CHICAGO- McDonald's Corp's U.S. restaurants will gradually stop buying chicken raised with antibiotics vital to fighting human infections, the most aggressive step by a major food company to change chicken producers' practices in the fight against dangerous 'superbugs.'

The world's biggest restaurant chain announced on Wednesday that within two years, McDonald's USA will only buy chickens raised without antibiotics that are important to human medicine. The concern is that the overuse of antibiotics for poultry may diminish their effectiveness in fighting disease in humans. McDonald's policy will begin at the hatchery, where chicks are sometimes injected with antibiotics while still in the shell.

"We're listening to our customers," Marion Gross, senior vice president of McDonald's North American supply chain, told Reuters. She said the company is working with its domestic chicken suppliers, including Tyson Foods Inc, to make the transition.

Veterinary use of antibiotics is legal. However, as the rate of human infections from antibiotic-resistant bacteria increases, consumer advocates and public health experts have become more critical of the practice of routinely feeding antibiotics to chickens, cattle and pigs.

Scientists and public health experts say whenever an antibiotic is administered, it kills weaker bacteria and can enable the strongest to survive and multiply. Frequent use of low-dose antibiotics, a practice used by some meat producers, can intensify that effect. The risk, they say, is that so-called superbugs might develop cross-resistance to critical, medically important antibiotics.

Superbugs are linked to an estimated 23,000 human deaths and 2 million illnesses every year in the United States, and up to $20 billion in direct healthcare costs, according to the U.S. Centers for Disease Control and Prevention.

Poultry producers began using antibiotics in the 1940s, not long after scientists discovered that penicillin, streptomycin and chlortetracycline helped control outbreaks of disease in chickens. The drugs offered an added benefit: They kept the birds' digestive tracts healthy, and chickens were able to gain more weight without eating more food.

Gross said McDonald's expects its suppliers will treat any animals that become ill, using antibiotics when prescribed. McDonald's, however, will not buy those treated chickens, she said. The poultry industry's lobby takes issue with the concerns of government and academic scientists, saying there is little evidence that bacteria which do become resistant also infect people.