Antibiotic resistance is a natural phenomenon which has been around for millions of years. Indeed, bacteria and fungi produce antibiotics naturally, to protect themselves or to fight each other. The first antibiotics used in medicine, including penicillin, were extracted from fungi. However, the use of large quantities of such substances in human and animal health care since the mid-1940s has greatly amplified a natural process: each time that an antibiotic is used, resistant bacteria multiply and then prosper because they are the only ones able to do so under those particular circumstances.
When bacteria multiply – most are able to do this several times per hour – they must replicate all their genes (DNA). As these copies are made, the bacteria are also subject to errors (mutations), some of which might help them survive the onslaught of an antibiotic in their surroundings. They can also acquire this ability by receiving small pieces of DNA (plasmids) from other bacteria – which may or not be of the same species.