The 'ewww!' on your shoe

Mother was right. Our shoes are filthy, and we'd be smart to leave them at the door, like they do in Japan.

Even microbiologist Charles P. Gerba was surprised to discover what we track into the house on our footwear.

"I'm starting to make myself paranoid," he said. "It seems like we step in a lot more poop than I thought."

Gerba is a professor in the University of Arizona's Department of Soil, Water and Environmental Science. He's spent years studying how humans spread microbes around and "share" them wherever they go.

But when he was asked by the Rockport Co. whether he thought throwing shoes in the washer made hygienic sense, he told them, "I don't know. Nobody's ever studied it."

For the record, Rockport makes shoes - including a new line of washable shoes. And animal-health experts for years have demonstrated the role of footwear in transporting animal diseases from farm to farm, Gerba said.

But scientists have not spent as much effort looking at the same disease vectors off the farm. So Rockport commissioned Gerba to look at what we pick up on our shoes and track into our homes.

His findings have not been peer-reviewed or published, but the inquiry for Rockport fit nicely with work he has been doing for years on the role of carpeting, flooring, mobile phones, vacuums and other objects in the transmission of pathogens. He hopes to add the shoe study to his list of publications.

In his initial test, Gerba swabbed for bacteria on 26 shoes

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worn by test subjects for three months or more. He cultured the samples and identified nine microbial species that can cause intestinal, urinary, eye, lung, blood and wound infections.

Coliform bacteria - originating in fecal matter - were found on the outside of all but one of the shoes, and the samples averaged 421,000 bacterial units per square centimeter sampled. (Each unit is enough bacteria to reproduce and grow a new colony.) Seven of the shoes had picked up Escherichia coli (E. coli) bacteria.

The insides of the shoes were much cleaner, averaging 11,000 bacterial units per square centimeter. He found no coliform bacteria inside the shoes.

"Public restrooms have got a lot of fecal bacteria on the floor," Gerba said. And they're shared by more people than ever before as our society becomes more mobile.

We pick up still more intestinal bacteria walking where dogs, birds and other critters have relieved themselves. Then we bring it all home.

The bacteria on our shoes "are surviving for long periods of time," Gerba said. "We're tracking them around for quite some distance."

In the open air, viruses that cause diarrhea can survive several days to weeks. Bacteria can survive one to three days, but they do better with food around - and we provide that, too.

As the crud builds up on our shoes, Gerba said, we add food and other debris that bacteria can feed on, and "our shoes become a bacteria cafeteria."

Back home, we trail the stuff across our floors and carpets, where the microbes find a new residence.

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