Gas discharge plasma inactivation of microorganisms at low (close to ambient) temperature is a promising area of investigation that is attracting widespread interest. This paper describes atmospheric-pressure, nonthermal plasma (NTP) methods for cold sterilization of liquids and thermal sensitive surfaces. These methods are based on the use of direct current (DC) gas discharge plasma sources fed with steady-state high voltage. Parameters characterizing the plasma sources used (plasma-forming gas, gas flow rate, electric power consumed, etc.) are given. The results for plasma sterilization of different microorganisms (vegetative cells, spores, fungi, biofilms) are presented. An empirical mathematical approach is developed for describing NTP inactivation of microorganisms. This approach takes into account not only the destruction of different components of the cells, but their reparation as well.
Non-thermal plasma jet formed by self-running pulsed-periodical high-current spark generator (PPSG) was used for atmospheric pressure inactivation of microorganisms including biofi lms. A distinctive feature of the PPSG is a formation of transient hot plasma clouds (plasma bullets) periodically fl ying out to the target. We experimented with model biofi lms of E. coli and Bacillus subtilis monocultures which were grown on agar and surfaces of steel and polypropylene coupons. High effi ciency of plasma inactivation was demonstrated. This effect is associated primarily with an interaction of transient hot plasma clouds with biofi lms. Besides complete or partial degradation of the cell membrane, weakening of the cell wall of E.coli culture by active plasma was found.
The effect of cold plasma on E. coli cells was studied. It was shown that the treatment of E. coli cells with cold plasma caused partial or total disruption of the plasma membrane integrity, which was accompanied by a release ofintracellular substances into the extracellular environment. A quantitative assessment of the extent of the damage to the cell membrane showed that a loss of no more than 23.6% of intracellular substances (calculated by the proportion of the intracellular nucleotide release) is sufficient to lead to cell death. The use of media with different ionic strength levels to create osmotic shock showed that the treatment of E. coli cells with cold plasma significantly decreased the cell wall strength.
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