Characterization of Antimicrobial Effects of Plasma-Treated Water (PTW) Produced by Microwave-Induced Plasma (MidiPLexc) on Pseudomonas fluorescens Biofilms

Abstract

For the decontamination of surfaces in the food production industry, plasma-generated compounds such as plasma-treated water or plasma-processed air offer many promising possibilities for future applications. Therefore, the antimicrobial effect of water treated with microwave-induced plasma (MidiPLexc) on Pseudomonas fluorescens biofilms was investigated. A total of 10 mL deionized water was treated with the MidiPLexc plasma source for 100, 300 and 900 s (pretreatment time) and the bacterial biofilms were exposed to the plasma-treated water for 1, 3 and 5 min (post-treatment time). To investigate the influence of plasma-treated water on P. fluorescens biofilms, microbiological assays (colony-forming units, fluorescence and XTT assay) and imaging techniques (fluorescence microscopy, confocal laser scanning microscopy, and atomic force microscopy) were used. The colony-forming units showed a maximum reduction of 6 log10 by using 300 s pretreated plasma water for 5 min. Additionally, a maximum reduction of 81% for the viability of the cells and a 92% reduction in the metabolic activity of the cells were achieved by using 900 s pretreated plasma water for 5 min. The microscopic images showed evident microbial inactivation within the biofilm even at the shortest pretreatment (100 s) and post-treatment (1 min) times. Moreover, reduction of the biofilm thickness and increased cluster formation within the biofilm was detected. Morphologically, the fusion of cell walls into a uniform dense cell mass was detectable. The findings correlated with a decrease in the pH value of the plasma-treated water, which forms the basis for the chemically active components of plasma-treated water and its antimicrobial effects. These results provide valuable insights into the mechanisms of inactivation of biofilms by plasma-generated compounds such as plasma-treated water and thus allow for further parameter adjustment for applications in food industry.

Plasma-treated water affects Listeria monocytogenes vitality and biofilm formation

“…It demonstrated a reduction of 6 log 10 steps compared to a control bio lm with a CFU of 10 log 10 . Mechanistically, a clear removal of the bio lm material by the PTW treatment could be proven as well as cell death within that work (41). In the present study, a reduction of about 5 log 10 steps compared to a control bio lm of 8 log 10 was detectable.…”

“…It is well established that EPS has a protective effect for the bio lm during plasma treatment with plasma gas or direct contact with the plasma e uent. However, the results of this work in combination with our previous work showed a rather antagonistic effect (41). This could be due to the high water binding capacity of the bio lm (85)(86)(87).…”

“…It has been shown that multispecies bio lms of L. monocytogenes showed a higher resistance against plasma gas than monospecies bio lms. In our work, we demonstrated that L. monocytogenes bio lms have a much higher resistance against PGCs than other bacteria like P. uorescens (41).…”

“…Nevertheless, the detailed mechanisms of the PGC effects remain unclear, but recent studies revealed a coherence of the antimicrobial potential with the concentration of reactive oxygen (ROS) and nitrogen (RNS) species (39,41).…”

“…In contrast to the MiniMIP, this plasma source can be operated with compressed air instead of argon gas, which leads to lower operation costs. In addition, it is possible to treat different amounts of liquids with the microwave-induced plasma gas, because of its integrated bottle adapter (39) (41). The MidiPLexc was operated with compressed air as working gas and a forward power of 80 W and a reverse power of 20 W. After 30 min running time to ensure a stable gas ow and e uent, the generation of plasma-treated water (PTW) was started.…”