Bacteria survive being zapped

That lunchroom advice has been put to the test by researchers who looked for bacteria inside microwave ovens and found a surprisingly diverse ecosystem that is resistant to the appliances’ heat.

Microwaves work by causing water molecules inside food to vibrate, producing frictional heat that cooks the food and kills the bacteria it might contain. But there is not enough water inside microbes living on surfaces inside the microwave to have this same effect—so they survive being zapped.

“It’s not the same thing to warm up fish or pasta, and then to warm up these tiny microorganisms that may be mixed with some fat in a very thin layer on top of this glass tray that is inside the microwave," said Manuel Porcar, a researcher at the University of Valencia and chief executive of Darwin Bioprospecting Excellence, a Spanish biotechnology firm.

Porcar and colleagues took swabs from 30 microwaves, cultured the results and analyzed the DNA of the microbes they found. One-third of the samples came from household microwaves, a third from shared office microwaves and the rest from microbiology laboratories, where the appliances were used to heat chemical solutions.

Altogether, the researchers reported 747 kinds of bacteria.

The domestic microwaves contained bacteria such as staphylococcus that are commonly found on human skin and could pose a risk to health, according to the study. Lab microwaves—which are turned on for longer periods—were home to bacteria resistant to radiation and extremely high temperatures, similar to bacteria found living on solar panels or near nuclear waste sites.

The researchers usually work with microorganisms that live in exotic environments, such as hot springs, tropical rainforests or arid deserts, to find genetic material that might be useful in industrial applications or the food industry—for example in developing a mouthwash that doesn’t kill beneficial microorganisms.

The colleagues turned their expertise to microwaves out of curiosity.

“We wanted to take a look at the microwave as a microbial niche and understand which kind of microorganisms are there independent of whether you are eating a fish or chicken or milk or whatever," Porcar said.

The kitchen microwaves had a greater mass of microbes, they found, while the laboratory microwaves hosted greater diversity.

To rid a microwave of the germs, Porcar said using soap or diluted bleach will do the trick.

“Microwaves are as clean or as dirty as the surface of your kitchen table," he said. “This means that you must not forget to clean it."

The findings were published in August in the journal Frontiers in Microbiology.

Write to Eric Niiler at eric.niiler@wsj.com