Antibiotics are substances that kill bacteria. Their development and widespread use has greatly reduced the threat of infectious disease and saved countless lives. But many pathogenic bacteria have developed resistance to antibiotics, particularly in environments where antibiotics are routinely used, such as hospitals, livestock operations, and fish farms. In these environments where antibiotics are continually present, the only bacteria that survive are those that possess antibiotic resistance. Released from competition with other bacteria, resistant bacteria multiply quickly and spread. In this way, the presence of antibiotics selects for resistant bacteria, reducing the effectiveness of antibiotic treatment for infections.

Antibiotic resistance in bacteria frequently results from the action of genes located on R plasmids, small circular plasmids that can be transferred by conjugation. Some drug-resistant R plasmids convey resistance to several antibiotics simultaneously. Ironic but plausible sources of some of the resistance genes found in R plasmids are the microbes that produce antibiotics in the first place. R plasmids can spread easily throughout the environment, passing between related and unrelated bacteria in a variety of situations.