Sponges and sinks and rags, oh my! Where microbes lurk and how to rout them - includes related article on foodborne disease reporting

Science News, Sept 14, 1996 by Janet Raloff

Rags, Oh My! Where microbes lurk and how to rout them

Think household germs, and chances are you'll think of the bathroom. . Yet when scientists from the University of Arizona in Tucson sample surfaces from kitchens and bathrooms in the same house, "consistently, kitchens come up dirtier," notes microbiologist Carlos Enriquez. This trend holds even for disease-causing germs spread by fecal contamination, such as the Escherichia coli coliform bacteria.

"We have swabbed the toilet rim, for instance, and seldom do we find fecal coliform bacteria there, surprising as that may sound," he observes. Enter the kitchen, though, and they're everywhere-in the sponges, dish towels, sink, even on countertops. "So my boss usually jokes about it being safer eating dinner in the bathroom," he says.

But kitchen pathogens are no laughing matter. In the United States, the diseases they cause kill an estimated 9,000 persons each year-mostly the very young, the very old, and those with severely weakened immune systems. The cost of treating foodborne infections ranges from $5 billion to $22 billion annually, according to an analysis released in May by the U.S. General Accounting Office.

Though state and federal agencies compile records on widespread or highly publicized cases-like the E. coli deaths traced to hamburger served at Jack-in-the-Box restaurants in early 1993-they have little information on cases involving just one or two individuals, especially when the ensuing stomach cramps, vomiting, or diarrhea don't lead to hospitalization. However, when researchers have attempted to tally homespun outbreaks, the numbers have proved staggering, notes food safety expert Elizabeth Scott of Newton, Mass.

In the January Journal of Applied Bacteriology, she reviewed European data on disease that could be traced to food eaten at home. For 1989 to 1991 in England and Wales, for instance, 86 percent of the 2,766 reported outbreaks of salmonella infection involving one or more persons appeared to stem from household exposure.

Says Enriquez, these data indicate that "even though we usually feel more secure eating at home, it doesn't necessarily mean it's safer." He and researchers in a few other labs around the country are now investigating where kitchen bugs lurk, with an eye toward making home cooking safer.

Sponges provide an ideal way to spread disease, a discovery the Arizona researchers stumbled upon while swabbing kitchen surfaces daily in several homes.

Bacteria tend to be concentrated in the sink, its drain, and the sponge, Enriquez and his colleagues found. In one home they examined, however, everything from the countertops to refrigerator handles bore consistently heavy contamination-until the sixth day, when most surfaces suddenly turned up virtually germfree. It turned out the family had simply begun using a new sponge.

That was a few years ago. At the American Society for Microbiology meeting in New Orleans last May, Enriquez and his coworkers reported finding that most of the 75 dishrags and 325 sponges from home kitchens that they have sampled harbor large numbers of virulent bacteria (SN: 5/25/96, p. 326), including E. coli and strains of Salmonella, Pseudomonas, and Staphylococcus.

They measure bacteria in colony-forming units-one or more cells that, when cultured, generates a clump of bacteria. In wet areas around the sink, and especially its drain, Enriquez's group has measured up to 10,000 colony-forming units per milliliter of moisture sampled. "And we've found up to 10 million colony-forming units in 1 ml of the liquid wrung from a sponge," he told Science News.

"Initially, we were surprised," he says. In retrospect, the microbiologists realized that continually moist cellulose sponges provide "a very hospitable environment" for bacteria. Key to their survival is a surface easy to cling to, a steady supply of nutrients-even microscopic scraps of food-and moisture.

If a sponge stays moist, the number of live microbes doesn't decrease for 2 weeks. Bacteria can even survive for at least 2 days, Enriquez finds, in a damp sponge gradually drying in the air.

On dry surfaces, resident bacteria survive no more than a few hours. However, Enriquez points out, that's long enough to infect another source of food, or a person's hands during meal preparation.

Though bacteria may love sponges, they happily colonize even stainless steel, notes Edmund A. Zottola of the University of Minnesota in St. Paul. Metal that appears smooth to the naked eye is, from a microbe's perspective, "full of all kinds of nooks and crannies, canyons, gullies, and hills," he observes.

Whenever bacteria find a site harboring moisture and food, he says, "they will set up housekeeping and grow."

His studies have shown that if they aren't sent packing quickly, the microbes produce an organic goo with threadlike tendrils "that literally cements the cells to the surface." This allows them to weather the elements-fast-flowing sprays of water, a little rubbing, or a weak detergent solution. Eventually, unrelated families of microbes move in. The resulting cosmopolitan community forms biofilms that further protect its inhabitants.