Analysis of factors that contribute to and interfere with bactericidal properties of low-temperature atmospheric pressure plasmas

Bacterial resistance to antibiotics continues to be a significant concern globally. In the search for more effective antibacterial treatments, low temperature plasmas (LTPs) have arisen as an attractive alternative to traditional therapies. LTPs generate a cocktail of reactive nitrogen and oxygen species (RNOS), ultraviolet (UV) photons, electrons and electromagnetic fields. They therefore offer the potential to facilitate the localised delivery of bactericidal agents without contact to the treatment site. However, whether the distribution of RNOS in the gas phase, the presence of organic matter during treatment and the heterogeneity of bacterial populations affect the treatment outcome was unknown. Single-cell analysis was undertaken throughout to investigate the heterogeneous response of the model system Salmonella enterica to treatment with atmospheric-pressure plasma jets. In conjunction with the electrical, optical, and chemical studies, this enabled key mechanisms that drive plasma-bacteria interactions to be explored. My research has demonstrated three main points: (a) The level of DNA damage induced in single cells is determined by the spatial distribution of RNOS in the plasma effluent. This was found to be a characteristic of LTPs generated in open air. (b) The contribution of UV radiation solely to bacterial elimination and induction of DNA damage is minimal. Therefore, the bactericidal action of LTPs can be ascribed to the RNOS generated in the plasma, although the role of charged species and electric fields cannot be ruled out. (c) Preferential redox reactions between plasma-generated RNOS and external biomolecules in the environment decrease the efficacy of the treatment. This study evidences the importance of the aforementioned conditions for the development of successful antimicrobial plasma therapies. It highlights the usefulness of single cell analysis to assess heterogeneous responses in a bacterial population in response to LTPs treatments. These results were made possible only due to interdisciplinary quantitative approaches used in this project.