CHICAGO – Consumers are demanding more antibiotic-free meat. At McDonald's, so is a group of nuns.

The world's largest burger chain and its fast-food brethren have made commitments to remove antibiotics from chicken, but plans to curb the use of antibiotics in beef and pork have been far less common. It's a far more complex and expensive proposition, and fast-food chains are largely taking a wait-and-see approach before changing the way their burgers and bacon are made.

KFC, a holdout in the wave of major fast-food restaurants vowing to curb the routine use of antibiotics in chicken, jumped on board earlier this month in response to pressure from animal and environmental groups. A number of major restaurant chains and producers have made promises, of varying degrees, to only use antibiotics on chicken when they're sick, a step back from widespread use of the drugs.

But promises to curb the use of antibiotics in cows and pigs have been slower coming. A renewed push by a group of socially conscious nuns asking McDonald's to announce a plan for antibiotic-free pork and beef highlights the hurdles that the industry will have to jump to meet consumers' growing appetite for "clean" meat.

In a regulatory filing, McDonald's, headquartered in suburban Chicago, revealed that the Benedictine Sisters of Boerne, Texas, intend to submit a proposal at the company's annual meeting in May, asking it to set goals and timelines to phase out the routine use of antibiotics in pork and beef. The sisters have pushed McDonald's for similar promises in recent years. Antibiotics are often given to healthy animals in industrial or so-called factory farms to prevent disease in close quarters.

Antibiotic resistance is a growing problem across the world. More than 70 percent of antibiotics important to human medicine in the U.S. are sold for livestock use, according to the Food and Drug Administration. And the Centers for Disease Control and Prevention and the World Health Organization have said the overuse of antibiotics has led to a global health crisis because the more people are exposed to them, the less effective they are. Every year in the U.S., at least 2 million people become infected and 23,000 people die after becoming infected with drug-resistant bacteria, according to the CDC.

McDonald's recommends shareholders vote against the proposal. The burger chain said it's too early to set timelines for committing to a rollout of antibiotic-free pork and beef because it doesn't purchase the entire cow or pig as it does with chicken, which the company says limits its "ability to directly influence change."

Second, McDonald's said complex sourcing makes it more difficult to trace the animals' exposure. Animals may move from farm to farm during their lives, McDonald's said, and "there is currently a lack of traceability" in some cases between those farms in the U.S. and other countries from which McDonald's sources beef and pork.

McDonald's said it could also see meat shortages if it moves too quickly to antibiotic-free pork and beef. While chickens are slaughtered in just weeks, cattle can be 2 years old when they go to market, leaving far more time in which an animal can get sick. That raises the likelihood that a farmer would need to use antibiotics, even if they weren't using them routinely in the animal's feed as is commonplace in many industrial farms.

"We need to ensure that any change is purposeful, acceptable to customer preferences, and is designed for a continuous supply of products for McDonald's restaurants," the company said in its response to the shareholder proposal.