Medical/biological study (experimental study)

Specific electromagnetic effects of microwave radiation on Escherichia coli med./bio.

By: Shamis Y, Taube A, Mitik-Dineva N, Croft R, Crawford RJ, Ivanova EP

Published in: Appl Environ Microbiol 2011; 77 (9): 3017-3022

Aim of study (acc. to author)

To study the effects of microwave irradiation on Escherichia coli applied under a sub-lethal temperature.

Background/further details

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.

Endpoint

cell viability/cell division/proliferation: cell viability of Escherichia coli
morphol./histopathol. changes: morphology and membrane permeability of Escherichia coli

Exposure

- 18 GHz
- microwaves
- CW continuous wave

Exposure	Parameters
Exposure 1: 18 GHz Exposure duration: 60 s on - 120 s off - repeated three times	electric field strength: 300 V/m absorbed power: 1500 kW/m³

Exposure 1

Main characteristics

Frequency	18 GHz
Type	electromagnetic field
Exposure duration	60 s on - 120 s off - repeated three times

Exposure setup

Exposure source	multi-mode microwave cavity
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

Exposure source

multi-mode microwave cavity

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

Parameters

Measurand	Value	Type	Method	Mass	Remarks
electric field strength	300 V/m	-	measured	-	-
cf. remarks	-	-	calculated	-	absorbed power: 1500 kW/m³

Reference articles

Shamis Y et al. (2008): Development of a microwave treatment technique for bacterial decontamination of raw meat

Exposed system:

Methods Endpoint/measurement parameters/methodology

cell viability/cell division/proliferation: cell viability of Escherichia coli (colony forming units)
morphol./histopathol. changes: morphology and membrane permeability of Escherichia coli (cell surface: scanning electron microscopy; cell membrane and permeability: via FITC-Dextran and confocal microscopy)

Investigated system:

Time of investigation:

before exposure
after exposure

Main outcome of study (acc. to author)

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.

Study character:

medical/biological study
experimental study
full/main study

Study funded by

not stated/no funding

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Shamis Y et al. (2008): Development of a microwave treatment technique for bacterial decontamination of raw meat
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