In 1967, the National Academy of Sciences reported that the practice of adding antibiotics to animal food, while producing greater yields, may leave traces of antibiotics in meat, thus increasing drug resistance among bacteria. That study prospectively followed bacteria in farm animals and farm workers after the introduction of tetracycline-supplemented feed. Within 2 weeks, the bacteria found in the guts of animals fed the tetracycline supplement were nearly all tetracycline-resistant.