gms | German Medical Science

Objectives: Silver nanoparticles (Ag-NP) represent an important nanomedicine-based approach in the fight against multiresistant bacteria. Silver compounds have already been used in medical applications, including wound dressings, surgical instruments, and as additives to bone substitute biomaterials, e.g., silver-containing calcium phosphate cements. It is generally understood that Ag-NP under oxidative conditions release silver ions (Ag⁺) and up to know free Ag⁺ are believed to act as the reactive silver species. Recently, it was shown that Ag⁺ released from Ag-NP into chloride-containing media (such as biological fluids) rapidly precipitates as poorly soluble silver chloride nanoparticles [1]. Therefore, in this study stable silver chloride nanoparticles (AgCl-NP) were prepared and compared with metallic Ag-NP at same silver content. The antibacterial activity towards Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) and the cell compatibility towards mammalian cells, i.e. human mesenchymal stem cells (hMSC) and human peripheral blood mononuclear cells (PBMC), were evaluated.

Methods: Poly(N-vinylpyrrolidone) (PVP)-coated AgCl-NP and metallic Ag-NP were synthesized as described in the literature [1], [2]. The minimal inhibitory concentration (MIC) for prokaryotic cells (S. aureus and E. coli) was determined in liquid culture medium, and the minimal bactericidal concentration (MBC) was determined by plating the pre-incubated bacteria. The viability of the incubated leukocytes (PMBC) was quantified by flow cytometry using 7-AAD. The viability and the morphology of the incubated hMSC were analyzed by calcein-AM staining.

Results and Conclusion: AgCl-NP have a higher toxicity towards eukaryotic cells and bacteria compared to metallic Ag-NP at a similar total silver dose. Antibacterial effects were observed for AgCl-NP in the concentration range of 2.0-5.0 µg mL^-1, and for Ag-NP in the concentration range of 12.5-50 µg mL^-1. AgCl-NP and metallic Ag-NP were easily taken up by eukaryotic cells. Cytotoxic effects towards hMSC and PBMC were observed for AgCl-NP in the concentration range of 2.5-5.0 µg mL^-1, and for Ag-NP in the concentration range of 25-50 µg mL^-1. We found that these biological effects on eukaryotic and prokaryotic cells were also dependent on protein content and cell number.

Our results demonstrate a crucial role of in-situ generated silver chloride particles for antibacterial activities which suggests a future application of AgCl-NP in the development of new anti-infective biomaterials. Currently, the mechanism of the cytotoxicity of dispersed AgCl-NP is further analysed to elucidate their potential application as novel antibacterial coating.