By the 1980s, however, common bacterial pathogens, including certain strains of Staphylococcus aureus, were increasingly found to be resistant to antibiotics. A particularly pernicious strain, methicillin-resistant Staphylococcus aureus, has become a widely recognized cause of serious disease and death. Well-known pathogens like Streptococcus pneumoniae (the causative agent of many pneumonias) and Salmonella are more frequently found to be resistant, and difficult-to-treat infections with these pathogens are becoming more common. Dr. Margaret Chan, director-general of the World Health Organization (WHO), did not mince words when she warned last year, "the world is heading toward a post-antibiotic era, in which many common infections will no longer have a cure and, once again, kill unabated."

Epidemiologists now contend that we have entered the next transition -- one marked by, among other things, the increasing resistance of bacteria to antibiotics.

UNDERSTANDING RESISTANCE

When a physician prescribes an antibiotic to treat an infection, he or she will select the specific antibiotic and dose that are most likely to cure the disease. If the wrong antibiotic is chosen or the dose is too low to treat the disease, some bacteria -- those most resistant to the drug used -- will survive and reproduce, allowing the disease to return or prolonging the resolution of the infection.

Most of the food animals fed antibiotics are not diseased. Nor would the amounts they receive be high enough to treat disease if they were. Rather, food animals are continuously fed small quantities of antibiotics throughout their lives to make them grow faster. Today, the overwhelming majority of antibiotics in this country -- nearly 80 percent -- are sold for use in food animals, not humans.

The development of resistance is Darwinian evolution on a microscopic scale. Bacteria compete with each other for limited resources. Some bacteria are more susceptible to antibiotics than others, and become less able to compete when exposed to an antibiotic. Repeated exposure to an antibiotic will continue to make resistant bacteria better competitors, allowing them to increase their numbers relative to the susceptible organisms -- especially when continuously exposed to the low doses of antibiotics typically used in industrial food animal production. This practice facilitates and promotes the development of antibiotic resistance -- and threatens public health.

Resistant bacteria have been shown to remain in the animal as it is raised and slaughtered, and to persist on the animals' meat following slaughter. Exposure to these bacteria can lead to infections that do not respond to treatment, as evidenced by recent outbreaks of foodborne illness. Hannaford grocery stores recently recalled ground beef after 19 people in seven states who ate the beef were sickened by resistant Salmonella Typhimurium. Late last year, Cargill Meat Solutions recalled more than 36 million pounds of ground turkey after 136 people in 34 states fell ill with resistant Salmonella Heidelberg.