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- Abteilung für Mikrobiologie und Molekularbiologie (3) (remove)
The here presented dissertation investigated the molecular mechanisms, by which the food industry model bacteria Pseudomonas fluorescens and Listeria monocytogenes, grown either as planktonic cultures, were inhibited by plasma treated water (PTW) produced by a microwave-induced plasma source (MidiPLexc). As a starting point, optimal operating parameters were determined with 5 standard liters per minutes(slm)compressed air during the treatment of 10 ml deionized water within a treatment time of up to 15 min (pre-treatment time). Treatment times of 1, 3 and 5 min were selected (post-treatment time). In addition to physical parameters, i.e. temperature measurements at different spots at the plasma source during the production of the PTW, the chemical composition of PTW was determined by pH measurements, chronoamperometry (determination of the H2O2 concentration), ion chromatography (determination of the NO2-, NO3- and ONOO- concentrations) and mass spectrometry (qualitative determination of the molecules). In addition, concentration changes of reactive species over a period of 3 h indicated a decrease of the NO2- concentration as well as an increase of the NO3- and ONOO- concentration in the PTW. Microbiological assays, i.e. quantification of colony-forming units (CFU), fluorescence and XTT assays, revealed a significant reduction of the proliferation ability of the cells, membrane damages and metabolic activity have been demonstrated for planktonic cultures as well as mono- and multispecies biofilms. PTW effects on biofilm structures were investigated using microscopic methods such as fluorescence microscopy, confocal laser scanning microscopy (CLSM), atomic force microscopy (AFM), and scanning electron microscopy (SEM), as well as physical methods such as contact angle measurements. Significant changes in the biofilm structure have been shown, which indicate an ablation of the biofilm mass from top to bottom by approximately 2/3 of the biofilm mass and a destruction of the extracellular matrix (ECM) by the reactive species within the PTW. Subsequently, fresh-cut lettuce has been treated with PTW produced by up-scaled plasma sources. Apart from qualitative parameters of the lettuce after PTW treatment such as texture and color, the concentration of PTW reactive species have been determined. These experiments showed that the composition of the reactive species were slightly different from that of the laboratory-scaled plasma source MidiPLexc. Notably, the PTW treatment did not cause significant changes in texture and color of the fresh-cut lettuce. Finally, a synergistic effect of PTW treatment followed by plasma-processed air (PPA) drying was demonstrated application-specific.
Summary
The susceptibility of Candida albicans biofilms to a non‐thermal plasma treatment has been investigated in terms of growth, survival and cell viability by a series of in vitro experiments. For different time periods, the C. albicans strain SC5314 was treated with a microwave‐induced plasma torch (MiniMIP). The MiniMIP treatment had a strong effect (reduction factor (RF) = 2.97 after 50 s treatment) at a distance of 3 cm between the nozzle and the superior regions of the biofilms. In addition, a viability reduction of 77% after a 20 s plasma treatment and a metabolism reduction of 90% after a 40 s plasma treatment time were observed for C. albicans. After such a treatment, the biofilms revealed an altered morphology of their cells by atomic force microscopy (AFM). Additionally, fluorescence microscopy and confocal laser scanning microscopy (CLSM) analyses of plasma‐treated biofilms showed that an inactivation of cells mainly appeared on the bottom side of the biofilms. Thus, the plasma inactivation of the overgrown surface reveals a new possibility to combat biofilms.