The effects of high-power microwaves on the ultrastructure of Bacillus subtilis

Aims: To investigate the microbicidal mechanisms of high-power microwave (2.0 kW) irradiation on Bacillus subtilis and to determine the effect of this procedure on the ultrastructure of the cell wall. Methods and Results: We performed viability test, examined cells using transmission electron microscopy (TEM), and measured the release of intracellular proteins and nucleic acids. The inactivation rate of B. subtilis by 2.0-kW microwave irradiation was higher than that of a domestic microwave (0.5 kW). Few proteins were released from either microwaved or boiled cells. However, the leakage of nucleic acids from 2.0-kW-microwaved cells was significantly higher than that of 0.5-kW-microwaved or boiled cells. Therefore, we examined ultrastructural alterations of microwaved or boiled cells to analyse the pattern of release of cytoplasmic contents. Although boiled cells did not show any ultrastructural changes on TEM, 2.0-kW-microwaved cells showed disruption of the cell wall. Conclusion: The microbicidal mechanisms of 2.0-kW microwave irradiation include damage to the microbial cell wall, breakage of the genomic DNA, and thermal coagulation of cytoplasmic proteins. Significance and Impact of the Study: TEM images showed that the cytoplasmic protein aggregation and cell envelope damage by microwave irradiation were different from the ultrastructural changes observed after boiling.