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.

Aims:  To investigate the sporicidal mechanisms of microwave irradiation on Bacillus licheniformis spores. Methods and Results:  We measured spore viability and the release of DNA and proteins, and performed transmission electron microscopy (TEM). A microwave oven (0·5 kW) was modified to output power at 2·0 kW, which allowed a shorter sterilization cycle. A 2·0 kW microwave treatment at the boiling temperature for 1 min did not kill all spores, but killed most spores. The spore inactivation rate was faster than that of boiling and 0·5 kW microwave oven. In contrast to boiling and 0·5 kW microwave treatments, the 2·0 kW microwave resulted in significant leakage of proteins and DNA from spores due to injury to the spore structure. TEM revealed that 2·0 kW microwave irradiation affected spore cortex hydrolysis and swelling, and ruptured the spore coat and inner membrane. Conclusions:  These results suggest that 2·0 kW microwave irradiation ruptures the spore coat and inner membrane, and is significantly different from boiling. Significance and Impact of the Study:  This study provides information on the sporicidal mechanisms of microwave irradiation on B. licheniformis spores.