In order to ensure that any effects caused by microwave exposure were not purely a result of thermal heating, a heat control with the same temperature gradients (20°C - 40°C - 20°C etc.) was used experienced by the bacteria during microwave exposure.
|Exposure duration||60 s on - 120 s off - repeated three times|
|Setup||frequency of 18 GHz chosen because its wavelength is comparable to the bacterial cell diameter; bacterial samples of 2 ml placed into micro Petri dishes with a diameter of 35 mm; samples placed individually on a ceramic pedestal in the exposure chamber; during the 60 s exposure temperature rise of 20°C/min from 20°C to 40°C; during the 120 s interval sample was cooled on ice back to 20°C with a cooling rate of 10°C/min|
Scanning electron microscopy analysis performed immediately after microwave exposure revealed that the Escherichia coli cells exhibited a significantly different cell morphology compared to that of the heat controls. This microwave 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 microscopy revealed that FITC-conjugated dextran dye was taken up by the microwave exposed cells, suggesting that pores had formed within the cell membrane.
Cell viability experiments showed that the microwave exposure was not bactericidal, since 88% of the cells recovered after irradiation.
It is proposed that one of the effects of exposing Escherichia coli cells to microwave irradiation under sub-lethal temperature conditions is that the cell surface undergoes a modification that is electro-kinetic in nature, resulting in a reversible microwave-induced poration of the cell membrane.