NEW YORK - Faced with mounting evidence that the routine use of antibiotics in livestock may diminish the drugs' power to cure infections in people, the U.S. Food and Drug Administration has begun a major revision of its guidelines for approving new antibiotics for animals and for monitoring the effects of old ones.

The goal of the revision is to minimize the emergence of bacterial strains that are resistant to antibiotics. Such resistance makes them difficult or even impossible to kill.

Drug-resistant infections, some fatal, have been increasing in people in the United States, and many scientists attribute the problem to the misuse of antibiotics in both humans and animals. Of particular concern to scientists is that recent studies have found bacteria in chickens that are resistant to fluoroquinolones, the most recently approved class of antibiotics and one that scientists had been hoping would remain effective a long time.

A crucial component of the new guidelines will be the requirement that manufacturers test certain new livestock drugs for a tendency to foster the growth of resistant bacteria that could prove harmful to people. Testing will be required both before a drug is approved and after.

The use of antibiotics as growth stimulators in livestock is a concern of many consumer groups in America. The fear that resistance to disease and bacteria can be reduced in the human population has lead the European Union to ban the use of human antibiotics in livestock production.

Many scientists believe that giving low doses of antibiotics to animals over long periods brings out resistance. Scientists say that resistant bacteria from animals can make people sick in several ways. A person can become ill from contact with an animal carrying a disease-causing resistant germ, or from handling contaminated meat or eating it when it has not been cooked enough to kill the bacteria. The infection may be difficult or even impossible to treat.

In some cases, the resistant bacteria may themselves be harmless, but live on in the gut and cause trouble later by passing their genes for antibiotic resistance to other bacteria, ones that do cause disease. Or, if a person's immune system is weakened by illness or chemotherapy, the otherwise harmless bacteria can turn dangerous.

At the Centers for Disease Control and Prevention, researchers said they had been detecting increases in the levels of drug-resistant bacteria found in people with gastrointestinal illness from the microbes salmonella and campylobacter, which are most commonly contracted from contaminated meat or eggs (International Herald Tribune).