Microwave ovens have become an essential part of the modern kitchen, yet their potential as a reservoir for bacterial colonization and the microbial composition within them remain largely unknown. In a new study, microbiologists from the University of Valencia and Darwin Bioprospecting Excellence SL investigated the bacterial communities within microwave ovens and compared the microbial composition of domestic microwave ovens, microwaves used in large shared spaces, and laboratory microwaves. The bacterial populations in microwave ovens were 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 microwave ovens was similar to that of kitchen surfaces, while laboratory microwave ovens contained a higher proportion of species known to tolerate microwave radiation, high temperatures, and dryness.

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

Natural extreme environments are an excellent source of new microbial species as well as new secondary metabolites for biotechnological applications, but we don’t have to go very far to find them.

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

The application of electromagnetic waves in the range of 300 MHz to 300 GHz to a dielectric medium such as food (also known as microwave heating) produces heat that reaches a lethal temperature that inactivates most microorganisms. E. coli, Enterococcus faecalis, Clostridium perfringens, Staphylococcus aureus, Salmonella and Listeria Genus

Recent studies have shown that cellular inactivation is associated with inactivation of genes controlling oxidation, DNA damage, increased permeability, and reduced cell membrane integrity.

Despite this extensive characterization of the biological effects of microwave radiation on foodborne pathogens, there have been no reports of microwaves being a microbial niche, an environment in which a specifically adapted microbiota can form through specific selective pressures (in this case heat shock, microwave radiation, and desiccation).

“Our results reveal that domestic microwaves harbor a more 'humanised' microbiome, similar to kitchen surfaces, while laboratory microwaves harbour bacteria that are more resistant to radiation,” said Dr Daniel Trent, Darwin Bioprospecting Excellence SL researcher.

For the study, Dr Trent and his colleagues sampled microorganisms from inside 30 microwaves: 10 from household kitchens, 10 from shared household spaces such as corporate centers, science labs and cafeterias, and 10 from molecular biology and microbiology laboratories.

The aim of this sampling plan was to determine whether these microbial populations were influenced by interactions with food and by user habits.

They used two complementary methods to explore microbial diversity: next-generation sequencing and culturing 101 strains in 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.

The researchers found that the composition of the general microbial community partially overlapped between shared and domestic microwave ovens, but was quite different in laboratory microwave ovens.

Diversity was lowest in home microwaves and highest in laboratory microwaves.

Members of the genus Acinetobacter, Bhargavaea, Brevibacterium, Brevundimonas, Dermatococcus, Klebsiella, Pantoea, Pseudoxanthomonas and Rhizobium It has only been found in domestic microwave ovens.

Arthrobacter, Enterobacter, Yanibacter, Methylobacterium, Neobacillus, Nocardioides, Novosphingobium, Paenibacillus, Peribacillus, Planococcus, Russia, Sporosarcinaand Teribacillus This was only seen in items shared within the household.

Nonomura The bacteria was isolated only from laboratory microwaves. Delftia, Micrococcus, Deinococcus Unidentified genera of the phylum Cyanobacteria were also common and were found at a significantly higher frequency than in Japan.

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

As expected, the microbiome inside the microwave was similar to that present on typical kitchen surfaces.

“Some genera found in domestic microwave ovens include Klebsiella, Enterococcus and Aeromonaswhich could pose a health risk to humans,” Dr Trent said.

“However, it is important to note that the microorganisms living in microwave ovens do not pose any unique or increased risk compared to other common kitchen surfaces.”

But it also had similarities to the microbiome in industrial environments, namely on solar panels.

The scientists proposed that the constant heat shock, electromagnetic radiation and desiccation in this highly radioactive environment repeatedly selected for more resistant microorganisms, just as occurs in microwave ovens.

“We encourage both the public and laboratory personnel to regularly disinfect their microwave ovens with a diluted bleach solution or a commercially available disinfectant spray,” Dr. Trent said.

“In addition, it's important to wipe down interior surfaces with a damp cloth after each use to remove any residue and wipe up any spills immediately to prevent bacterial growth.”

of result Published in the journal The cutting edge of microbiology.

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Alba Iglesias others2024. The microwave bacteriome: biodiversity in domestic and laboratory microwave ovens. Front. Microbiol 15;doi: 10.3389/fmicb.2024.1395751