Antibiotic resistance is the ability of a microorganism to withstand the effects of an antibiotic. Antibiotic resistance evolves naturally via natural selection through random mutation, but it could also be engineered by applying an evolutionary stress on a population. Once such a gene is generated, bacteria can then transfer the genetic information in a horizontal fashion (between individuals) by plasmid exchange (plasmid: A circular, double-stranded unit of DNA that replicates within a cell independently of the chromosomal DNA. Plasmids are most often found in bacteria and are used in recombinant DNA research to transfer genes between cells.) If a bacterium carries several resistance genes, it is called multiresistant or, informally, a superbug. The antibiotic action is an environmental pressure; those bacteria which have a mutation allowing them to survive will live on to reproduce. They will then pass this trait to their offspring, which will be a fully resistant generation.
Bacteria, like all living organisms, change over time in response to environmental challenges. Because of the widespread use and misuse of antibiotics, bacteria are constantly exposed to these drugs. Although many bacteria die when exposed to antibiotics, some develop resistance to the drugs' effects. For example, 50 years ago, Staphylococcus aureus (a common cause of skin infections) was very sensitive to penicillin. But over time, strains of this bacteria developed an enzyme able to break down penicillin, making the drug ineffective. Researchers responded by developing a form of penicillin that the enzyme could not break down, but after a few years, the bacteria adapted and became resistant to this modified penicillin. Other bacteria have also developed resistance to antibiotics.
Medical research continues to develop drugs to combat bacteria. But patients and doctors can help prevent the development of resistance in bacteria. Taking antibiotics only when necessary can help. That is, people should take antibiotics only for infections caused by bacteria, not for those caused by viruses such as a cold or the flu. Also, taking antibiotics for the complete time prescribed helps limit the development of resistance.