Destruction of bacteria using an atmospheric-pressure dielectric capillary electrode discharge plasma

Summary form only given. The interaction of plasmas with chemical and biological agents, in particular in the context of sterilization and decontamination has received much attention in recent years. Particular emphasis has been on the utilization of atmospheric-pressure plasmas as they do not require operation in costly vacuum enclosures and thus facilitate the convenient and low-cost treatment of large surface areas. However, atmospheric-pressure discharge plasmas are highly susceptible to instabilities and the generation and reliable maintenance of uniform, large-volume discharge plasmas at or near atmospheric pressure remain formidable challenges. A new concept to generate and maintain atmospheric-pressure plasmas over a wide range of operating conditions was developed at Stevens Institute of Technology and subsequently licensed to PlasmaSol for development in various areas of applications. The atmospheric-pressure plasma is produced using a patented capillary dielectric electrode discharge concept that employs dielectric capillaries that cover one or both electrodes of the discharge reactor. The capillaries serve as plasma sources, which produce jets of high-intensity plasma at atmospheric pressure in a variety of carrier gases under the right operating conditions. Spore-forming bacteria, in particular bacteria of the genera Bacillus, among the most resistant microorganisms. The species Bacillus subtilis has received particular attention, as these bacteria are easy to grow in a reproducible fashion under chemically well-defined conditions. As a result, Bacillus subtilis has been the species of choice in many sterilization experiments in the past. In this paper, we report the first experiments aimed at the quantitative determination of the destruction of spore-forming bacteria, which are believed to be among the most resistant micro-organisms, using a novel atmospheric-pressure plasma shower reactor whose design utilizes a patented atmospheric-pressure dielectric capillary electrode discharge plasma. We established a straightforward protocol to prepare and characterize various bacteria including Bacillus subtilis on either glass or aluminum surface supports and analyze the samples after treatment by atmospheric-pressure plasma jets emanating from the plasma reactor using either in He or air (N/sub 2//O/sub 2/ mixture) as a carrier gas at varying power levels and exposure times. We used several Bacillus subtilis strains such as Bacillus subtilis var. niger ATCC 9372 in its three different colonial morphologies, Bacillus subtilis var. niger W 0235, and Bacillus subtilis W 0228 as prototypical examples of spore-forming bacteria. In some cases, we also used non-spore-forming bacteria (Pseudomonas fluorescens ATCC 1474) for selected experiments for reasons of comparison. We found significant reductions in colony-forming units ranging from 10/sup 4/ (He plasma) to 10/sup 8/ (air plasma) for plasma exposure times of less than 10 minutes. We also measured the UV/VIS absorption spectrum of the spore suspension before and after several minutes of plasma treatment. The UV absorption spectrum of a suspension of Bacillus subtilis showed a marked increase in the absorption of the plasma-treated sample below 300 nm with a local maximum around 260 nm. This is attributed to the presence of extracellular compounds that are released during the plasma treatment, most likely DNA, RNA, and proteins and thus verifies the destruction of the cell by the plasma. The utilization of our plasma device in other sterilization and decontamination applications is currently also being studied.