Bio-decontamination of biofilms on surfaces by cold plasma

In this PhD thesis, biomedical applications of low-temperature plasmas at atmospheric pressure are investigated. In particular, bio-decontamination of planktonic bacteria and bacterial biofilms on both flat and complex surfaces by direct current (DC) air corona discharges and by pulsed discharges in argon is considered. We first provide a theoretical overview of biofilms, plasmas, and the current state of knowledge on bio-decontamination by low-temperature plasma, as well as methods presently used for biofilm decontamination in the plasma medicine domain. We characterize three plasma sources for the decontamination of Escherichia coli bacteria and biofilm. DC corona discharges in air - positive streamer corona and negative Trichel pulses were used for decontamination of planktonic bacteria and bacterial biofilms. In some experiments water was electro-sprayed onto samples from a high voltage (HV) electrode. Stable reactive species were identified in the gas phase by Fourier transform infrared spectroscopy (FTIR) and the concentration of reactive oxygen and nitrogen species was measured in the electro-sprayed water. Bio-decontamination of bacterial biofilms was carried out on glass cover slides. Within 15 min of the plasma treatment, most of the bacteria were rendered uncultivable. Some of the uncultivable bacteria remained viable, only bacteria in the top layers of the biofilm were killed, as demonstrated by a confocal laser scanning microscopy (CLSM) of biofilms stained by live/dead viability kit. Water electro-spray through the corona discharges significantly improves the biofilm inactivation and disintegrates the polymeric protective matrix of the biofilm. The second plasma source investigated was the pulsed corona discharge propagated inside long narrow quartz tubes, in which dry argon or argon with water vapor were flowing at atmospheric pressure. This type of discharge has a potential application in decontamination of inner surfaces of catheters or other long tubular devices, and could be able to deliver low-temperature plasma over longer distances inside the human body. This pulsed corona discharge was characterized by its electrical parameters, such as the discharge propagation velocity and the mean electric field in the plasma channel. Optical emission spectroscopy of such plasma identified UV B emission form excited hydroxyl radicals, especially with humid argon working gas. The effect of this UV B was tested on planktonic bacteria sensitive to UV damage (recA-), and was found to cause a substantial damage even to bacteria placed far downstream in the tube or on the outer surface of the tube. The third plasma source tested was an argon jet based on cylindrical dielectric barrier discharge with dry, humid or water saturated argon as a working gases. This discharge was used for biofilm decontamination, where we obtained similar results as with DC corona discharges - strong inactivation of biofilm bacteria determined from the cultivability tests and biofilm thickness shrinkage shown by CLSM. We used this discharge to test the effect of vacuum UV on bacterial survival, but the effect was neither confirmed nor disproved. (author)