After repeated exposure to antibiotics, bacteria can mutate to resist drug treatment. Officials are concerned that widespread use of antibiotics on the farm may result in people acquiring tough-to-treat infections from their food.

If the FDA determines that using a drug in livestock was likely to risk human health, the agency could deny approval or impose limits on giving the drug to animals, officials said.

In the new guidelines, the FDA suggested that animal-drug makers evaluate the probability that a particular drug would prompt emergence of resistant bacteria in animals, the likelihood that people would ingest the resistant bacteria, and the chances that exposure would harm human health.

"Resistance to the antimicrobial drugs needed to treat human illnesses is a serious public health threat, and we intend to use the best science-based methods to prevent it," FDA Commissioner Mark McClellan told reporters.

Livestock are fed with antibiotics, in many cases the same ones used to treat people, to fight infections and to promote faster growth.

The Animal Health Institute, which represents Bayer, Pfizer Inc and other animal-drug makers, welcomed the FDA plan, saying it was better than Europe's decision to phase-out antibiotics for growth promotion.

The FDA's approach will allow the agency to "protect the public health from the potential transfer of antibiotic resistant bacteria from animals to humans. At the same time it will preserve the important benefits of using antibiotics to protect animal health," said Alexander Mathews, the group's president.

The guidelines apply to new applications for antibiotics intended for use in food-producing animals.

The agency also is reviewing the public health impact from on-farm use of currently marketed antibiotics, officials said.

Consumer groups praised the plan for new antibiotics but voiced concern that the FDA would not move quickly to protect the public from risks posed by drugs already widely used in agriculture.