NEWS
08 August 2024

Your microwave oven has its own microbiome

Survey of bacteria living inside household and laboratory appliances finds a robust ecosystem.

By Alix Soliman

Extremophiles’ are organisms that can survive, and even thrive, in the harshest of environments, including inside scorching hydrothermal vents, sub-zero Antarctic ice and the crushing pressures of Earth’s crust. Now, they’ve been discovered in a more pedestrian setting: microwave ovens.

Although previous studies found distinct communities of microbes in kitchen appliances such as dishwashers1 and coffee machines2, this is the first time that the microwave oven has been investigated for having its own microbiome. The research, published today in Frontiers in Microbiology3, adds to existing work challenging a common misconception: that microwave radiation heats up and completely kills bacteria that cause food-borne illness, such as Escherichia coli and Salmonella.

“We’ve all been taught, from like the 1980s, that if you use a microwave oven, it heats everything up — it kills everything,” says Jason Tetro, a freelance microbiologist, known as ‘The Germ Guy’, in Edmonton, Canada. This study is “important”, he says, because it shines a spotlight on potential pathogens in these appliances, especially shared ones.

All that’s zapped is not killed

Alba Iglesias, a microbiologist at the University of Valencia in Spain, and her colleagues swabbed 30 microwave ovens — including some in households; some shared in large spaces, such as offices; and some used in laboratories to heat specimens and chemical solutions. The team then cultured its samples in Petri dishes and determined the genera of the microbes that grew. They also sequenced the DNA in the material swabbed from the microwave ovens to get a sense of the bacterial diversity inside the appliances.

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ORIGINAL RESEARCH article
Front. Microbiol., 07 August 2024
Sec. Extreme Microbiology
Volume 15 - 2024 | https://doi.org/10.3389/fmicb.2024.1395751

The microwave bacteriome: biodiversity of domestic and laboratory microwave ovens

Alba Iglesias, Lorena Martínez, Daniel Torrent, Manuel Porcar

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 this study, we investigated the bacterial communities in microwave ovens and compared the microbial composition of domestic microwaves, microwaves used in shared large spaces, and laboratory microwaves, using next-generation sequencing and culturing techniques. 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 laboratory microwaves had a higher abundance of taxa known for their ability to withstand microwave radiation, high temperatures and desiccation. These results suggest that different selective pressures, such as human contact, nutrient availability and radiation levels, may explain the differences observed between domestic and laboratory microwaves. Overall, this study provides valuable insights into microwave ovens bacterial communities and their potential biotechnological applications.

Introduction
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 (Shu and Huang, 2022). However, one does not need to travel that far in search for extreme environments.

As a result of human activity and modernization, many different man-made artificial devices were built in the last century. Many studies have described the microbial populations present in highly anthropized artificial environments such as elevator buttons (Kandel et al., 2014), the underground (Gohli et al., 2019), and small electronic devices (Lax et al., 2015). Other works have unveiled that some man-made devices, machines, and appliances, despite being in constant contact with humans or human activities, have their own microecosystems with their own selective pressures and conserved microbiomes. This is the case, for example, of coffee machines (Vilanova et al., 2015) or dishwashers (Raghupathi et al., 2018).

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 coli, Enterococcus faecalis, Clostridium perfringens, Staphylococcus aureus, Salmonella spp. and Listeria spp. (Woo et al., 2000; Kubo et al., 2020). 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 (Cao et al., 2018; Shaw et al., 2021). Despite this extensive characterization of the biological effects of microwave radiation on foodborne bacteria, to our knowledge 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.

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