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Ecoli cows need fiber { June 6 2001 }

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DISCOVER Vol. 22 No. 6 (June 2001)
Table of Contents

High-Fiber Steak
Like humans, cows need a fiber-rich diet -- or they'll make us sick as well.
By Tim Stoddard

Cattle diets are under intense scrutiny. Grains destined for cows' dinners are screened for animal products that could transmit mad cow disease. But James Russell, a microbiologist with the U.S. Department of Agriculture, says that the grain itself may be a health problem. Fattening cattle on corn products yields nicely marbled beef, but it also causes digestive disorders that make cattle sick, and that promotes the growth of pathogenic bacteria that ultimately infect humans, Russell says.

For 3 to 5 months before they are slaughtered, cattle are fed diets of 50- to 90-percent grains that are low in fiber, but high in starches. The grain diet accelerates weight gain, but it also radically alters the ecology of microorganisms within the animals' digestive tracts, which are evolutionarily set up for meals of mostly hay and grass. Cattle can't make fiber-digesting enzymes on their own, but have evolved symbiotically with bacteria, fungi, and protozoans that inhabit their rumens, or the first of four stomachs in their digestive tract. In return for a warm, wet habitat, and a constant supply of food, the microbes convert fibers into proteins, vitamins, and fatty acids that their hosts then absorb. But a few species of the rumen bacteria can also ferment starches. So when a cow first gobbles up grain, this bacterial minority grows explosively, dominating the rumen ecosystem. In their starch-fermenting frenzy, the bacteria also produce lactic acid, which lowers the pH of the otherwise neutral rumen. Undigested starch that passes to the animal's gut sets off the same process there.

Russell and Jennifer Rychlik, a graduate student at Cornell University, have found that this lowered pH in the gut promotes the growth of acid-resistant Escherichia coli, that can then pose a health threat to consumers. In the suddenly acidic environment, E. coli survives by playing a trump card: It switches on an acid-resistance gene. Once it is turned on, this gene stays on, and the E. coli can then survive a trip through the acidic human stomach and colonize the human gut, Russell says. Most strains of E. coli are harmless, but some pathogenic strains can cause excessive intestinal bleeding. When cattle are slaughtered, it's virtually impossible to keep the carcasses from being contaminated with intestinal material. If the meat is properly cooked, the pathogenic E. coli will be destroyed. But about 62,000 Americans suffer food-borne E. coli illnesses every year as a result of improper handling of uncooked meat.

As if that weren't enough of a problem, there are potential long-term effects of the cows' diet. Unlike the human stomach, the inner lining of the cow's rumen doesn't feature a protective layer of mucus, so the lactic acid irritates the tissue, forming ulcers. Acid-loving bacteria infect the ulcers, migrate into the blood and lodge in the liver. With antibiotics, the health effects are relatively minor: 13 percent of the livers have to be thrown out, but less than 0.3 percent of the 25 million beef cattle in feedlots actually die from grain-related problems every year. The antibiotics for treating liver-abscesses are the same ones that veterinarians use to treat other animals. The fear, says Russell, is that the cattle are promoting resistance to antibiotics that may render most drugs useless.

It's unlikely that the beef industry will ever abandon grain feeding, because the energy-dense diet speeds up production and increases profit. But Russell and Rychlik have proposed some moderate changes that might reduce the prevalence of acid-resistant E. coli. Previously, Russell found that flushing the cow's digestive tract with a high-fiber diet just before slaughter returns the pH of the gut to normal, so the offspring of acid-resistant bacteria don't switch on their acid resistance gene. With the fast life cycle of bacteria, the level of acid-resistant organisms quickly drops. "Then you can kill them with a pH 2 shock that mimics the human gastric stomach," says Russell. While another study has since confirmed this, the beef industry has little economic incentive to experiment with hay-purging. "If the cattle is finished on grass, it does not give the intramuscular fat that's desired for taste and associated with tenderness," said Fred Owens, senior ruminant researcher with Dupont Specialty Grains in Desmoines, Iowa. Owens also noted that pathogenic E. coli will continue to infect many Americans regardless of whether or not their acid-resistance is activated. "There are many older individuals that take antacids, and some people lack normal acid production in the stomach," he said. "And in that case it doesn't matter whether the organism is acid-resistant or not."

Posted 05/16/01

"Factors that alter rumen microbial ecology." James B. Russell and Jennifer Rychlik Science 292:1119-22 See

"Grain Feeding and the Dissemination of Acid-Resistant Escherichia coli from Cattle." Francisco Diez-Gonzalez, Todd R. Callaway, Menas G. Kizouliz, James B. Russell. Science 281: 1666-68 See

Copyright 2001 The Walt Disney Company. Back to Homepage.

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