NEW YORK (AP) — McDonald's says it plans to require chicken suppliers to stop using antibiotics important to human medicine within two years.

The company says its suppliers will still be able to use a type of antibiotic called ionophores that keeps chickens healthy and isn't used in humans. Later this year, McDonald's also said it will no longer serve milk from cows treated with a particular artificial growth hormone.

Many cattle, hog and poultry producers give their livestock antibiotics to make them grow faster and ensure they are healthy. The practice has become a public health issue, with officials saying it can lead to germs becoming resistant to drugs so that the drugs are no longer effective in treating a particular illness in humans.

Chipotle and Panera already say they serve chicken raised without antibiotics, but the announcement by McDonald's is notable because of its size; the company has more than 14,000 U.S. locations. Chipotle has nearly 1,800 locations, while Panera has almost 1,900 locations.

"This really does move the ball quite a bit," said Gail Hansen, a senior officer with the antibiotic resistance project with The Pew Charitable Trusts. Hansen said that ionophores, the antibiotics that will be allowed by McDonald's, are not considered medically important for humans.

Hansen noted the poultry industry had already been moving away from the use of antibiotics used in human medicine, which likely made the decision by McDonald's easier.

McDonald's noted that while suppliers can still use medically important antibiotics to treat sick animals, those animals would not be used in its food supply Marion Gross, senior vice president of McDonald's North America's supply chain, said the change will cost the company more but noted the increase won't necessarily be passed on to customers because several factors are used to determine restaurant prices.

"I think you will hear more from us as it relates to our food," Gross added.

The announcement comes as McDonald's Corp. struggles to transform its image amid intensifying competition from smaller rivals positioning themselves as more wholesome alternatives.

The company has long battled negative perceptions about its food, but that has become a bigger vulnerability as more people shift toward options they feel are made with ingredients that are higher quality or meet standards on social responsibility.

After seeing customer visits to U.S. stores decline two years in a row, McDonald's had recently hinted that changes could be on the way. Franchisees were told of the upcoming chicken and milk announcement Tuesday at a "Turnaround Summit" in Las Vegas.

Scott Taylor, a McDonald's franchisee who was at the conference, said ingredients are "becoming more and more important" to customers and that McDonald's has suggested more changes could be in store.

In a statement, chicken supplier Tyson Inc. said it looks forward to working with McDonald's to meet its new standards. Tyson noted it has reduced the use of antibiotics effective in humans by more than 84 percent since 2011. The National Chicken Council also said in a statement that chicken producers have been working to phase out the use of antibiotics important in human medicine to promote growth in animals.

Caroline Smith DeWaal, food safety director at the Center for Science in the Public Interest, said McDonald's plan to sell milk produced without rBST was also a good step because the artificial growth hormone can cause health problems in dairy cows.

As McDonald's fights to hold onto customers, the company has also made a number of leadership changes. The pressures reached the top of the company in late January, when McDonald's said CEO Don Thompson would be replaced by Steve Easterbrook, its chief brand officer.

The CEO change officially took effect this week, and Easterbrook was at the franchisee summit in Las Vegas.

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