Plasma-deposited nanocomposite polymer-silver coating against Escherichia coli and Staphylococcus aureus: Antibacterial properties and ageing

Abstract

A plasma-deposited coating, containing silver nanoparticles embedded in an organosilicon matrix, was synthesized, using AISI 316L stainless steel as the underlying substrate. The coating antibacterial property was evaluated on the Gram-negative Escherichia coli K12 MG1655 and the Gram-positive Staphylococcus aureus MW2 strains, by combining indirect (plate counting) and in situ (fluorescent bacteria labelling) methods. Both approaches were shown to be highly complementary and converged on a maximal antibacterial efficacy against E. coli, as plate counts showed a decrease of 6 and 1 Log and dead bacteria represented 25% and 2% of the total adhering bacteria for E. coli and S. aureus, respectively. The coating antibacterial potential was then determined over time on E. coli biofilm. Whatever the biofilm age, growth inhibition was observed due to silver-mediated bacteriostatic effect. The coating bactericidal activity was initially strong. However, differences between coated and bare stainless steel surfaces tended to collapse above a 2-day contact time. A thorough characterization of the film properties after ageing in biological suspension or saline solution (short and longer-term exposures) revealed an oxidation of both the organosilicon matrix and the silver nanoparticles, accompanied by silver release at the extreme surface. However, a silver reservoir was still present and potentially active in the deep layers of the coating.