Abstract
A growing body of literature has recognized the non-thermal effect of pulsed microwave radiation (PMR) on bacterial systems. However, its mode of action in deactivating bacteria has not yet been extensively investigated. Nevertheless, it is highly important to advance the applications of PMR from simple to complex biological systems. In this study, we first optimized the conditions of the PMR device and we assessed the results by simulations, using ANSYS HFSS (High Frequency Structure Simulator) and a 3D particle-in-cell code for the electron behavior, to provide a better overview of the bacterial cell exposure to microwave radiation. To determine the sensitivity of PMR, Escherichia coli and Staphylococcus aureus cultures were exposed to PMR (pulse duration: 60 ns, peak frequency: 3.5 GHz) with power density of 17 kW/cm2 at the free space of sample position, which would induce electric field of 8.0 kV/cm inside the PBS solution of falcon tube in this experiment at 25 °C. At various discharges (D) of microwaves, the colony forming unit curves were analyzed. The highest ratios of viable count reductions were observed when the doses were increased from 20D to 80D, which resulted in an approximate 6 log reduction in E. coli and 4 log reduction in S. aureus. Moreover, scanning electron microscopy also revealed surface damage in both bacterial strains after PMR exposure. The bacterial inactivation was attributed to the deactivation of oxidation-regulating genes and DNA damage.
Highlights
A growing body of literature has recognized the non-thermal effect of pulsed microwave radiation (PMR) on bacterial systems
Woo et al, reported that non-thermal microwave radiation in Escherichia coli and Bacillus subtilis cell suspensions resulted in a dramatic reduction of the viable counts, as well as an increase in the amounts of DNA and proteins released from the cells[9]
It might be possible that the electric field of 8 kV/cm generated by PMR in PBS solution induced oxidative stress and intracellular reactive oxygen species (ROS) in bacteria
Summary
A growing body of literature has recognized the non-thermal effect of pulsed microwave radiation (PMR) on bacterial systems. Cells of S. aureus irradiated by microwaves exhibited a greater metabolic imbalance than conventionally heated c ells[7] During microwave propagation, both thermal and non‐thermal effects can alter the intracellular components of the microorganisms. Woo et al, reported that non-thermal microwave radiation in Escherichia coli and Bacillus subtilis cell suspensions resulted in a dramatic reduction of the viable counts, as well as an increase in the amounts of DNA and proteins released from the cells[9]. The inactivation of bacterial spores by heavy ions resulted in inactivation (determined from loss of colony formers), mutagenesis (reversion to histidine prototrophy), and inhibition of DNA repair It is still not clear whether the repair systems are inactivated, or merely that heavy-ion lesions are less r epairable[13,16]. It has been reported that the bacteriophage PL-1, which is specific to Lactobacillus casei, is sensitive to microwave r adiation[21,22,23,24]
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