Six of the largest U.S. school districts are switching to antibiotic-free chicken, officials said on Tuesday, pressuring the world's top meat companies to adjust production practices in the latest push against drugs used on farms. The move by districts in New York City, Los Angeles, Chicago, Dallas, Miami-Dade County and Orlando County is intended to protect children's health amid concerns about the rise of so-called superbugs," bacteria that gain resistance to conventional medicines. The change may raise costs for schools. The six districts ... hope to limit costs by combining their purchasing power. Under the new standards, all chicken products served in the districts must come from birds that were never fed antibiotics. School officials are demanding the change after meeting with industry experts and "really understanding how this affects the human body overall and our future with antibiotic resistance," said Leslie Fowler, executive director of nutrition support services for the Chicago Public Schools. The switch is expected to take several years. Companies like Tyson Foods Inc and Pilgrim's Pride Corp have said they will not be able to change production systems quickly. A Reuters investigation in September found that major U.S. poultry firms were administering antibiotics to their flocks far more pervasively than regulators realized.