The present study investigated the effects of microwave (MW) radiation on Escherichia coli applied under a sub-lethal temperature. The experiments were conducted at a frequency of 18 GHz and performed at a temperature below 40oC to avoid the thermal degradation of bacterial cells during exposure. The absorbed power was calculated to be 1500 kW/m3 and the electric field was determined to be 300 V/m. Both values were theoretically confirmed using CST Microwave Studio 3D Electromagnetic Simulation Software. As a negative control, E. coli cells were also thermally heated up to temperatures up to 40oC using Peltier Plate heating. Scanning Electron Microscopy (SEM) analysis performed immediately after MW exposure revealed that the E. coli cells exhibited a significantly different cell morphology compared to that of the negative controls. This MW effect, however, appeared to be temporary, as following a further 10 minute elapsed period, the cell morphology appeared to revert to a state that was identical to that of the untreated controls. Confocal Laser Scanning Microscopy (CLSM) revealed that FITC-conjugated dextran (150 kDa) was taken up by the MW treated cells, suggesting that pores had formed within the cell membrane. Cell viability experiments revealed that the MW treatment was not bactericidal, since 88% of the cells were recovered after radiation. It is proposed that one of the effects of exposing E. coli cells to MW radiation under sub-lethal temperature conditions is that the cell surface undergoes a modification that is electro-kinetic in nature, resulting in a reversible MW-induced poration of the cell membrane.
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Shamis, Y., Taube, A., Mitik-Dineva, N., Croft, R. J., Crawford, R. & Ivanova, E. (2011). Specific electromagnetic effects of microwave radiation on escherichia coli. Applied and Environmental Microbiology, 77 (9), 3017-3022.