3.11. Action of Metal NPs against bacteria
The external side of the cell wall of both Gram-positive and Gram-negative bacteria bears negative charges due to the presence of functional groups like carboxyl and phosphate and hydroxyl (Ashmore et al., 2018). Gram-positive bacteria possess a thick peptidoglycan layer, which resides in linear chains alternating residues of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked together by a sequence of 3 to 5 amino acids that cross-link each other, giving rise to a cohesive mesh. Additionally, negatively charged teichoic acids (with high levels of phosphate groups) are spread from the cell wall to the surface of most Gram-positive bacteria (Scheme S4.a) . In contrast, Gram-negative bacteria display more complex structure with a thinner layer of peptidoglycan and a phospholipid outer membrane with partially phosphorylated lipopolysaccharides (LPS) that contribute to raise the negative surface charge (Scheme S4.b) (Stensberg et al., 2011).
Negatively charged bacterial cell walls interact with positively charged particles such as metal cations via electrostatic interactions. Cations may act through diverse pathways among which two seem to prevail in aqueous media: (i) strong electrostatic interaction that alter bacteria membrane equilibrium, and (ii ) Lewis acid-base interaction with water molecules that generates Bronsted acidity (Mn+ + xH2O = [M(H2O)(x-1)OH](n-1)++ H+) that may alter bacteria membrane. The metal ions are then free to interact with cellular structures (e.g.,proteins, membranes, DNA), disrupting cell functions (Ashmore et al., 2018). In contrast, MNPs are supposed to interact via strong LAB interaction with atoms bearing available electron pairs (O, S, and N) that act as Lewis base. Such interactions are assumed to affect the normal cell exchange through bacteria membrane. Other mechanisms such MNP diffusion inside the cell and disrupt biological processes (Stensberg et al., 2011). Inside the cell, both metal cation and nanoparticles can generate reactive oxygen species (ROS) like hydrogen peroxide (H2O2, superoxide anion (· O2 ), and hydroxyl radicalOH (Gordon et al., 2010; Yun’an Qing et al., 2018). These species are assumed to bind to phosphate groups inhibiting protein phosphorylation frequently involved in enzymatic activation. This is expected to inhibit bacterial growth and cell cycle through the dephosphorylation of some important proteins for enzymatic activities (Dakal, Kumar, Majumdar, & Yadav, 2016). Given that metals bind to biomolecules through non-specific interactions, MNPs generally exhibit a wide variety of processes against bacteria (Scheme S4.c) (Yuan, Ding, Yang, & Xu, 2018).
Once inside the cell, both AgNPs and Ag+ ions interact with diverse species resulting in cell dysfunction. Reportedly, AgNPs can act through four main mechanism pathways: (i) attraction on bacterial surface; (ii) destabilization of the bacterial cell wall and increase in membrane permeability even for larger AgNPs (Losasso et al., 2014); (iii) genesis of ROS and free radicals that induce toxicity and oxidative stress; (iv ) modification of signal transduction pathways (Dakal et al., 2016). AgNP adsorption on the bacterial surface can be followed by diffusion of smaller particle inside the cell and retention of larger ones on the external side of the bacteria membrane. In spite of their antibacterial activity (Ávalos, Haza, Mateo, & Morales, 2013), AgNPs were found to be less performant that cations (El Badawy et al., 2011). The latter are much more attracted by the negative charges of bacterial walls (Slavin, Asnis, Häfeli, & Bach, 2017). However, AgNPs may also act through partial dissolution into Ag+ cations, as reported by many works. The Ag+ cations act differently by binding to the cell membrane inducing changes in the membrane potential and proton leakage (Losasso et al., 2014). Reportedly, Ag+ cation may intercalate DNA segments generating complexes with nucleotides and disrupting H-bonds between base pairs (Yun’an Qing et al., 2018). Similar observations were made for CuNPs where a Cu2+ release was found to be the main contribution to the high antibacterial activity (Chatterjee, Chakraborty, & Basu, 2014; Sistemática, Gabriela, Daniela, & Helia, 2016). As for silver, CuNP action may also involve diverse mechanisms, the most reported by the literature being: (i ) CuNP concentration and dissolution in the bacterial membrane inducing potential and permeability changes, with unavoidable leak in lipopolysaccharides, membrane proteins, intracellular biomolecules and protons (Amro et al., 2000; Azam, Ahmed, Oves, Khan, & Memic, 2012). (ii) production of ROS, MNPs oxidation and dissolution into Cu2+ cation, with other detrimental oxidative processes (Fenton, Harber-Weiss) processes (Applerot et al., 2012; Fang, Lyon, Wiesner, Dong, & Alvarez, 2007); (iii) Accumulation of Cu2+ cation with decay in intracellular ATP production and disruption of DNA replication (Kim et al., 2007; Sondi & Salopek-Sondi, 2004). These pathways should confer higher activity to CuNPs against both bacteria compared with AgNPs (Chudobová & Kizek, 2015).