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When irradiated with microwaves, Escherichia coli cells briefly change from smooth to prune-like rods, according to Elena P. Ivanova of Swinburne University in Melbourne, Australia, and her collaborators.
Moreover, once irradiated, the cells ingest sugar molecules from the surrounding medium, suggesting that this treatment opens pores that allow sugar polymers to cross the cellular membrane, the researchers report.
Nonetheless, about 90% of the cells survive microwave treatment. Details appear in the May 2011 Applied and Environmental Microbiology (AEM) (77:3017-3022).
"For decades, it has been debated whether microwave frequency exposure can affect bacterial cells independent of the microwave-associated temperature enhancement," Ivanova says. In general, earlier experimenters exposed microbes to high-intensity microwaves that boosted temperatures beyond 45°C, the point where heat damage becomes pronounced.
She and her collaborators sidestepped this problem by irradiating test cells with continuous microwave energy, which subjects them to lower peak energies. Further, she carefully controlled the temperature of cells that are not subject to microwaves, holding them within a range between 20 and 40°C, corresponding to what happens to those cells that are exposed to microwaves.
"Temporary pores form within the E. coli cell membrane, leading to leakage of cellular components and causing the cell shape to appear shrunken and dehydrated," Ivanova says, noting that these changes do not occur among the control microbes. Moreover, she adds, "Within 10 minutes following microwave processing, the cellular components re-enter the cell and the cell shape returns to normal." Cell shape changes were followed both via scanning electron microscopy and confocal laser scanning microscopy.
Meanwhile, the researchers at Swinburne used dextran sugar polymers, tagged with a fluorescent dye, to show that neutral macromolecules can freely enter the microwave-irradiated bacteria while they rehydrate. Moreover, the dextran polymers remain trapped inside those cells, showing that the pores remain open only briefly, Ivanova says. Because the dextran polymers fall within a specific molecular weight range, using them helps to establish the size of the pores, and they fall within the expected range for pores that form in microbial membranes, according to the AEM report.
"To my knowledge, this is the first time that cell permeabilization is reported using microwave radiation," says Isabelle Lagroye of the French National Center for Scientific Research (CNRS), Bordeaux, who praises the quality of the research. The pores need to be better characterized to confirm their existence, she adds. Microwave irradiation of bacterial cells could lead to research and medical applications, according to Ivanova.
Source
http://www.microbemagazine.org/
(MDN)
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