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His findings have not been peer-reviewed or published, but the inquiry for Rockport fit nicely with work he has been doing for years on the role of carpeting, flooring, mobile phones, vacuums and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 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.
Gerba ran tests to see how efficiently our shoes deposit bacteria on what had been clean floor tiles. Very efficiently, it turns out: 90 percent to 95 percent of the colonies on the shoes found a home on the tiles.
"Every step they took, we sampled after them - 10 to 20 steps," he said. "We could still find plenty of organisms on every footstep."
Once the microbes are in a home, anyone can pick them up - especially young children. While adults may have immunity to some of the pathogens, others will be new, especially to children. And germs are changing all the time.
"Kids are pretty intimate with the floor," Gerba said. He cited studies that found children younger than 2 bring hands, toys or something else to their mouths 80 times an hour as they play. Kids ages 2 to 5 do it 50 times an hour.
"They're shoving a lot in their face, right where the germs want to be," Gerba said.
So, does throwing shoes in the washing machine make sense? Rockport thought so. The company has been making and selling washable shoes since 2005, mostly overseas. This is the first year it has pushed them hard in the U.S.
The company's marketers noticed that more people were wearing moccasins and boat shoes without socks.
"It became kind of a global thing," said Bob Settino, Rock- port's product development director. But the idea of millions of bare feet stewing inside hot, damp shoes led to visions of billions of luxuriating bacteria and fungi stinking up the joint.
"That led us to entertain the idea of making a washable shoe," he said.
The problem: When leather absorbs moisture and dries out, it can shrink, and "a size 9 becomes a size 8 1/2 ," Settino said. It can also become brittle.
The company came up with a tanning process that fills moisture-absorbing cavities in shoe leather with a blend of oil and silicone. That preserved the leather's shape and strength through at least 10 washings in company tests. High-tech glues and stitching held it all together.
But does machine washing really make the shoe more hygienic?