Chain reaction

日期:2017-12-01 07:28:33 作者:闾丘钟 阅读:

By Jonathan Knight FEEDING cattle on hay immediately before slaughter could make beef safer, claim microbiologists in New York state, as potentially harmful gut bacteria from hay-fed cattle do not thrive in the human stomach. But farmers may resist moves to stop feeding grain to cattle, fearing this could reduce their profit margins. The acid in our stomachs destroys many of the harmful bacteria in food. In this hostile environment, most pathogenic bacteria are reduced to harmless levels unless more than 100 000 cells are consumed at once. But Escherichia coli O157 is much more tolerant of acid. As few as 10 cells of this strain can be enough to start a lethal infection. James Russell and his colleagues at Cornell University in Ithaca, New York, say the practice of raising cattle on grain is partly to blame for the resilience of E. coli. Cattle are deficient in enzymes that break down starch, so most of the maize, wheat and barley they eat ferments, producing acidic by-products. “They did not evolve as grain-eating beasts,” says Russell. The acids lower the pH in the cows’ colons, where E. coli bacteria reside. The researchers compared the manure of cattle fed hay with those on a diet of 90 per cent grain. The grain diet lowered the pH of the manure from a neutral 7.3 to a mildly acidic 5.3. They also found that the grain-fed animals had 1000 times more E. coli in their manure, and that nearly 10 per cent of these bacteria could survive in a culture broth of pH 2—the equivalent of human stomach acid. Only 0.01 per cent of the E. coli from hay-fed animals survived this high acidity. The researchers say in the latest issue of Science (vol 281, p 1667) that a high-grain diet produced a million times as many acid-resistant E. coli as a diet of hay. None of the cattle in Russell’s study was infected with the O157 strain. But in laboratory cultures, Russell found that E. coli O157 also greatly increased its survival at pH 2 after first being exposed to a mild acid. He concludes that giving cattle hay for several days before slaughter could vastly reduce the risk of contaminating meat. “This opens up a whole new research area for us,” says Terry Klopfenstein, a meat industry specialist at the University of Nebraska in Lincoln. But he warns that the cattle industry may resist the idea of using hay, as animals fed on hay rather than grain for even a few days would gain less weight and fetch a lower price. Also, since cattle find hay harder to digest than grain, there would be a large increase in intestinal waste—which is known as paunch—at slaughter. “That’s a big disposal problem for the packing plant,” he says. The British beef industry may be more receptive, as it badly needs to restore public confidence in its product after successive health scares, says Hugh Pennington, a microbiologist at the University of Aberdeen. “They’d have to study the economic implications,” he says,