Antibiotics in agriculture: When is it time to close the barn door?

Everybody knows that bacterial resistance to antibiotics is a bad thing, at least for humans and animals, if not for bacteria. Drugs that were effective for treating community- and hospital-acquired infections are no longer so because the target bacteria are resistant to their action. To be sure, it may be some time before we really enter the predicted “postantibiotic era” in which common infections are frequently untreatable. Even now, however, the consequences of resistance in some bacteria can be measured as increases in the term and magnitude of morbidity, higher rates of mortality, and greater costs of hospitalization for patients infected with resistant bacteria relative to those infected with sensitive strains (1). Dozens of new antimicrobial compounds have been licensed in the U.S. during the last half century, but almost all “new antibiotics” introduced in the last 40 years have been relatively minor chemical variants of compounds to which bacteria have already developed resistance. As a result, bacteria have rapidly adapted existing resistance mechanisms to evade the new compounds. Indeed, only a single chemically novel class of antibacterial agents, the oxazolidinones, has been introduced into clinical use since the 1970s.