McDonald's is no longer serving chicken raised on antibiotics that are important to human medicine. The company made the pledge last year, and now reports that it has completed its transition to the new antibiotic policy ahead of schedule.

As we've reported, many scientists are concerned that the more an antibiotic is given to food animals, the more quickly bacteria could adapt and become resistant to it. And the FDA has urged farmers and ranchers who raise livestock to reduce antibiotic use.

Chicken served at McDonald's won't necessarily be raised completely without antibiotics. The company's suppliers are still allowed to used ionophores, a type of medicine that is not used to treat people.

In a press release, Lena Brook of the Natural Resources Defense Council writes: "The fact remains that 70 percent of medically important antibiotics (classes of drugs also used in human medicine) in the U.S. are sold for use on animals, not people."

While Brook applauds McDonald's for implementing its new policy, she calls on the company to expand its efforts to include "meaningful antibiotics policies that also apply to pork and beef."

McDonald's isn't the first to make this shift. Panera and Chipotle began moving toward antibiotic-free meat more than a decade ago. And Chick-fil-A made a commitment back in 2014 to work with suppliers to remove all antibiotics (including ionophores) from its chicken supply.

NRDC and other groups have been calling on the chicken chain KFC to adopt a new policy on antibiotics, but Brooks says they "have yet to see any meaningful movement from this iconic fried chicken chain." Earlier this year, Taco Bell, which is owned by Yum! Brands, Inc, the same parent company that owns KFC, issued a statement committing to stop using antibiotics important to human medicine in its chicken by 2017.

McDonald's announced additional changes including hamburger buns made without high fructose corn syrup and the removal of artificial preservatives from its Chicken McNuggets and from its breakfast items, including pork sausage patties and biscuit breakfast sandwiches.

Analysts have pointed to investments in food quality and ingredients as one factor in the company's improved financial performance over the last year.