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September 11, 1998

E. Coli Bacteria Can Be Eliminated From Cattle, Researchers Find


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    Microbiologists at Cornell University have found a way to virtually rid cattle of harmful strains of E. coli bacteria, including the bacteria that have caused scores of deaths and sickened thousands of consumers of undercooked hamburgers.

    Their studies, described in Friday's issue of the journal Science, demonstrated that the grain-based feedlot diet usually fed to cattle before slaughter fosters the growth of E. coli bacteria, some of which can cause disease. Among the dangerous strains that can survive when cattle are fed grain is E. coli O157:H7, the most deadly E. coli known.

    The findings were met with enthusiasm by both the cattle industry and food safety experts.

    Agriculture Secretary Dan Glickman said in a statement, "This deceptively simple finding, if confirmed by further research, has the potential to greatly assist our efforts to fight food-borne illness."

    Grain feeding helps make beef tender and fatty and is considered the most economical way to feed cattle before slaughter. But the researchers found that if cattle were switched to a diet of hay or fresh grass for five days before slaughter, the possibility of infection from meat contaminated with the deadly strain could all but be eliminated.

    Dr. James Russell, an Agriculture Department microbiologist who teaches at Cornell, explained that when cattle ate grain, the acid level of their colon rose and fostered the growth of acid-resistant bacteria, which could include the highly virulent E. coli O157:H7. This organism could remain alive in the animal's fecal matter and accidentally contaminate meat during slaughter.

    Acidity in the animal's colon, Russell said, "turns on E. coli's acid-resistant genes, which allows them to survive in the human stomach." Normally, stomach acid is an effective barrier to infection by food-borne pathogens because the organisms die in an acid environment.

    Most cases of infectious food poisoning require consumption of about 10,000 microorganisms, but as few as 10 E. coli O157:H7 cells can make a person deathly ill, Russell said. The organism is made even more dangerous by its capacity to survive the journey through the acid juices in the human stomach.

    Bacteria on the surface of meat are destroyed by the heat of cooking. But if the organisms get mixed into foods like hamburgers, they can survive the cooking process unless the meat is well done throughout. Other occasional sources of infection by E. coli O157:H7 are vegetables grown in fields fertilized by cattle manure, unpasteurized fruit juices and contaminated water from wells, lakes, water parks and swimming pools.

    Dr. Robert Buchanan, a microbiologist at the Food and Drug Administration, said the proposed change in feeding could be "an extremely practical, cost-effective intervention" but also emphasized the need for further study.

    E. coli are normal inhabitants of the human digestive tract, but only certain strains can cause illness. E. coli O157:H7 produces a toxin that attacks the gastrointestinal tract and can cause severe cramping, abdominal pain, watery or bloody diarrhea, vomiting or fever. It can also cause kidney failure, which is fatal in as many as 30 percent of cases.

    In the Cornell studies, conducted by Dr. Francisco Diez-Gonsalez, Todd Callaway and Menas Kizoulis along with Russell, researchers showed that when cattle were fed grains until slaughter, the acid level rose in the colon enough to permit the growth of about 100 million E. coli cells, about a million of which were acid-resistant and able to withstand the acidity of the human stomach.

    But when the animals were fed hay for just five days, only about 10,000 E. coli cells were present, and virtually none had the potential to become acid resistant, Russell said in an interview. The researchers wrote that "virtually all" the surviving bacteria were killed by a dose of acid comparable to what is found in the human stomach.

    Russell said the colon contents of cattle became acidic on a diet of grain because the animals incompletely digested the starch in grains. This permits some starch to reach the colon where bacteria can ferment it and produce fermentation acids. But on a diet of hay, there is no residual starch to be fermented in the colon. Thus, the acid level remains low and the E. coli remain acid-sensitive. The result is a million-fold fewer acid-resistant E. coli cells in hay-fed cattle, the researchers found.

    Russell pointed out that only a very small percentage of cattle -- "only about 1 to 2 percent" -- harbor the deadly E. coli, but that "when animals go to slaughter, no one knows which ones they are."

    Dr. Gary Weber, executive director for regulatory affairs at the National Cattlemen's Beef Association in Washington called the findings "exciting and very promising," but added that further study was needed "to see how it might work in full-scale farm and ranch feeding systems" and whether other techniques might work on acidity.

    If further research confirms the value of the early findings, the Department of Agriculture could recommend changes in the way the cattle are fed, but it now lacks the authority to mandate such a change, the association said.

    Although it may seem easier to control E. coli infections by improving the hygiene in slaughter houses, Russell said that even in the most hygienic establishments it is nearly impossible to prevent all fecal contamination of meat.

    In their journal report, the researchers noted that "because grain feeding promotes both the production and efficiency of cattle, it is unlikely that American cattle will ever be fed diets consisting only of hay." But, they added, based on their results "cattle could be given hay for a brief period immediately before slaughter to reduce the risk of food-borne E. coli infection."

    Furthermore, this brief diet of hay "should not affect either carcass size or meat quality," said Dr. Donald Beermann, professor of animal science at Cornell. Beermann added that the dietary change would involve "minimal expense and inconvenience to feedlot operators."



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