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Nonthermal plasma generated by parallel-plate dielectric-barrier discharge with 60-kHz high-voltage power was used to sterilize the bacteria in atmospheric air. Two kinds of typical bacteria, gram-negative E. coli (ATCC8099) and grampositive S. aureus (ATCC6538), were used as test strains. Bacteria cells held by cover-glass were placed on the bottom electrode. By adjusting the applied voltage, gap spacing, and treatment time, the effects of plasma and electric field on bacteria inactivation were investigated. The transmission electron microscope was used to observe the damage of ceils treated by plasma. The concentrations of [K.sup.+], protein, and nucleic acid leaked from cells were measured for detecting the cytoplasm status after plasma treatment. Experimental results showed that almost 100% of S. aureus and E. coli strains were killed in less than 10- and 7-s plasma treatment, respectively. It is concluded that the reactive oxygen species (ROS) in plasma play a dominant role in the inactivation process but not the electric field. It is supposed that the ROS can oxidize the cell membrane and then damage the protein and nucleic acid inside the cells and, thus, kill the bacteria. Index Terms--Bacterial inactivation, dielectric-barrier discharge (DBD), E. coli, reactive oxygen species (ROS), S. aureus.

A dielectric barrier discharge was used to generate low-temperature plasma to treat Candida albicans. When the gap spacing was 3 nun, the killing log value (KLV) of the plasma on Candida albicans within 20 s of exposure was more than five, and for 4 and 5 nun gap spacing, the KLV within 25 s was more than five. With the extension of exposure time, the decrease in velocity of the number of living Candida albicans was fastest for the gap spacing of 3 mm, and then for 4 and 5 mm spacing. With 60 s of electric field treatment, as the applied voltage increases, the survival number of Candida albicans has no significant difference from that of the control sample. As for the inactivation mechanism of plasma on Candida albicans on the molecular microbiology side, transmission electron microscopic examination and the results of protein, nucleic acid and [K.sup.+] detection in the extracellular environment showed that the plasma destroyed the outer structure of Candida albicans. Cytoplasm was also released, which caused Candida albicans to be dead. On the plasma physics side, the role of the electric field during Candida albicans inactivation by plasma is considered negligible. Therefore, charged particles and reactive species in plasma might play a dominant role in the process of destroying the outer structure of Candida albicans. Index Terms--Candida albicans, dielectric barrier discharge (DBD), low-temperature plasma.

Dielectric barrier corona discharge (DBCD) is a robust plasma reactor with high efficiency and large scalable area. Nonthermal plasma is generated by DBCD in open condition under atmospheric pressure. Products of plasma are analyzed by using spectroscopy technology. The DBCD plasma is used to treat three kinds of typical microorganisms such as Escherichia coli, Staphylococcus aureus, and Candida albicans. Then, the inactivation effects of the plasma are discussed and compared with our formerly published results of parallel-plate dielectric barrier discharge (DBD) plasma. The results show that the DBCD plasma damages microbe cells by reactive species (RS) and lower pH values, while the parallel-plate DBD plasma works mainly through microdischarge channel and RS. In addition, different cell wall configurations result in different treatment effects. [ABSTRACT FROM PUBLISHER]