WASHINGTON (AP) -- The Food and Drug Administration is asking drug companies to help limit the use of antibiotics given to farm animals, a practice which scientists say has contributed to a surge in drug-resistant bacteria.

Antibiotics are mixed with animal feed to help animals put on weight and stay healthy in crowded barns. Scientists have warned that this routine use leads to the growth of antibiotic-resistant germs that can be passed to humans.

Under the guidelines, the agency recommends antibiotics be used only when they are necessary to keep animals healthy. The agency also wants to require a veterinarian to prescribe the drugs. Currently the drugs can be purchased over-the-counter by farmers.

The FDA is asking drugmakers to voluntarily change the labeling on their drugs to limit how they can be used.