Comparison of Direct and Indirect Effects of Non-Thermal Atmospheric Pressure Plasma on Bacteria and Mechanisms of Such Interaction

Summary form only given. Non-thermal plasmas are already well-known for their sterilization ability; however, the mechanisms of this sterilization are under debate. Short and long-living active species and radicals produced by plasma, ultraviolet (UV) radiation in VUV and UVC bands, local thermal effects of micro-discharges, and bombardment by charged particles are all listed as potential candidates for sterilization of various surfaces. Biochemical and physical mechanisms of plasma interaction with biological materials are proposed and discussed. Direct interaction, where a surface of a microorganism is used as one of the plasma-generating electrodes, is compared with indirect interaction, where plasma is generated elsewhere and the plasma-treated gas is carried off to a remote location for microorganism treatment. Under these treatment conditions, the authors show that: Direct treatment by plasma is orders of magnitude faster than indirect treatment by plasma products; bombardment of the surface of a microorganism by charged particles is the primary inactivation mechanism. Microorganisms selected for this study were those commonly present on skin: Streptococcus, Staphylococcus, and yeast. Bacteria were collected from human cadaver tissue, bacterial specie identification was performed at the Hahnemann hospital microbiology lab, and the samples were cultured on Ttypticasetrade soy agar with 5% sheep blood. Samples were treated by floating electrode dielectric barrier discharge plasma in either continuous-wave (sinusoidal, 4-20 kHz), microsecond pulse (5 mus pulse duration, 0.1-1 kHz repetition rate), or nanosecond pulse modes (3 kV/ns rise time, 10-40 ns pulse duration, 0.1-1 kHz repetition rate). Microorganisms were either treated directly by plasma or plasma afterglow and by-products were utilized. Separately considered are the effects of: UV (in VUV and UVC bands), short and long-lived active species and radicals, local heating (or "micro-thermalldquo) effects, local and global (applied) electric fields, and the effects of charged species. Mechanisms of these interactions are discussed.