Soft Matter Composites Interfacing with Biomolecules, Cells, and Tissues - Bioinspired Approaches for Human-Centric Technologies - page 64

respect, it has been demonstrated that some particular strains of E. coli and
Salmonella spp. already possess a peculiar operon, named sil , encoding for different
proteins responsible for silver resistance (Gupta et al. 1998 , 1999 , 2001 ). In
particular, the sil gene cluster codifies for periplasmic silver-binding proteins and
molecular efflux pumps, which work in cooperation for expelling Ag + ions from the
cytoplasm (or even the periplasmic space) to the extracellular space. Finally, the
uncontrolled environmental release of silver (in the form of bulk, Ag + , and AgNPs)
is increasing the chance of exposure to humans (with unpredictable toxicity con-
sequences), as well as it represents a serious risk from an ecological viewpoint, a
topic that will be discussed with more details in the following paragraph.

2.10.1 Implications for the Environment and Human Risk

Exposure

The environmental release of silver, in all its forms (i.e., ions, nanoparticles, and
clusters), is constantly rising, and it is actually quantified to be c.a. 20 tons per year
(Gottschalk et al. 2009 ). Hence, several research efforts aimed to understand the
potential ecological consequences of silver release, in terms of investigating the
toxic effects to the different organisms populating specific ecosystems. Also in this
case, there is a significant data disagreement, since several works labeled nanosilver
as a potential polluting agent, while other data considered it as negligible and not
dangerous (Hansen and Baun 2012 ; Grieger et al. 2012 ; Blaser et al. 2008 ; Musee
et al. 2011 ; Nowack et al. 2012 ). For instance, the release of silver in the soil has
been proved to induce a dramatic decrease in the reproduction potential of the
nematode Caenorhabditis elegans as a consequence of increased oxidative stress
(Roh et al. 2009 ). Other environmental model organisms, such as the green alga
Chlamydomonas reinhardtii or Danio rerio , displayed toxicity effects upon AgNPs
treatments (Navarro et al. 2008 ; Asharani et al. 2008 ), suggesting that nanosilver is
a potential pollutant. However, it should be highlighted that the ecotoxicology
assays are usually performed by means of model experiments (i.e., in laboratory),
which are not similar to real conditions. Here, in fact, the physicochemical charac-
teristics of silver are quite unpredictable, in terms of particles size, shape, and
agglomeration state. Hence, understanding the real effects of nanosilver on a
specific fauna could represent, most probably, an extremely difficult challenge,
due to the high and complex variables characterizing the system.
The rise in environmental presence of AgNPs is also increasing the possibility of
human risk exposure. For this reason, many studies focused on exploring the
potential adverse effects of nanosilver on eukaryotes (nanotoxicity assessment)
(Christensen et al. 2010 ; Ahamed et al. 2010 ). Inhalation of vapors, aerosols, or
particulates and oral or skin adsorption are the major routes of entry of silver
compounds into the body. Upon inhalation, AgNPs may deposit in the respiratory
tract, causing damage through direct contact with tissues. Then, they can reach the