McDonald’s restaurants across the United States plan to change the ingredients of one of their most popular items. The fast-food company said late Tuesday that its Chicken McNuggets will no longer contain human antibiotics and artificial growth hormones within the next two years, and its milk will be free of those ingredients beginning later this year. Suppliers, however, would still be able to use chicken antibiotics called ionophores to treat sick poultry.

"This really does move the ball quite a bit," said Gail Hansen, a senior officer with the antibiotic resistance project of the Pew Charitable Trusts, according to the Associated Press. Hansen added that ionophores do not medically affect humans and were not even considered antibiotics in Europe.

Human antibiotics previously used to boost chicken growth for Chicken McNuggets will be phased out as officials have said their use could lead to germs becoming resistant to antibiotics, leading to the possibility of antibiotics becoming ineffective when used as treatment for humans. The use of bovine growth hormone rBST will also be stopped for cows providing milk to the chain.

"We're listening to our customers," Marion Gross, senior vice president of McDonald’s North America supply chain, told business news site GreenBiz. "At the end of the day, our customers are telling us that things have changed for them, and their expectations have changed in terms of how their food is sourced."

The announcement follows McDonald’s worst sales slump in more than a decade and the departure of its chief executive, Don Thompson, who stepped down in January. McDonald’s has been trying to revive its image as it battles negative impressions over how its food is made. The company had launched an image-rebranding campaign last year, including addressing questions about whether its nuggets were being made by “pink slime” (no, they were not) and whether its hamburger buns contain azodicarbonamide, a chemical used to make yoga mats (yes, they do, but it is a U.S. Food and Drug Administration-approved dough conditioner).

Franchises were informed of the chicken and milk changes in a “Turnaround Summit” in Las Vegas on Tuesday. The fast-food chain has more than 14,000 U.S. locations. Scott Taylor, a McDonald's franchisee who was at the conference, said ingredients were "becoming more and more important" to customers, according to AP. He said the company was suggesting it needs to "be where our consumers want and need us to be."