The aim of this study was to develop a microbial assay system for easy testing possible effects of the exposure to 900 MHz radiofrequency electromagnetic fields on genome stability of different Escherichia coli strains proficient or defective in methyl-directed mismatch repair system.
In Escherichia coli the methyl-directed mismatch repair system plays an important role in correction of errors made during DNA replication and genetic recombination.
Different Escherichia coli strains were used transformed with an erythromycin (antibiotics)-resistance plasmid (allowing determination of mutation frequencies).
Exposure duration: continuous for 3 hr, 6 hr, 12 hr or 24 hr
|Exposure duration||continuous for 3 hr, 6 hr, 12 hr or 24 hr|
|Setup||cells placed inside the planar transmission line between the electrodes; line width = 10 cm, line length = 40 cm, distance between electodes = 1.5 cm; two BNC connectors mounted on the longitudinal ends of the line; B- and E-field direction in a plane perpendicular to the propagation direction of the wave|
|Sham exposure||A sham exposure was conducted.|
|electric field strength||66 V/m||maximum||measured||-||-|
|electric field strength||40 V/m||minimum||measured||-||-|
|magnetic flux density||156 nT||minimum||calculated||-||-|
|magnetic flux density||260 nT||maximum||calculated||-||-|
|SAR||0.22 mW/kg||mean||estimated||-||at E = 1.1 V/m d: bei|
|SAR||24 µW/kg||mean||estimated||-||at E = 0.37 V/m d: bei|
|SAR||2.7 µW/kg||mean||estimated||-||at E = 0.12 V/m d: bei|
|SAR||0.3 µW/kg||mean||estimated||-||at E = 0.04 V/m d: bei|
Using the present reporter gene system, a slight anti-mutagenic effect of the radiofrequency exposure was demonstrated. An effect was only found in the mismatch repair-proficient strain. Precisely, in the proficient strain, exposure for 24 h to radiofrequency electromagnetic fields (at a SAR value of 2.2 x 10-4 W/kg) decreased the mutation frequencies to erythromycin resistance of about 2.8 fold in comparison to the sham exposed cells. The anti-mutagenic effect of radiofrequency exposure increased with the exposure time (peaking in 12 and 24 h exposed cells). The effect was also observed at a SAR value of 2.4 x 10-5 W/ kg, whereas it was not detectable at SAR values of 2.7 x 10-6 or 3.0 x 10-7 W/kg, demonstrating the existence of dose-response relationship.
In contrast, in mismatch repair-defective strains, the mutation frequency was unaffected by radiofrequency electromagnetic field exposure.
The data indicate that the anti-mutagenic effect of the 900 MHz radiofrequency exposure might be due to an improved efficiency of the mismatch repair system. The mechanism by which the radiofrequency electromagnetic field may affect the activity of the mismatch repair system is unclear.