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Grappling With E. coli

Only 8 years ago, a killer foodborne bacterial disease was relatively unknown in the United States.

In fact, when USDA's Economic Research Service published a report on the economic impact of major U.S. foodbome diseases in 1987, Escherichia coli O157:H7 was not even included.

Then, in January 1993, four children died and dozens of children and adults became ill from eating undercooked hamburgers contaminated with E. coli O157:H7 bacteria at a fast-food restaurant in Seattle, Washington. It wasn't the first time food scientists recognized O157:H7 infections, but the incident focused research attention on the problem.

In March of 1993, a team of ARS scientists at the National Animal Disease Center (NADC) in Ames, Iowa, began studying the relationship between E. coli O157:H7 and other microbes that normally live in a cow's first stomach, or rumen. Their findings will provide the basis for future studies on the impact of pre-slaughter livestock feeding schedules on the presence of pathogenic microbes such as E. coli O157:H7.

The members of this ARS team—microbiologist Mark A. Rasmussen, veterinary medical officer Brad T. Bosworth, and microbiologists William C. Cray, Jr., and Thomas Casey—have translated their research findings into recommendations for livestock producers and marketers.

"We used data collected by Australian scientists who have studied the effect of rumen fermentation on Salmonella bacteria and applied this information to the O157:H7 problem," says Rasmussen. "Poorly managed cattle that are subjected to dietary stress during transport and marketing represent a high-risk group and may carry an unusually high number of O157:H7."

This stress occurs when cattle refuse feed or the feed is offered only intermittently.

"In animals deprived of feed, the normal rumen microorganisms aren't as active as they would be under everyday conditions of digestion," says Rasmussen. "The normal microbial activity is what keeps the bad bugs at bay."

He presented these findings and recommendations to the National Livestock and Meat Board's blue-ribbon task force on E. coli O157:H7 in Atlanta in November 1993.

Rasmussen recommends that producers and marketers minimize pre-slaughter fasting stress by feeding animals regularly. This practice could do a lot to maintain the normal balance of rumen microbes and suppress bacteria like E. coli O157:H7.

In other NADC studies, ARS microbiologist William C. Cray, Jr., has developed an experimental model to study E. coli O157:H7 in healthy cattle. In the last 2 years, he has established the pattern and duration of fecal shedding of O157:H7 from cattle.

Cray's research has shown that O157:H7 doesn't make cattle sick and that calves can shed more bacteria for longer periods of time than mature cows.

"The potential for calves to serve as reservoirs of E. coli O157:H7 needs to be studied more thoroughly," says Cray.

Meanwhile, to quickly identify O157:H7, Bosworth and Casey are using molecular biology to develop a PCR (polymerase chain reaction) method. PCR has already been proved a useful tool in identifying several viruses, viroids, and mycoplasma-like organisms directly from nucleic acid extracts. The method could be useful in checking several samples simultaneously and in differentiating O157:H7 from other E. coli strains. -- By Linda Cooke, ARS

Thomas Casey and Mark A. Rasmussen are in the ARS-USDA Pre-Harvest Food Safety and Enteric Diseases Research Unit , National Animal Disease Center, Ames, Iowa 50010.


"Grappling With E. coli" was published in the July 1995 issue of Agricultural Research magazine.


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