McDonald's US 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 4 March 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.

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.

Superbugs are linked to an estimated 23,000 human deaths and two million illnesses every year in the United States, and up to $20bn (£13.1bn) in direct healthcare costs, according to the US Centers for Disease Control and Prevention.

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.

"By starting down the path with chicken, getting rid of these antibiotics and hormones, that's a first step forward and then eventually they can work towards improving the quality the food and the ingredients through their supply chain for other products on the menu. So, it's a very important first step for them and certainly consumers will respond very positively toward this change," said Darren Tristano, executive vice president at the food consulting firm Technomic.

There are exceptions to McDonald's new policy. The company will buy chickens from farmers who "responsibly use" ionophores, an animal antibiotic not used in human medical treatment.

The phase-out applies only to McDonald's roughly 14,000 US restaurants. It currently does not affect the company's approximately 22,000 international restaurants.

"Clearly, it's going to take some time for them to get all of these antibiotics out of the system and for suppliers to catch up with the great demand. But, it's going to really change the way producers, chicken producers, look at how they're treating these animals and what the demand looks like," added Tristano.

"Overall, I think this change is going to be good, not only for McDonald's and its customers but for the rest of the industry because McDonald's leads and generally sets an example."

The action by McDonald's, which has been fighting to win back diners and bolster sagging US sales, is in step with consumer demand for food made with "clean" and more "natural" ingredients. But it falls short of similar policies at smaller chains such as Chipotle Mexican Grill and Panera Bread, which ban the use of all antibiotics.

A Reuters investigation last year revealed that some of the nation's largest poultry producers routinely fed chickens an array of antibiotics, not just when sickness strikes, but as a standard practice over most of the birds' lives.