Oxidative DNA damage caused by pulsed discharge with cavitation on the bactericidal function

Abstract

Plasma-based techniques are expected to have practical use for wastewater purification with a potential for killing contaminated microorganisms and degrading recalcitrant materials. In the present study, we analysed oxidative DNA damage in bacterial cells treated by the plasma to unveil its mechanisms in the bactericidal process. Escherichia coli cell suspension was exposed to the plasma induced by applying an alternating-current voltage of about 1 kV with bubbling formed by water-cavitation, termed pulsed discharge with cavitation. Chromosomal DNA damage, such as double strand break (DSB) and oxidative base lesions, increased proportionally with the applied energy, as determined by electrophoretic and mass spectrometric analyses. Among the base lesions identified, the yields of 8-hydroxyguanine (8-OH-G) and 5-hydroxycytosine (5-OH-C) in chromosomal DNA increased by up to 4- and 15-fold, respectively, compared to untreated samples. The progeny DNA sequences, derived from plasmid DNA exposed to the plasma, indicated that the production rate of 5-OH-C exceeded that of 8-OH-G, as G:C to A:T transitions accounted for 65% of all base changes, but only a few G:C to T:A transversions were observed. The cell viabilities of E. coli cells decreased in direct proportion to increases in the applied energy. Therefore, the plasma-induced bactericidal mechanism appears to relate to oxidative damage caused to bacterial DNA. These results were confirmed by observing the generation of hydroxyl radicals and hydrogen peroxide molecules following the plasma exposure. We also compared our results with the plasma to those obtained with 137Cs γ-rays, as a well-known ROS generator to confirm the DNA-damaging mechanism involved.

Plasma generation in water using a compact electrode with a semi-covered structure

“…In the previous study [13], cavitation bubbles were generated by the nozzle, and the H 2 O 2 concentration was 3.5 μg l −1 and the energy yield of H 2 O 2 was 8.5 mg kWh −1 when the product was discharged for 96 s with a treated water volume of 2000 ml, a flow rate of 20 l min −1 , and a discharge power of about 30 W. The energy yield was greatly improved up to about 700 times higher on this paper in contrast to previous study. Since the power supply, reactor and electrode structures are different, it is not possible to make a strict comparison, but it is presumed that the main factor for this yield is due to the improvement of the electrode structure.…”

“…In our research group, we proposed underwater plasma for water purification by using underwater cavitation, and we have then reported the results of the research [11][12][13][14]. In this method, since a nozzle is used to generate underwater cavitation, both the pressure loss in the pipeline and the power consumption of the water pump are high.…”