Specific Electromagnetic Effects of Microwave Radiation on Escherichia coli

Abstract

The present study investigated the effects of microwave (MW) radiation applied under a sublethal temperature on Escherichia coli. The experiments were conducted at a frequency of 18 GHz and at a temperature below 40°C to avoid the thermal degradation of bacterial cells during exposure. The absorbed power was calculated to be 1,500 kW/m 3 , 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 to temperatures up to 40°C using Peltier plate heating. Scanning electron microscopy (SEM) analysis performed immediately after MW exposure revealed that the E. coli cells exhibited a cell morphology significantly different from that of the negative controls. This MW effect, however, appeared to be temporary, as following a further 10-min 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 fluorescein isothiocyanate (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 sublethal temperature conditions is that the cell surface undergoes a modification that is electrokinetic in nature, resulting in a reversible MW-induced poration of the cell membrane.The effects of MW radiation on microorganisms have been studied and debated for more than half a century (3,4,10,12,17,20,28,29,35). The nature of the debate surrounding this interaction has often referred to the existence of so-called specific microwave (MW) effects that are nonthermal in nature (4,10,13,17,20,28,29). Much has been published supporting the notion that a range of specific MW effects exist and can be identified in terms of their manifestations on cell physiology (2,4,10,13,27,28). For example, Dreyfuss and Chipley examined the effects of MW radiation (2.45 GHz) at sublethal temperatures on the metabolic activities of a range of enzymes expressed by the bacterium Staphylococcus aureus (10). These results suggested that MW radiation affected S. aureus cells in a way that could not have been explained solely by thermaleffect theories. It has also been found that Burkholderia cepacia bacteria could be wholly inactivated using MW radiation at sublethal temperatures at a frequency of 20 GHz (2). Samarketu et al. (25) examined the effects of MW radiation at a frequency of 9.575 GHz on the physiological behavior of Cyanobacterium dolium (Anabaena dolium). The authors suggested that MW radiation nonthermally induced different biological effects by changing the protein structures by differentially partitioning the ions and altering the rates and/or directions of biochemical reactions (25)...

Differences in Physical and Biochemical Properties of Thermus scotoductus SA-01 Cultured with Dielectric or Convection Heating

“…S4A and B, respectively, in the supplemental material). These observations are in direct contrast to the observations published for mesophilic E. coli and S. cerevisiae cells exposed to MW radiation (11,23). In E. coli, changes in cell morphology (described as a "dehydrated appearance") were reported after 540 s of MW exposure.…”

“…In E. coli, changes in cell morphology (described as a "dehydrated appearance") were reported after 540 s of MW exposure. The change was temporary, and after 10 min the cells recovered and were indistinguishable from control cells (11). S. cerevisiae cells also showed reversible cell membrane damage after low doses of MW radiation (100 W, 90 s) and irreversible cell membrane damage after higher doses (220 W, 90 s) (23).…”

“…A separate study found that MW heating, but not conventional heating, reversibly damaged the membranes in Escherichia coli (11). The authors concluded that there are specific nonthermal effects on bacteria that are induced by exposure to MW radiation (11). While the designs and results of these two studies were different, the authors agreed on one fundamental point, i.e., that it is difficult to distinguish between the effects of MWs and temperature on living microorganisms.…”

“…The results demonstrated that mesophiles can survive MW exposure, but the growth of these organisms during MW exposure was not addressed. A separate study found that MW heating, but not conventional heating, reversibly damaged the membranes in Escherichia coli (11). The authors concluded that there are specific nonthermal effects on bacteria that are induced by exposure to MW radiation (11).…”

Are Mobile Radiations Harmful for Bacteria? A Case Study.

“…When cells are exposed to 18 GHz frequency of 1500KW/m 2 energy, it causes inactivation of E.coli cells as a result of maximum increase in the porosity of plasma membrane. Thus it could be expected that higher frequency exposure might cause increase in the porosity of cell membrane leading to their inactivation 14 . The study of structural changes of the bacterial cell membrane upon higher frequency exposure could thus be an effervescent topic of investigation.…”