About half of the raw chicken breasts in a nationwide sampling, carried antibiotic-resistant "superbug" bacteria, a U.S. consumer group has said, calling for stricter limits on the use of the medicine on livestock. It could be more difficult to treat people if they became ill after eating chicken with the antibiotic-resistant bacteria, said Consumer Reports, which describes itself as the world's largest independent product-testing organization. The group said it tested for six types of bacteria in 316 raw chicken breasts purchased from retailers nationwide during July. Almost all of the samples contained potentially harmful bacteria, it said. Some 49.7 percent carried a bacterium resistant to three or more antibiotics, according to the group, and 11 percent had two types of bacteria resistant to multiple drugs. Resistance was most common for the antibiotics used for growth promotion and disease treatment of poultry. Consumer Reports urged passage of a law to restrict eight classes of antibiotics for use only to treat humans and sick animals. The law would be more effective, it said, than the Food and Drug Administration's plan, announced last week, to phase down the non-medical use of antibiotics in livestock over three years.