Friday, August 16, 2024

Microwaves don't kill all bacteria

Biologists Find Diverse Bacterial Communities in Microwave Ovens - not really a good thing

By Enrico de Lazaro

Microwaves have become an essential part of the modern kitchen, but their potential as a reservoir for bacterial colonization and the microbial composition within them remain largely unexplored.

In a new study, microbiologists from the University of Valencia and Darwin Bioprospecting Excellence SL investigated the bacterial communities in microwave ovens and compared the microbial composition of domestic microwaves, microwaves used in shared large spaces, and lab microwaves.

The microwave oven bacterial population was dominated by Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes, similar to the bacterial composition of human skin.

Comparison with other environments revealed that the bacterial composition of domestic microwaves was similar to that of kitchen surfaces, whereas lab microwaves had a higher abundance of species known for their ability to withstand microwave radiation, high temperatures and desiccation.

Iglesias et al. showed that microwaves harbor a specialized community of locally adapted microbial genera, which resembles that reported on kitchen surfaces and in another extreme, highly irradiated habitat: on solar panels.

Microorganisms that thrive in ecosystems characterized by extreme environmental conditions have been well studied to elucidate the evolutionary mechanisms that have favored their adaptation.

Natural extreme environments represent an exceptional source of novel microbial species, as well as a source of novel secondary metabolites with biotechnological applications. However, one does not need to travel that far in search for extreme environments.

Microwave irradiation has been used for decades to reduce the presence of microorganisms in food and extend food shelf life.

The application of an electromagnetic wave in the range of 300 MHz to 300 GHz to a dielectric medium such as food, also known as microwave heating, generates heat to reach lethal temperatures that inactivate most microorganisms, such as Escherichia coliEnterococcus faecalisClostridium perfringensStaphylococcus aureusSalmonella spp. and Listeria spp.

Recent work has shown that cell inactivation is associated with deactivation of oxidation-regulating genes, DNA damage and increased permeability and disrupted integrity of cell membranes.

Despite this extensive characterization of the biological effects of microwave radiation on foodborne bacteria, there are no reports of microwaves as microbial niches, that is, environments where specific selective pressures (in this case, thermal shock, microwave radiation, and desiccation) can shape a specifically adapted microbiome.

“Our results reveal that domestic microwaves have a more ‘anthropized’ microbiome, similar to kitchen surfaces, while lab microwaves harbor bacteria that are more resistant to radiation,” said Dr. Daniel Torrent, a researcher at Darwin Bioprospecting Excellence SL.

For the study, Dr. Torrent and his colleagues sampled microbes from inside 30 microwaves: 10 each from single-household kitchens, another 10 from shared domestic spaces, for example corporate centers, scientific institutes, and cafeteria, and 10 from molecular biology and microbiology laboratories.

The aim behind this sampling scheme was to see if these microbial communities are influenced by food interactions and user habits.

They used two complementary methods to inventorize the microbial diversity: next generation sequencing and cultivation of 101 strains on five different media.

In total, the authors found 747 different genera within 25 bacterial phyla. The most frequently encountered phyla were Firmicutes, Actinobacteria, and especially Proteobacteria.

They found that the composition of the typical microbial community partly overlapped between shared domestic and single-household domestic microwaves, while lab microwaves were quite different.

The diversity was lowest in single-household microwaves, and highest in lab ones.

Members of genera AcinetobacterBhargavaeaBrevibacteriumBrevundimonasDermacoccusKlebsiellaPantoeaPseudoxanthomonas and Rhizobium were found only in domestic microwaves.

ArthrobacterEnterobacterJanibacterMethylobacteriumNeobacillusNocardioidesNovosphingobiumPaenibacillusPeribacillusPlanococcusRothiaSporosarcina, and Terribacillus were found only in shared-domestic ones.

Nonomuraea bacteria were isolated exclusively from lab microwaves. There, DelftiaMicrococcusDeinocococcus and one unidentified genus of the phylum Cyanobacteria were also common, found in significantly greater frequencies than in domestic ones.

The researchers also compared the observed diversity with that in specialized habitats reported in the literature.

As expected, the microbiome in microwaves resembled that found on typical kitchen surfaces.

“Some species of genera found in domestic microwaves, such as KlebsiellaEnterococcus and Aeromonas, may pose a risk to human health,” Dr. Torrent said.

“However, it is important to note that the microbial population found in microwaves does not present a unique or increased risk compared to other common kitchen surfaces.”

However, it was also similar to the microbiome in an industrial habitat: namely, on solar panels.

The scientists proposed that the constant thermal shock, electromagnetic radiation, and desiccation in such highly irradiated environments has repeatedly selected for highly resistant microbes, in the same manner as in microwaves.

“For both the general public and lab personnel, we recommend regularly disinfecting microwaves with a diluted bleach solution or a commercially available disinfectant spray,” Dr. Torrent said.

“In addition, it is important to wipe down the interior surfaces with a damp cloth after each use to remove any residue and to clean up spills immediately to prevent the growth of bacteria.”

The results were published in the journal Frontiers in Microbiology.

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Alba Iglesias et al. 2024. The microwave bacteriome: biodiversity of domestic and laboratory microwave ovens. Front. Microbiol 15; doi: 10.3389/fmicb.2024.1395751