Elemental silver nanoparticles are an effective antibacterial substance and are found as additive in various medical applications. Gold nanoparticles are used due to their optical properties in microscopy and cancer therapy. These advantages might be combined within alloyed nanoparticles of both elements and thereby open new fields of interest in research and medical treatment. In this context, laser ablation of solid alloys in liquid gives access to colloidal silver-gold alloy nanoparticles with a homogeneous ultrastructure. Elemental and alloy silver-gold nanoparticles with increasing molar fractions of silver (50, 80, and 100 %) were produced and stabilized with citrate or albumin (BSA). Particles were embedded in agar at concentrations of 3-100 μg cm −3 and tested on clinical relevant Staphylococcus aureus regarding their antibacterial properties. Cytotoxic effects were measured within the same particle concentration range using human gingival fibroblasts (HGFib). As expected, a reduced fraction of silver in the nanoalloys decreased the antibacterial effect on S. aureus according to the evaluated minimal inhibitory concentrations. However, this decrease turned out stronger than expected by its relative mass per particle, due to the electrochemical, disproportionally high effect of gold on the bioresponse to silver within silver-gold nanoalloy particles.BSA was able to stabilize all colloids and maintain antibacterial activity, whereas sodium citrate reduced antibacterial effects and cytotoxicity even at high nanoparticle concentrations. The alloying of silver with gold by laser ablation in liquid produced nanoparticles with both reduced antibacterial and cytotoxic properties in comparison to silver nanoparticles but still retains the application spectrum of both elements combined in one colloid. In particular, alloying with gold may render silver nanoparticles more biocompatible, and allows bioconjugation via established thiol chemistry.

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The inhibition of bacterial growth through effective non‐toxic antimicrobial substances is of great importance for the prevention and therapy of implant infections in various medical disciplines. For the evaluation of a therapeutic window of silver nanoparticles (AgNPs), their bactericidal properties were tested in agar composites and colloids on four medical relevant bacteria. Therefore, we produced AgNPs using high‐power nanosecond laser ablation in water showing a log‐normal particle diameter distribution centered at 17 nm. Bacteria were incubated with AgNP concentrations ranging from 5 to 70 µg · mL−1 and the growth rate was recorded. Additionally, cytotoxic effects of AgNPs on human gingival fibroblasts were examined. The experiments demonstrated that laser‐synthesized AgNPs resulted in a significant bacterial growth inhibition of more than 80% at the indicated concentrations in a solid agar model (Pseudomonas aeruginosa 10 µg · mL−1, Streptococcus salivarius 10 µg · mL−1, Escherichia coli 20 µg · mL−1, Staphylococcus aureus 70 µg · mL−1). In a planktonic bacteria model, the growth of the tested bacteria was significantly delayed by the addition of AgNPs at a concentration of 35 µg · mL−1. The cytotoxic assays showed limited adverse effects on human fibroblasts at concentrations of less than 20 µg · mL−1. The present study illustrates the strong antibacterial effects of ligand‐free, laser‐generated AgNPs that exhibit moderate cytotoxic effects, resulting in a therapeutically applicable concentration of AgNPs for medical purposes between 10 and 20 µg · mL−1.

IntroductionThe purpose of the present study was to investigate and compare early biofilm formation on biomaterials, which are being used in contemporary fixed orthodontic treatment.MethodsThis study comprised 10 healthy volunteers (5 females and 5 males) with a mean age of 27.3 +–3.7 years. Three slabs of different orthodontic materials (stainless steel, gold and ceramic) were placed in randomized order on a splint in the mandibular molar region. Splints were inserted intraorally for 48 h. Then the slabs were removed from the splints and the biofilms were stained with a two color fluorescence assay for bacterial viability (LIVE/DEAD BacLight–Bacterial Viability Kit 7012, Invitrogen, Mount Waverley, Australia). The quantitative biofilm formation was analyzed by using confocal laser scanning microscopy (CLSM).ResultsThe biofilm coverage was 32.7 ± 37.7% on stainless steel surfaces, 59.5 ± 40.0% on gold surfaces and 56.8 ± 43.6% on ceramic surfaces. Statistical analysis showed significant differences in biofilm coverage between the tested materials (p=0.033). The Wilcoxon test demonstrated significantly lower biofilm coverage on steel compared to gold (p=0.011).Biofilm height on stainless steel surfaces was 4.0 ± 7.3 μm, on gold surfaces 6.0 ± 6.6 μm and on ceramic 6.5 ± 6.0 μm. The Friedman test revealed no significant differences between the tested materials (p=0.150). Pairwise comparison demonstrated significant differences between stainless steel and gold (p=0.047).ConclusionOur results indicate that initial biofilm formation seemed to be less on stainless steel surfaces compared with other traditional materials in a short-term observation. Future studies should examine whether there is a difference in long-term biofilm accumulation between stainless steel, gold and ceramic brackets.

Implant-associated infections still pose serious problems in modern medicine. The development of fabrication processes to generate functional surfaces, which inhibit bacterial attachment, is of major importance. Sharklet™-like as well as grooves and grid micro-structures having similar dimensions were fabricated on the common implant material titanium by ultra-short pulsed laser ablation. Investigations on the biofilm formation of Staphylococcus aureus for up to 24 h revealed similarly reduced bacterial surface coverage on all micro-structures investigated compared to smooth titanium surfaces. This study is a prove-of-principle and could serve as basis for further investigations towards a structure-based biofilminhibiting implant.