Particle-Cell Contact Enhances Antibacterial Activity of Silver Nanoparticles

Abstract

BackgroundIt is generally accepted that antibacterial properties of Ag nanoparticles (AgNPs) are dictated by their dissolved fraction. However, dissolution-based concept alone does not fully explain the toxic potency of nanoparticulate silver compared to silver ions.Methodology/Principal FindingsHerein, we demonstrated that the direct contact between bacterial cell and AgNPs' surface enhanced the toxicity of nanosilver. More specifically, cell-NP contact increased the cellular uptake of particle-associated Ag ions – the single and ultimate cause of toxicity. To prove that, we evaluated the toxicity of three different AgNPs (uncoated, PVP-coated and protein-coated) to six bacterial strains: Gram-negative Escherichia coli, Pseudomonas fluorescens, P. putida and P. aeruginosa and Gram-positive Bacillus subtilis and Staphylococcus aureus. While the toxicity of AgNO3 to these bacteria varied only slightly (the 4-h EC50 ranged from 0.3 to 1.2 mg Ag/l), the 4-h EC50 values of protein-coated AgNPs for various bacterial strains differed remarkably, from 0.35 to 46 mg Ag/l. By systematically comparing the intracellular and extracellular free Ag+ liberated from AgNPs, we demonstrated that not only extracellular dissolution in the bacterial test environment but also additional dissolution taking place at the particle-cell interface played an essential role in antibacterial action of AgNPs. The role of the NP-cell contact in dictating the antibacterial activity of Ag-NPs was additionally proven by the following observations: (i) separation of bacterial cells from AgNPs by particle-impermeable membrane (cut-off 20 kDa, ∼4 nm) significantly reduced the toxicity of AgNPs and (ii) P. aeruginosa cells which tended to attach onto AgNPs, exhibited the highest sensitivity to all forms of nanoparticulate Ag.Conclusions/SignificanceOur findings provide new insights into the mode of antibacterial action of nanosilver and explain some discrepancies in this field, showing that “Ag-ion” and “particle-specific” mechanisms are not controversial but, rather, are two faces of the same coin.

Heteroaggregation of nanoparticles with biocolloids and geocolloids

“…Heteroaggregation of NPs with microbial cells such as bacteria and algae is important for fate, transport, transformation, and toxicity of NPs in both aquatic and terrestrial environments [106][107][108][109][110][111][112][113]. Heteroaggregation could occur between a single particle and a cell, or via particle aggregate-cell interactions [114] -with important implications in the natural environment.…”

“…For instance, NPs-microorganism heteroaggregation can lead to increased removal of NPs from wastewater during secondary treatment [106,[115][116][117][118]. Interactions between cell walls of microorganisms and NPs are also thought to precede uptake and toxicity [107,108,[110][111][112]116,119], and may be important for trophic transfer of NPs in both aquatic and terrestrial environments [108,[119][120][121].…”

“…Bondarenko et al [110] showed that Pseudomonas aeruginosa had a higher affinity for surface interactions with Ag NPs than five other bacteria (from same or different genera), demonstrating that heteroaggregation may vary between different microbial cells. Similarly, a stronger NP-cell association was observed between nC 60 and Gram negative E. coli (155-342 ng-C 60 /g-dry cell mass) than with Gram positive Bacillus subtilis (21-41 ng-C 60 /g-dry cell mass) [123].…”

Lignosulfonic Acid Based Hydrogel Nanocomposites for Antimicrobial Application

“…The size of the nanoparticle implies that it has a large surface area to come in to contact with the bacterial cells; therefore, it will have higher percentage of interaction when compared to bigger particles (Agnihotri et al, 2014;Bondarenko et al, 2013). The formation of Ag-NCHs in the copolymeric hydrogel networks can be expected in our current strategy because the silver salts loaded hydrogels are readily reduced by AAm-LSA hydrogels, which immediately turn into dark brown colour.…”

“…One of the advantages of the hydrogels is fabrication in their capability of undergoing first-order phase transition under the change of some external parameters such pH (Melendez-Ortiz et al, 2015), ionic strength (Paula et al, 2002), and temperature (Lorenzo et al, 2002). There are several possible mechanisms leading to in situ gel formation: solvent exchange, UV-irradiation, ionic cross linkage, pH change, and temperature modulation (Chaudhary et al, 2014). Generally, polymer nanocomposites are the result of the combination of polymers and inorganic/ organic fillers at the nanometer scale.…”