Particle-Cell Contact Enhances Antibacterial Activity of Silver Nanoparticles

Background: It 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 Findings: Herein, 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, similar to 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/Significance: Our 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.