A five-day hay diet for beef cattle just before slaughter could radically reduce the chances the animals could transmit deadly Escherichia coli bacteria when they reach hamburger status, researchers report in a new study.

"We think this is a doable procedure that could be adopted by the American cattle industry to help remedy the problem," said James Russell, a microbiologist with the U.S. Department of Agriculture and a professor at Cornell University, who headed the research team. The results were published recently in the journal Science.Currently, most American beef cattle are fed a diet that's 90 percent grain to fatten them for several months until they go to market. Russell, microbiologist Francisco Diaz-Gonzalez and colleagues at Cornell found that this feed is digested in such a way that it encourages growth in the gut of E. coli strains that can escape our stomach defenses and sicken humans.

One strain in particular, E. coli O157-H7, has resulted in a number of food poisoning cases and the recall of millions of pounds of ground beef in the past several years. Four children died in 1993 after eating contaminated hamburgers.

The Centers for Disease Control and Prevention estimate that 20,000 people are infected by E. coli O157 each year and that about 250 die as a result. The bacterium doesn't kill directly but throws off toxins that damage human organs, resulting in bleeding colitis and kidney failure.

E. coli bacteria are present in the intestinal tracts of most animals and humans, and most types are not harmful. But several strains can do severe damage to people without affecting cattle. These strains, for reasons that aren't clear, seem to infect only about 2 percent of cattle.

Food contamination can come directly from meat or via manure that's used to fertilize other crops. But researchers have been puzzled about why there has been an increase in the number of E. coli outbreaks in recent decades.

Russell, who grew up on a dairy farm and is also an expert in animal nutrition, wondered if the answer might lie in the bovine diet. Since World War II, mature cattle have increasingly been "finished" on feedlots where they consume a diet that's 80 percent to 90 percent starchy grains rather than mainly grass or hay.

But because a cow's digestive tract digests starch poorly, some undigested grain reaches the animal's colon, where it ferments and creates an acidic environment. This in turn results in E. coli that are resistant to the acids in human stomachs, which form our last line of defense before pathogens reach the intestines.

The researchers tested dung from 90 percent grain-fed cows and found as many as 1 million acid resistant E. coli per gram. "With a hay diet, we couldn't detect any, and the limit of detection was as little as 10 cells," Russell said. The cells were tested for acid resistance in the lab by being exposed to an acid bath similar to that delivered by the human stomach.

"And when we switched cattle from grain-based diets to hay for only five days, we couldn't find any acid resistant bacteria from them."

While none of the test animals hosted the E. coli O157 strain, Russell's team ran further lab tests to confirm that it becomes acid resistant in the same way.

Donald Beerman, a Cornell professor of animal science who was not part of the research team, said a brief period of hay-feeding just before slaughter "should not affect either carcass size or meat quality" and could be adopted with minimal expense or disruption to feedlot operators.

Although many scientists have been working on ways to block or neutralize E.coli infections, including a vaccine, the Cornell team's approach "looks to be a relatively inexpensive, potentially important intervention that farmers can do," said Robert Buchanan, a senior food safety expert with the Food and Drug Administration.

Beef industry officials were enthusiastic, although they are still to be convinced that a sudden hay binge wouldn't cause digestive problems and weight loss for cattle. "We think it's an exciting finding. We look forward to seeing if it will work under real world field conditions," said Gary Weber of the National Beef Cattlemen's Association.

Weber said more research is needed before any changes in production practices can be recommended.

Lee Bowman covers health and science for Scripps Howard News Service.