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Hay fight ecoli { September 19 1998 }

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September 19, 1998
Hay! What a way to fight E coli
Since a lethal food-poisoning outbreak at a fast-food chain catapulted Escherichia coli O157:H7 into the news 5 years ago, microbiologists and food scientists have been scrambling to find ways to curb its contamination of hamburger and other animal products. Last week, a team of scientists at Cornell University and the U.S. Department of Agriculture reported one palatable solution: Feed cows hay.

Federal estimates indicate that E. coli infections cause 20,000 cases of food poisoning in the United States each year—some 200 of which prove fatal. The most notorious strain of E. coli is O157:H7, which is so virulent that ingestion of just 10 cells can trigger a syndrome characterized by bloody diarrhea and the risk of deadly kidney failure.

Livestock, especially cattle, serve as a reservoir of E. coli. These microbes can set up housekeeping in the bovine gastrointestinal tract, causing no harm to the host. A small share of the bugs will, however, be shed in the feces. At slaughter, meat can become contaminated with traces of these feces—and cause disease unless the tainted meat is fully cooked. Potentially even more problematic, fruits and vegetables can become a source of disease if they are fertilized with tainted manure.

Though "most bacteria are killed by the acid of [human] stomach juice," E. coli can generally withstand such a caustic environment, notes James B. Russell, a USDA microbiologist on Cornell’s faculty. Their acid tolerance means that these E. coli can not only survive but thrive to cause gut-wrenching disease.

However, Russell’s team reports evidence in the Sept. 11 Science that by tinkering with a cow’s diet, farmers can diminish E. coli’s acid resistance, rendering it susceptible to degradation in the human stomach.

When a lean cuisine is best

Russell’s team studied 61 cows that had been eating a diet dominated by rolled corn—an energy-dense, protein-rich grain. After periodically sampling partially digested food from the animals’ forestomach, also known as a rumen, they added various amounts of hay to the diets of some of these animals.

They found that a rumen’s pH varied with the diet. When the cows were eating at least 90 percent rolled corn, their rumen ran a somewhat acidic 5.9. When those grain rations fell to just 60 percent, forcing cows to make up the difference with hay, the pH climbed to a nearly neutral 6.9. When the cows dined solely on a lean cuisine of hay, the pH climbed slightly more, to just above 7—or the pH of distilled water.

At the same time the pH rose, the number of E. coli present per gram of digesting food fell, as did the number of those bacteria shed in feces. Sugars and starches in the hay do not reach the intestines, observes Todd R. Callaway, a coauthor of the new paper. This appears to explain why the microbes appeared to proliferate more slowly in hay-fed cows.

Most importantly, his team found, as rumen pH climbed, the E. coli isolated from the these animals also grew less tolerant of acid. Although there were no O157:H7 bacteria isolated from these animals, the researchers point out that there is no reason why that strain of the bacteria should react differently.

What do these findings mean for food-poisoning risks. To find out, Russell’s team plunged the microbes into a highly acid bath—one with a pH of 2, equal to that of the human stomach. While E. coli extracted from the high-grain diet survived this caustic bath, the vast majority of those bacteria from hay-fed animals succumbed within 5 days to an "acid shock."

Over the past 40 years, U.S. farmers have increasingly been feeding grain to their beef cattle for the 3 months preceding slaughter—both to fatten the animals up and to make their meat more tender. Under this relatively new regimen of animal husbandry, Russell observes, natural selection appears to have favored the growth of E. coli strains possessing the genes to tolerate acid.

"Now that we know where the acid-resistant E. coli are coming from," he observes, "we can control them with a relatively inexpensive change in diet"—perhaps just a 5-or 6-day weaning from grain.

Shining a light on a related problem

While this dietary alteration may reduce the risk that meat harbors pathogenic E. coli , meat inspectors can’t count on that—at least not yet. Moreover, there are a host of other pathogens that feces may contribute to meat during slaughter and processing. At USDA’s National Animal Disease Center at Iowa State University (ISU), Mark E. Rasmussen and his colleagues are developing a new hand-held laser technology to find and highlight—quite literally—all fecal contamination.

Each year, U.S. meat processors slaughter some 37 million cattle, 93 million hogs, 4 million sheep, 300 million turkeys, and 7.8 billion chickens. It’s the responsibility of plant operators and meat inspectors to visually scan each carcass for signs of feces. However, Rasmussen notes, the meat zips down assembly lines fairly rapidly. So unless the quantity of feces is substantial, it may be missed. Moreover, he points out, "packers have told me they’ll often see a spot and not know what it is. It could be feces, a spot of grease, or dried blood."

What these processors want is "an on-the-spot argument settler"—something that can resolve whether or not the spot is feces, which is outlawed. "And that’s what we’re developing," he says.

The current prototype resembles the hand-held metal detector employed by airport security crews for travelers approaching departure gates. In this case, the scanner irradiates the target with a laser beam whose wavelength has been selected for its ability to make feces fluoresce.

While the light emitted by a tiny speck of contamination would not be visible to the naked eye, "we can design the instrument to give some positive feedback," Rasmussen says—such a beep, a flashing red light, or a needle that sweeps across some dial in response to detection of the fluorescence. Should any contamination be identified, he says a processor could just home in on the spot by waving the scanner, then slice off the affected tissue.

Earlier this year, ISU and USDA applied for a patent on the system. "As we learn more about the engineering and design of this," Rasmussen says, "we can scale up to larger—even whole carcass—systems." Right now, however, test models appear a bit crude and are "cobbled together with tape." But it shouldn’t be long before a commercial version debuts in meat-packing houses, Rasmussen told Science News Online. Indeed, he notes, "packers told us that if we had an instrument ready, they’d buy it tomorrow."

Related Readings:

Diez-Gonzalez, T.R. Calloway, . . . and J.B. Russell. 1998. Grain feeding and the dissemination of acid-resistant Escherichia coli from cattle. Science 281(Sept. 11):1666.

Raloff, J. 1998. Ka-Boom! Science News 366(June 6):366.

_____. 1998. Wash-Resistant Bacteria Taint Foods. Science News 153(May 30): 340.

_____. 1998. Staging germ warfare in foods. Science News 153(Feb. 7):89.

_____. 1997. Cutting through the cutting board brouhaha. Science News Online(July 12).

_____. 1996. To disinfect your salad. Science News Online (Sept. 28).

_____. 1996. Sponges and sinks and rags, oh my! Science News 150(Sept. 14):172.

_____. 1996. Tracking and tackling foodborne germs. Science News 149(May 25):326.

1996. Food safety: Information on foodborne illnesses. Report RCED-96-96 (May). U.S. General Accounting Office, Washington, DC 20548-0001.


Mark Rasmussen
National Animal Disease Center
P.O. Box 70
2300 Dayton Avenue
Ames, Iowa 50010

Todd R. Calloway
Division of Biological Sciences
Section of Microbiology
Cornell University and Agricultural Research Service
U.S. Department of Agriculture
Ithaca, NY 14853-8101

James B. Russell
Division of Biological Sciences
Section of Microbiology
Cornell University and Agricultural Research Service
U.S. Department of Agriculture
Ithaca, NY 14853-8101

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