A surprisingly wide range of bacteria live inside microwaves

Microwave ovens in homes, offices and laboratories are home to a surprising diversity of bacteria.

Although microwaves are widely used to heat up food and sterilise samples, the radiation they emit is non-ionising, which means that it doesn’t damage biological molecules. They heat things up by making water molecules vibrate, but this only kills bacteria if they reach a high enough temperature.

Nevertheless, repeated bouts of heating and drying mean that microwaves were thought to be a difficult environment for microorganisms to survive in.

Manuel Porcar at the University of Valencia, Spain, and his colleagues took samples from 30 microwave ovens: 10 from single-household, domestic kitchens; 10 from shared kitchens such as corporate centres, scientific institutes and cafeterias; and 10 from molecular biology and microbiology laboratories.

In total, the researchers found 747 different genera of bacteria within 25 bacterial phyla. Diversity was lowest in single-household microwaves, and highest in laboratory appliances.

Many of the bacteria found in shared, domestic microwaves and the single-household microwaves overlapped, and were similar to the bacteria often found on human hands and in other parts of the kitchen. This suggests microbes don’t need special adaptations to survive in a microwave, perhaps because food fragments protect them from radiation, says Porcar.

However, those in labs, where food was not cooked, were more distinct and were similar to the microbiome found in other extreme, dry, hot and irradiated environments, such as on solar panels.

The researchers noted that some bacteria found in domestic microwaves, such as Klebsiella, Enterococcus and Aeromonas, may pose a risk to human health. However, they say the microbial population found in microwaves does not present a unique or increased risk compared with other common kitchen surfaces.

“What is clear is that one cannot trust a microwave to be a cleaner environment in terms of microorganisms compared to another kitchen surface. They must be cleaned like the rest of the kitchen,” says Porcar.

Belinda Ferrari at the University of New South Wales, Australia, says she is not surprised at all that the researchers found bacteria living in microwaves. “They can survive in almost any extreme-exposure environment and they can adapt to everything,” she says.

Ferrari recommends regularly cleaning microwaves with disinfectant products. “Some workplace microwaves are disgusting and no one cleans them,” she says.

She would like to see more information in the study about when the microwaves were last cleaned. “If I was doing this experiment, I would also like to study the biome before and after cleaning,” she says.