How different aspects of plasma treated liquids (PTLs) influence their antimicrobial properties

This work studies the decontamination efficacy of different plasma treated liquids (PTLs) on bacteria from the genera Staphylococcus and Pseudomonas, both commonly associated with various infections. Clinical isolates and reference strains were used, to ensure the relevance to real-life applications. Bacterial suspensions were exposed to studied PTLs and to different comparative solutions that help dissect the mechanisms behind the observed antimicrobial effects. These comparative solutions comprised standard solutions of major reactive species (hydrogen peroxide, nitrites, nitrates) typically found in PTLs, solutions simulating the chemical composition of PTLs, and solutions adjusted to different pH levels to isolate the role of acidity in bacterial inactivation. The antimicrobial effects of studied solutions were examined at various contact times from 10 min up to 24 h. This allowed for a comprehensive understanding of both immediate antimicrobial effects and the persistence of PTL´s activity over time. The findings of this research demonstrate a superior antimicrobial efficacy of plasma treated liquids compared to the other studied solutions. Neither the individual standard solutions of reactive species, the solutions simulating the chemical composition of PTLs, nor pH-adjusted solutions were able to match the antimicrobial efficacy of the tested PTLs. Although it has been found that for some bacterial species, pH of the PTL may play a key role in the decontamination efficacy. The study also shows that different PTLs vary in their antimicrobial efficacy, depending on the specific formulation and the type of targeted microbial species. These differences in bacterial response may be influenced by factors such as cell wall structure, antioxidant capacity, and pH tolerance. In conclusion, this work supports the potential of indirect cold plasma treatment (via PTLs) for antimicrobial purposes. It highlights the complex interplay of factors involved in microbial inactivation and offers deeper insight into the differing responses of gram-negative and gram‑positive bacterial species to various PTLs. Furthermore, the study provides an overview of the antimicrobial effects of individual components present in PTLs across a wide range of concentrations and pH conditions. This may help other researchers compare the efficacy of different antimicrobial agents and explore potential mechanisms of inhibition.