Selectively Antibacterial Silver-Gold Alloy Nanoparticles Conjugated with Target Specific Aptamer Sequences

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Implant- associated infections occur in many different medical disciplines and are one of the most important challenges in modern medicine. These infections are caused by bacteria organised in complex biofilms which are highly resistant to antibiotics and to the host's immune system. This can often lead to loss of the implant and - in some disciplines - even to life-threatening complications. Antibacterial implant surfaces and active substances have been used for prophylaxis or therapy. However, these have the disadvantage that bacterial cells cannot be selectively and effectively treated without causing long-term damage to human cells too. Consequently, there is great need for innovative anti-infective strategies with a broadened therapeutic window.In this project, targeting silver-gold alloy nanoparticles (AgAu-NP) with specific activity against selected pathogenic bacteria will be developed. This involves combining the properties of antibacterial but also cytotoxic Ag-NP and highly biocompatible Au-NP. Here silver is responsible for the bactericidal activity by Ag+ ion release, whereas gold is meant to increase particle stability, to control ion release and to allow conjugation with target-specific aptamers via gold-thiol chemistry. To this end stable AgAu-NPs are synthesised by using an advanced laser-based method, which allows synthesis of these alloy nanoparticles with controlled composition, while the resulting NPs are conjugated ex situ with thiolated aptamer sequences. In this context the influence of particle composition (AgAu) and particle size on metal ion release, surface coverage and conjugation efficiency during bioconjugation as well as the formation of soluble and insoluble silver species in different media will be systematically examined. Due to specific binding of these AgAu-NP-conjugates to the bacterial membrane of relevant bacteria like S. aureus and P. gingivalis the pathogens are exposed to a high local Ag+ concentrations found in the vicinity of the particles which selectively inhibits the growth of this bacterium, even in the presence of other strains. In order to deduce the ideal NP composition and to achieve maximal antibacterial activity combined with minimal toxicity, a variety of systematic molecular biological assays and cell culture tests will be performed. This includes co-culture models of different bacteria strains as well as of relevant bacteria parallel to human cells. Selective binding experiments will explore the possibility of a specific association of the AgAu-NP-conjugates to the bacteria, while, furthermore, cellular uptake is examined in order to elucidate the mechanism of bactericidal activity. The planned experiments provide a highly innovative procedure for the treatment of bacterial infections and will elucidate basic scientific knowledge on how bioconjugation is affected by NP composition.