This sad history exemplifies the co

This sad history exemplifies the coevolutionary treadmill of drug resistance. A drug is released; its success leads to widespread use; bacteria evolve resistance to it, rendering it useless; another drug is introduced; and the cycle repeats. It happens quickly and for good reasons. Bacterial populations are huge; they have access through horizontal gene transfer to pre-evolved libraries of genetic information on resistance; they generate vast numbers of genetic variants through mutation; and their generation times are short, measured in hours or days. By creating huge differences in the reproductive success of resistant and nonresistant clones, antibiotic use generates very strong selection. Resistance is the trait that varies, that variation has a genetic basis, and selection is strong. The process is rapid and efficient. Within a few years, resistance has evolved against every new antibiotic that has been introduced. We are starting to move into a postantibiotic era in which treatment of bacterial infections is more difficult and surgery is riskier because postsurgical intections are becoming more difficult to treat than previously.