Influence of Salmonella enterica’s lipopolysaccharide chain length and the surface polarity of tetrapodal zinc oxide on its antimicrobial behaviour compared to spheric zinc oxide nanopowder

Monocrystalline, tetrapod-shaped zinc oxide (t-ZnO) represents a promising material for biomedical applications, providing selective antimicrobial properties against bacteria and an easily functionalizable surface. However, the mechanisms behind its toxicity are not fully understood. Here, t-ZnO was compared to commercially available s-ZnO nanopowder using established antibiotic testing principles. Salmonella enterica mutants served as model organism to assess a possible influence of LPS chain configuration on ZnO toxicity towards bacteria. While bacterial killing was dependent on particle concentration, no statistically relevant correlation was found between the LPS chain length and antibacterial activity for either morphology of ZnO. A second series of experiments revealed setup weaknesses, but also provided evidence for completely different toxicity mechanism. Scanning electron microscopy was performed on the t-ZnO and revealed that using a phosphate-buffered solution led to formation of biologically inactive zinc phosphate. Differences between the two materials lie in their crystal structure and particle geometry. For s-ZnO, the release of Zn2+ into the medium and ROS formation were likely causing the toxic effects in our study. At given concentrations, t-ZnO provided much smaller surface area and a higher degree of atomic order, which potentially limited Zn2+ release and increased the formation of ROS on the particle surface. Additionally, piercing of the bacterial cell wall as a mechanical effect was discussed. An electrostatic interaction between bacteria and tetrapods remains a hypothesis for future experiments. These results provide evidence that differences in the crystal structure lead to distinct biomechanical properties, offering an alternative to common antibiotics in the future. T-ZnO may replace s-ZnO in certain applications. However, more fundamental research is needed to fully understand t-ZnO-mediated toxicity.

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