title: Particle-cell contact enhances antibacterial activity of silver nanoparticles.
abstract: 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.
  
  
  RESULTS
  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, ∼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
  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.
authors.affiliation: Laboratory of Environmental Toxicology, National Institute of Chemical Physics and Biophysics, Tallinn, Estonia. olesja.bondarenko@kbfi.ee
authors.forename: Olesja
authors.initials: O
authors.lastname: Bondarenko
authors.forename: Angela
authors.initials: A
authors.lastname: Ivask
authors.forename: Aleksandr
authors.initials: A
authors.lastname: Käkinen
authors.forename: Imbi
authors.initials: I
authors.lastname: Kurvet
authors.forename: Anne
authors.initials: A
authors.lastname: Kahru
mesh_terms: D000900:Anti-Bacterial Agents / Q000737:chemistry / Q000494:pharmacology; D001682:Biological Availability; D002448:Cell Adhesion; D011232:Chemical Precipitation; D004305:Dose-Response Relationship, Drug; D006090:Gram-Negative Bacteria / Q000166:cytology* / Q000187:drug effects*; D006094:Gram-Positive Bacteria / Q000166:cytology* / Q000187:drug effects*; D042541:Intracellular Space / Q000187:drug effects; D053768:Metal Nanoparticles; D012834:Silver / Q000737:chemistry* / Q000494:pharmacology*; D018030:Silver Compounds / Q000737:chemistry / Q000494:pharmacology; D012835:Silver Nitrate / Q000737:chemistry / Q000494:pharmacology; D013045:Species Specificity
chemical_list: D000900:Anti-Bacterial Agents; D018030:Silver Compounds; D012834:Silver; C013892:collargol; D012835:Silver Nitrate