Mechanism Of Microorganism Inactivation By Atmospheric Plasma

Mechanism of Microorganism Inactivation by Atmospheric PlasmaTo date, most researches on the interaction of nonequilibrium discharge plasmas with microorganisms have concentrated on the germicidal effects and validated on the contributing active radicals using different work gases, energized powers and discharge setups and produced a variety of conclusions. Little attention is given to the biochemical mechanism when microbial cells are exposed to plasmas. This biology and biochemistry-based study will compliment inactivation studies based on plasma chemistry characterization.Firstly, baker's yeast (Saccharomyces cerevisiae) is chosen as a microbial model to study biochemical effects triggered by their exposure to helium dielectric barrier discharge (DBD) at atmospheric pressure. The possible inactivation mechanism of plasma on yeast is given based on four kinds of biochemical impacts, including the mitochondrial activity, protein concentration in the supernatant, morphological changes of cells and acidity of baker's yeast suspension and its culture. The remarkable rupture of the treated yeast cells is investigated using scan electron microscopy. An increase of protein concentration in the suspension, measured by the Bradford method, will then be used to confirm the cellular rupture of this yeast exposed in plasma. The mitochondrial activity, evaluated by the 3-(4, 5-dimethylthiazol-2-yl) 2, 5-diphenyl-tetrazolium bromide (MTT) method, decreases sharply while cells are exposed in helium DBD plasma for 1.0-2.5 min. The pH of yeast suspension and its culture decrease with the plasma exposure time extension. This change can keep more than 2 h after plasma treatment. The severe loss of cellular structural integrity is considered to be due to the great and durative acidity changes on yeast culture.Secondly, the survival of Escherichia coli K12 (E. coli K12) deposited on a membrane filter surface and subsequently exposed to an atmospheric helium plasma jet depends on the cell surface density. As this increases from 10⁷ to 10¹¹ cells/cm², the maximum inactivation rate constant modelled by Baranyi inactivation model decreases from 19.59 to 1.03/min. Cells from late exponential growth phase is more sensitive than those from mid-exponential phase to the effects of the plasma. Optic emission spectroscopy confirms that there is no heat effect while this plasma jet acting on cells. SEMs reveal that the integrity of E. coli K12 cells is damaged. It can be confirmed that the cell concentration of E. coli K12 is a very important factor on plasma inactivation efficiency. The results modelled by Baranyi model consist with experimental results well.