Effect of silver content on the structure and antibacterial activity of silver-doped phosphate-based glasses

Abstract

Staphylococcus aureus can cause a range of diseases such as osteomyelitis as well as colonize implanted medical devices. In most instances the organism forms biofilms that are not only resistant to the body’s defence mechanisms but also display decreased susceptibility to antibiotics. In the present study, we have examined the effect of increasing silver content in phosphate-based glasses to prevent the formation of S. aureus biofilms. Silver was found to be an effective bactericidal agent against S. aureus biofilms and the rate of silver ion release (0.42-1.22 µg.mm-2. h-1) from phosphate-based glass was found to account for the variation in its bactericidal effect. Analysis of biofilms by confocal microscopy indicated that they consisted of an upper layer of viable bacteria together with a layer (~20µm) of non-viable cells on the glass surface. Our results showed that regardless of the silver contents in these glasses (10, 15 or 20 mol%) the silver exists in its +1 oxidation state which is known to be a highly effective bactericidal agent compared to other oxidation states (+2 or +3). Analysis of the glasses by 31P NMR and HEXRD showed that it is the structural rearrangement of the phosphate network that is responsible for the variation in silver ion release and associated bactericidal effectiveness. Thus an understanding of the glass structure is important in interpreting the in vitro data and also has important clinical implications for the potential use of the phosphate-based glasses in orthopaedic applications to deliver silver ions to combat S. aureus biofilm infections.