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Controlling Microbes: Not Too Hot to Handle |
Chapter: 9
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Connections Antibiotics such as penicillin, streptomycin, and tetracycline have revolutionized medicine while providing wonder drugs to agriculture as well. Today, about two-thirds of cattle and virtually all poultry, hogs, and veal calves in the United States are raised on feed that includes antibiotics. According to a recent study, animals consume almost 8 million pounds of drugs annually, nearly 40 percent of all that is produced in the United States. The antibiotics keep the animals healthy in pens and, for reasons not yet clear, appear to speed up growth.
After decades of largely uncritical acceptance, scientists are questioning the use of antibiotics in animal feeds. The trouble stems from the observed increase in the resistance of bacteria to drugs. Researchers believe that drug-resistant mutant strains are emerging and when meat and poultry products transfer mutant strains to humans, they cause disease that defies antibiotic treatment.
Evidence for this belief was largely circumstantial until 1983. Early that year, 18 people in four Midwestern states became infected with Salmonella newport. This Gram-negative rod grows in the human intestine and causes extensive diarrhea. Tetracycline and two forms of penicillin proved useless in treatment, and 11 patients were hospitalized and one died.
Scientists from the federal Centers for Disease Control and Prevention (CDC) began an exhaustive search for the origin of the disease. In the ensuing weeks they traced the Salmonella back to a beef herd in South Dakota and learned that the farmer regularly added a handful or so of tetracycline to a ton of feed to promote growth. The farmer had won local awards for cleanliness, but he offered that several of his calves died of diarrhea some months before and that he sent samples of one calf to a federal laboratory in Iowa. The investigators called the lab and secured samples of the calf tissue. Not only did they isolate Salmonella newport, they also found plasmids in the Salmonella biochemically identical to plasmids from the Salmonella in victims.
Now the search intensified. Researchers determined that some of the cattle were slaughtered in Minnesota, processed in Nebraska, and shipped as hamburger meat to certain Minneapolis–St. Paul supermarkets. Coincidentally, eight of the Salmonella victims shopped for their meat at these stores, and all had eaten ground beef in the days before their illness. Two other patients purchased hamburger meat at Iowa supermarkets also supplied by the processor.
Though Salmonella newport was never found in supermarket beef, the investigators connected enough dots to persuade skeptics that the cattle were the biological factories in which drug-resistant Salmonella newport emerged. To many scientists, the findings provided the smoking gun linking the use of antibiotics in animal feed to human illness. |
