For as many decades as antibiotic resistance has thwarted the cure of bacterial infections, scientists have pondered the origins of resistance genes and how they became such a problem. Fingers were pointed squarely at the overprescription of antibiotics in human medicine—and rightly so, as early on, these drugs were liberally utilized for every imaginable malady without concern for the possible consequences. Not long after their discovery, however, these miracle drugs were applied not only to sick humans and animals, but to healthy ones as well.

Nowhere is this practice more prevalent and controversial than in animal husbandry, where animal feeds laced with small amounts of antibiotic are provided over extended periods of rearing. Labeled as “growth promotion” and employed primarily in large, concentrated feedlots for poultry, swine, and cattle, this nontherapeutic application appeared to fatten the animals faster, prevent rampant herd disease, and help bring healthy animals to market more quickly.