A surprising variety of bacteria lives in microwave ovens

Microwaves in homes, offices, and laboratories harbor a surprising variety of bacteria.

Microwaves are widely used to heat food and sterilize samples, but the radiation they emit is non-ionizing and does not damage biological molecules. Microwaves heat objects by vibrating water molecules, but bacteria are only killed if a high enough temperature is reached.

However, repeated heating and drying processes meant that microwaves were considered to be a difficult environment for microorganisms to survive.

Manuel Polker Researchers from the University of Valencia in Spain sampled 30 microwaves: 10 from private kitchens, 10 from shared kitchens such as corporate centers, scientific laboratories and cafeterias, and 10 from molecular biology and microbiology laboratories.

In total, the researchers found 747 different genera of bacteria within 25 bacterial phyla, with diversity lowest in domestic microwave ovens and highest in laboratory devices.

Many of the bacteria found in shared and single-family microwaves overlapped and were similar to bacteria commonly found on people’s hands and elsewhere in the kitchen, suggesting that microbes don’t need special adaptations to survive in microwaves, perhaps because food particles protect them from radiation, Polker said.

But the microbiome found in the lab, where food was not cooked, was more distinctive and resembled those found in extremely dry, hot and irradiated environments, such as solar panels.

The researchers found that some of the bacteria found in household microwave ovens include: Klebsiella, Enterococcus and Aeromonaswhich may pose a risk to human health, but the microbial populations found in microwave ovens do not pose any unique or elevated risk compared to other common kitchen surfaces, the researchers said.

“What’s clear is that the microwave cannot be trusted to be a cleaner environment in terms of microbes than the rest of the kitchen, and it should be cleaned just like the rest of the kitchen,” Polker says.

Belinda Ferrari A researcher from the University of New South Wales in Australia says she’s not at all surprised that researchers found bacteria that can live in microwaves. “Bacteria can survive almost any extreme exposure and can adapt to anything,” she says.

Ferrari recommends regularly cleaning your microwave with a disinfectant: “Some microwaves in workplaces are filthy and no one cleans them,” she says.

She would like to see more detailed information about when microwaves were last cleaned in her research: “If we were to do this experiment, we would also like to study the biome before and after cleaning,” she says.