Federal drug regulators announced Wednesday that farmers and ranchers must restrict their use of a critical class of antibiotics in cattle, pigs, chickens and turkeys because such practices may have contributed to the growing threat of bacterial infections in people that are resistant to treatment.

The medicines belong to a class of antibiotics known as cephalosporins and include such brands as Cefzil and Keflex. They are among the most common antibiotics prescribed to treat strep throat, bronchitis, skin infections and urinary tract infections. Surgeons also often use them before surgery to prevent bacterial infections.