Animals in crowded feedlots are constantly exposed to bacterial infections, so producers put low doses of antibiotics into their feed. The measure may boost profits, but it also favours the survival of bacteria with genes that help them resist the antibiotics. Antibiotic resistance genes (ARGs) can migrate between bacteria, creating superbugs that are hard or impossible to treat and that can infect people.

The European Union banned antibiotic growth promoters for this reason in 2006. But they are still permitted in the US, where recently released figures show around half of all samples of the food-poisoning bacterium salmonella, taken from retail poultry meat, are resistant to at least three antibiotics.