so destroy the cell.

condensation resulting in cell death [28–29].

**3.3 Silver nanoparticles internalization: effects on DNA**

*Silver Nanoparticles Offer Effective Control of Pathogenic Bacteria in a Wide Range of Food… DOI: http://dx.doi.org/10.5772/intechopen.89403*

In a previous study conducted by Biao et al. [21], chitosan was combined with silver nanoparticles to form composites. They found that chitosan-silver colloid has a high inhibition ratio against the prokaryotes *E. coli* and *Staphylococcus aureus* (*S. aureus*) and the eukaryote *Candida albicans* (*C. albicans*). They concluded that the chitosan-silver colloid had a broad spectrum of antimicrobial activity.
